Technical Basis Document No. 8: 
Colloids 
Revision 2 

September 2003

1. INTRODUCTION 
This technical basis document provides a summary of the conceptual understanding relevant to 
the projected behavior of radionuclides potentially attached to colloids as they are transported 
through the engineered and natural features of the Yucca Mountain repository system. This is 
one in a series of technical basis documents that are being prepared for the different components 
relevant to predicting the likely postclosure performance of the Yucca Mountain repository. The 
relationship of colloid formation, stability, and transport to the other components is illustrated in 
Figure 1-1. 
Figure 1-1. Components of the Postclosure Technical Basis for the License Application 
Colloids and colloidal transport processes are treated as an integrated component of the 
postclosure technical basis because aspects of their behavior could impact the performance of the 
various engineered and natural features of the repository system. Therefore, although colloids 
are not a feature of the repository system in the same way as the waste package, unsaturated zone 
and saturated zone are, treating colloids as an integrated component assures proper consistency 
of their treatment across these features. 
The information presented in this technical basis document, along with the associated references, 
provides a summary-level synthesis of the relevant aspects of colloid-facilitated radionuclide 
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transport and forms an outline of the ongoing development of the postclosure safety analysis that 
will be included in the license application (LA). This information is also used to respond to open 
Key Technical Issue (KTI) agreements made between the U.S. Nuclear Regulatory Commission 
(NRC) and the U.S. Department of Energy (DOE). Placing the DOE responses to individual KTI 
agreement and additional information needed requests within the context of the overall colloid 
transport processes, allows for a more direct discussion of the relevance of the agreement. 
This technical basis document and appendices are responsive to agreements made between the 
DOE and the NRC during Technical Exchange and Management Meetings on Evolution of the 
Near-Field Environment (Reamer 2001a), Radionuclide Transport (Reamer and Williams 2000), 
and Total System Performance Assessment and Integration (Reamer 2001b). The appendices to 
this document are designed to allow for a transparent and direct response to each KTI agreement. 
This technical basis document presents a summary and synthesis of the detailed technical 
information presented in the analyses and model reports and other technical products that are 
used as the basis for the description of the waste form, invert, unsaturated zone, and saturated 
zone features and the incorporation of those features into the postclosure performance 
assessment. Several analyses, model reports, and other technical products support this summary: 
 Site-Scale Saturated Zone Transport (BSC 2003a) 
 Saturated Zone Colloid Transport (BSC 2003b) 
 Saturated Zone Flow and Transport Model Abstraction (BSC 2003c) 
 Saturated Zone In-Situ Testing (BSC 2003d) 
 Site-Scale Saturated Zone Flow Model (BSC 2003e) 
 EBS Radionuclide Transport Abstraction (BSC 2001a) 
 Radionuclide Transport Models Under Ambient Conditions (BSC 2001b) 
 Unsaturated Zone Flow Models and Submodels (BSC 2001c) 
 Waste Form and In-Drift Colloids Associated Radionuclide Concentrations: 
Abstraction and Summary (BSC 2003f) 
 Dissolved Concentration Limits of Radioactive Elements (BSC 2003g) 
 In-Package Chemistry Abstraction (BSC 2003h) 
 Particle Tracking Model and Abstraction of Transport Processes (BSC 2003i) 
 In Situ Field Testing of Processes (BSC 2001d) 
 Advection Versus Diffusion in the Invert (BSC 2003j) 
 Engineered Barrier System: Physical and Chemical Environment Model (BSC 2003k). 
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1.1 SUMMARY OF CONCEPTUAL UNDERSTANDING 
Colloids are very fine particles ranging in size from 1 nm to 10 m in characteristic dimension. 
Colloid transport may affect the rate of migration of contaminants in the subsurface environment 
relative to the transport of the contaminants as dissolved species. Potentially faster movement of 
colloids is the result of complex physical, chemical, electrical, and hydrodynamic phenomena 
acting on the colloids. These are phenomena that typically have not been associated with the 
transport of dissolved species. The potentially faster transport characteristic of colloids is 
generally referred to as the hydrodynamic chromatography effect (Haber and Brenner 1993). 
Radioactive colloids (i.e., colloids that carry radionuclides) have been classified as either Type I 
colloids (either true or intrinsic) or Type II colloids (pseudocolloids). This classification was 
adopted to distinguish colloids resulting from nucleation or precipitation of dissolved 
radionuclides under chemically saturated conditions, which generate radionuclides in particulate 
form (true colloids), from colloids carrying radionuclides due to sorption of dissolved 
radionuclides onto natural or other colloids suspended in groundwater (pseudocolloids). This 
classification is used in this technical basis document. 
There are several potential sources of colloids in groundwater, such as clay minerals, metal 
oxides, viruses, bacteria, and humic macromolecules (Chrysikopoulos and James 2003). It has 
been suggested (Haber and Brenner 1993) that dissolved groundwater contaminants could have a 
large affinity to sorption onto the surface of suspended colloids. As a result, colloids can 
possibly act as fast carriers for contaminants in subsurface environments and can be significant 
contributors to the rate of migration of contaminants, such as radionuclides potentially released 
from a repository system. 
In order for colloids to provide a significant means for the transport of radionuclides from the 
waste form to the biosphere and, consequently, be a major contributor to dose, colloidal 
suspensions must: (1) contain high enough concentrations of colloids to carry a significant 
amount of radionuclides, either embedded in the colloids or attached on the surface of the 
colloids, relative to the concentration of dissolved radionuclides; (2) be stable for distances in the 
order of tens of kilometers and for time periods in the tens of thousands of years; and (3) not be 
appreciably filtered, reversibly or irreversibly, by the host rock. If one or more of these 
conditions are not met, colloids will not be significant to system performance of a high-level 
radioactive waste repository. 
The main conclusion of this technical basis document is that colloid-facilitated radionuclide 
transport is not an important contributor to predicted system performance compared to transport 
of dissolved radionuclides. This conclusion is consistent with the opinions expressed by the 
International Review Team, which conducted a peer review of the total system performance 
assessment (TSPA) for the site recommendation. In its report, the International Review Team 
concluded that overly conservative assumptions may have been used in the modeling of colloid 
transport through the unsaturated zone (OECD and IAEA 2002, p. 39) and the saturated zone 
(OECD and IAEA 2002, p. 43). As a result of the conservatisms used in the modeling of colloid 
transport of radionuclides, the International Review Team concluded that the importance of 
colloid transports contribution to dose could have been over rated. 
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1.2 PURPOSE 
The purpose of this technical basis document is to present a screening of processes important to 
colloid-facilitated transport of radionuclides in the Yucca Mountain engineered and natural 
barrier subsystems that will demonstrate that the presence of colloids will not lead to the early 
and/or significant discharge of radionuclides to the biosphere, and hence, will not play a major 
role in the system performance. 
Screening arguments will be presented for each of the subsystems, from the waste forms within 
the waste package to the boundary between the saturated zone and the biosphere. The technical 
basis supporting the process screening comprises the general theory of colloid stability, filtration, 
and radionuclide partitioning as documented in the scientific literature, and the results of Yucca 
Mountain Project (YMP) science activities that have studied the relevant physical/chemical 
processes for colloid generation in the waste package and transport of colloids from the waste 
package through the invert, unsaturated, and saturated zones. 
1.3 SUMMARY CONCLUSIONS 
To determine the importance of colloid-facilitated radionuclide transport to system performance, 
the pertinent processes regarding colloid transport in four repository subsystems are examined: 
waste package, invert, unsaturated zone and saturated zone. Some radionuclides 
(e.g., radionuclides of plutonium and americium) may be transported primarily in colloidal form 
during the 10,000-year regulatory period, while others are primarily transported as dissolved 
species (e.g., technetium, iodine, and neptunium). 
As supported by the discussions in Sections 3, 4, 5, and 6 of this technical basis document, 
different processes impact the concentration of colloids in different repository subsystems. 
Within the drift environment, the key controlling factors are those that determine the stability of 
the colloidal suspensions. These factors are related to chemical conditions (such as high ionic 
strengths for the smectite colloids and a pH-ionic strength combination for the iron oxyhydroxide 
colloids) that cause the suspensions of both types of those colloids to be unstable during the 
first 2,000 to 3,000 years following waste emplacement. The instability of the colloid 
suspensions results in the colloids settling out. These same factors will also be the main 
contributors to the instability of colloid suspensions in the invert and the perturbed near field. 
Within the unsaturated zone and the saturated zone, filtration mechanisms will be the major 
contributors to reducing the concentration of colloids. Within the unsaturated zone, the interface 
between adjacent geologic units with sharply contrasting hydrologic properties will act as a 
barrier inhibiting colloids from moving through the downstream unit. Figure 1-2 shows 
qualitatively the impact of the different phenomena that lead to a decrease in the number of 
colloids in the repository subsystems, from the waste package to the biosphere. 
The impact of the reducing concentration of colloids on radionuclide releases to the biosphere is 
shown qualitatively in Figure 1-3. Among all of the radionuclides, americium and plutonium are 
the ones of biggest concern with respect to colloidal transport because of their high affinity to 
attach to particle/solid surfaces. In Figure 1-3, the dissolved concentration of these two 
radionuclides is qualitatively compared to their concentration associated with colloids for the 
different repository subsystems, from the drift to the saturated zone. Because radionuclides that 
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adsorb to natural colloids may also adsorb to immobile material coated with secondary minerals, 
these radionuclides are eventually removed from the transporting fluid. The colloids of concern 
represent only a small fraction of waste degradation or rock alteration products. Figure 1-3 
shows that the concentration of smectite colloids qualitatively drops significantly within the 
waste package until it reaches a constant value. This constant value qualitatively represents the 
natural concentration of smectite colloids available for reversibly sorbing radionuclides. The 
drop in the concentration of smectite colloids within the waste package is due to higher 
temperature and high ionic strength rendering the colloids unstable. Mills et al. (1991) 
concluded that metals strongly adsorbing onto natural colloids may also strongly adsorb onto the 
immobile matrix. It is thus anticipated that dissolved radionuclides, such as technetium and 
iodine, which are highly soluble, will be significantly more important to system performance 
than americium or plutonium, which are strongly associated with colloids. The screening 
arguments discussed in Sections 3, 4, 5, and 6 of this technical basis document support the 
conclusion that colloid-facilitated transport of radionuclides during the regulatory period is not as 
significant as transport of dissolved species for each of the four subsystems discussed. 
Consequently, colloid-facilitated radionuclide transport will not be significant to system 
performance. 
The discussion in this technical basis document comprises a screening argument for processes 
governing colloid-facilitated transport of radionuclides at the Yucca Mountain repository. 
However, it is not suggested that colloid-facilitated transport be excluded from the TSPA. 
Rather, it is concluded that, on a risk basis, colloid-facilitated transport is not a significant 
contributor to system performance; therefore, uncertain technical issues associated with this 
transport mode are of relatively low importance compared to uncertain technical issues related to 
radionuclide transport as dissolved species. It is also concluded that, for purposes of the TSPA, 
the current treatment of colloid-facilitated transport of radionuclides is adequate. 
1.4 ORGANIZATION OF TECHNICAL BASIS DOCUMENT 
In addition to this introduction, this technical basis document consists of the following sections: 
 Section 2Colloid Stability, Filtration, and Radionuclide Partitioning: General Theory 
 Section 3Waste Package 
 Section 4Invert 
 Section 5Transport of Colloids in Unsaturated Zone 
 Section 6Colloid Transport in the Saturated Zone 
 Section 7Summary 
 Section 8References 
 Appendix AExclusion of Entrained Colloids in Thermal-Chemical Alteration 
(Response to ENFE 1.06, ENFE 4.04 and GEN 1.01 (Comment #35)) 
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 Appendix BSensitivity Analysis of Colloid Transport Parameters (Response to ENFE 
4.06 AIN-1 and GEN 1.01 (Comments 35 and 37)) 
 Appendix CScreening Out Coupled Thermal-Hydrologic-Chemical Effects (Response 
to ENFE 4.03 and GEN 1.01 (Comments 35 and 37)) 
 Appendix DContrasting Colloid Concentrations in the Engineered Barrier System and 
Saturated Zone (Response to TSPAI 3.30 and GEN 1.01 (Comments 43 and 46)) 
 Appendix ESensitivity Studies to Test Importance of Colloid Transport Parameters 
and Models (Response to RT 3.07 and GEN 1.01 (Comments 35, 43, and 46)) 
 Appendix FTransport of Dissolved and Colloidal Radionuclides Through Invert 
(Response to TSPAI 3.17 and GEN 1.01 (Comments 36 and 38)) 
 Appendix GScreening Criteria for Attachment of Radionuclides to Colloids 
(Response to RT 1.03 AIN-1, ENFE 3.05 AIN-1 and ENFE 4.05 AIN-1) 
 Appendix HChanges in Colloid Concentrations due to Shifts in pH and Ionic Strength 
(Response to TSPAI 3.42) 
1.5 NOTE REGARDING THE STATUS OF SUPPORTING TECHNICAL 
INFORMATION 
This document was prepared using the most current information available at the time of its 
development. This technical basis document and its appendices providing KTI agreement 
responses that were prepared using preliminary or draft information reflect the status of the 
Yucca Mountain Projects scientific and design basis at the time of submittal. In some cases, this 
involved the use of draft Analysis and Model Reports (AMRs) and other draft references whose 
contents may change with time. Information that evolves through subsequent revisions of the 
AMRs and other references will be reflected in the LA as the approved analyses of record at the 
time of LA submittal. Consequently, the Project will not routinely update either this technical 
basis document or its KTI agreement appendices to reflect changes in the supporting references 
prior to submittal of the LA. 
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Figure 1-2. Processes Impacting Colloid Concentration from Drift to Biosphere Resulting in a Consistent Decrease in Colloid Concentration 
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Figure 1-3. 
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Schematic Representation of Radionuclide Concentrations (Plutonium and Americium) as 
Dissolved Species and Colloids from Drift to Biosphere 
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2. COLLOID STABILITY, FILTRATION, AND RADIONUCLIDE PARTITIONING: 
GENERAL THEORY 
In this section, the general theory on the stability, transport and filtration, and sorption of 
radionuclides onto colloids is presented. A summary section is included presenting an overall 
picture of the potential impact of colloids on the performance of the repository system. This 
Section 2 is primarily based on a review of publications in the open literature. As appropriate, 
relevant information from YMP science activities is included to support the conclusions 
presented. It should be noted that the open literature, as well as that pertaining to repository 
systems has dealt mainly with colloid transport through saturated porous and fractured media. 
2.1 STABILITY OF COLLOIDAL SUSPENSIONS 
A key condition for colloid-facilitated transport of radionuclides to be a significant contributor to 
repository performance is the ability of the colloidal suspension to remain stable for distances of 
tens of kilometers and over tens of thousands of years. 
The stability of colloidal suspensions against flocculation (also referred to as agglomeration or 
coagulation) of the colloids and deposition is governed by intercolloidal repulsive electrical 
forces, and these are a function of colloid number, density, size, composition, and surface 
chemistry (Haber and Brenner 1993), and attractive London-van der Waals forces that depend on 
the intercolloid separation distance and colloid size (Guzy et al. 1983; Elimelech and OMelia 
1990). In the 1940s and 1950s, the Derjaguin and Landau, and Verwey and Overbeek 
(Elimelech and OMelia 1990) theory was used to explain colloid stability. The Derjaguin, 
Landau, Verwey and Overbeek theory defined the total interaction energy between colloids as a 
function of the separation distance between colloids. The total interaction energy is the sum of 
the energy of the electric double layer and the London-van der Waals energy. In addition, 
physical effects, such as flow rate and gravitational forces, and chemical conditions (the latter 
determining the colloid surface charge) also influence the intercolloid interactions that could lead 
to flocculation and gravitational settling of the particles (Guzy et al. 1983; Elimelech and 
OMelia 1990). 
While gravitational settling is directly proportional to the third power of particle size, larger 
colloids tend to form more stable suspensions in natural systems due to size-exclusion effects 
(Haber and Brenner 1993). However, this should not be interpreted to mean that colloids, 
particularly in the higher end of the size spectrum (i.e., about 10 m in diameter), will be stable 
over the temporal and spatial scales of relevance to repository performance. Several researchers 
have concluded that it is difficult to envision colloidal suspensions remaining stable, and 
therefore mobile, for large distances and long times (Mills et al. 1991; van der Lee et al. 1994). 
Elimelech and OMelia (1990) have concluded that even for very stable colloidal suspensions, 
the attachment of the colloids to the wall of the flow channel may be significant. 
The interaction between colloids is governed by the sum of the electric repulsion force due to 
identical surface charges (known as the electric double-layer force) and the attractive 
London-van der Waals force (Elimelech and OMelia 1990). The electric double-layer force 
stems from approaching colloids having equal surface charges. Each colloid is surrounded by a 
layer of oppositely charged ions that counterbalances the colloid surface charge. The 
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London-van der Waals attractive force is a function of distance between approaching colloids 
and the colloids size. It has been shown that the strength of the London-van der Waals force is 
inversely proportional to the distance between the colloids and the colloid size (Guzy et al. 
1983). That is, the closer the colloids are to each other and the smaller they are, the higher the 
strength of the attractive force. 
There are several factors that affect the strength of the repulsive electric double-layer force; the 
zeta potential (a measure of the surface charge), colloid size, and ionic strength and temperature. 
Ionic strength and temperature have a considerable impact on the thickness of the electrical 
double layer (Guzy et al. 1983). The double-layer thickness is inversely proportional to the 
square root of temperature and to the square root of the ionic strength (Guzy et al. 1983, 
Equations 47 and 48). Thus, higher ionic strengths and higher temperatures lead to thinner 
electric double layers and hence weaker repulsive forces between colloids. As temperature and 
ionic strengths increase, the repulsive force decreases allowing the attractive London-van der 
Waals forces to dominate colloidal particle interactions. 
These results are consistent with others reported in the literature. For example, Langmuir (1997, 
p. 439) states that a colloidal suspensions stability ratio is inversely proportional to temperature. 
In Waste Form and In-Drift Colloids Associated Radionuclide Concentrations: Abstraction and 
Summary (BSC 2003f), experimental results demonstrate that colloid suspensions become more 
unstable with increasing ionic strength. 
As discussed in Section 3, during the first one-third of the 10,000-year postclosure regulatory 
period, the high temperature will result in water evaporation and increased ionic strengths 
rendering colloids important to radionuclide transport unstable. This will decrease the 
concentration of colloids either carrying or able to carry radionuclides for the waste package. 
Even after 10,000 years, the temperature within the emplacement drifts is expected to be 20 to 
25C higher than the host-rock ambient temperature (BSC 2003l, Figure 6.5.2-1). 
2.2 COLLOID FILTRATION 
As colloids travel along the flow path from the repository to the biosphere, they will undergo 
interactions with the host rock. Larger colloids are likely to be physically trapped within the 
pores in the rock matrix. If colloids are bigger than the pore size through which they are 
attempting to pass, the particles will not be able to get past the pore entrance. Smaller colloids 
that can enter the pore are likely to come in very close proximity with each other to overcome 
any possible repulsive forces between the colloids and flocculate, thus, forming larger particles 
that will then get trapped within the pores. The smaller colloids (i.e., diameter less than 1 m) 
will be subjected to Brownian deposition onto the rock matrix (Haber and Brenner 1993), thus, 
reducing the number of colloids entrained in the flow. 
Colloid filtration is also called colloid attachment or colloid deposition. There are many 
different studies regarding the transport of and deposition of colloidal particles through fractures 
under saturated flow conditions; some examples are Haber and Brenner (1993), van der Lee et al. 
(1994), Chrysikopoulos and James (2003), Chrysikopoulos and Abdel-Salam (1997), and James 
and Chrysikopoulos (1999, 2000, 2003). There have been other studies aimed at understanding 
the kinetics of colloid interactions leading to deposition on the surfaces of immobile phases in 
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porous media. Some examples are Guzy et al. (1983), Elimelech and OMelia (1990), and 
Adamczyk et al. (1992a, 1992b). Even though the latter works were for porous media, the 
kinetic models apply equally to the interactions between colloids and fracture surfaces because 
(1) the ratio of the characteristic dimension of the colloids to the characteristic dimension of the 
immobile phase was considered to be much smaller than one, and (2) the interactions between 
colloids and the immobile phases are only important at separation distances much smaller than 
the colloid characteristic dimension. Consequently, as the colloids approach the immobile phase, 
the immobile phase looks and behaves like a planar surface from the perspective of the colloids. 
As colloids move down a fracture, they will be affected by (1) hydrodynamic and physical forces 
in the directions parallel to the fracture walls and (2) external physical, chemical, and electrical 
forces in the direction normal to the fracture walls. Hydrodynamic forces account for the drag 
that the flow exerts on the colloids leading to entrainment of the colloids in the flow and 
gravitational effects. External forces include gravitational effect, London-van der Waals 
attractive force, and electric double-layer force. In addition, the external forces account for 
lubrication effects that colloids are subjected to as they approach the immobile phase. 
Finally, colloids are subjected to Brownian motion, which is typically modeled using an 
equivalent diffusivity. The Brownian diffusivity is directly proportional to temperature and 
inversely proportional to colloid size (Guzy et al. 1983). The effects of Brownian motion are 
more pronounced for colloids of less than 1 m in characteristic dimension. 
The theoretical studies regarding colloid transport through porous and fractured media are 
generally consistent regarding the effect of colloid size on transport times. Larger colloids are 
transported at a faster rate than smaller ones, with the latter showing a larger propensity to 
deposit onto the immobile rock. Comparisons of experimental and theoretical results for colloid 
deposition reported by Elimelech and OMelia (1990) show increased deposition rates measured 
experimentally than predicted theoretically. Those experimental results also suggest that in 
natural waters colloid deposition is independent of size. Elimelech and OMelia (1990) attribute 
this discrepancy to failure of the Derjaguin, Landau, Verwey and Overbeek theory to account for 
the dynamic interactions between colloids and the immobile phase. 
Colloid filtration will be an important phenomenon resulting in the decrease of the colloid 
concentration, particularly within the unsaturated zone directly below the repository. 
2.3 RADIONUCLIDE PARTITIONING BETWEEN COLLOIDS AND DISSOLVED 
SPECIES 
Radionuclides can potentially be transported from the waste form to the biosphere as either 
dissolved species or in colloidal form. In order for colloid transport to be a significant 
contributor to the discharge of radionuclides to the biosphere, colloids must carry a significant 
mass of radionuclides. 
As discussed in Section 3, the most likely forms of radioactive colloids under the expected 
conditions at the Yucca Mountain repository are: (1) colloids with embedded radionuclides 
resulting from the degradation of defense high-level radioactive waste glass, referred to as 
waste-form colloids; (2) colloids formed as a result of corrosion of steel components within the 
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repository, referred to as iron oxyhydroxide colloids; and (3) natural groundwater colloids, 
referred to as smectite colloids. The formation of colloids due to hydrolysis or polymerization 
of dissolved actinides (referred to as true or intrinsic colloids) is not considered because 
recent experiments have demonstrated that these colloids dissolve under the expected repository 
conditions (see Section 3 of this technical basis report). First, in the tests performed at Argonne 
National Laboratory for degradation of defense high-level radioactive waste glass (Ebert 1995, 
Sections 6.2.1 and 6.2.2), no evidence was found of the formation of true colloids. Second, 
degradation of commercial spent nuclear fuel (CSNF) may form true colloids close to the fuel 
surface where the fluid may be saturated with respect to uranium, but these true colloids are 
likely to dissolve in unsaturated fluid. 
Besides the mass of radionuclides that can be embedded in waste-form colloids from the 
degradation of defense high-level radioactive waste glass, dissolved radionuclides can be 
adsorbed onto the surface of colloids to form pseudocolloids. Only plutonium and americium 
are expected to be embedded in colloids resulting from the degradation of defense high-level 
radioactive waste glass. Thus, the discussion herein focuses on the sorption of dissolved 
radionuclides onto the surface of available colloids. 
A key consideration in the assessment of colloid contribution to radionuclide transport is 
determining the partition coefficients between interstitial solvent (i.e., dissolved in the 
groundwater), the immobile phase of the host rock (i.e., sorption within the porous rock and the 
fracture surfaces), and the surfaces of the mobile colloids (Haber and Brenner 1993). 
Transport of groundwater colloids is likely to be preferentially through fractures in the 
unsaturated zone (see Section 5) and in the fractured volcanics within the saturated zone 
(Section 6). The stability of a colloidal suspension increases with particle size because of the 
reduced number of particles per unit volume that can collide (i.e., the larger the colloids the more 
stable the suspension will be and the higher the probability that the colloids will remain entrained 
in the groundwater flow) (Haber and Brenner 1993). However, larger colloids are unlikely to be 
effectively transported through the unsaturated zone and the volcanic fractures in the saturated 
zone in the vicinity of Yucca Mountain due to simple geometric considerations (BSC 2003m, 
Section 6.18). Larger colloids are more likely to be physically trapped in the pore constrictions 
and do not penetrate the porous matrix. As noted in Radionuclide Transport Models Under 
Ambient Conditions (BSC 2003m), colloids are discharged to the unsaturated zone directly into 
fractures for the model elements corresponding to the repository. 
During transport through fractures as both dissolved species and in colloidal form, radionuclides 
undergo (1) sorption of dissolved radionuclides onto the fracture surfaces, (2) sorption of 
dissolved radionuclides onto the porous matrix following diffusion, (3) sorption of dissolved 
radionuclides onto the surface of colloids, (4) attachment of radioactive colloids onto the surface 
of fractures (filtration), (5) transport as dissolved species and as colloids, (6) diffusion of 
dissolved radionuclides from the fractures into the porous matrix, and (7) decay. The 
conservation equation for each radionuclide needs to account for all of these phenomena to 
adequately estimate the amount of radioactive species discharged to the biosphere. Diffusion of 
the larger, more stable colloids into the rock matrix is not considered an important phenomenon 
because of the very low probability of the larger colloids being able to physically penetrate into 
the pores (BSC 2003m, Section 6.18). 
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Sorption of dissolved radionuclides onto the fracture surfaces and onto the rock matrix after 
diffusion has been traditionally treated using an equilibrium isotherm model and defined by a 
distribution coefficient. Diffusion of dissolved species from the fractures into the rock matrix 
has been treated using a flux boundary condition at the fracturematrix interface based on Ficks 
law of diffusion. These modeling approaches are well established and accepted (de Marsily 
1986). 
Sorption of radionuclides onto the surface of colloids depends on a number of phenomena; 
namely: (1) electrostatic forces, (2) ion exchange, (3) surface reactions, and (4) co-precipitation 
(BSC 2003f). These are complex phenomena, and it has been assumed that the effect of these 
phenomena can be captured using the linear isotherm model for reversible sorption (BSC 2003f). 
However, for this assumption to be applicable, some important conditions must exist. First, the 
dissolved radionuclidecolloid system must be in thermodynamic equilibrium. Second, the rate 
of radionuclide sorption must be linearly correlated to the concentration of dissolved 
radionuclides. Third, the rate of sorption must be independent of other solutes present in the 
groundwater. Fourth, the entire colloid surface area must be available for sorption. The 
likelihood that these conditions are met under repository conditions is not particularly high 
(BSC 2003f). 
Haber and Brenner (1993) investigated the sorption of dissolved contaminants onto the surface 
of colloidal particles under both equilibrium and nonequilibrium conditions. They developed a 
mathematical expression for the rate of solute sorption from the solvent onto the surface of 
colloids for nonequilibrium conditions (Haber and Brenner 1993, Equation 2.9b). That 
expression demonstrates that, under nonequilibrium conditions, the rate of sorption increases 
linearly with the solute concentration and decreases linearly with the concentration of solute 
already adsorbed onto the surface of colloids. The key parameter in the Haber and Brenner 
expression for nonequilibrium sorption is a mass transfer coefficient. The asymptotic limit of the 
Haber and Brenner expression for high values of the mass transfer coefficient is the linear 
isotherm model. 
The Haber and Brenner nonequilibrium sorption expression indicates that under nonequilibrium 
conditions the use of the linear isotherm model to describe the sorption of solutes onto the 
surface of colloids will overestimate the mass of solutes adsorbed. 
In Waste Form and In-Drift Colloids Associated Radionuclide Concentrations: Abstraction and 
Summary (BSC 2003f) it was concluded that equilibrium conditions are not expected at the 
Yucca Mountain repository. This conclusion, combined with the work of Haber and Brenner 
(1993), shows that the mass of radionuclides that can be transported as pseudocolloids will be 
overestimated by using a linear isotherm model to describe the partitioning of radionuclides 
between dissolved species and colloids. 
The mass of dissolved radionuclides that can be adsorbed onto the surface of colloids is a 
function of the colloids surface area per unit volume (BSC 2003f). Larger colloids, the ones 
that could transport farther in the fractures, have a smaller surface-area-to-volume ratio than the 
smaller colloids. The smaller colloids are more susceptible to attachment to the fracture walls. 
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The mass of radionuclides that can adsorb onto colloids will also be a function of the colloid 
concentration. The higher the concentration of colloids, the higher the colloidal surface area 
available to compete with the immobile rock surfaces for sorption of radionuclides. Analyses of 
colloid concentrations and size distributions in Yucca Mountain groundwaters have found 
colloid concentrations to be too low for significant colloid-facilitated transport of radionuclides 
(DOE 2002, pp. 4-335 to 4-336). Moreover, Triay et al. (1996), in a compendium of colloid 
concentration as a function of water chemistry, found that under thermal and chemical conditions 
similar to those expected at Yucca Mountain, colloid concentrations are unlikely to be high 
enough for colloids to effectively compete with the immobile rock for the sorption of dissolved 
radionuclides, particularly when the immobile rock is coated with secondary minerals. 
As dissolved radionuclides travel from the repository towards the biosphere, their concentration 
is expected to decrease due to the various retardation processes that the radionuclides undergo, 
such as diffusion on the rock matrix and sorption onto the surfaces of the fractures and within the 
rock matrix. These processes act as effective mass sinks for dissolved radionuclides. This 
means that there will be lesser amounts of dissolved radionuclides available for sorption onto 
colloids and a lower likelihood for the formation of radioactive colloids. It is possible that far 
away from the repository desorption of radionuclides from the surface of colloids will take place, 
as discussed in the following paragraph. Based on conclusions reached by Mills et al. (1991), 
van der Lee et al. (1994) questioned whether colloid-facilitated transport of radionuclides would 
be significant at a nuclear waste disposal site. 
Mills et al. (1991) evaluated the transport of colloids and metals in porous media. They analyzed 
the transport of only dissolved species (solutes) under a variety of transport conditions and 
repeated the analyses under the same conditions including colloids. The basic system Mills et al. 
(1991) analyzed consisted of a contaminant source located in an unsaturated zone and the 
transport of the contaminants to a receptor downgradient in a saturated zone. The solute-colloid 
transport analyses considered uncertainty in several key parameters: source duration (1,000 and 
100,000 years), natural colloid concentration (0, 10, and 100 mg/l), infiltration rate (0.007 and 
0.35 m/year), partition coefficient for adsorption of solutes onto colloids (103, 104, and 105 l/kg) 
and partition coefficient for adsorption of solutes to the immobile soil matrix (0, 10, 102, 103, 
104, and 105 l/kg). Mills et al. (1991, p. 206) concluded that, As colloids move through 
subsurface media, and if adsorbed metal species are present, then the colloids reach a clean 
portion of the aquifer, solutes are rapidly adsorbed to the aquifer-soil matrix and rapidly 
desorbed from the colloids to reestablish a new equilibrium between solute and colloid-adsorbed 
species. Thus, the colloids are rapidly stripped of adsorbed metal species and continue migrating 
with relatively little metal adsorbed. According to Mills et al. (1991), this desorption 
phenomenon explains their results in which, for many of the cases they analyzed, the impact of 
colloid transport on the total mobile phase concentration (dissolved metal concentration plus 
metal concentration adsorbed to mobile colloids), in terms of either increased concentration or 
early arrival at the receptor location, was not remarkable. Mills et al. (1991, p. 206) further 
concluded that, Typically, when travel times to a source are thousands of years in the absence of 
colloids, the travel times still remain on the order of thousands of years in the presence of 
colloids. 
September 2003 2-6 No. 8: Colloids
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2.4 SUMMARY AND CONCLUSIONS 
While the potential exists for fast transport and early release of radioactive colloids to the 
biosphere at the Yucca Mountain repository, several conditions must simultaneously be met for 
transport of radioactive colloids to be a significant contributor to dose. Those conditions are: 
 Colloids must be present in high enough concentrations and provide high surface area 
per unit volume to carry a significant amount of radionuclides, either embedded in the 
colloids or attached on the surface of the colloids, relative to the concentration of 
dissolved radionuclides. 
 Colloidal suspensions must be stable for distances in the order of tens of kilometers and 
for time periods in the tens of thousands of years. 
 Radioactive colloids must not be appreciably filtered, reversibly or irreversibly, by the 
host rock. 
If any one of these conditions is not met, the likelihood that colloid transport provides a 
significant transport mechanism for the release of radionuclides to the biosphere is low. 
The higher temperatures and higher ionic strength within and in the vicinity of the repository 
during the first one-third of the postclosure regulatory period are expected to result in unstable 
colloidal suspensions. 
As radionuclides adsorb onto the surface of colloids, they may neutralize negative colloid surface 
charges, thus rendering the colloids more susceptible to filtration by attachment to the surface of 
fractures, particularly if the fractures are coated with secondary minerals. Therefore, 
radionuclide sorption may have a beneficial effect on decreasing the potential for fast transport 
of radioactive colloids. 
As radionuclides move farther away from the repository, their concentration as dissolved species 
decreases; therefore, there will be less mass of dissolved radionuclides available for sorption 
onto colloids. Diffusion into and sorption onto the rock matrix lead to the decrease in the 
concentration of dissolved radionuclides. It has been concluded (Mills et al. 1991) that over long 
distances and long travel times, a decrease in dissolved contaminants concentration far away 
from the source can result in desorption of contaminants from the surface of colloids. This 
potential desorption process has led some researchers to conclude that when contaminant travel 
times are thousands of years in the absence of colloids, they are also thousands of years in the 
presence of colloids. 
Most contaminant transport studies reported in the literature pertain to saturated conditions. 
There is little information regarding the transport of colloids under unsaturated conditions. This 
seeming lack of information for unsaturated systems notwithstanding, some intuitive arguments 
based on physics can be made regarding colloid transport in unsaturated conditions. Assuming 
that the flow in the unsaturated zone below the proposed repository horizon is primarily films of 
groundwater adjacent to the fracture walls, then only colloids of a size considerably smaller than 
the films could be transported with the groundwater flow. Also, as stated earlier, the smaller 
September 2003 2-7 No. 8: Colloids
colloids are more susceptible to Brownian deposition and, hence, to attachment to the fracture 
walls. Finally, the colloids within the groundwater film could be physically closer to the fracture 
walls to enhance attachment to the latter by other mechanisms. 
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September 2003
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3. WASTE PACKAGE 
This section of the technical basis document summarizes the current understanding of the 
formation of radionuclide-laden colloids, prediction of their concentration within the waste 
package and their release into the invert. The waste package is the first of the repository system 
subfeatures examined to determine the importance of colloid-facilitated radionuclide transport 
with respect to the postclosure performance of the Yucca Mountain repository, as shown in 
Figure 1-1. This section provides a summary-level synthesis of the processes and phenomena 
governing the formation, prediction of their concentration, and transport of radionuclide-carrying 
colloids within the waste package. The modeling assumptions to be used in the TSPA for the LA 
(TSPA-LA) are discussed. In preparing this section, information has been summarized from the 
following pertinent analysis and model reports, as well as other technical products: 
 Multiscale Thermohydrologic Model (BSC 2003n) 
 Engineered Barrier System: Physical and Chemical Environment Model (BSC 2003k) 
 In-Package Chemistry Abstraction (BSC 2003h) 
 Dissolved Concentration Limits of Radioactive Elements (BSC 2003g) 
 Waste Form and In-Drift Colloids Associated Radionuclide Concentrations: 
Abstraction and Summary (BSC 2003f). 
3.1 DESCRIPTION OF RELEVANT PROCESSES AROUND WASTE PACKAGE 
The relevant physical/chemical processes for colloid generation and transport inside waste 
packages prior to any disruption by igneous intrusion are shown in Figure 3-1. 
3.1.1 In-Drift and In-Package Chemistry 
The evolution of general physical and chemical environments is important for understanding 
colloid generations and stabilities inside the drift and the waste package. Detailed analyses of indrift 
thermal-hydrologic conditions are documented in the Multiscale Thermohydrologic Model 
(BSC 2003n, Figure 6.3-12). A typical evolution of temperature, relative humidity, and liquidphase 
saturation degree over time is illustrated in Figure 3-2. Relative humidity decreases to low 
values shortly after emplacement of the waste packages. As the waste packages cool over time, 
relative humidity begins to rise until it reaches 100 percent after a few thousand years. During 
the period with relative humidity less than 98 percent, any dilute species in groundwater flowing 
into the drift become concentrated by evaporation, which results in a high ionic strength solution, 
which, in turn, destabilizes colloid suspensions. The ionic strength of some representative 
seepage waters as a function of relative humidity is shown in Figure 3-3 (BSC 2003k). As long 
as the relative humidity is less than 98 percent, the representative solutions have an ionic strength 
greater than the upper limit of colloid stability (0.05 M) (BSC 2003f, Sections 6.3.2.2 and 
6.3.2.3). Therefore, the formation of a stable colloid suspension during the first thousand years 
after closure is unlikely. Colloid transport can be possible only after the effects of evaporation 
cease to be significant after the thermal period when temperatures are below 90C (Figure 3-2). 
September 2003 3-1 No. 8: Colloids
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Furthermore, the degradation of the waste form and the waste package can lead to the formation 
of concentrated solutions. In process modeling, in-package chemistry has been predicted for two 
scenarioswater vapor condensation and seepage water drippingwith an assumption of no 
water evaporation. For the scenario of seepage water, the reaction-path calculations show that 
the initial seepage-water compositions are quickly modified by waterpackage chemistry 
produced as components within the waste package degrade (BSC 2003h). Depending on the 
conditions, in most cases, the resulting solution can attain ionic strength higher than 0.05 M as 
the waterwaste package reaction progresses (Figure 3-4). Under such conditions, a stable 
colloid suspension, if there is any, will become unstable. This prediction is consistent with the 
observation in the static-saturated tests that colloid concentrations increase with time, up to the 
point where the colloid concentrations reach maximum values and start to decrease (Figure 3-5). 
In summary, the formation of a stable colloid suspension during the first few thousand years of 
the regulatory period is unlikely because of evaporation during the thermal period. Colloids 
could be generated at a late stage of the regulatory period, when evaporation becomes less 
effective. However, these colloids can be destabilized by high ionic strength environments that 
can be potentially created during waste degradation. Therefore, the general physical/chemical 
environments have imposed restrictive constraints on colloid stability inside the drift. It is 
expected that in-drift colloid transport can be possible only for limited time periods and limited 
combinations of physical and chemical conditions. 
September 2003 3-2 No. 8: Colloids
Figure 3-1. Relevant Physical and Chemical Processes for Colloidal Generation and Transport from Waste Forms and Waste Packages 
Revision 2 
September 2003 3-3 No. 8: Colloids
Source: BSC 2003n, Figure 6.3-12. 
NOTE: These waste packages bracket the entire range of temperature at this location. 
Figure 3-2. 
No. 8: Colloids 
Thermal-Hydrologic Conditions for the Mean Infiltration-Flux Case for a Range of Waste 
Packages at the P2WR8C8 Location in the Tptpmn (tsw34) Unit 
Revision 2 
September 2003 3-4
Source: BSC 2003k, DTN: MO0308SPAPCESA.001. 
Figure 3-3. 
Source: BSC 2003h, Figure 11. 
Figure 3-4. 
No. 8: Colloids 
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Ionic Strength (in Molality) of Some Representative Seepage Waters as a Function of 
Relative Humidity 
An Example of Ionic Strength Evolution during the Degradation of Commercial Spent 
Nuclear Fuels under the Assumption of No Water Evaporation 
September 2003 3-5
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Source: BSC 2003f, Figure 3. DTN: LL991109751021.094. 
Figure 3-5. 
No. 8: Colloids 
Concentrations of Plutonium and Colloids as a Function of Defense High-Level Radioactive 
Waste Glass Corrosion Test Duration 
3.1.2 Waste Types and Potential Colloid Types 
As explained later in Section 3.4, three types of colloids are modeled in the current waste 
package model abstraction: 
 Natural colloids in seepage water/groundwater include mineral fragments, humic 
substances, and microbes. Humic substances are not sufficiently abundant in 
Yucca Mountain groundwater to impact transport (Minai et al. 1992). Microbes are 
susceptible to filtration due to their large sizes (~1 to 10 m). (See Section 3.2.6.). The 
mineral colloids are represented as smectite colloids (BSC 2003f). 
 Corrosion product colloids are derived from the corrosion of waste package and 
metallic invert materials. These colloids are primarily composed of iron 
oxyhydroxides. The concentration range of the colloids is estimated based on 
experiments performed at the University of Nevada, Las Vegas on scaled-down 
miniature waste packages (DTN: MO0212UCC034JC.002; BSC 2003f). 
 Waste form colloids are formed from the corrosion of defense high-level radioactive 
waste glass. Glass waste forms tested include Savannah River Laboratory and West 
Valley Demonstration Project glasses (CRWMS M&O 2001a; BSC 2003f). (Glass is 
not available from the yet-to-be-built Hanford vitrification plant.) Colloids produced 
from both waste forms are primarily smectite clays containing measurable plutonium, 
as well as discrete radionuclide-bearing phases including brockite (thorium-calcium 
orthophosphate) and an amorphous thorium-titanium-iron silicate, similar to thoriutite. 
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Long-term corrosion testing of CSNF and DOE spent nuclear fuel (DSNF) under hydrologically 
unsaturated, oxidizing conditions has been conducted. Results from this testing show both very 
low colloid concentrations and low fractions of uranium in the colloid mass (Mertz et al. 2003). 
Thus, colloids from both CSNF and DSNF are excluded in the colloidal modeling. Note that 
CSNF accounts for the major fraction of the wastes by metric tons of heavy metal to be 
emplaced in the Yucca Mountain repository. 
The formation of true or intrinsic colloids depends on the degree of saturation of the solution 
(i.e., the solution must be supersaturated) with respect to the corresponding radionuclide-bearing 
mineral phase. There has been no evidence of their formation in the tests performed at Argonne 
National Laboratory for degradation of defense high-level radioactive waste glass. Degradation 
of CSNF may form true or intrinsic colloids (e.g., schoepite), but they are likely to dissolve in 
groundwater in the unsaturated zone. In the Argonne National Laboratory tests (Mertz et al. 
2003), it was observed that these metastable colloids dissolved upon the introduction of J-13 
groundwater. J-13 groundwater was assumed to be a lower bound with respect to the 
temperature and chemistry of water inside the drift. Typical water inside the drift is expected to 
be at a higher temperature and have higher ionic strength than J-13 water. Consequently, 
colloids will dissolve more readily in the water inside the drift than in J-13 groundwater. 
Therefore, the possibility of formation of intrinsic colloids is eliminated from further 
consideration in the colloid modeling (BSC 2003f, Section 6.3.1). 
3.1.3 Radionuclide Sorption onto Colloids 
The effectiveness of colloid-facilitated transport depends on the sorption capability of colloids 
for radionuclides of interest. For reversible sorption processes, a linear isotherm model is used: 
(Eq. 3-1) K m m = d d c 
d is the distribution coefficient. The ranges of Kd values are 
where mc is mass of radionuclide adsorbed on a unit of mass of solid; md is the concentration of 
dissolved radionuclide; and K 
derived from literature data, which are obtained mostly from noncolloidal systems. 
The results of experiments with plutonium and americium with colloidal hematite and goethite 
show that the rates of desorption of these radionuclides are significantly lower than the rates of 
sorption. Over a time period up to 150 days, the extent of desorption is considerably less than 
that of sorption. Based on these data (Lu et al. 1998) and observations from field studies (Brady 
et al. 2002), 90 to 99 percent of sorbed plutonium and americium onto iron oxyhydroxides are 
modeled as irreversibly adsorbed (BSC 2003f). 
Colloid FiltrationColloids generated within the defense high-level radioactive waste glass and 
at its outer surfaces can be filtered. Removal of suspended colloids within fractured and granular 
material occurs because of two types of phenomena: (1) physical effects, and (2) surface 
chemical effects. Physical filtration of colloids generally means the retention of colloids moving 
with the suspending fluid in pores, channels, and fracture apertures that are too small or dry to 
allow passage of the colloids. Two types of physical filtration are recognized in the unsaturated 
areas (Wan and Tokunaga 1997): conventional straining and film straining. Conventional 
straining, sieving, and pore-clogging will filter colloids if they are between 5 and 10 percent of 
September 2003 3-7 No. 8: Colloids
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the size of grains in the media and thus likely larger than a pore throat diameter or fracture 
aperture. Where water saturation is low, colloids may be filtered by film straining if their size is 
greater than the thickness of the adsorbed water film coating the grains of the rock. The rate of 
colloid transport through thin water films depends upon the colloid size relative to the film 
thickness. Therefore, physical straining because of low saturation and surface chemical effects 
such as adhesion and flocculation through chance interception and diffusion will dominate. 
However, these processes are more important in the extensive unsaturated and saturated zones. 
Within the waste package, the filtration process is implicitly included in the colloid generation 
source term. 
Colloid Sorption at the AirWater InterfaceBoth hydrophilic and hydrophobic colloids may 
be sorbed at the gaswater interface for partially saturated conditions (Wan and Wilson 1994). 
Similar to physical straining at low saturation, the concentration of colloids sorbed depends on 
the saturation degree. However, other factors include the affinity of colloids for the gaswater 
interface, the electrostatic charge, and the salinity of the aqueous phase (BSC 2003f, 
Section 5.7). Hydrophobic colloids have higher affinities for the airwater interface. Colloids 
with low negative charge exhibit a stronger affinity. Also, high salinity of the aqueous phase 
generally promotes sorption. Buck et al. (2003) found that hydrophobic colloids of meta-studtite 
and meta-schoepite generated from degradation of CSNF tend to irreversibly attach to the air 
water interface. These colloids contain moderate levels of neptunium and plutonium as well as 
high levels of strontium, cesium, and technetium. Colloid sorption to stationary airwater 
interfaces generally retard colloid transport, except in cases of relatively high saturation where 
bubbles with attached colloids could be transported. However, high-saturation conditions are 
expected to be unlikely in the drift and the unsaturated zone. Therefore, within the waste 
package, the colloid sorption at the airwater interface is not considered in the colloid generated 
source term. 
3.2 MODELING ASSUMPTIONS 
3.2.1 Colloids from the Corrosion of Commercial and DOE Spent Nuclear Fuel 
Long-term corrosion testing of CSNF and DSNF under unsaturated, oxidizing conditions has 
been performed to examine the release of dissolved radionuclides, as well as radionuclides 
associated with colloids (BSC 2003f). Testing was designed to simulate a variety of Yucca 
Mountain repository relevant water-exposure conditions for several spent nuclear fuels with a 
range of fuel burnup and composition. Results from the unsaturated testing of CSNF and DSNF 
at Argonne National Laboratory indicated formation of alteration products containing very low 
concentrations of uranium-based colloids and dissolution of the uranium-based colloids in less 
than several months (BSC 2003f, Section 6.3.1.2). Uranium-based spent nuclear fuels will be 
prevalent in the repository, and the colloidal properties of a mixture of two uranium minerals, 
meta-schoepite [(UO2)4(OH)6.5H2O] and UO2+x, have been examined (BSC 2003f, 
Section 6.3.1.2.3). These colloids are stable under short duration tests with respect to dissolution 
and interparticle interactions at near neutral and higher pH values in solutions saturated with the 
respective mineral phases. Furthermore, these colloids dissolve after introduction to J-13 
groundwater in short duration tests. Natural analog studies also suggest that colloidal uranium 
will not be a significant contributor to radionuclide transport under oxic pH-neutral environments 
September 2003 3-8 No. 8: Colloids
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in uranium deposits (BSC 2003f, Section 6.3.1.2.4). Thus, colloids from CSNF and DSNF were 
eliminated from further consideration in the colloid modeling and abstraction. 
Commercial Spent Nuclear Fuel 3.2.1.1 
Assessment of the importance of potential colloid formation from CSNF is based on four major 
observations: (1) very low colloid concentrations were observed in the CSNF degradation tests, 
at least an order of magnitude less than concentrations observed in the defense high-level 
radioactive waste glass degradation tests (based on dynamic light-scattering measurements) 
(Mertz et al. 2003); (2) the fraction of uranium in the colloid mass was uniformly low in the 
CSNF tests, the only deviation from this occurring immediately following one of two changes in 
vessel configuration in which the uranium fraction increased but rapidly decreased to the 
approximate level of earlier values (Mertz et al. 2003); (3) suspensions of meta-schoepite and 
UO2+x colloids in J-13 groundwater appear to dissolve in short-term saturated tests (their stability 
in unsaturated solutions has not been tested (Mertz et al. 2003)); and (4) field studies at 
uranium-bearing deposits indicate generally that under oxidizing conditions at near-neutral pH, 
colloid particles contain little uranium, and there is little sorption of uranium complexes to 
colloids (BSC 2003f). 
One of the reasons hypothesized for the low colloid release in the CSNF tests was the test 
configuration in which the Zircaloy-4 support for the fuel fragments had 7-micron holes. 
However, the results from the unirradiated UO2 tests also show few colloids after the formation 
of alteration products (Wronkiewicz et al. 1997). The unsaturated tests on unirradiated UO2 had 
a test configuration with large 2 to 3 mm holes at the holder base allowing for the spallation of 
UO2+x particulate during initial corrosion. However, the formation of a dense mat of alteration 
products during the UO2 corrosion apparently reduced particulate release by trapping particulate 
in the altered products (Wronkiewicz et al. 1997). A similar mechanism whereby the alteration 
products minimize particulate release may be applicable to the CSNF unsaturated tests. 
The concentration of released particulates or colloids from the CSNF tests is very low except 
during movement of the fuel samples from one setup to another (Mertz et al. 2003). In that case, 
colloid and particulate concentrations increased temporarily but returned to very low 
concentrations after the disruption (Mertz et al. 2003). While this indicates that disruptive events 
may contribute to the release of particulates and colloids from CSNF, it also indicates that the 
longevity of the colloids in the leachate is very short relative to the regulatory period. 
DOE Spent Nuclear Fuel 3.2.1.2 
Of the approximately 250 different types of fuel in the DSNF inventory, metallic uranium fuel 
comprises approximately 85 percent (by weight of heavy metal) of that inventory and, as the 
only waste form in significant quantities distinct from the other spent fuel types, was selected for 
corrosion testing. An irradiated uranium metal fuel from the N-Reactor at Hanford was tested in 
an experimental setup similar to that used at Argonne National Laboratory for testing CSNF. 
Additional details on the testing can be found elsewhere (DTN: MO0306ANLSF001.459). 
Corrosion testing of metallic uranium samples resulted in rapid oxidization (within a few 
months) of the uranium primarily to an oxide sludge consisting of UO2 and higher oxides of 
uranium (DTN: MO0306ANLSF001.459). Although the uranium fuel disintegrated rapidly, 
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corrosion testing was continued to determine the effect of groundwater leaching on the fuel 
sludge. Results from the corrosion tests showed that the composition of the DSNF colloids 
evolve over time from an initially UO2-rich population, to a mixed colloid population containing 
UO2 and higher oxides of uranium as well as smectite clays, to a population that appears to be 
dominated by uranium-containing smectite clays. 
After approximately one year of testing, the total quantity of uranium in the sludge represented 
approximately all the original uranium fuel sample. The uranium associated with the colloids 
corresponds to 0.002 to 0.006 weight percent of the original uranium fuel sample. The quantity 
of uranium in the fraction attached to the stainless steel vessel was 0.1 to 0.3 weight percent of 
the original fuel sample (DTN: MO0306ANLSF001.459). The attached material is measured by 
washing the stainless steel vessel in HNO3; the attached material includes sorbed solutes, sorbed 
colloids, and precipitates (DTN: MO0306ANLSF001.459). 
During DSNF corrosion, plutonium is predominantly associated with the colloidal, particulate, 
and sorbed size fractions. The 239Pu/238U ratios in the colloid fraction are significantly larger 
than those in the other fractions (sorbed, particulate, and dissolved) and is the only fraction that 
showed enrichment of plutonium in comparison to that in the fuel prior to corrosion 
(DTN: MO0306ANLSF001.459). Results from the testing suggest that plutonium is 
significantly adsorbed to the surface of colloids (such as corrosion products of the waste package 
or groundwater clays). However, it does not form as an embedded radionuclide in waste form 
colloids (DTN: MO0306ANLSF001.459). 
3.2.2 Filtration of Colloids 
Near the waste, colloids may form at corroded waste fuel pellets and at its outer surfaces. They 
could be filtered within fractures in fuel pellets or trapped at grain boundaries. Colloids forming 
within fuel rods whose cladding has been breached could be filtered at perforations in the 
cladding. Colloids formed and spalled from the defense high-level radioactive waste glass could 
be filtered at perforations in the stainless steel high-level radioactive waste canister. No 
differentiation was made between colloid filtration and generation in the waste degradation 
experiments. Physical and chemical filtration of colloids is thus implicitly part of the generation 
process of colloids for radioactive waste. This filtration is a possible reason for the observed 
lack of waste form colloids in leachate leaving the CSNF and DSNF degradation experiments at 
Argonne National Laboratory noted above in Sections 3.2.1.1 and 3.2.1.2. 
Colloids reaching the interior of the waste package (after escaping from fuel-rod cladding and 
high-level radioactive waste containers) could be filtered at perforations in the skin of the waste 
package. However, further filtration within the waste package and the drift is excluded from 
modeling. There have been no comprehensive studies of colloid filtration within the defense 
high-level radioactive waste glass. Hence, meaningful analysis of colloid filtration separate from 
colloid generation within the waste package is currently not feasible. Therefore, a conservative 
assumption is made: all colloids formed within the waste (the calculated colloid source term) are 
assumed to exit the waste package and invert (see Section 4 for a discussion of the invert) and 
enter the unsaturated zone without filtration (BSC 2003f). 
September 2003 3-10 No. 8: Colloids
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3.2.3 Microbes and Colloidal Organic Components 
2, 
The occurrence of microbes in the repository has been evaluated in In-Drift Microbial 
Communities (CRWMS M&O 2000a). In assessing the potential effects on microbial 
populations within the engineered barrier system, In-Drift Microbial Communities (CRWMS 
M&O 2000a) considered the drift mineralogy; drift physical parameters; metals, alloys, and 
cement used in engineered barrier system components; waste dissolution rates and quantities; 
groundwater compositions and infiltration rates; and compositions and fluxes of gases (e.g., CO 
water vapor). Environmental limits on microbial activity considered include redox conditions, 
temperature, radiation, hydrostatic pressure, water activity, pH, salinity, available nutrients, and 
others. The abundance of water and phosphorous were found to be the two environmental 
components which, could limit the development of microbial communities in the Yucca 
Mountain repository under the probable physicochemical conditions likely to be present in the 
proposed repository configuration. However, water and phosphorous in the repository would 
potentially be available in sufficient quantities to allow for the growth of microbial communities 
at certain stages of the postclosure period (CRWMS M&O 2000a). 
Microbes can accelerate or retard the transport of radionuclides in a number of ways. First, in 
some systems, microbes can passively or actively bioaccumulate radionuclides across the cell 
membrane. If the microbes are mobile, they can facilitate transport of the radionuclides. 
Alternatively, the microbes may be readily filtered by the rock or may form biofilms, a part of 
biomass attached to rocks, in which case they retard transport. Second, microbes may produce 
exudates, for example organic complexants, which may enhance the solubility of radionuclides 
and affect their sorption characteristics if complexes are formed. Third, in the course of 
extracting energy or nutrients, microbes may generate colloids by degrading materials or may 
actually destroy colloids by consuming them or by facilitating agglomeration. For example, 
microbial oxidation of metallic iron can produce iron-oxide colloids and aggregates. Conversely, 
microorganisms can decrease the concentration of stable colloids by aggregating colloidal 
material that they use as a food source. This has been shown to result in a significant decrease in 
colloid concentrations (Hersman 1995). Fourth, in some systems, microbes are able to reduce 
the oxidation states of some multivalent radioelements (e.g., uranium, neptunium, and 
plutonium). Typically, reduced forms of radionuclides are less soluble and more strongly 
sorptive. They may also impact local groundwater chemistry in and around the waste package. 
Microbes and inorganic colloids in close proximity to one another may result in collisions, which 
allow mutual adhesion or adsorption and result in particle size growth (agglomeration) (Buffle 
et al. 1998; Hersman 1995). This increase in particle size can have several effects on the 
potential for colloids to facilitate radionuclide transport within the repository: (1) for a given 
range of pore sizes in the transport medium, the larger composite particles may become filtered 
more readily than the individual microbe or colloid; (2) the larger particles will tend to diffuse 
more slowly and/or may precipitate through gravitational settling; and (3) the interactions of 
relatively large populations of colloids and microbes can result in agglomerated particles 
sufficiently large that the suspension becomes unstable and the microbe and colloid particles 
flocculate. Reduced transport from increased particle size is even more significant in the 
situation where water is present as thin films within the pore spaces and on the engineered barrier 
system components. In all of these processes, the net result is particle size increase and reduced 
transport of colloids and associated radionuclides. Hersman (1995) conducted experiments with 
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Yucca Mountain-native bacteria and bentonite clay. In one test, agglomeration of clay colloids 
in a sterile microbial growth medium was compared to agglomeration in a medium into which a 
bacterium was introduced. The test results showed greater agglomeration in the growth medium 
inoculated with bacteria. In a second test, the bacteria were cultured in the medium and the test 
repeated with those bacteria, and similar results were noted. 
The development of biofilms on a substrate has been shown to result in an increased tendency to 
attract suspended inorganic colloids. Sprouse and Rittmann (1990) and Rittmann and Wirtel 
(1991) demonstrated that biofilm colonization increased the colloid cohesion efficiency, , in a 
system where the collector was granular activated carbon in a methanogenic fluidized bed and 
the colloids were milk solids approximately one micron in diameter. Lo et al. (1996) concluded 
from an experiment with Fe(III) oxide colloids in a biofilm reactor that the deposition of Fe(III) 
oxide colloids increased slightly with biofilms present. Further iron deposition on surfaces 
increased with increasing particle size, suggesting interception of the colloids and/or 
sedimentation. 
Although uncertainty surrounds the microbial effect, the experiments with bacteria suggest that 
microbial action will tend to increase the sizes of inorganic colloids, and promote gravitational 
settling and filtration. Thus, not including the effects of microbes in the colloid source term and 
transport analysis is considered conservative with respect to total system performance. 
3.2.4 Intrinsic Colloids 
Intrinsic colloids are colloidal-sized assemblages (between approximately 1 nm and 1 m in 
longest dimension) formed from the hydrolysis and polymerization of actinide ions dissolved in 
solution (BSC 2003f). They may form in the waste package and engineered barrier system 
during waste-form degradation and radionuclide transport. Intrinsic colloids are also called 
primary colloids, Type I colloids, Eigenkolloide, real colloids, and true colloids. 
The formation of intrinsic colloids is solubility limited and, thus, based on the solution 
chemistry. This fact prevents significant introduction of intrinsic colloids to the environment 
(CRWMS M&O 2001a, Section 6.1). There has been no evidence of their formation in Argonne 
National Laboratory defense high-level radioactive waste glass degradation tests (Ebert 1995, 
Sections 6.2.1 and 6.2.2). Degradation of CSNF may form intrinsic colloids (e.g., schoepite-like 
uranium hydroxides) close to the fuel surface where the fluid may be saturated with respect to 
uranium, but they are likely to dissolve in unsaturated fluid. Because experimental data supports 
the conclusion that intrinsic colloids are negligible, intrinsic colloids are not considered in the 
TSPA-LA. 
3.2.5 Effect of Temperature on Colloids 
Coupled thermal, hydrologic, and chemical effects are likely to result in unstable colloid 
suspension and, therefore, reduce the number concentration of colloids in suspension. With 
increasing temperature and increasing ionic strength, both conditions expected as a result of 
thermal, hydrologic, and chemical effects, colloid suspensions become less stable. Specifically, 
the coupled thermal-chemical perturbation will take place in the first few thousand years after 
repository closure (Figure 3-2). The thermal event produced by radiation heat will create a 
September 2003 3-12 No. 8: Colloids
Revision 2 
drying-out zone and induce water evaporation within the drift and the surrounding area, resulting 
in the increase in the ionic strength in the percolating solution. The chemical degradation of 
waste package will further concentrate the solution (Figure 3-4). The increase in ionic strength 
will reduce the stability of both smectite and iron oxyhydroxide colloids inside the drift. 
For example, the rate of particle coagulation is described by the stability relationship, W, defined 
as (BSC 2003f): 
(Eq. 3-2) W = exp(Vmax/kT) 
where, Vmax is the height of the energy barrier preventing particle coagulation; k is the Boltzman 
constant; and T is temperature. Thus, increasing the temperature will cause a colloid suspension 
to become less stable. This is because the elevated temperature will enhance Brownian motion 
of the particles and therefore increase the probability of interparticle collisions. 
In addition, the thermal event and its induced chemical perturbations are limited in space and 
transient in time. Because of the drying-out effect, a significant release of radionuclides from 
waste packages can be possible only after the thermal event, when the coupled thermal-chemical 
perturbations become completely attenuated and enough seepage water becomes available for 
leaching radionuclides. It is possible that some chemical perturbations may result in an increase 
in colloid concentration. However, such perturbations are expected to be transient and will 
attenuate rapidly as the colloid suspension moves away from the disposal room. 
This conclusion is consistent with the results reported by Triay et al. (1996) who found that, 
under the perturbed conditions expected at Yucca Mountain, colloid concentrations are unlikely 
to be high enough to effectively compete with the immobile rock for the sorption of 
radionuclides. High temperatures and high ionic strengths are not favorable to colloid transport. 
Therefore, the screening out of coupled thermal-hydrologic-chemical effects on the transport of 
radioactive colloids in the TSPA is a reasonable assumption. 
3.2.6 Colloid Sorption at the AirWater Interface 
The concentration of colloids sorbed at the gaswater interface is a function of the following 
conditions (BSC 2003f, Section 5.7): 
 The interface surface area available for colloid uptake, which is a function of the total 
gas saturation 
 The affinity of colloids for the gaswater interface (hydrophobic colloids have higher 
affinities than hydrophilic colloids) 
 The electrostatic charge on the colloidcolloids with lower negative charge exhibit a 
stronger affinity 
 The salinity of the aqueous phase with higher salinity promoting sorption. 
Empirical evidence suggests that the sorption affinity of colloids at the gaswater interface may 
be stronger than to the rock matrix (Wan and Wilson 1994). 
September 2003 3-13 No. 8: Colloids
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Partially saturated conditions may be classified by considering degrees of saturation. At low 
water saturations, the surface area of the gaswater interface approximates that of the rock 
matrix. Overall, colloid migration is retarded, although colloids may still move through the 
adsorbed water films. At intermediate water saturations, there is still an interconnected gas 
phase, although gas flux may be lower. The interface may act as a static sorbing surface, but 
estimating the geometry and surface area is complicated. At high water saturations, the majority 
of the gas is present as small gas bubbles that may migrate, transporting sorbed colloids. 
Colloid migration rates depend more strongly on colloid size as lower saturation states are 
considered (CRWMS M&O 2001b; McGraw 1996). To examine the influence of colloid size on 
transport, McGraw (1996) investigated transport of monodisperse colloids (five different sizes, 
between 20 nm and 1,900 nm) under both saturated and unsaturated conditions in a quartz sand. 
The results indicated that under saturated conditions the time required for breakthrough of 
50 percent of the original colloid concentration was the same as that of the breakthrough of a 
nonreactive tracer, indicating no relationship of colloid size to migration under saturated 
conditions. However, the times required for breakthroughs of the colloids under highly 
unsaturated conditions exhibited a strong relationship between the colloid breakthrough and the 
colloid size with fairly complete breakthrough of the 20-nm colloid and little or no breakthrough 
of the 1,900-nm colloid. 
Another set of experiments (McGraw 1996) compared four sets of hydrophobic and hydrophilic 
colloids (modified latex microspheres). The results indicated that transport of hydrophobic 
colloids depends on colloid size, water film thickness, and colloid charge density. In contrast, 
hydrophilic colloids were not affected by these variables and were rapidly transported through 
the system even under very low moisture contents. It was concluded that for hydrophobic 
colloids, the cumulative mass of colloids recovered relative to column input was logarithmically 
dependent upon the ratio of the water film thickness to colloid diameter. In contrast, for 
hydrophilic colloids, the cumulative mass of colloids recovered relative to the column input was 
linearly dependent upon the ratio of the water film thickness to colloid diameter; similar, but 
more pronounced than, the effect with the nonreactive tracer. These findings suggest that 
unsaturated porous media may not completely impede colloid migration when a water film is 
present, even for relatively large colloids, however, larger colloids will tend to be retarded more 
than smaller ones. 
Although the potential effects of degree of saturation on colloid transport are varied and 
complex, on balance colloids would be somewhat retarded under low-saturation both inside and 
outside the waste package. Because of this conclusion, sorption at the airwater interface is 
conservatively omitted within the waste package. 
3.2.7 Selection of Radioisotopes 
Transport of radioisotopes on colloids is potentially important for radioisotopes that (1) have 
long half-life and low solubility; (2) can be entrained in, or sorbed onto, waste forms, engineered 
barrier materials, or geologic barrier materials that generate colloidal particles; (3) represent a 
major portion of the inventory; and (4) have large dose conversion factors. Considering these 
four criteria as part of radionuclide screening, Waste Form and In-Drift Colloids-Associated 
Radionuclides Concentrations: Abstract and Summary (BSC 2003f) evaluated eight 
September 2003 3-14 No. 8: Colloids
Revision 2 
radionuclides for sorption onto colloids: plutonium, americium, thorium, cesium, protactinium, 
neptunium, uranium, and strontium. Considering these four criteria as part of radioisotope 
screening, the colloidal concentration abstraction concluded that five radioisotopes attached to 
colloids should be evaluated: plutonium, americium, thorium, cesium, and protactinium. Four 
of these were analyzed in TSPA for site recommendation (CRWMS M&O 2000b; Leigh and 
Rechard 2001). Cesium has been added for TSPA-LA based on an updated screening analysis 
(BSC 2003f). Plutonium, americium, thorium, cesium, protactinium are assumed to be attached 
to colloids reversibly using a linear isotherm model. Plutonium and americium are assumed to 
be predominantly attached to iron oxyhydroxide colloids and smectite waste form colloids 
irreversibly. The following discussion summarizes the rationale used to include or exclude these 
radionuclides from the model for reversible attachment to colloids (BSC 2003f, Section 6.3.3.1). 
Rationale for Including Five Radioisotopes 3.2.7.1 
PlutoniumPlutonium meets all four criteria. A large quantity of plutonium will exist in the 
repository (criterion 3). Plutonium is sparingly soluble (criterion 1) but sorbs strongly to oxide 
mineral surfaces (generally less strongly to silicates). Plutonium is observed to sorb strongly to 
soil mineral, and laboratory investigations have shown that it sorbs readily to colloids as well 
(criterion 2). Finally, plutonium has a large dose conversion factor. 
AmericiumAmericium also meets all four criteria. Americium will be a significant contributor 
to radioactivity during the first 10,000 years. Like plutonium, americium is sparingly soluble but 
strongly sorbs to mineral surfaces, including colloids. Laboratory investigations have shown that 
it sorbs strongly to colloids. 
ProtactiniumProtactinium will be a significant contributor to radioactivity during the first 
10,000 years. Because of this, and the fact that relatively little is known of the colloid behavior 
of protactinium, it was included in this analysis. 
ThoriumThorium will be a significant contributor to radioactivity during the first 10,000 years. 
Because of this, and the fact that there is evidence that thorium sorbs strongly to oxides, it was 
included in this analysis. There is relatively little known of the colloid-related behavior of 
thorium. 
Cesium135Cs has a long half-life and can attach strongly to certain sheet silicates (including 
clays) by means of ion exchange. For this reason, cesium has been observed to sorb to soil 
minerals, and it could potentially form pseudocolloids particularly with groundwater and defense 
high-level radioactive waste glass-derived clay colloids. 
Rationale for Excluding Neptunium, Uranium, and Strontium 3.2.7.2 
NeptuniumBecause neptunium will be the most significant contributor to radioactivity beyond 
the first 10,000 years, it was considered for inclusion in this analysis. Neptunium is more 
soluble under anticipated repository conditions than many of the other important radionuclides, 
and it sorbs considerably less strongly than, for example, plutonium and americium (see 
Table 3-1). The typical Kd values for neptunium sorption on Yucca Mountain-vicinity colloids 
are less than 100 mL/g. As demonstrated in Section 3.5, as long as Kd is less than 5000 mL/g, 
colloid-facilitated transport is not likely to be significant relative to transport of dissolved 
September 2003 3-15 No. 8: Colloids
species. It would appear then that the mobility of neptunium is influenced mostly by its 
solubility. For these reasons, and to simplify the modeling, neptunium was not included in the 
reversible-sorption portion of the colloid-associated radionuclide transport analysis. 
UraniumUranium will be by far the most abundant radioactive element in the repository and 
primarily for this reason was considered for the analysis. Uranium is more soluble under 
anticipated repository conditions than many of the other important radionuclides, and it sorbs 
considerably less strongly than, for example, plutonium and americium. Kd values for uranium 
sorption on Yucca Mountain-vicinity colloids typically are less than about 1,000 mL/g 
(Table 3-1). As demonstrated in Section 3.5, as long as Kd is less than 5,000 mL/g, colloidfacilitated 
transport should not be significant relative to transport of dissolved species. As with 
neptunium, the mobility of uranium is thus influenced mostly by its solubility. Field 
observations at uranium deposits and mine sites have indicated that little or no colloid uranium 
transport occurs. For these reasons, and to simplify modeling, uranium was not included in the 
reversible-sorption of the colloid-associated radionuclide transport analysis. 
StrontiumBecause of its very short half-life, strontium was not considered important in the 
groundwater pathway either as a dissolved species or attached to colloids. 
Modeled Kd for Plutonium, Americium, Thorium, Neptunium, and Uranium Sorption onto 
Kd Values, mL/g 
1 to 6  102 
1  101 to 1  102 
1  103 to 1  104 
2  103 to 9  104 
1  104 to 1  107 
September 2003 
Table 3-1. 
Yucca Mountain-Vicinity Colloids 
3.2.7.3 
Radionuclide Sorbate and Oxidation 
States at YMP 
U(VI) 
Np(V) 
Pu(V) 
3-16 
Th(IV) 
Am(III) 
Source: BSC 2003f, Table 9. 
Rationale for Irreversible Adsorption 
Defense high-level radioactive waste glass degradation experiments show that plutonium is 
probably irreversibly attached to smectite colloids generated during the experiments. Further, 
evidence from sorption experiments with plutonium and americium (Lu et al. 2000) with 
colloidal hematite and goethite show that the rates of desorption (backward rate) of plutonium 
and americium are significantly slower than the rates of sorption (forward rate). More 
importantly, over a significant time period (up to 150 days in some experiments), the extent of 
desorption is considerably less than the extent of sorption. Plutonium and americium are 
considered so strongly sorbed to colloids that, in essence, they can be considered irreversibly 
sorbed and are modeled in this manner within the engineered barrier system. Plutonium 
transport velocities in soils reflect the fact that plutonium binds strongly to soils, leaving very 
little, if any, soluble plutonium available for groundwater transport or plant uptake. Coughtrey 
et al. (1985) estimate exchangeable plutonium to be less than one percent. At Rocky Flats, 
plutonium in soil is largely bound to soil metal hydroxides. Litaor and Ibrahim (1996) used 
0.01M CaCl2 as an extractant and measured plutonium in Rocky Flats soil to be 0.04 to 
0.08 percent exchangeable. Bunzl et al. (1995) measured exchangeable 239Pu and 240Pu (0.5 to 
No. 8: Colloids 
Revision 2
Revision 2 
1 percent) and 241Am (1.5 to 15 percent) from fallout-contaminated soils in Germany using 1M 
C2H7NO2 (ammonium acetate NH4C2H3O2) as the extractant. Laboratory experiments of 
plutonium sorption onto iron oxides have shown that only approximately one percent of the 
initially sorbed plutonium can be desorbed into solution, even after months of time have elapsed 
(Lu et al. 2000), which is broadly consistent with field observations. For these reasons, 
plutonium and americium are modeled as irreversibly attaching to corrosion (iron oxyhydroxide) 
colloids. No other radionuclides are considered to be irreversibly attached to colloids. 
3.3 SOURCE OF DATA AND TESTING 
3.3.1 Tests on Defense High-Level Radioactive Waste Glass 
Tests on defense high-level radioactive waste in borosilicate glass have been conducted at 
Argonne National Laboratory using two modes of water contacting the waste (CRWMS M&O 
2001a). In the static-saturated test, glass samples were immersed in fluid for more than 
four years. In the dripping tests, fluid was dripped at specified rates onto glass samples. Fluid 
used was J-13 water and deionized water. It was observed in the static-saturated tests that 
colloids developed and increased in concentration with time, up to the point where the colloid 
concentration reached a maximum value and then decreased (Figure 3-5). From Figure 3-5, it is 
estimated that 1  10-7 M plutonium is equivalent to 5 ppm of total radionuclide-embedded 
colloids when both concentrations reach their maximum values. 
3.3.2 Tests of Waste Package Corrosion 
Maximum colloid concentration values are estimated based on experiments performed at 
University of Nevada, Las Vegas. In these experiments, scaled-down miniature waste packages 
were exposed to J-13 groundwater in either a bathtub mode or a flow-through mode. The 
experimental results obtained indicate that the iron corrosion products are mainly composed of 
magnetite (Fe3O4), lepidocrocite (FeOOH), and goethite (FeOOH) 
(DTN: MO0302UCC034JC.003). The cumulative results have yielded average concentrations 
of colloidal size materials in the range of 20 mg/L within the initial four weeks of the 
experiments (DTN: MO0212UCC034JC.002). 
3.3.3 Iron Oxyhydroxide Colloids in Nature 
Few data for iron oxyhydroxide colloids concentrations in nature have been found in the 
scientific literature, and the colloid concentrations reported vary greatly. At an iron rich ore 
body in South America, the Morro de Ferro natural analog site, the concentration was near 
1 mg/L. Values ranging from 0.6 mg/L to 260 mg/L were measured in the vicinity of a mined 
uranium ore formation at Cigar Lake in Northern Saskatchewan (Vilks et al. 1993). 
Two Swedish groundwater colloid concentrations were measured to be 0.02 mg/L and 
0.043 mg/L for saline and nonsaline groundwater samples, respectively (Laaksoharju et al. 
1995). The experiments performed at University of Nevada, Las Vegas on miniature waste 
packages produced colloids ranging in concentration from near 0 mg/L to approximately 
50 mg/L. As mentioned in Section 3.4, this latter information was used to define a uniform 
distribution between 0.05 and 50 mg/L. 
September 2003 3-17 No. 8: Colloids
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3.3.4 Stability of Smectite and Iron Oxyhydroxide Colloids 
The zero-point of charge of smectite is about pH 2 (Figure 3-6), below possible pH values 
anticipated within waste packages (BSC 2003h). Therefore, smectite colloids would remain 
stable if the ionic strength of the solution is low (less than 0.05 M). In the defense high-level 
radioactive waste glass tests conducted at Argonne National Laboratory, which were run at pH 9 
and 11.5, smectite colloids were detected (Buck and Bates 1999). Tombacz et al. (1990) 
investigated the stability of smectite (also referred to as montmorillonite) suspensions as a 
function of pH and ionic strength in an NaCl solution. The results are summarized in Figure 3-6. 
At a neutral pH, iron oxyhydroxide colloids tend to be unstable and agglomerate. The related 
stability diagram is displayed in Figure 3-7. 
Source: BSC 2003f, Figure 4. 
NOTE: zpc = zero-point of charge. 
Figure 3-6. 
No. 8: Colloids 
Experimental Determination of Montmorillonite (a Variety of Smectite) Stability as a 
Function of pH and Ionic Strength (M) 
September 2003 3-18
Revision 2 
Source: BSC 2003f, Figure 7. 
NOTE: zpc = zero-point of charge. 
Figure 3-7. 
No. 8: Colloids 
Schematic Representation of Iron Oxyhydroxide Colloid Stability as a Function of pH and 
Ionic Strength (M) 
Note that the pH of groundwater at Yucca Mountain is generally close to neutral. Within this pH 
range, iron oxyhydroxide colloids would have a minimal surface charge, thus reducing mutual 
repulsive forces and resulting in flocculation, even at a low ionic strength. This fact explains 
why iron oxyhydroxide colloids may occur in low concentrations in groundwater and is one of 
the reasons why smectite colloids are used to represent seepage/groundwater colloids. This fact 
also implies that any iron oxyhydroxide colloids released from waste packages into groundwater 
may become unstable and flocculate as they move through the disposal system. 
3.3.5 Colloid Concentrations in Groundwater 
The range of colloid concentration in seepage/groundwater was derived based on groundwater 
sampling from the Yucca Mountain area. Literature data (Degueldre et al. 2000) and 
groundwater sampling at the Idaho National Engineering and Environmental Laboratory 
(BSC 2003f) was used as additional technical information that corroborated the site-specific 
data. The results are summarized in Figures 3-8 and 3-9. Practically no colloids were detected 
for the ionic strength above 0.05 M. The upper limit of colloid concentration is about 200 ppm, 
with a median value around 0.1 ppm (Figure 3-9). 
September 2003 3-19
Source: BSC 2003f, Figure 11. 
NOTE: The ordinate values are in particles per milliliter (pt/mL). 
Figure 3-8. Groundwater Colloid Concentration Data Collected in the Vicinity of Yucca Mountain 
Compared with Data Collected from Groundwaters around the World 
Source: BSC 2003f, Figure 12. 
Figure 3-9. Cumulative Distribution Function Showing the Probability of Occurrence of Colloid 
Concentration Levels (ppm or mg/L) in Groundwater Samples in the Yucca Mountain Area 
and Idaho National Engineering and Environmental Laboratory 
No. 8: Colloids 
Revision 2 
September 2003 3-20
Revision 2 
3.3.6 Radionuclide Distribution Coefficients (K ds) 
The effectiveness of colloid-facilitated transport depends on the sorptive affinity of radionuclides 
to a substrate, which can be described by the distribution coefficient (Kd) (Eq. 3-1). The Kd 
values used in the colloid transport modeling were obtained from various data sources and 
measurements including those from National Cooperative for the Disposal of Radioactive Waste 
(Switzerland), U.S. Environmental Protection Agency, and Yucca Mountain-specific projects 
(BSC 2003f). Based on these sources, professional judgment was used to develop uncertainty 
distributions (Table 3-2). 
The work of Lu et al. (1998) needs to be mentioned specifically, because it provides the best 
direct evidence suggesting the irreversible sorption of plutonium onto colloidal particles. The 
work shows that Pu(IV) and Pu(V) were rapidly adsorbed by colloids of hematite, goethite, 
smectite, and silica in natural and synthetic groundwater. After five days, hematite colloids 
sorbed all Pu(IV) and Pu(V) present in the solution, goethite sorbed 97 to 100 percent of 
plutonium, smectite sorbed 94 to 100 percent plutonium, and silica sorbed 46 to 86 percent of 
plutonium. Desorption of plutonium from colloids of hematite, goethite, and smectite was much 
slower than the sorption process. After 150 days, less than 0.02 percent Pu(V) was desorbed 
from hematite colloids. Little colloidal Pu(IV) was desorbed from hematite colloids during 
150 days, even using sequential extraction under vigorous shaking conditions. Although 
desorption of Pu(V), as well as Pu(IV), from goethite and smectite colloids is relatively faster, 
only less than 1 percent of plutonium was desorbed from goethite, and 1.5 percent Pu(V) and 
2.5 percent to 11 percent of Pu(IV) were desorbed from smectite after 150 days. 
3.4 MODEL FOR COLLOID-FACILITATED TRANSPORT IN WASTE PACKAGE 
This section summarizes the algorithm used to incorporate the colloids source term abstraction in 
the TSPA-LA model, using a simplified model intended to retain the important principles and 
processes of the analyses. The logic implemented in the TSPA-LA model is provided, although 
specific programming details are not. 
September 2003 3-21 No. 8: Colloids
Table 3-2 Kd Values Used for Reversible Radionuclide Sorption on Colloids in Calculations for the Total 
System Performance Assessment for the License Application 
Radionuclide 
Pu 
Am, Th, Pa 
Cs 
Source: BSC 2003f, Table 10. 
NOTE: The Kd values for Tc and I are very low and not listed here. 
No. 8: Colloids 
Colloid 
Iron Oxyhydroxide 
Smectite 
Iron Oxyhydroxide 
Smectite 
Iron Oxyhydroxide 
Smectite 
Kd Value Range 
(mL/g) 
104 to 106 
103 to 106 
105 to 107 
104 to 107 
101 to 103 
102 to 104 
3-22 
Kd Value Intervals 
(mL/g) 
<1  104 
1  104 to 5  104 
5  104 to 1  105 
1  105 to 5  105 
5  105 to 1  106 
>1  106 
<1  103 
1  103 to 5  103 
5  103 to 1  104 
1  104 to 5  104 
5  104 to 1  105 
1  105 to 5  105 
5  105 to 1  106 
> 1  106 
<1x 105 
1  105 to 5  105 
5  105 to 1  106 
1  106 to 5  106 
5  106 to 1  107 
>1  107 
<1  104 
1  104 to 5  104 
5  104 to 1  105 
1  105 to 5  105 
5  105 to 1  106 
1  106 to 5  106 
5  106 to 1  107 
>1  107 
<1  101 
1  101 to 5  101 
5  101 to 1  102 
1  102 to 5  102 
5  102 to 1  103 
>1  103 
<1  102 
1  102 to 5  102 
5  102 to 1  103 
1  103 to 5  103 
5  103 to 1  104 
>1  104 
Revision 2 
Kd Value Interval 
Probabilities 
0
0.15 
0.2 
0.5 
0.15 
0
0
0.04 
0.08 
0.25 
0.2 
0.35 
0.08 
0
0
0.15 
0.2 
0.55 
0.1 
0
0
0.07 
0.1 
0.23 
0.2 
0.32 
0.08 
0
0
0.13 
0.22 
0.55 
0.1 
0
0
0.2 
0.25 
0.5 
0.05 
0 
September 2003
Revision 2 
3.4.1 Sources of Colloids 
As already noted in Section 3.1, three sources of colloids are considered (Figure 3-10): 
(1) natural colloids in the seeping groundwater, (2) colloids generated from degradation of the 
waste package and other metallic materials in the repository, and (3) colloids generated from 
degradation of high-level radioactive waste. Colloids formed from the defense high-level 
radioactive waste glass and the naturally occurring groundwater colloids are represented by 
smectite colloids as explained in Sections 3.3.1 and 3.3.5. The three colloid types can transport 
reversibly sorbed radioisotopes, but plutonium and americium radioisotopes attached on iron 
oxyhydroxides colloids can also transport as irreversibly attached radioisotopes (Figure 3-10). 
Furthermore, colloids generated from spallation of the high-level radioactive waste glass have 
plutonium and americium radioisotopes engulfed during formation and are thus irreversibly 
attached as well. The colloidal concentration model uses concepts developed for the Compliance 
Certification Application for the Waste Isolation Pilot Plant (DOE 1996; Larson 2000; Stockman 
et al. 2000), in particular, categories of colloids tracked, mode of radioisotope uptake, and 
colloid stability. 
Figure 3-10. Embedded, Reversibly, and Irreversibly Attached Radioisotopes on Colloids 
September 2003 3-23 No. 8: Colloids
Revision 2 
3.4.2 Algorithm Overview 
The algorithm in the colloidal concentration abstraction for TSPA-LA consists of three steps 
(BSC 2003f) that are executed at each time step in the calculations (Figure 3-11). First, the 
component determines the potential colloidal mass available, usually as a function of ionic 
strength. Second, the component determines the stability of the colloidal mass as a function of 
ionic strength and pH, where ionic strength and pH are determined by the in-package chemistry 
(BSC 2003h) of the waste form model (CRWMS M&O 2000c; Rechard and Stockman 2001). 
When the colloids are unstable, a low (nonzero) limit of colloid concentration is used. Third, the 
component determines the fraction of the available mass of the dissolved inventory of cesium, 
americium, plutonium, protactinium, and thorium to be adsorbed onto the colloids. The 
dissolved inventory of the five radioisotopes is determined by the dissolved concentration 
component model (BSC 2003g) of the waste form model (CRWMS M&O 2000c; Rechard and 
Stockman 2001). The colloidal mass generated in Step 1 and its stability as a function of ionic 
strength and pH in Step 2 are generally bounding, but some uncertainty is included in parameter 
values for sorption in Step 3. The specific methods employed for these three steps depend on the 
source of the colloids. The description follows. 
The abstraction calculates the concentration of dissolved radionuclides, the concentration of 
plutonium and americium embedded in defense high-level radioactive waste glass smectite 
colloids, the concentration of radionuclides sorbed reversibly onto the three colloid types, the 
concentration of radionuclides sorbed irreversibly onto iron-hydroxide colloids, and the mass 
concentration of each colloid type (Figure 3-11). 
The colloidal concentration abstraction assumes that colloids are in the groundwater that enters a 
breached waste package and adsorb radioisotopes. Groundwater colloids potentially present in 
the repository include (1) microbes, (2) organic macromolecules (humic and fulvic acids), and 
(3) mineral colloids (primarily clay minerals, silica, and iron oxyhydroxide minerals). Microbial 
colloids were disregarded because the relatively large size of microbes makes them susceptible 
to filtration by the geologic media as already described in Section 3.2.3. Organic 
macromolecules were also disregarded, based on their low capacity to form chemical complexes 
at Yucca Mountain. Specifically, Minai et al. (1992) studied humic substances from J-13 well 
water and found that the complexation capacity was only about 2.3  10-10 eq/L (equivalents per 
liter) at pH 6.9 and 2.7  10-11 eq/L at pH 8.2, because of the presence of calcium in J-13 well 
water. 
September 2003 3-24 No. 8: Colloids
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September 2003 
Source: BSC 2003f, Figure 17a. 
Figure 3-11. Algorithm for Determining the Availability and Stability of Colloids within the Waste Package 
3-25 No. 8: Colloids
Revision 2 
Mineral colloids were considered because quartz, feldspars, silica, cristobalite, fused silica, 
amorphous silica, aluminosilicates, layer silicates, zeolites, plagioclase, carbonate, smectite clay, 
hematite, and goethite colloids occur in groundwater in the vicinity of Yucca Mountain 
(Kingston and Whitbeck 1991). The mineral colloids were represented by smectite clays 
because smectite was the dominant colloid type observed and strongly adsorbs radioisotopes. 
As discussed in Section 3.3, the YMP has compiled the colloid concentration in groundwater 
from igneous rock in saturated and unsaturated regimes from the vicinity of Yucca Mountain. 
This relationship was used for Step 1 of the procedure to estimate the groundwater colloidal 
mass concentration (Figures 3-9 and 3-11). 
For Step 2, the colloidal concentration determined in Step 1 was maintained provided the pH and 
ionic strength were below the approximately linear stability function measured by Tombacz et al. 
(1990), based on known properties of smectite colloidal suspensions (Section 3.3.4, Figure 3-6). 
Otherwise, the colloidal concentration was reduced to a minimum value of 10-6 mg/L if ionic 
strength is less than 0.05 M. The threshold value of the ionic strength of the 0.05 M in the waste 
package used to calculate the concentration of groundwater colloids is identical to the upper 
threshold used for determining stability and concentration of waste form colloids, since both 
waste form and groundwater colloids are modeled as smectite. 
To determine the mass of radioisotopes (plutonium, americium, thorium, protactinium, cesium) 
reversibly sorbed by the groundwater colloids for Step 3, a linear Freundlich model (Eq. 3-1) 
(Langmuir 1997) was used, using the dissolved concentration of radionuclides within the waste 
package as calculated by the solubility component of TSPA-LA model. The sorption coefficient 
(Kd) values for cesium, plutonium, and americium (and, by analogy, protactinium and thorium) 
were assigned piecewise uniform distributions, based on sorption experiments on smectite (Table 
3-2). For cesium, the values range from 102 to 104 mL/g; plutonium ranged from 103 to 
106 mL/g; for americium, the values range from 104 to 107 mL/g (Table 3-2). 
3.4.3 Colloids from Iron Oxyhydroxide 
The colloidal concentration component assumes that iron oxyhydroxide (rust) colloids (primarily 
goethite, ferrihydrite, and hematite) form during corrosion of waste packages. For Step 1, 
a uniform distribution was assigned with a range of 50 mg/L and 0.05 mg/L, based on the 
miniature colloid experiments conducted at University of Nevada, Las Vegas (Section 3.3.2). 
For Step 2, the stability of iron oxyhydroxide colloids has been measured (Liang and Morgan 
1990) and was used for developing the stability region as a function of pH and ionic strength. At 
and near the zero-point of charge, colloids are unstable, even at low ionic strengths. For iron 
oxyhydroxide colloids, the zero-point of charge ranges from about pH 5.5 to about pH 8.5 
depending on the mineralogy and water chemistry. Colloids are stable at higher and lower pH 
values, similar to other mineral colloids, provided the ionic strength is less than 0.05 M. The 
result is a typical U-shaped stability curve (Figure 3-7). The minimum concentration of rust 
colloids in the unstable region was 10-3 mg/L. 
For Step 3, two types of sorption are modeled, irreversible sorption of a large fraction of 
plutonium and americium onto iron oxyhydroxide corrosion colloids and fixed corrosion 
September 2003 3-26 No. 8: Colloids
Revision 2 
products; and reversible sorption of a small fraction of plutonium and americium and reversible 
sorption of protactinium, thorium, and cesium onto iron oxyhydroxide colloids. 
Plutonium and americium are considered so strongly sorbed to colloids that, in essence, can be 
considered irreversibly sorbed and are modeled in this manner within the engineered barrier 
system (BSC 2003o, Attachment II). Plutonium transport velocities in soils reflect the fact that 
plutonium binds strongly to soils, leaving very little, if any, soluble plutonium available for 
groundwater transport or plant uptake. Coughtrey et al. (1985) estimate exchangeable plutonium 
(soil available, best estimate, Table 2a, p. 119) to be less than one percent. At Rocky Flats, 
soil plutonium is largely bound to soil metal hydroxides. Litaor and Ibrahim (1996) used 0.01M 
CaCl2 as an extractant and measured plutonium in Rocky Flats soil to be 0.04 to 0.08 percent 
exchangeable. Bunzl et al. (1995) measured exchangeable 239+240Pu (0.5 to 1 percent) and 241Am 
(1.5 to 15 percent) from fallout-contaminated soils in Germany using 1M C2H7NO2 (ammonium 
acetate NH4C2H3O2) as the extractant. Laboratory experiments of plutonium sorption onto iron 
oxides have shown that only about 1 percent of the initially sorbed plutonium can be desorbed 
into solution, even after five months of time have elapsed as already mentioned in Section 3.3.6 
(Lu et al. 2000), which is broadly consistent with field observations. 
In the colloid abstraction, the difference in specific surface areas between the colloids and the 
stationary corrosion products is taken into account as well as the total surface areas (based on 
total masses). For example, plutonium will tend to attach to the colloids with higher specific 
surface areas, while the much larger mass of stationary corrosion products relative to total 
colloid mass results in most of the dissolved plutonium attaching to the stationary corrosion 
products. Honeyman and Ranville (2002) incorporated a stationary phase in their analysis of the 
effects of colloids on radionuclide retardation. The stationary phase competes strongly with the 
colloids for radionuclide sorption. By far most of the americium mass was calculated as attached 
to the stationary phase; at most, 0.001 percent of the total americium was associated with 
colloids (Honeyman and Ranville 2002, p. 140). In order to accommodate the field and 
laboratory observations, distribution of plutonium between colloids and the stationary corrosion 
products was implemented such that a large fraction of total plutonium is sorbed to corrosion 
products, a small fraction to colloids, and a very small fraction remains dissolved in the fluid. 
The distribution of irreversibly and reversibly sorbed plutonium and americium onto fixed and 
colloidal substrates is performed using a kinetic model that is described in EBS Radionuclide 
Transport Abstraction (BSC 2003o). Figure 3-12 shows the two domains of the conceptual 
model of radionuclide sorption onto the iron oxyhydroxide colloidal and stationary phases. The 
upstream domain is assumed to be degraded fuel rods, including secondary phases, in 
equilibrium with the aqueous phase at the radionuclide solubility limit (CRNdiss) predicted by the 
solubility component (BSC 2003g). The radionuclides of concern are the plutonium and 
americium isotopes, but since the material balance equations are written as a mass balance, the 
equations are valid for any solute species. There is no sorption considered in the upstream 
September 2003 3-27 No. 8: Colloids
Revision 2 
domain. Plutonium and americium are transported by both advection and diffusion downstream 
into the reaction mixing cell domain, where they can be involved in six1 separate reactions: 
1. Reversible plutonium and americium sorption onto iron oxyhydroxide colloidal 
particles 
2. Reversible plutonium and americium sorption onto the stationary phase iron 
oxyhydroxide corrosion products 
3. Irreversible plutonium and americium sorption onto iron oxyhydroxide colloidal 
particles 
4. Irreversible plutonium and americium sorption onto the stationary phase iron 
oxyhydroxide corrosion products 
5. Reversible plutonium and americium sorption onto waste form colloids 
6. Reversible plutonium and americium sorption onto groundwater colloids. 
Note: SPu refers to dissolved plutonium from spent fuel, other parameters as defined in figure. 
   
1 Because plutonium and americium are also embedded into waste spalling from the high-level radioactive waste, 
they actually are involved in a total of eight separate reactions, but the embedded plutonium and americium are 
= 
determined separately and not included in the kinetic model. 
Figure 3-12. First Two Computational Cells of Conceptual Model of Reversible and Irreversible Sorption 
(Eq. 3-3) . 
September 2003 No. 8: Colloids 
on Mobile Iron Oxyhydroxide Colloids and Immobile Rust 
The mass in the fluid exiting the reaction mixing cell is constrained such that the mass of 
plutonium sorbed both reversibly and irreversibly onto colloids is between 90 percent and 
99 percent of the total mass of plutonium exiting the system in all forms.aqueous, reversibly 
sorbed, and irreversibly sorbed. This conforms to observations in nature, such as the transport of 
plutonium from the Benham test site (Kersting et al. 1999). This flux ratio is expressed as: 
_mass 0.90 0.99 _ flux_out 
_ flux_out 
colloid 
total _mass 
3-28
Revision 2 
Also of interest is the ratio of the mass flux leaving the mixing cell to the mass flux entering the 
mixing cell. This ratio of mass out to mass in is given by 
 (Eq. 3-4) flux _ out 
flux _ in 
mass _ 
mass _ 
= 
and is a measure of the retardation due to the corrosion products. It is expected that most of the 
plutonium mass entering the mixing cell is sorbed to the stationary iron oxyhydroxide phase and 
only a small fraction of it flows downstream to the unsaturated zone. 
Reversible sorption onto the colloids is calculated from the sampled mass concentration of 
corrosion colloids, Kd values between the fluid and iron oxyhydroxide colloids, and the dissolved 
concentration of radionuclides. The Kd values are defined with piecewise-uniform distribution 
describing the distribution of radionuclides (BSC 2003f, Table 10): the Kd for plutonium ranges 
between 104 and 106 mL/g. The Kd for americium, thorium, and protactinium ranges between 
105 and 107 mL/g. The Kd for cesium ranges between 101 and 103 mL/g. 
3.4.4 Colloids from High-Level Radioactive Waste 
The laboratory evidence suggests that as high-level radioactive waste glass degrades, colloids are 
generated and often contain embedded or irreversibly attached plutonium and americium 
(CRWMS M&O 2001a). The embedded plutonium and americium are likely isolated from the 
aqueous system and thus not in equilibrium with the surrounding aqueous environment. 
Therefore, some plutonium or americium in the high-level radioactive waste is assumed to be 
embedded in the high-level radioactive waste colloids. On the other hand, few colloids have 
been observed in laboratory tests of CSNF, and those colloids observed did not have 
radioisotopes irreversibly attached (CRWMS M&O 2001a). Furthermore, DSNF is assumed to 
behave similarly to CSNF (Section 3.2.). 
Step 1CFor Step 1, colloidal concentration was set to a bounding value above the maximum 
value observed in the high-level radioactive waste experiments (1  10-7 M) at low ionic strength 
(CRWMS M&O 2001a). At ionic strength greater than 0.05 M, colloidal concentration was set 
at a low value (10-11 M). At intermediate ionic strength (between 0.01 and 0.05 M), irreversible 
colloidal concentration was calculated by interpolating between concentrations of 10-11 and 
8  10-8 M. 
Step 1aCThe radionuclides plutonium and americium are modeled in Step 1a as embedded 
within waste form colloids, which are produced from corrosion of defense high-level radioactive 
waste glass. The concentrations are defined by ionic strength, based on experimental 
observations. 
In TSPA-LA, Ihi-thresh,coll,wf and Ilo-thresh,coll,wf are threshold values of the ionic strength of the fluid 
in the waste package used to calculate the concentration of embedded plutonium used in the 
abstraction. Americium is calculated as the product of the plutonium concentration times the 
Am/Pu ratio in the inventory, which is determined at each time step of the TSPA-LA model 
calculations. Stability of the defense high-level radioactive waste glass-derived colloids is 
September 2003 3-29 No. 8: Colloids
Revision 2 
determined in Step 1b. Colloid mass concentrations are calculated from the amount of 
plutonium embedded in smectite waste form colloids in Step 1c. 
Step 1bThe stability of waste form colloids from defense high-level radioactive waste glass is 
determined on the basis of the fluid ionic strength and pH, based on known properties of smectite 
colloidal suspensions. 
The Ihi-thresh,coll,wf and Ilo-thresh,coll,wf parameters are the same threshold values of the ionic strength 
of the fluid in the waste package used to determine plutonium concentration (Step 1a). They are 
used here to determine, along with pH, whether or not the colloids are stable. These threshold 
values may vary due to specific chemical and environmental conditions and are therefore 
sampled over a range. 
Step 1cCalculation of the mass concentration of waste form colloids from defense high-level 
radioactive waste glass is done on the basis of experimental observations.
RNcoll,wf,embed,max. It is an 
The colloid mass concentration is related to the embedded plutonium concentration 
(Equation 3-3). With reference to Equation 3-3, Mcoll,wf,both,max is the maximum colloid mass 
concentration corresponding to the maximum plutonium concentration, C 
intermediate result based on the experimental data showing the relationship between embedded 
plutonium concentration and colloid mass concentration. Mcoll,wf,both is a parameter that 
represents the total mass concentration of defense high-level radioactive waste glass colloids 
with both embedded and reversibly sorbed radionuclides. 
Step 1dThe calculation of the concentration of radionuclides (plutonium, americium, thorium, 
protactinium, cesium) reversibly sorbed on waste form colloids from defense high-level 
radioactive waste glass is based on the mass concentration of waste form colloids, Kd values 
describing the distribution of radionuclides between the fluid and smectite colloids, and the 
dissolved concentration of radionuclides as calculated by the TSPA-LA model. 
The CRNdiss is the concentration of radionuclide RN, determined by the TSPA-LA model as 
output from the solubility concentration model, which is used as an input to the colloid 
abstraction. Kd,RN,wf is a parameter derived from several sources and is an equilibrium sorption 
coefficient used to approximate the partitioning of dissolved radionuclide RN between colloids 
and fluid. 
Step 2For Step 2, the simple linear stability relationship that was used for groundwater colloids 
(primarily smectite) was also used to determine the stability of the irreversible high-level 
radioactive waste colloid concentration as a function of pH because waste form colloids are 
composed primarily of smectite clay minerals (CRWMS M&O 2001a). When in the unstable 
region, the colloid concentration was set to 10-11 M. 
Step 3For Step 3, the algorithm is more complicated for high-level radioactive waste because 
the radioisotopes can be both irreversibly and reversibly attached to the high-level radioactive 
waste colloids. The algorithm makes several assumptions: (1) only americium and plutonium 
are irreversibly attached; (2) the irreversibly bound isotopes of plutonium and americium are 
partitioned according to the isotopic mass fraction calculated at the time the colloids are formed; 
(3) no initial mass of decay daughters are assumed irreversibly bound; and (4) in any one time 
September 2003 3-30 No. 8: Colloids
Revision 2 
step, americium and plutonium do not attach reversibly until the maximum available mass of 
americium and plutonium in the high-level radioactive waste is irreversibly attached. That is, the 
concentration of irreversible americium and plutonium is first determined; then, the reversible 
available mass (including any available radioisotopes from DSNF) is calculated as the total mass 
available times the ratio of the irreversible concentration and the maximum irreversible 
concentration (Figure 3-11). The reversible concentration is determined with the linear 
Freundlich model. Both plutonium and americium used Kd values defined with piecewiseuniform 
distributions (Section 3.4.3). 
3.5 UNCERTAINTY AND SENSITIVITY 
3.5.1 Uncertainty of Colloid Concentration 
The parameters developed for the colloid transport model have various degrees of uncertainty. 
The sources of uncertainty arise due to (1) uncertainty associated with the use of non-sitespecific 
data in some cases to develop parameter values; (2) uncertainty associated with the 
prediction of actual physical and chemical conditions in the repository; (3) uncertainty associated 
with site- and environment-specific testing of colloid formation, stability, and transport; and 
(4) uncertainty associated with the use of supporting technical and corroborative information that 
may not have been developed specifically for these physical and chemical conditions. These 
uncertainties must be appropriately captured in the TSPA calculations by choosing conservative 
model formulations or appropriate upper/lower limits for parameter values. 
In the implementation of a TSPA, it is often necessary to use experimental data gathered over a 
few days to a few years and field observations recorded over a few days to a few decades to 
arrive at conclusions on what is anticipated to occur over the 10,000-year regulatory compliance 
period The parameters relating to irreversible association of radionuclides with colloids, which 
is influenced by kinetics, must be used cautiously when speculating about future conditions. 
Similarly, the extrapolation of laboratory-scale results to the scale of the repository setting also 
needs to be cautious. Corroboration with field data is generally required. Kersting et al. (1999) 
measured the total plutonium concentration at the ER-20 site, which is 1.3 km from the Nevada 
Test Site. The maximum plutonium concentration measured was about 10-14 M. Given the 
strong affinity of plutonium for solid surfaces, Kersting et al. (1999) argued that this low 
concentration indicated that a small fraction of plutonium could be potentially transported by 
colloids over a long distance. Whether this mechanism is plausible or relevant to the repository 
conditions (note that the physical/chemical conditions at Nevada Test Site can be very different 
from repository conditions), the low concentrations from Kersting et al. (1999) indicate that 
plutonium transported by groundwater can be attenuated very quickly over a short distance (~1.3 
Km). The plutonium concentration was about six orders of magnitude lower than the solubility 
limits of 10-8 M experimentally determined as likely to be present in Nevada Test Site 
groundwater. Therefore, these observations indicate that there would be a potential for longdistance 
colloid-associated plutonium transport in the Yucca Mountain environment (given the 
fact that plutonium tends to be strongly sorbed on solid surfaces), but the contribution of such 
transport to system performance is unlikely to be significant. 
Uncertainties associated with the determination of colloid concentration in seepage/groundwater 
derive from (1) field sampling techniques, including differences in pumping rates at each well 
September 2003 3-31 No. 8: Colloids
Revision 2 
during extraction of the water samples, (2) other unknown factors affecting the quantities of 
particles suspended in the water samples, including the types of additives introduced in the wells 
during the drilling process itself, and (3) errors inherent to the laboratory methods used to 
measure the quantities of colloids suspended in the water samples (e.g., filter ripening, 
interference and detection limitations for dynamic light scattering measurement techniques). The 
sampling perturbation may result in over-estimating colloid concentrations. 
With respect to the spatial scale, there is the issue of the appropriateness of extrapolation of 
specific water sample measurements to represent the colloid concentrations in waters over a 
wider region or area. Temporal scaling issues could include potential seasonal variations in the 
quantities of colloids suspended in water samples extracted, or even shorter time-scale changes 
in colloid concentrations in water samples during the sampling of wells. To define this 
uncertainty, a cumulative distribution function was developed based on groundwater samples 
extracted in the vicinity of Yucca Mountain (Section 3.3.5). To evaluate the reasonableness of 
this distribution for the site-specific data, the distribution function compared with colloid mass 
concentrations reported in the literature and at Idaho National Engineering and Environmental 
Laboratory (Figure 3-9). As shown in Figure 3-9, the probabilistic distribution of colloid 
concentrations developed for YMP is reasonable. 
3.5.2 Uncertainty in Stability of Colloids 
Uncertainty in the stability of colloids (smectite and iron oxyhydroxide) as a function of pH and 
ionic strength is associated mostly with the extrapolation of laboratory data reported in the 
literature or project-supported experimental work to actual conditions (i.e., solution chemistry) 
that would be present in the repository environment over the regulatory compliance period. 
Much of this uncertainty is accommodated by establishing conservative, but reasonable, 
bounding values and ranges of parameter values. This uncertainty is propagated through the 
TSPA-LA model by stochastic sampling of these distributions during Monte Carlo simulations 
employed in the model calculations. 
3.5.3 Uncertainty in Partition Coefficients 
Uncertainty is associated with the development of sorption partition coefficients (Kd values) to 
describe the degree of sorption of specific radionuclides to colloids. Values reported in the 
literature are primarily the result of experimental work designed to establish Kd values for 
contaminants sorbed onto rocks, soils and other minerals, but literature specific to colloid-size 
minerals is not readily available. For this reason, the Kd value parameters established for 
smectite and iron oxyhydroxide colloids rely on limited experimental work conducted at Los 
Alamos National Laboratory (Lu et al. 1998). Corroborative data reported in the literature were 
evaluated to augment the Los Alamos National Laboratory data (Table 3-2). 
The transport of a radionuclide in the presence of a constant colloid concentration can be 
described by: 
  .    ) ( ) ( ) 1 [( ) ] ( ) ( ( m m V ) m cV c d c d s s c d + + = .   D D 
 
 
 
x 

 
 
 
 
 
cm
t 
m
x 
c m
x 
m
x  t x 
.. 
. . . 
. 
.. 
. + . 
. 
x 
(Eq. 3-5) 

 
3-32 
 
t 
No. 8: Colloids 
 
 
September 2003
Revision 2 
where  is the porosity; md is the concentration of dissolved radionuclide (M); c is the colloid 
concentration (g/L); mc is the moles of radionuclide sorbed on colloids (mol/g); s 
is the density 
of the immobilized adsorbing phase (g/dm3); ms is the moles of radionuclide sorbed on the 
immobilized adsorbing phase (mol/g); V is the flow velocity (dm/y); D is the dispersion 
coefficient of the dissolved species (dm2/y); t is the time (y); and x is the spatial coordinate (dm). 
Substituting Equation 3-1 into Equation 3-5 for mc, one can obtain: 
)V cK )D cK 2m d d d d d = (Eq. 3-6) 2 
m
x 
m
t    

 ) 1 ( + 1 ( . ) 1 ( ) 1 ( cK K cK K x d d d s d 
.
d 
 
cK ) ' (Eq. 3-7) 

 
d s 
by: 
 
Q . 
s 
V T  ' (Eq. 3-8) L 
V T < ' 
1 ( + 
cK ) d 
1 ( +
+ 
d 
include technetium, iodine, and cesium. 
For 1 cK >> , Equation 3-8 is reduced to: d 
1 ( +
+ 
V 
d 
VT . L for .. 
.
V  'T L for 
.  
L (Eq. 3-9) Q
1 ( +
) + 
Therefore, the actual velocity of the advancement of the adsorption front ( ' V ) can be calculated 
by: 
V = 
Assuming that a fluid percolates through a porous medium of a length of L (dm), the total 
radionuclide release from the system over a time period T (y) can be approximately calculated 
AF 
V 
where A is the cross-section area of the system; and F is the incoming flux (mol/dm2/y). For 
L , Q = 0, i.e., no radionuclide will be released from the system. 
For the colloid-facilitated radionuclide transport to be negligible compared to the dissolved 
species transport, it is required that cK << 1. Using the upper colloid concentration limit of 
200 ppm in the Yucca Mountain groundwater, it can be calculated that any radionuclide with a 
d K less than 5,000 mL/g can be ignored for colloid-facilitated transport. These radionuclides 
.. 
. 
. AF 
V
1 ( +
.. 
. 
colloids to the immobilized adsorbing phase, 
+ 
.. 
. 
Therefore, as long as Kd is large enough (greater than 5000 mg/L), the total radionuclide release 
from the system becomes insensitive to Kd and only depends on the concentration ratio of 
c /(1. )  , which is typically very small. s 
1 ( + 
cK ) + (1. ) K 
cK ) d 
K 1 ( . d s )  
VT . 
c 
c + (1. ) s 
3-33 September 2003 No. 8: Colloids
3.6 MODEL CONFIDENCE BUILDING 
Sensitivity studies conducted to prioritize work for the LA (BSC 2002a) showed that the colloids 
had a small effect on the dose. Therefore, a relatively low level of confidence is required for the 
colloid model. 
The types and characteristics (including stability and concentration) of colloids formed from the 
degradation of the waste forms as used in the model abstraction is based on extensive 
observations of colloids from testing programs and natural groundwater. 
The validation activities for this model analysis and abstraction took into account the criteria 
established in the Yucca Mountain Review Plan, Final Report (NRC 2003). Post-modeldevelopment 
validation is accomplished through corroboration of model predictions and data 
used with data published in referred journals or literature and through corroboration by 
comparison to data from natural analog sites. Detailed validation arguments are documented in 
Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and 
Summary (BSC 2003f). Corroborating/supporting data and information used to develop and 
validate the parameters are listed in Table 3-3. It is concluded that the validation activities 
performed for building confidence in the model elements and data developed for YMP colloidal 
transport have sufficient scientific basis and that the acceptance criteria documented in the Yucca 
Mountain Review Plan, Final Report (NRC 2003) will be met satisfactorily. 
Supporting (Corroborating) Information Used to Build Confidence in the Colloid Model Table 3-3. 
Supporting (Corroborating) 
Information Source 
Tombacz et al. 1990 
Mertz et al. 2003 
Short et al. 1988 
Payne et al. 1992 
Znker et al. 2000 
Vilks et al. 1993 
Brady et al. 2002 
DTN LA0002SK831352.003 
DTN LA0002SK831352.004 
Coughtrey et al. 1985 
Litaor and Ibrahim 1996 
Bunzl et al. 1995 
No. 8: Colloids 
Revision 2 
Data/Information 
Stability of smectite (which is used as a surrogate mineralogy for defense high-level 
radioactive waste glass colloids) at full range of conditions anticipated in TSPA-LA 
calculations 
Corroborative data supporting conclusion of no significant colloids generated from 
CSNF degradation 
Corroborative information and data regarding low colloid-associated uranium 
concentrations in the vicinity of mines 
Corroborative information and data regarding low colloid-associated uranium 
concentrations in the vicinity of mines 
Corroborative information and data regarding low colloid-associated uranium 
concentrations in the vicinity of mines 
Corroborative information and data regarding low colloid-associated uranium 
concentrations in the vicinity of mines 
Corroborative information and data regarding limited extent of dissolved uranium 
plumes 
Corroborative (non-Q) data for qualified groundwater data 
Corroborative information regarding irreversibility of plutonium on mineral particles 
Corroborative information regarding irreversibility of plutonium on mineral particles 
Corroborative information regarding irreversibility of plutonium on mineral particles 
September 2003 3-34
Revision 2 
3.7 SUMMARY 
Three different types of colloids are considered in the modeling of the radionuclide source term: 
 Waste-form colloids resulting from corrosion of defense high-level radioactive waste 
glass with embedded discrete phases of plutonium and americium and represented by 
smectite colloids 
 Iron oxyhydroxide colloids resulting from the corrosion of waste package and metallic 
invert materials 
 Natural colloids in seeping groundwater, including mineral fragments, humic substances 
and microbes and represented by smectite colloids. 
Humic substances are not considered to be sufficiently abundant in Yucca Mountain 
groundwater to be of significant impact to radionuclide transport. Also, microbes, due to their 
relatively large size (about 1 m) within the colloid-size range, are highly susceptible to 
filtration. Smectite colloids are used to represent natural colloids at the Yucca Mountain site, 
because of their abundance and relatively high affinity for radionuclides. 
True or intrinsic colloids are eliminated from consideration in TSPA analyses for several 
reasons: (1) long-term corrosion testing with CSNF and DSNF under unsaturated, oxidizing 
conditions have shown very low colloid concentrations and small amounts of uranium within the 
colloid mass; and (2) the same test indicates that meta-schoepite and UO2+x colloids readily 
dissolve upon the introduction of J-13 groundwater. 
Defense high-level radioactive waste glass degradation experiments show that plutonium can be 
embedded in smectite colloids. Further, evidence from sorption experiments show that 
plutonium and americium are strongly adsorbed onto iron oxyhydroxide colloids and the rates of 
desorption of the two radionuclides are significantly slower than the rate of sorption. In the 
model it is assumed that over 90 percent of the adsorbed plutonium and americium is irreversibly 
adsorbed to both corrosion colloids and stationary corrosion products. The sorption of all other 
radionuclides is assumed to be reversible. 
Colloid suspensions are likely to be unstable partly because of evaporation during the thermal 
period and waste degradation reactions may lead to ionic strengths above 0.05 M. For the 
seepage-water dripping scenario, the most relevant in-package chemistry scenario with regard to 
colloid transport, reaction-path calculations indicate that the initial seepage water composition is 
rapidly overtaken by water-package interactions (BSC 2003h). Depending on the seepage flux, 
as the chemical reaction progresses, the water can reach an ionic strength more than 0.05 M, the 
upper ionic-strength limit for stable colloid suspension. Therefore, under such conditions, 
colloid suspensions tend to be unstable. 
Once formed and stably suspended, colloids could be filtered out. Notwithstanding the potential 
colloid filtration processes that would reduce the amount of radioactive colloids released from 
the waste package, it has been assumed that the waste package does not provide a barrier to the 
release of radioactive colloids. Therefore, the dominant mechanism leading to low 
September 2003 3-35 No. 8: Colloids
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concentrations of colloids in suspension is the expected lack of stability of the colloids due to 
chemical conditions that lead to high ionic strengths. 
September 2003 3-36 No. 8: Colloids
Revision 2 
4. INVERT 
This section of the technical basis document summarizes the current understanding of the 
formation of radionuclide-carrying colloids within the invert and their release into the 
unsaturated zone. The invert is the second of the repository system subfeatures examined to 
determine the importance of colloids with respect to the postclosure performance of the Yucca 
Mountain repository system, as shown in Figure 1-1. This section provides a summary-level 
synthesis of the processes and phenomena governing the formation, prediction of their 
concentration, and transport of pseudocolloids within the invert. Modeling assumptions to be 
used in the TSPA-LA are discussed. In preparing this section, information has been extracted 
and summarized in pertinent analysis and model reports as well as other technical products that 
are used: 
 Multiscale Thermohydrologic Model (BSC 2003n) 
 Advection Versus Diffusion in the Invert (BSC 2003j) 
 EBS Radionuclide Transport Abstraction (BSC 2003o) 
 Engineered Barrier System: Physical and Chemical Environment Model (BSC 2003k). 
4.1 DESCRIPTION OF RELEVANT PROCESSES AND MODELING ASSUMPTIONS 
FOR INVERT 
4.1.1 Collapse of Pallet Supporting Waste Package 
The invert, which is primarily composed of crushed tuff rocks, is 0.8 m thick. The relevant 
physical/chemical processes for colloid generation and transport in the invert are summarized in 
Figure 4-1. The waste package sits on a pallet composed of Alloy 22. The pallet, in turn, sits on 
an invert. Degradation of the pallet is not considered since it has been assumed to have collapsed 
by the time the waste package has breached. Hence, the waste package is assumed to be in 
intimate contact with the invert below. When releases of colloidal material are primarily through 
diffusion, as in the nominal case without any dripping seepage water, neglecting the pallet 
shortens the radionuclide transport path and, thus, is a conservative assumption as discussed in 
the feature, event, or process (FEP) screening-out argument for FEP 2.1.06.05.0A (BSC 2003o). 
In the following discussion, the invert also provides a large diffusion path for colloids in this 
situation as well. Either diffusion barrier by itself is probably sufficient to remove all colloids. 
Thus, it is necessary only to model one barrier. Because the pallet is reasonably omitted in the 
other two scenarios considered in the TSPA (the scenario where an igneous dike intrudes into the 
repository and the scenario where a seismic event severely shakes the repository), the pallet was 
not considered. 
September 2003 4-1 No. 8: Colloids
Revision 2 
Figure 4-1. Relevant Physical/Chemical Processes for Colloid Generation and Transport 
4.1.2 Water Flow 
All flux that may originate from the waste package flows to the invert, independent of patch/pit 
location on the waste package. The flux that is diverted around the waste package and the drip 
shield also flows into the invert. In addition, seepage water may also enter the invert by a 
wicking process. Finally, evaporation may take place in the upper portion of the invert during 
the thermal pulse over the first few thousand years after repository closure (BSC 2003o). 
Evaporated water may condense on the inside of the drip shield, and drip onto the waste package. 
All mass flux into the invert, except for the evaporative flux, is released into the unsaturated 
zone. During the regulatory period, the type of water flow through invert depends on the 
scenario under consideration (nominal, seismic, or volcanic), as explained further in 
Section 4.3.3. For the most part, the crushed tuff invert will maintain unsaturated conditions 
under the waste package and thereby reduce advective transport of colloids out of the engineered 
barrier (see Figure 3-2). 
4.1.3 Invert Chemistry 
Because of the generally slow seepage of water into the invert previously described, any seepage 
water entering the invert will have time to react with the crushed tuff. The resulting solution will 
tend toward neutral to mildly basic pH. Any seepage water entering the invert will combine with 
the flux from the waste package, and likely alter the chemistry of the flux from the waste 
package, as well. As a result, the iron oxyhydroxide colloids may become unstable (as shown in 
Figure 3-7). Also, the evaporation process may concentrate the solution, which may further 
September 2003 4-2 No. 8: Colloids
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increase the instability of colloids in the invert. As explained further in Section 4.3.3, the 
chemistry of the water used in the invert was dependent on the scenario. 
4.1.4 Generation of Additional Colloids 
In addition to those colloids directly input from the waste package and the seepage water, similar 
colloids could be present in the invert and generated from the corrosion of pallet and steel 
supports within the invert. The corrosion of invert support material and electrical conductors 
will generate iron and copper oxides, which are generally strong adsorbents for various 
radionuclides. Although the mass of material in the drift including the steel mesh, ribs, invert 
support, and drip shield (~1.8 Mg/m) is about the same as in a CSNF waste package 
(~1.6 Mg/m), additional colloids present in the drift and the sorption effect were not included as 
important phenomena. First, the waste package material is in closer proximity to the actinides. 
Second, the spatial distribution of the corrosion products in the invert will tend to be localized 
and relatively widely spaced. Hence, seepage water from the waste package could completely 
miss corrosion products in the invert. Third, the iron oxyhydroxide colloids, representing the 
corrosion products, may be unstable in the invert water chemistry, as noted above. Finally, the 
additional sources are not expected to change the upper limits of colloid concentrations estimated 
in Section 3 from systems that have achieved metastable equilibrium states. 
4.1.5 Colloidal Filtration 
The extent of colloidal filtration cannot be expressed with certainty since physical properties of 
the crushed tuff have not been specified. Because of the large difference in colloid particle size 
and crushed tuff in the invert, physical filtration such as straining will likely be unimportant in 
the invert. Within the invert, the filtration process is not included because of the small 
dimension of the invert (0.8 m maximum) and, thus, limited opportunity for sufficient interaction 
of particles with the media. As an example of a model applied to nuclear waste disposal, Aguilar 
et al. (1999) modeled the filtration potential in an analysis for the performance assessment of the 
Waste Isolation Pilot Plant in Carlsbad, New Mexico. Their model predicted substantial 
filtration of colloids suspended in Waste Isolation Pilot Plant brines, often in distances of several 
meters. Based on typical parameter ranges and using a similar approach, Rechard et al. (2003) 
have shown that on average only minor filtration would occur in the invert due to the short 
filtration path of 0.8 m (maximum) and it has been ignored in the TSPA. However, filtration is 
modeled in the unsaturated and saturated zones in Sections 5 and 6, because of its greater extent. 
4.2 SOURCE OF DATA AND TESTING 
Diffusion coefficients used by the YMP for the invert material are tabulated in Table 4-1. 
Corroborative information is also available since the diffusion coefficient of ions has been 
determined specifically for crushed tuff invert materials as a function of volumetric moisture 
content (m3 water/m3 bulk rock), using electric conductivity measurements (BSC 2003o, 
Table 41). The data are tabulated in Table 4-2. The diffusion coefficient for colloids is 
estimated from the Stokes-Einstein relationship (see Equation 4-1 in Section 4.3). 
September 2003 4-3 No. 8: Colloids
Table 4-1. Diffusion Coefficient for Granular Materials at Various Volumetric Moisture Contents 
Volumetric Moisture Content 
(%) 
66.3 
49.0 
40.0 
32.3 
30.9 
20.4 
17.5 
10.1 
9.24 
5.40 
3.70 
Source: BSC 2003o, Table 6. DTN: MO0007MWDIDD31.001. 
Diffusion Coefficient of Crushed Tuff Invert Materials Table 4-2. 
Volumetric Moisture Content 
(%) 
32.13 
18.15 
9.26 
5.41 
3.64 
0.20 1.25  10-10 
Source: BSC 2003o, Table 7. DTN: MO0002EBSDDC02.003. 
No other specific tests have been performed simulating the configuration of the invert; however, 
tests on the invert material, iron and tuff, have been performed. The results of these tests, as 
discussed in Section 3.3, are used. For example, for the radionuclide partitioning, the same 
distribution coefficients tabulated in Table 3-2 are applied. 
4.3 MODEL FOR COLLOID-FACILITATED TRANSPORT IN INVERT 
This section summarizes the algorithm used to reevaluate the colloid stability. The logic 
implemented in the TSPA-LA model is provided, although specific programming details are not. 
No. 8: Colloids 
Diffusion Coefficient 
(cm2/s) 
1.8  10-5 
6.09  10-6 
6.90  10-6 
4.60  10-6 
1.51  10-6 
4.19  10-6 
1.10  10-5 
3.51  10-7 
2.55  10-7 
7.60  10-8 
3.70  10-8 
Diffusion Coefficient 
(cm2/s) 
2.02  10-6 
5.40  10-7 
4.05  10-8 
9.97  10-10 
5.00  10-10 
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4.3.1 Two Flow Regimes in Invert 
The crushed tuff in the invert in the current LA design is characterized by intergranular pore 
space (pore space in between the aggregate) and intragranular pore space (pore space in the 
matrix aggregate) (BSC 2003j; BSC 2003k; BSC 2003o). The intragranular pore space has a 
relatively low intrinsic permeability and porosity whereas the intergranular pore space has a 
relatively high permeability and porosity. After the thermal period the invert should achieve 
hydrologic equilibrium with the host rock through capillary action or imbibition, that is, water 
saturations in the intragranular pore space will reach approximately 90 percent (same as host 
rock matrix) and water saturations in the intergranular pore space will remain near zero. During 
the thermal period the pore space in the invert will be dry or near dry and water will gradually 
enter the intragranular pore space as the invert cools. 
4.3.2 Algorithm Overview 
The algorithm for the invert cell (BSC 2003k) divides the invert into these two pore spaces and 
implements the chemistry that corresponds to each pore space. That is, first, waste-package 
releases are partitioned into two fractions (Figure 4-2): (1) the intergranular pore space release 
fraction and (2) the intragranular pore space fraction. An important simplification is made by 
assuming no mass transfer occurs between the two pore spaces. The partition of the flow in the 
two flow regimes in the algorithm differs for the three main scenarios evaluated: nominal 
scenario, seismic scenario, and igneous scenario. The flow fraction entering the intragranular 
pore space is transported by diffusion using chemistry (i.e., ionic strength ionic strength and pH) 
appropriate for the tuff matrix, as evaluated by the drift-scale thermal-hydrologic-chemical 
model (BSC 2003p) and then evolved by the physical and chemical environment model 
(BSC 2003k). The flow fraction entering the intergranular pore space is transported by 
advection and diffusion using chemistry appropriate for the fractures using the tables developed 
by the physical and chemical environmental model (BSC 2003k). 
Second, the abstraction determines the stability of the colloidal mass as a function of ionic 
strength and pH, using the same stability curves as developed for the colloidal source-term 
discussed in Section 3. Third, the amount of radioisotopes reversibly sorbed on colloids is 
recalculated based on the radioisotope solubility for cesium, americium, plutonium, 
protactinium, and thorium adjusted for the appropriate invert chemistry. Finally, the colloids are 
transported via diffusion or advection through the invert. 
4.3.3 Partition of Contaminated Water Between Inter- and Intraporosity of Invert 
The distribution of water contaminated with dissolved radioisotopes and colloids with 
radioisotopes attached to the inter- and intraporosity of the invert depends on whether the 
nominal, seismic, or igneous scenario is being evaluated. In turn, the appropriate water 
chemistry to evaluate the stability of colloids is dependent upon whether the colloids enter the 
inter- or intraporosity of the invert. 
September 2003 4-5 No. 8: Colloids
Source: BSC 2003k. 
4.3.3.1 
No. 8: Colloids 
Figure 4-2. Colloidal Transport Algorithm within the Invert 
Nominal Scenario 
The nominal scenario represents the collection of FEPs most likely to occur. Based on corrosion 
analysis, the drip shield will remain intact for the regulatory period and, thus, any seepage water 
that enters the drifts will be diverted away from the waste package. Hence, the only source of 
water is condensation or dripping of condensation onto the waste package. With this limited 
amount of water, in the nominal scenario the waste packages do not fail through generalized or 
localized corrosion or stress corrosion cracking within the 10,000-year regulatory period. This is 
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similar to the model treatment in the TSPA for the site recommendation (DOE 2002). Hence, for 
the nominal scenario, releases can only occur because a small fraction of the waste packages 
have small manufacturing defects. In this situation, all the releases occur via diffusive pathways 
through the manufacturing defects in the waste packages. Within the shadow of the drip shield, 
most radionuclides will be transported via diffusion through the intragranular pore space. 
Diffusive transport through the intergranular pore space will be insignificant since water 
saturations in the upper portion of the invert will be near zero. Diffusive transport of colloidal 
material through the intragranular pore space of the invert will likely be small, since the diffusion 
coefficient for colloids is at least 100 times smaller than for dissolved species based on the 
relative size of colloids to dissolved species. The colloidal diffusion coefficient can be estimated 
from the following Stokes-Einstein relationship (BSC 2003o). 
ion (Eq. 4-1) ion Dcoll = D r
rcoll 
 . 
. 
100 times smaller than that of a dissolved ion. 
(Eq. 4-2) T = 
Seismic Scenario 4.3.3.2 
. .. 
. 
where Dcoll is the diffusion coefficient for a colloidal particle of radius rcoll and Dion is the 
diffusion coefficient of an ion of radius rion. Given a typical ion radius and colloidal particle 
radius of 0.1 and 1 nm, respectively, the diffusion coefficient of a colloidal particle is generally 
Furthermore, the characteristic time (T) for dissolved radionuclides to diffuse through the invert 
of thickness L can be calculated by: 
D L / 2 
where D is the diffusion coefficient. For an invert transport thickness of 0.8 m, the characteristic 
time T for colloid diffusion is ~3000 years at 90 percent saturation (i.e., the diffusion coefficient 
is 6.09  10.8 cm2/s for colloids for 90 percent saturation, which corresponds to a moisture 
content of 49 percent in Table 4-1, assuming porosity equal to 0.54 for the tuff). 
Any mixing that occurs between waters in the shadow of the drip shield will be by the slow 
process of diffusion of anions and cations. The water chemistry in this shadow will remain 
similar to matrix pore water as ions migrate and react with the water constituents and minerals in 
the matrix. Thus, the chemistry for evaluating the stability of the colloids corresponds to the 
matrix pore water chemistry as determined by the physical and chemical environment model 
(BSC 2003k). 
In the seismic scenario evaluated out to 10,000 years, an earthquake damages both the drip shield 
and the waste package. Hence, some advective transport of colloids can occur for those fractions 
of packages that encounter dripping water. Colloidal releases from diffusion for those packages 
that either do not have damaged drip shields or do not encounter dripping water will likely be 
negligible as described for the nominal scenario. In those packages encountering dripping water, 
colloidal releases can also occur through diffusion into the inter- and intragranular porosity of the 
invert. Unlike the nominal scenario, intergranular diffusion could occur since the invert 
saturation will not be zero for waste packages with dripping water. Consequently, the stability of 
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colloids that are advected through the invert or diffuse via the intergranular porosity must be 
evaluated using the fracture water chemistry as estimated by the physical and chemical 
environment model (BSC 2003k). 
Igneous Scenario 4.3.3.3 
In the igneous scenario, an igneous dike is envisioned to intersect the repository. Two distinct 
zones are formed. In Zone 1 where the disposal drifts are intersected by the dike, the drip shields 
and packages are assumed to be disrupted and breached from either the shock wave or heat from 
the igneous intrusion. In Zone 2, no damage is assumed to occur, and, thus, the same conditions 
used in the nominal scenario are assumed. Degradation of the waste and formation of colloids in 
both Zones 1 and 2 is assumed to occur once the drift cools. In Zone 1, the drift is assumed to be 
completely filled and the host rock and waste package disrupted enough that the drift no longer 
influences water percolating through Yucca Mountain. Hence, a uniform percolation flux 
through drifts and waste is hypothesized. Consequently, advective transport of colloids from all 
the packages through the invert will occur in Zone 1. The water chemistry for evaluating the 
stability of the colloids is modeled as that formed from the pore water equilibrated with basalt. 
4.3.4 Re-evaluation of Colloidal Stability and Solubility 
Based on the appropriate ionic strength and pH of the seeping water carrying the colloids in the 
invert, the colloid stability is reevaluated. Because the material is the same, the stability 
functions developed and used for the source-term (Section 3) are used in the invert as well. 
Irreversibly attached plutonium and americium are modeled as a separate dissolved species. If 
the water chemistry is such that the colloids are unstable, colloids would flocculate and settle 
out. This situation is modeled as if the species precipitated. If at a later time step, the water 
chemistry is such that the colloids are stable, then the irreversibly attached and embedded 
colloids are assumed to be resuspended and modeled as if the species redissolved. 
The situation is different for colloids with reversibly attached radioisotopes. If the colloids are 
unstable, then they would flocculate and settle out. However, the radioisotopes are still assumed 
to be reversibly attached and able to desorb if the solution is below the solubility limit. Then, the 
solubility of the radioisotopes is reevaluated based on the invert water chemistry. 
4.3.5 Re-evaluation of Adsorption 
In the final step, the dissolved radioisotope mass is redistributed between the solution and total 
number of colloids that are available based on the colloid stability functions and the reversible 
adsorption coefficients (Kds). The total number of colloids is used since the conceptual model 
does not exclude iron oxyhydroxide (rust) colloids with irreversible attached radioisotopes from 
also reversibly adsorbing radioisotopes. 
A distinct difference, however, is that irreversible adsorption on iron rust colloids is no longer 
active in the invert. Adsorption on immobile tuff material can occur in the invert, but is probably 
only minor. The adsorption coefficients for tuff are small (three orders of magnitude less than 
iron rust) and so adsorption on the 0.8 m maximum thickness of the invert will be small 
compared to potential adsorption in about 300 m of unsaturated tuff. Steel support structures 
will be placed throughout the invert to support the steel rails on which the waste packages and 
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drip shields will be transported, and on which the emplacement pallets will be placed. When this 
support corrodes, much of the iron rust will remain in the invert. In addition, communication 
and electrical cables will eventually corrode, leaving copper oxides in the invert. Although these 
oxides are capable of sorbing substantial amounts of radioisotopes, the oxides will be highly 
localized and widely separated in relation to the 5-m-long waste packages. For example, the 
transverse support beams in the invert are spaced 1.5 m apart (BSC 2003o). Thus, the oxides 
will reside in a strip several centimeters wide separated by 1.5 m of crushed tuff containing little 
corrosion products. In short, these materials are out of the general flow paths under the waste 
package, and they are of small spatial extent. Thus, the chance of radioisotopes being released 
from the waste package and passing through the adsorbing oxides is not great. 
4.3.6 Advective and Diffusive Transport 
The final step is the evaluation of the advective transport of radioisotopes attached to colloids out 
of the invert cell based on the fluid velocity through the cell, and the diffusive transport of 
colloids based on the colloid concentration gradient and the colloid diffusion coefficient (Dcoll). 
The colloid diffusion coefficient is set 100 times smaller than the diffusion coefficient of 
dissolved radioisotopes based on the 100 times difference between the ionic radius of a 
radioisotope and the radius of a colloidal particle. The actual calculation involves the solution of 
a similar one-dimensional differential equation used when evaluating transport within the waste 
package mentioned in Section 3. 
Intergranular pore space water saturation during the postclosure period is calculated based on an 
analysis that defines intergranular pore space saturation as a function of dripping flux. The 
saturation of the intragranular porosity is assumed equal to the tuff matrix saturation, based on 
the Multiscale Thermohydrologic Model (BSC 2003n). 
4.4 UNCERTAINTY AND SENSITIVITY 
4.4.1 Uncertainty in Chemical Environment 
Within the invert, the stability of colloids and the solubility of radioisotopes is a function of the 
chemical environment. For the nominal and seismic scenarios, the uncertainty in composition of 
seepage water entering the emplacement drifts and the invert during the postclosure period is 
represented by five different seepage water groups. The time-dependent compositions of these 
five seepage groups are provided by the drift-scale thermal-hydrologic-chemical model (BSC 
2003p) and the in-drift physical and chemical environment model (BSC 2003k). Each seepage 
water group includes two water types, matrix water and fracture water. Fracture water is 
assumed to enter the emplacement drift by dripping at the crown of the drift. Matrix water enters 
the drift by imbibition from the host rock matrix into the intragranular pore space occupying the 
invert ballast material. The capillary pressures in the relatively coarse intergranular pore space 
are too small to produce imbibition. 
When crown seepage drips onto the invert, some of it may imbibe into the intragranular matrix 
and some of it will flow downward through the intergranular pore space. The dripping water 
will tend to travel through the intergranular pores since the permeability of the matrix is much 
lower than the intergranular permeability. Because of the low matrix permeability, advection 
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through the matrix will be minimal even when seepage is occurring. Water that imbibes into the 
matrix will react with the matrix water and the minerals in the matrix. Hence, the mixed water in 
the matrix over time will tend to become similar to matrix water. 
The time-dependent fracture and matrix water compositions for each of the five different seepage 
water groups are used as input to the physical and chemical environment model. The physical 
and chemical environment model evaporates these different waters over a range of representative 
environment conditions for the engineered barrier system (e.g., temperature, partial pressure of 
CO2, and relative humidity) and the results for both water typesmatrix water and fracture 
waterare abstracted into the form of lookup tables for use by the TSPA-LA model. Each 
realization selects a water and the corresponding lookup tables for invert and crown seepage 
chemistry are accessed. The five uncertain water groups are given equal probability weighting. 
4.4.2 Uncertainty in Adsorption 
The reversible coefficients for adsorption on colloids in the invert are the same coefficients as 
used in the waste package. Hence, the uncertainty discussed in Section 3.5 is applicable here as 
well. 
4.5 SUMMARY 
The distribution of water contaminated with colloids with radioisotopes attached to the inter- and 
intraporosity of the invert depends on whether the nominal, seismic, or igneous scenario is being 
evaluated. For the nominal scenario, only diffusive releases can occur and thus the potential for 
colloidal transport is negligible. For the seismic and igneous scenarios, some advective transport 
can occur. In these scenarios, the appropriate water chemistry to evaluate the stability of colloids 
is dependent upon whether the colloids enter the inter- or intraporosity of the invert. Based on 
the appropriate ionic strength and pH of the seeping water carrying the colloids in the invert, the 
colloid stability is reevaluated. Because the material is the same, the stability functions 
developed and used for the source-term (Section 3) are used in the invert as well. In the final 
step, the dissolved radioisotope mass is redistributed between the solution and total number of 
colloids that are available, based on the colloid stability functions, and the reversible adsorption 
coefficients (Kds). Irreversible adsorption on iron rust colloids is no longer active in the invert. 
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5. TRANSPORT OF COLLOIDS IN UNSATURATED ZONE 
This section of the report summarizes the current state of knowledge concerning the transport of 
radionuclide-carrying colloids in the unsaturated zone, and their release into the saturated zone. 
The unsaturated zone is the third of the repository system subfeatures examined to determine the 
importance of colloid transport with respect to the postclosure performance of the Yucca 
Mountain repository, as shown in Figure 1-1. Modeling assumptions to be used in the TSPA-LA 
are discussed below. 
The information presented in this section primarily draws on the discussions contained in the 
following documented sources: 
1. Radionuclide Transport Models Under Ambient Conditions (BSC 2003m). 
2. Unsaturated Zone Colloid Transport Model (CRWMS M&O 2000d), which is used 
for reference only in the discussion of the alternate conceptual model of colloid 
transport, Section 5.5. 
This chapter is organized in the following way: (1) Section 5.1 describes the processes that 
could occur during the transport of colloids; (2) Sections 5.2 through 5.6 describes the 
unsaturated zone process model, which includes a subset of the processes described in 
Section 5.1 along with input data and results; and (3) Section 5.7 describes the TSPA model 
abstraction. 
5.1 DESCRIPTION OF RELEVANT PROCESSES 
This section describes processes relevant to the transport of colloids through the unsaturated 
zone at Yucca Mountain. The discussion focuses on a realistic view of the transport and does not 
necessarily reflect the conceptual model or numerical models used to predict the transport of 
colloids through the unsaturated zone. Often in a model conservative assumptions are made 
when there is a lack of data to support what is believed to be more realistic. Assumptions made 
in moving to the conceptual model and numerical models used are discussed in Sections 5.2 
through 5.6. 
Many of the processes relevant to the transport of dissolved species are also relevant to the 
transport of colloids. However, there are processes unique to colloid transport. Figure 5-1 
presents a schematic illustration of colloidal transport processes in the relevant geologic units of 
the unsaturated zone. Transport below the repository may proceed by two very different means 
depending on whether devitrified, vitric or zeolitic tuff is encountered. When devitrified or 
zeolitic rock is encountered, fracture flow will be dominant due to the low permeability of the 
rock matrix, and colloids will tend to remain in the fractures. When vitric rock is encountered, 
matrix flow dominates leading to colloid transport mainly through the rock matrix, where 
colloids are more likely to be filtered or trapped. 
The common transport processes are summarized in Section 5.1.1. Transport processes 
specifically relevant to colloids are summarized in Section 5.1.2. Finally, current information 
pertaining to colloids at the Yucca Mountain site is summarized in Section 5.1.3. 
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Schematic Illustration of Colloidal Transport Processes in the Relevant Geologic Units of 
the Unsaturated Zone 
Figure 5-1. 
5.1.1 Common Transport Processes 
The following transport processes are relevant to both dissolved species and colloids throughout 
the unsaturated zone. 
AdvectionDissolved species and colloids will move by advective transport in groundwater. 
Advection in fractures is expected to be the dominant transport mechanism in many layers of the 
various hydrogeologic units. In a few hydrogeologic units, such as the CHv (Calico Hills vitric 
unit), pp2 (Prow Pass Unit 2), and pp3 (Prow Pass Unit 3) (BSC 2003m, Section 6.6.6), matrix 
flow is dominant, resulting in much slower transport velocities (compared to those in the 
fractures of other units) and longer radionuclide-matrix contact times (BSC 2003m, Section 6). 
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Colloids are much more likely to be filtered in matrix flow than fracture flow because of lower 
velocities and higher rock surface area to water volume ratios. 
Hydrodynamic DispersionHydrodynamic dispersion includes both mechanical dispersion 
arising from local velocity variations and molecular diffusion driven by concentration gradients. 
Dispersion of the radionuclides occurs both along (longitudinal) and transverse to the average 
flow direction. Hydrodynamic dispersion dilutes and smears sharp concentration gradients and 
reduces the breakthrough time of radionuclides to the water table. The dispersion coefficient is a 
function of dispersivity and flow velocity. 
Hydrodynamic dispersion is not expected to play an important role in unsaturated zone transport. 
First, significant dispersion effects are implicitly accounted for by the fracture-matrix 
dual-continuum approach, which explicitly models velocity variations between these continua. 
Second, the potential repository emplacement area is very broad (relative to the distance to the 
water table), which tends to make lateral dispersion effects less important (CRWMS M&O 
2000e, Section 6.2). 
SorptionSorption accounts for a combination of chemical interactions between the dissolved 
radionuclides and the solid phases. The solid phases may be the immobile rock matrix or 
colloids, which are mobile or immobile depending on their transport characteristics. By 
removing a portion of the dissolved species from the mobile liquid phase and transferring it to 
the immobile solid phase, sorption reduces the rate of advance of the concentration front 
(i.e., retards a dissolved or suspended species). However, the sorption of a dissolved species 
onto a mobile colloid can enhance the rate of advance of the species relative to its rate of 
advance as a solute. This process is called colloid-facilitated transport. 
Matrix DiffusionMatrix diffusion plays an important role in radionuclide exchange between 
the fractures and the rock matrix. Radionuclide diffusion into the rock matrix and away from the 
fracture surface is driven by a concentration gradient, and it will slow the advance of 
radionuclides by removing them from the faster flowing fractures. Matrix diffusion of colloids is 
modeled in the radionuclide transport process model using the Stokes-Einstein equation to 
represent colloid diffusion rates in water and a tortuosity factor to represent the geometrical 
effects of the pore structure (BSC 2003m, Section 6.2.6.2). 
Radioactive DecayRadioactive decay reduces the concentration of most radionuclides over 
time. However, the production of radioactive daughter products adds complexity because 
transport simulations must compute the total radioactivity distribution (i.e., the sum of the 
concentrations of all the members of the radioactive decay chain). This is especially significant 
if the daughters have long half-lives. The daughter products may have significantly different 
transport behavior than the parent radionuclide. These processes affect both dissolved 
radionuclides and those attached to colloids. 
5.1.2 Transport Processes Relevant to Colloids 
The following processes are relevant to colloid transport in the unsaturated zone. 
Colloid Deposition onto Rock SurfacesColloid deposition (physico-chemical filtration) during 
flow through a porous medium is commonly assumed to occur in two steps: (1) transport of 
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colloids to matrix surfaces by Brownian diffusion, interception, or gravitational sedimentation 
and (2) attachment of colloids to matrix surfaces. The attachment efficiency (i.e., the fraction of 
collisions resulting in attachment) is strongly influenced by short-range interactions between 
colloids and matrix surfaces, such as van der Waals and electric double-layer interactions, steric 
stabilization, and hydrodynamic forces (Kretzschmar et al. 1995, p. 435). The magnitude of 
these interactions is dependent on water chemistry, colloid and substrate composition and surface 
chemistry, and water flow rates. Kretzschmar et al. (1997, p. 1,129) demonstrated that colloid 
deposition generally follows a first-order kinetic rate law and experimentally determined the 
corresponding collision efficiencies. 
Colloid Interactions with Rock and RadionuclidesColloid attachment to the host rock is 
strongly dependent on electrostatic interactions. Once attached, colloid detachment (declogging) 
is generally slow to irreversible. Sorption of radionuclides on colloids is controlled by a number 
of chemical processes such as ion exchange, surface complexation, and organic complexation 
(EPRI 1999, p. 4-9). 
Colloid FiltrationWan and Tokunaga (1997, p. 2,413) distinguish two types of straining: 
conventional straining (if the colloid is larger than the pore throat diameter or the fracture 
aperture) and film straining (if the colloid is larger than the thickness of the adsorbed water film 
coating the grains of the rock). Wan and Tokunaga (1997, pp. 2,413 and 2,419) developed a 
conceptual model to describe colloid transport in unsaturated media as a function of water 
saturation, Sw. If the rock Sw exceeds a critical saturation value, Sc, colloids move through the 
system within locally saturated pore spaces. For Sw<Sc, colloids can only move in the thin film 
of water that lines the grain boundaries. Colloid transport through the water film depends on two 
parameters: the ratio of the colloid size to the film thickness, and the flow velocity. Airwater 
interfaces are assumed to be immobile and thus impart drag on the colloids as they move through 
the flowing water in the films. When the film thickness is small enough that the colloids come in 
contact with both the airwater interface and rock surfaces, the drag forces and surface 
interactions at both interfaces immobilize the colloid. Temporal variations in the Sw in the 
subsurface profile and in the infiltration rate can lead to strongly nonlinear colloid mobility in the 
vicinity of Sc. 
McGraw and Kaplan (1997, p. 5.2) investigated the effect of colloid size (from 52 to 1,900 nm) 
and Sw (from 6 percent to 100 percent) on colloid transport through unsaturated media in 
Hanford sediments. They showed a very strong dependence of filtration on the colloid size 
under unsaturated conditions. At a volumetric water content of 6 percent, colloid removal 
increased exponentially with colloid size. The decrease in colloid mobility at low volumetric 
water contents was attributed to resistance due to friction (as the colloids were dragged along the 
sand grains), which is essentially proposed as an alternative or additional film-straining 
mechanism. Colloid retardation increased as the ratio of the water film thickness to the colloid 
diameter decreased. 
Colloid Deposition onto AirWater InterfacesUnder unsaturated conditions, colloid transport 
may be either inhibited or enhanced (relative to saturated conditions) because of the presence of 
the airwater interface. Wan and Wilson (1994, pp. 857, 863) determined that retention of both 
hydrophilic and hydrophobic colloids increased with the gas content of the porous medium. 
They showed that colloids preferentially concentrate in the gaswater interface rather than on the 
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matrix surface. This tendency increases with the colloid surface hydrophobicity or contact angle, 
with hydrophobic colloids having the strongest affinity for the gaswater interface. Capillary 
forces keep the colloids immobilized at the interface. The implications of this colloid behavior 
may be important. Colloid affinity for the gaswater interface may retard their transport through 
the unsaturated zone if the interface is not moving. Conversely, if the subsurface conditions 
permit films to flow, as well as the stability and migration of bubbles, colloid transport may be 
enhanced. Deposition onto airwater interfaces is differentiated from film straining in that the 
former occurs when colloids are small relative to water film thickness, whereas the latter occurs 
only if colloids are large relative to film thickness. In a deposition situation, the airwater 
interface is essentially considered an additional surface to which colloids can attach in an 
otherwise saturated flow domain. Models of unsaturated colloid transport contain terms that 
account for film straining, attachment to airwater interfaces, and attachment to rockwater 
interfaces. 
The affinity of colloids for the airwater interface depends on their hydrophobicity and 
electrostatic charge. Hydrophilic colloids (such as organic colloids and microbes) have a low 
affinity for the interface, in contrast to hydrophobic colloids (such as mineral fragments). This 
affinity increases with the positive charge on the colloids (EPRI 1999, p. 6-11). The flow and 
saturation conditions in the unsaturated zone will determine whether this will enhance or retard 
transport. 
Colloid Detachment from Surfaces/InterfacesColloids that are deposited onto surfaces can 
later become detached and reenter the mobile phase. Detachment can occur as result of random 
fluctuations in short-range forces between the colloid and rock surface or between the colloid 
and airwater interface that cause the colloid to jump over an energy barrier that normally 
keeps it immobilized. Alternatively, detachment can be caused by mobile colloids or particles 
colliding with immobile colloids or by macro-scale transients in flow conditions (saturation or 
flow rates) and water chemistry, or even by large-scale energy inputs such as seismic events. 
Detachment is often described as a first-order kinetic process, although many studies have 
demonstrated that it may be an inherently nonlinear process (Dabros and Van de Ven 1982; 
Dabros and Van de Ven 1983). If colloid attachment and detachment are both linear first-order 
processes with rate constants given by katt and kdet, respectively, then a colloid retardation factor 
can be defined as Rcol = 1 + katt/kdet. 
Colloid Generation in the Unsaturated ZoneColloids can be generated in the unsaturated 
zone by several different processes. These include weathering, diagenesis, erosion, and 
dissolution of material that holds small particles in place on rock surfaces. Episodic flow may 
generate colloids by sweeping airwater interfaces, which colloids can attach to, across rock 
surfaces. Generation is important because it increases the mobile surface area for solution-phase 
radionuclides to adsorb to, and it can also mobilize radionuclides that are adsorbed to surfaces 
that become transformed into colloids. 
Gravitational SettlingBecause of their large size relative to solutes and their density contrast 
with ground water, colloids are subject to gravitational settling. Settling can affect the transport 
of colloids by enhancing colloid velocities in vertically-oriented fractures and increasing 
collision frequencies with rock surfaces in nonvertically oriented fractures. Settling velocities 
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are proportional to the difference in density between colloids and ground water and to colloid 
diameter squared, so settling is clearly more important for larger colloids. 
Film StrainingFilm straining occurs when colloids have a greater diameter than the water film 
thickness. The colloids get sandwiched between the airwater interface and the rock surface, 
which cause the colloids to become immobilized. 
5.1.3 Colloids at the Yucca Mountain Site 
Colloids can be categorized as waste-form colloids, corrosion produced colloids, and natural 
colloids. Waste-form colloids include glass (smectite) colloids with embedded plutonium and 
americium. For these colloids, the radionuclides are irreversibly attached onto the colloids. 
These radionuclides transport identically to the transport of the colloid onto which they are 
attached. Corrosion colloids include iron oxyhydroxide. Dissolved radionuclides may become 
attached to these colloids either reversibly or irreversibly. Natural colloids include clays, bioorganisms, 
and smectite. A natural colloid becomes a pseudocolloid when a dissolved 
radionuclide adsorbs onto it. True (intrinsic) Pu(IV) colloids (generated from the contaminant 
when its concentration exceeds its solubility) have been produced by the agglomeration of 
hydrolyzed Pu(IV) ions under acidic conditions (EPRI 1999, p. 3-2). 
The available data on colloid occurrence and concentrations at the Yucca Mountain site pertain 
to saturated zone studies. The current state of understanding is that (1) the natural colloid 
concentrations in ground water at the Yucca Mountain site are low (106 to 1,010 colloids/mL) 
and (2) waste-form colloids will be the dominant colloidal species of concern to transport studies 
(BSC 2003m, Section 6.18.1). 
Colloids diffuse more slowly than dissolved species because of their larger size (BSC 2003m, 
Section 6.2.5, Equation 6-23). For the largest colloids, diffusion is approximately three orders of 
magnitude slower than that for molecular species (Nuttall et al. 1991, p. 189). For example, the 
diffusion coefficient, D0, of a 0.1 m colloid (probably not the largest size) at 20C is 
4.29  10-12 m2/sec (BSC 2003m, Equation 6-23), while the D0 of Br is 2.08  10-9 m2/sec 
(Cussler 1984, p.147). 
Using mercury porosimetry, Roberts and Lin (1997, pp. 577 to 578) determined that the average 
pore diameters of welded and densely welded TSw tuff samples were 53.1 nm and 19.7 to 
21.4 nm, respectively. These extremely small pores are certain to exclude a significant 
proportion of colloids. It is possible for colloids to accumulate on the fracture walls and thus 
clog the matrix pores open to the fracture. This can lead to a reduction in the matrix 
permeability and a reduction in the colloid diffusion into the matrix. 
5.2 MODELING ASSUMPTIONS 
The current model for describing transport of dissolved and colloidal species in the unsaturated 
zone of Yucca Mountain is the unsaturated zone radionuclide transport model (BSC 2003m, 
Section 6). Only assumptions impacting colloid transport in the unsaturated zone are presented 
here. The complete list of assumptions for the radionuclide transport model and the discussion of 
the supporting rationale are in Radionuclide Transport Models Under Ambient Conditions 
(BSC 2003m, Section 6.1.5). 
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The following assumptions have been made: 
1. The individual and combined effects of diffusion (molecular and/or colloidal), surface 
diffusion, and hydrodynamic dispersion follow a Fickian model. Given the 
macroscopic continuum approach, this is a valid assumption (de Marsily 1986, pp. 228 
through 277). 
2. Transport is assumed to occur isothermally at 25C. This assumption of isothermal 
transport at the average ambient temperature of the unsaturated zone is consistent with 
the assumption of isothermal flow at the same temperature. The transport parameters 
affected by temperature are (1) the diffusion coefficient D0 of the dissolved or 
colloidal species and (2) the sorption parameters of the dissolved species or the 
filtration parameters of the suspended colloid. 
Natural temperature differentials in the undisturbed unsaturated zone profile occur 
because of the geothermal gradient. Substantial temperature increases over the 
ambient temperature are expected after radioactive waste emplacement in the 
repository (BSC 2003p, Section 6). An increasing temperature leads to a higher D0 
value (BSC 2003m, Section 6.1.2.9). Based on this relationship, an increase in 
temperature from 20C at the top of the unsaturated zone domain to 30C at the water 
table leads to an increase of D0 by about 30 percent. 
The effect of temperature on sorption and/or filtration is less well defined. The 
general effect of an increasing temperature is a decrease in the sorption of anionic 
species and an increase in the sorption of cationic species. Theoretical and 
experimental studies indicate an increase of the distribution coefficient Kd with 
temperature when sorption follows a linear equilibrium isotherm (BSC 2003p, 
Section 6). 
Colloid filtration (deposition) generally follows a kinetic process (BSC 2003m, 
Sections 6.2.3 and 6.16.2). An increase in temperature increases the forward filtration 
coefficient, +, indicating an increase in the filtration (deposition, clogging) rate (BSC 
2003m, Equation 6-31). There is no information on the effect of temperature on the 
reverse filtration coefficient, . Thus, an increasing temperature in the unsaturated 
zone enhances diffusion and increases sorption and/or filtration. This is a particularly 
important mechanism in species mass transfer from the flow-dominating fractures to 
the matrix. The cumulative effect is slower transport. 
The assumption of isothermal transport should not be viewed as an approximation of 
the prevailing conditions in the unsaturated zone, but rather as a condition that reflects 
a transport scenario that leads to conservative estimates of radionuclide travel times to 
the water table. 
3. The concentration of the radioactive solutes or colloids is too low to have any 
measurable effect on the flow regime. Colloids and radionuclides migrating from the 
repository are expected to occur at concentrations that are too low to affect the 
aqueous solution density (BSC 2003p, Section 6). 
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4. Filtration of colloids is assumed not to affect the medium porosity and permeability. 
Wasteform colloids and natural colloids under natural conditions are present in 
sufficiently small concentrations (BSC 2003m, Section 6.1.5.2) to make this a valid 
assumption. This reference also indicates that, as a result of conditions adverse to 
colloid stability (e.g., pH, ionic strength) in the immediate vicinity of the repository, 
radionuclides in colloidal form will be released in low concentrations for a long time. 
5. The dual-permeability model is a valid approximation for flow and transport 
simulations because it describes the matrix and fractures as separate but interconnected 
gridblocks and permits flow and transport between matrix gridblocks, fracture 
gridblocks, and fractures and matrix. A more detailed discussion on the rationale for 
this assumption can be found in (BSC 2003q, Section 6.1.5.3). 
A number of other conservative assumptions are made in the modeling of the transport of 
colloids through the unsaturated zone. These assumptions are justified as being conservative 
because their impact would be an overestimation of releases and dose. These are: 
6. The conditions for the creation and stability of solute and colloidal species exist, the 
radioactive tracers (solutes or colloids) are stable, unaffected by the near field 
conditions (thermal, geochemical, physical). This is a conservative assumption 
because the elevated temperature and high ionic strength expected in the near field will 
tend to destabilize colloid suspensions. However, the impact of this conservatism has 
not been evaluated. 
7. The radioactive tracers (solutes or colloids) are not subject to chemical immobilization 
(e.g., through irreversible sorption or precipitation) anywhere in the unsaturated zone. 
The impact of this conservatism has not been evaluated. 
8. The vertical fractures are assumed to be open and continuous throughout the 
unsaturated zone from the repository to the water table. This may not necessarily be 
important based on the results of a study that compared the impact of releasing 
radionuclides from the repository directly into faults and not releasing the 
radionuclides into the faults (BSC 2003m, Section 6.20). The analysis shows little 
difference in colloid concentration histories at the water table. The results indicate 
that faults are well connected and that colloids are able to make their way to these fast 
paths when they are released some distance away from the faults. 
9. The horizontal fractures are assumed to be interconnected and are also connected 
(directly or indirectly) to the vertical fractures. While this is a conservative 
assumption, its impact may not be significant for the reasons stated in the previous 
assumption. 
10. Sorption of colloids on fracture walls or in fractures is excluded. This may be a very 
conservative assumption. The C-Well tests indicated that retardation of colloids in 
fractures is significant (see Section 6.4.2 of this technical basis document). Applying 
the C-Wells data from saturated-zone volcanics to the unsaturated zone is conservative 
(see Section 5.7.2). Additionally, comparisons of results from the radionuclide 
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transport model with the alternate conceptual model using FEHM (see Section 5.6.3), 
which considers colloid retardation in the fractures, indicate that colloid retardation in 
the fractures can significantly delay colloid breakthrough. 
11. Film straining of colloids is excluded. This is a conservative assumption as colloids 
will not become immobilized by this process. The impact of this conservative 
assumption has not been evaluated. 
A summary of the processes in the process-level model for unsaturated zone colloid transport is 
given below: 
1. Advection 
2. Dispersion 
3. Matrix diffusion 
4. Colloid attachment and detachment in the matrix 
5. Colloid attachment and detachment in the fracture (assumed to be zero except for 
sensitivity cases) 
6. Pore size exclusion 
5.3 SOURCES OF DATA AND TESTING 
5.3.1 Data 
Only colloid-related data are summarized here. Details concerning other transport processes that 
may affect colloid-assisted radionuclide transport such as (1) infiltration rate, (2) dispersion, 
(3) sorption coefficients, (4) matrix diffusion, (5) fracture characteristics, (6) radionuclide decay, 
and (7) daughter product contributions are described in the specific input data sets and their 
associated data tracking numbers (DTNs) and sources listed in Table 4-1 of Radionuclide 
Transport Models Under Ambient Conditions (BSC 2003m). 
Colloidal Forms and PropertiesFour colloids of different sizes were considered (ranging from 
6 to 450 nm). Their sizes and their accessibility factors are shown in Table 6.18-1 of 
Radionuclide Transport Models Under Ambient Conditions (BSC 2003m). 
Colloid Filtration Model and CoefficientsPore-size exclusion (straining and filtration) was 
calculated using the colloid and formation-specific accessibility factors. The clogging (forward) 
kinetic coefficient + is computed using the formulations documented in Section 6.18.2 of 
Radionuclide Transport Models Under Ambient Conditions (BSC 2003m). No information 
exists on the kinetic declogging (reverse) coefficient -. Sensitivity results presented in Section 
5.4.3 of this report indicate that this is not a significant factor. 
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5.3.2 Testing 
Very limited field-testing or data exists at YMP or in the open literature concerning the transport 
of colloids through unsaturated media. The Busted Butte field test in nearby unsaturated rock 
was conducted in part to provide data, confirmatory evidence, and a validation basis for the 
transport of colloids through the unsaturated zone at Yucca Mountain. Additional details for the 
Busted Butte tests can be found in Unsaturated Zone and Saturated Zone Transport Properties 
(CRWMS M&O 2001b). 
The Busted Butte test facility is located in Area 25 of the Nevada Test Site approximately 
160 km northwest of Las Vegas, Nevada, and 8 km southeast of the potential Yucca Mountain 
repository area. The site was chosen based on the presence of a readily accessible exposure of 
the Topopah Spring Tuff and the Calico Hills Formation and the similarity of these units to those 
beneath the potential repository horizon. The colloids consisted of two sizes of fluorescent 
polystyrene microspheres of 0.3 and 1.0 m. 
Carboxylate-modified polystyrene latex (CML) microspheres were significantly attenuated in the 
unsaturated media at Busted Butte. However, the microspheres were typically injected in 
relatively high-ionic-strength solutions of solute tracers that would have tended to destabilize 
them and make them more susceptible to attachment to rock surfaces. Also, flow at Busted Butte 
was matrix dominated, not fracture dominated. For these reasons, the Busted Butte microsphere 
experiments probably do not provide a very good representation of colloid transport in fractures 
in the unsaturated zone. No evidence of microsphere breakthrough was observed in either test 
Phase 1B or Phase 2. Because of this, microspheres were eliminated from the experimental 
matrix early on in Phase 2. 
Because data and field testing of colloid transport in the unsaturated zone is limited and difficult 
to obtain, the data resulting from the C-Wells, conducted in the saturated zone, could serve as a 
conservative estimate in that it overestimates transport. These data are not used in the 
radionuclide transport model, which takes a conservative approach by not considering sorption 
of colloids in the fractures or on fracture walls. However, the TSPA-LA treatment of colloid 
transport in the unsaturated zone does consider the use of the C-Wells data to support colloid 
retardation in unsaturated zone fractures. The application of the C-Wells data from saturated 
zone volcanic fractures to unsaturated conditions is conservative (see Section 5.7.2, 
Assumption 6 and discussion in remainder of Section 5.7.2). The saturated zone used the 
C-Wells field test data (BSC 2003b), which results in considerable retardation of colloids in both 
the vitric and alluvial regions of the saturated zone (BSC 2003a). The characteristics observed in 
the C-Wells field test data are summarized below. 
Details for the C-Wells tests are documented most thoroughly in Saturated Zone In-Situ Testing 
(BSC 2003d). To estimate retardation of colloids in the fracture continuum, field experiments at 
the C-Wells complex near Yucca Mountain were examined, in which CML microspheres were 
used as analogs for colloids (BSC 2003d). Tests were conducted at the C-Wells (UE-25c#1, c#2, 
and c#3), which are located approximately 2 km southeast of the potential repository footprint, 
and completed in fractured volcanic tuffs. Microsphere tracer experiments were conducted in 
both the conductive Bullfrog Tuff and less conductive Prow Pass Tuff. The breakthrough curves 
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were fitted to obtain estimates of forward and reverse filtration rates. These were then used to 
calculate a retardation factor for colloid transport through saturated fractured rock (BSC 2003b). 
5.4 MODEL OF COLLOID-FACILITATED TRANSPORT IN THE UNSATURATED 
ZONE 
5.4.1 Purpose 
The purpose of this modeling is to evaluate the transport of radioactive solutes and colloids in the 
unsaturated zone under ambient conditions from the repository horizon to the water table at 
Yucca Mountain. 
The unsaturated zone radionuclide transport model (BSC 2003m, Section 6) considers: 
1. The transport of radionuclides through fractured tuffs. 
2. The effects of changes in the intensity and characteristics of fracturing among different 
hydrogeologic units. 
3. Colloid transport of irreversibly attached radionuclides. 
4. Physical and chemical retardation processes. 
5. The effects of different conceptual representations of the matrix-fracture system. 
6. The effects of different modeling approaches in the characterization of the transport 
problem. 
Radionuclides may interact with colloids in different ways. Sorption/desorption reactions for 
radionuclides onto colloids may be relatively fast leading to equilibrium between the aqueous 
and sorbed state. This type of interaction between radionuclides and colloids is termed 
reversible. Alternately, the interaction may be a result of fast sorption and relatively slow 
desorption such that the radionuclide may be considered permanently sorbed onto the colloid. 
Strong radionuclidecolloid interactions may also be a result of the radionuclide being embedded 
in the colloid material itself. This type of interaction between radionuclides and colloids is 
termed irreversible. For radionuclides irreversibly attached to colloids, the transport of the 
radionuclides and the colloids are equivalent. 
Because there are indications that colloid-facilitated transport for radionuclides with reversible 
interactions with colloids will not be significant, only irreversible colloids were considered in the 
radionuclide transport model simulations (BSC 2003m, Section 6.18.1). The irreversible 
colloids were taken to have the properties of PuO2 and are subject to radioactive decay. 
Four colloids of different sizes were considered. Note, however that colloid facilitated transport 
for radionuclides with interactions with colloids are included in the TSPA colloid facilitated 
radionuclide transport model (see Section 5.7 of this report). 
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5.4.2 Implementation 
Flux TermsThe mass balance equations that are solved in the radionuclide transport model are 
written in part in terms of species or contaminant flux. In general, the flux term has 
contributions from advective, diffusive, and dispersive transport processes. 
For the situation of evaluating colloid flux the following modifications are required: 
1. Surface diffusion does not occur in colloids. 
2. The Darcy velocities which are used to determine the dispersion tensor and ultimately 
the flux term are multiplied by the factor f (greater than 1), to reflect the observations 
that colloids move at a velocity greater than the average fluid velocity. 
3. The longitudinal and transverse dispersivities are generally different from those for 
solutes (Ibaraki and Sudicky 1995) and may be a function of the colloidal particle size. 
4. The term, D0, is the colloidal diffusion coefficient in water and is described by the 
Stokes-Einstein equation, according to Bird et al. (1960, p. 514). 
5. The fluxes are multiplied by the colloid accessibility factors fc at the interface of 
different media. The fc factor describes the portion of the colloidal concentration in a 
medium that is allowed to enter an adjacent medium of different characteristics, and 
quantifies pore size exclusion (BSC 2003m, Section 6.2.6.2). 
Because it is directly proportional to the water fluxes, advection in the fractures is by far the 
dominant mechanism of transport in the unsaturated zone of Yucca Mountain. Transport is 
further enhanced by longitudinal dispersion and molecular diffusion. Coupled with the fracture 
orientation and gravitational differentials, the result of the fracture and flow characteristics is a 
mainly downward migration of the radionuclides. Note that advection also occurs in the matrix 
(and is accounted for in the simulations), albeit at significantly lower rates. 
Lateral spreading of the contaminants can be achieved through transverse dispersion, and 
molecular diffusion. As sorption occurs from the liquid phase onto the solids, and fractures are 
by far the main conduits of water, retardation of sorbing radionuclides will be controlled by 
sorption of radionuclides onto fracture surfaces and the rate of radionuclide movement from the 
fractures to the sorbing matrix. This is represented by the sum of diffusive, dispersive, and 
surface diffusion fluxes, and results in the lateral migration of the contaminants. In the 
radionuclide transport process model sorption of colloids on fracture surfaces is conservatively 
not considered. 
Colloid Filtration TermColloidal filtration is included in the model application using a linear 
kinetic model by orapioglu et al. (1987, pp. 269 to 342). 
Filtration is one of the main mechanisms of radioactive colloid removal from the transporting 
liquid phase, the others being chemical destabilization due to pH and ionic strength changes and 
flocculation. Three types of filtration mechanisms may affect the transport of colloids through 
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the permeable fracture/matrix media: surface filtration, straining filtration, and physicalchemical 
filtration: 
1. Surface filtration occurs when particles are larger than the pores, in which case a filter 
cake is formed; colloids if they are small enough can enter the matrix by advective and 
diffusive processes. 
2. Straining filtration is determined by the ratio Rd = dg/dp, where dg is the diameter of 
the grains of the porous medium and dp is the suspended particle diameter. Herzig 
et al. (1970, p. 15) indicated that little straining was expected when Rd is greater than 
12, and calculated that when Rd = 50, only 0.053 percent of porosity would be 
occupied by particles. 
3. A detailed discussion of the physical-chemical filtration of colloids can be found in 
orapioglu et al. (1987, pp. 269 through 342). The physical-chemical colloidal 
filtration by the porous/fractured medium involves three mechanisms: (1) contact with 
the pore walls, (2) colloid fixation onto the walls, and (3) release of previously fixed 
colloids (Herzig et al. 1970; orapioglu et al. 1987). Contact with the pore walls and 
colloidal capture can be the result of: 
a. sedimentation (caused by a density differential between the colloidal and the 
carrier liquid), 
b. inertia (deviation of colloidal trajectories from the liquid streamlines because of 
their weight), 
c. hydrodynamic effects (caused by a variation in the velocity field of the liquid), 
d. direct interception (caused by collisions with the pore walls at convergent areas), 
and 
e. diffusion (Brownian motion causing colloids to move toward pore walls or 
dead-end pores). 
4. Fixation on the pore walls occurs at retention sites that include edges between 
two convex surfaces, pore throats smaller than the colloidal size, and dead-end pores 
or regions of near-zero liquid velocity. Fixation is caused by retentive forces, which 
include axial pressure of the fluid at constriction sites, friction forces, van der Waals 
forces, electrical forces and chemical forces (Herzig et al. 1970, pp. 4, 11 through 17). 
5. Remobilization of colloidal particles may be caused by a number of factors, including 
collision between a loosely held colloid and a moving particle, an increase in pressure 
as colloids constrict flow, and a change in the chemical environment. 
All three mechanisms are expected to be present in the unsaturated zone. The simulations of the 
unsaturated zone transport model can account for surface filtration by using a particle size versus 
pore size criterion, and by not allowing colloidal entry into media that do not meet this criterion. 
Straining filtration (pore-size exclusion) is described by using appropriate colloid accessibility 
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factors (BSC 2003m, Section 6.3.5), and physical-chemical filtration is represented by using 
appropriate kinetic parameters (BSC 2003m, Equations 6-14 or 6-15). 
Cake filtration and the effects of filtration on permeability and porosity are unlikely in the 
unsaturated zone because (1) natural pseudocolloids (such as clays) under natural conditions 
occur in small concentrations and (2) it is expected that, owing to chemical conditions adverse to 
colloid stability (e.g., pH, ionic strength) in the immediate vicinity of the repository, colloids will 
be released at low concentrations for a long time. 
Colloid-Assisted Transport TermsIn the radionuclide transport model, colloid assisted 
transport is considered for radionuclides that are irreversibly sorbed onto colloids. 
Radionuclides that are both irreversibly and reversibly sorbed onto colloids are considered in the 
TSPA-LA unsaturated zone transport model, see Section 5.7 of this report. 
The formulation used allows consideration of the whole size spectrum of a particular colloid, as 
each size would have different transport behavior and different sorption properties (resulting 
from different surface area). Of particular interest in unsaturated zone radioactive transport 
models is the potential of colloid-assisted transport to significantly enhance the migration of 
radionuclides whose normally strong sorbing behavior would confine them to the vicinity of the 
release point. Sorption of such a radionuclide (e.g., plutonium) onto pseudocolloids renders the 
colloid radioactive. The transport of such a radionuclide is no longer dictated by the strong 
radionuclide sorption behavior, but by the kinetics of colloid filtration. The kinetics of colloid 
filtration coupled with the fact that colloids can move at velocities up to 50 percent higher than 
the Darcy velocity, can result in travel times to the water table orders of magnitude shorter than 
the ones predicted by solute-based transport modeling. 
Daughter Products of Radioactive DecayRadioactive decay and the transport of daughter 
products is taken into account for both dissolved and colloid related radionuclides. 
If decay results in radioactive daughters, unsaturated zone simulations must compute the total 
radioactivity distribution (i.e., the sum of the concentrations of all the members of the radioactive 
decay chain). This is particularly important for daughters that have long half-lives. The 
formulation requires knowledge of the mass fraction of the decayed sorbed parent that remains 
sorbed as a daughter. This mass fraction is a function of the type of decay, as well as of the 
chemical form of the sorbed cation. In alpha decay (e.g., 237Np, 239Pu), it is 0. There is no 
information on the behavior of mass fraction for other types of decay. This mass fraction can 
have significant implications for the transport behavior of daughters if (1) the large sorbed 
masses of strongly sorbing parents are ejected back into the aqueous phase after decay, (2) the 
daughter is a much weaker sorber, and (3) the daughters sorption is kinetically controlled. Such 
a possibility could arise in kinetically dependent sorption of radionuclides onto pseudocolloids 
(CRWMS M&O 2001b, Section 6). For equilibrium isotherms this is not significant. 
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5.4.3 Results 
The results from the four colloid transport simulations using TOUGH2 V1.11MEOS9nTV1.0 
(Module EOS9nTV1.0) (LBNL 1999a) are discussed in detail in Section 6.18 and Attachment VI 
of Radionuclide Transport Models Under Ambient Conditions (BSC 2003m). Example results 
from case 1 (no declogging [colloid detachment]) and case 2 (strong declogging) are shown here 
in Figures 5-2 and 5-3, respectively. Each figure contains two sets of four curves, one for each 
colloid-size particle. The set labeled Water Table indicates the relative release rate at the 
unsaturated zone-saturated zone interface. The set labeled Repository indicates the decaying 
plutonium colloid source term for each of the colloid particle sizes considered as leaving the 
repository and entering the unsaturated zone. Note that the four curves labeled Repository 
exactly collapse into a single curve in both figures because the release from the repository is 
identical for all colloid sizes. The rapid drop in all of the curves is due to radioactive decay. 
Source: DTN: LB991220140160.017. 
No. 8: Colloids 
Figure 5-2. Example Results from Case 1 (No Declogging) 
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Source: DTN: LB991220140160.017. 
No. 8: Colloids 
Figure 5-3. Example Results from Case 2 (Strong Declogging) 
Colloid Transport Case 1This simulation assumes that no declogging (detachment) occurs 
once colloids are attached to the matrix. Two observations appear particularly important: 
1. The very fast breakthrough of the larger colloids (characterized by a rapid rise of the 
breakthrough curves), the rapidity of which is about the same for all the larger colloids 
(greater than or equal to 100 nm). 
2. The smallest colloid (6 nm) has the slowest breakthrough. It is really noteworthy that 
this colloid never reaches the 0.1 fraction level, indicating that it is sufficiently small 
compared to the larger colloids to enter the matrix and get attached to the fracture 
walls. This behavior results from a combination of the following factors: 
a. The larger colloids, because of their size, can only move in the center of 
pores/fractures where velocities are higher than the average water velocity. 
However, this will only result in colloids moving at most 1.5 times the average 
velocity. 
b. The inability of larger colloids to penetrate into the matrix from the fractures 
because of size exclusion. Thus, the colloid mass in the fractures is not reduced 
through colloidal diffusion and/or hydrodynamic dispersion, and since retardation 
in fractures is excluded, practically all of it moves exclusively in the fractures. 
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Colloid Transport Case 2In Case 2 there is a strong declogging factor, while in Case 1 there is 
no declogging. The differences in breakthrough curves between Case 1 and Case 2 are very 
small for all colloids, and are most prominent in the case of the 6-nm colloid. The relative 
insensitivity to the clogging model in the matrix indicates the dominant role of the fractures in 
the three-dimensional site scale system, with the matrix appearing to have a negligible 
contribution. 
Colloid Transport Mechanisms and Patterns for Case 1Details of the transport patterns are 
provided in the Radionuclide Transport Models Under Ambient Conditions (BSC 2003m, 
Section 6.18, Attachment VI and VII). The importance of fractures on the transport of the 6-nm 
colloid (Co0060) is evident in the results, which show colloid concentration contours after ten 
years (Figure 5-4). Despite the very early time, the colloid concentration in the fracture is 
substantial, and it approaches the release concentration. The reasons for the significant presence 
(in terms of areal extent and level of concentration) of the 6-nm compared to the dissolved 
colloid are that (1) diffusion into the matrix is limited because of its relatively large size 
(compared to solutes) and pore exclusion and (2) the tsw39 model layer is above the TSwCHn 
interface, where significant filtration occurs. The results also indicate that the colloids 
accumulate at the TSwCHn interface and, unable to cross it, move along the sloping interface in 
an easterly direction. This transport is more pronounced in the 6-nm colloid than in the solutes. 
Results of the matrix FR (relative filtered concentration) distributions at various times indicate 
more filtration in the northern part of the repository. Figure 5-5 illustrates this at 10,000 years. 
This is indicated by the large amount of areal spreading in the relative concentration (in green) 
about the higher fracture concentration (in light blue). Since the dominant pathway for the 
transport of colloids through the zeolitic rock in the north is through fractures, this spreading is 
caused by the inability of the colloids to migrate through the interface between the TSw and CHn 
layers. This is consistent with the permeability barriers that result in the perched water bodies at 
this location. The matrix-filtered relative concentrations are higher in the south (in dark blue) 
and this is due to the presence of vitric tuff where the matrix and fractures more equally share in 
the vertical transport of the 6-nm-size colloid and not due to lateral spreading at the interface. 
More details of these results and further explanations are presented in Section 6.18 of 
Radionuclide Transport Models Under Ambient Conditions (BSC 2003m). 
At t = 1,000 years (Figure 5-6), the concentration in the fractures exceeds that in the water 
released from the repository. This is caused by pore-exclusion (straining) at the TSwCHn 
interface, which leads to the accumulation of the 6-nm colloid in the tsw39. Results showing 
mass fraction concentration at the water table at 1,000 years follow patterns analogous to those in 
the tsw39 layer (Figure 5-7). The fracture XR (relative mass fraction) above the water table 
exceeds that in the water released from the repository, and the differences between the two 
increases with time. This occurs because of the fast transport of the colloids in the fractures and 
their accumulation. This accumulation is caused by the very significant pore-size exclusion and 
kinetic (physical-chemical) filtration at the water table, which prevent colloids from entering the 
saturated zone while allowing water to flow into it. This occurs because the saturated zone 
behaves as a porous (rather than a fractured) medium with the properties of the TSw matrix. 
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Source: BSC 2003m, Figure VI.1. 
Figure 5-4. Distribution of the Relative Mass Fraction, XR, of the 6-nm 239PuO2 Colloid in the Fractures 
of the tsw39 Layer at 10 Years for the Mean Present-Day Infiltration Rate and Continuous 
Release 
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September 2003 5-18
Source: BSC 2003m, Figure VI.12. 
Figure 5-5. Distribution of the Relative Filtered Concentration, FR, of the 6-nm 239PuO2 Colloid in the 
Fractures of the tsw39 Layer at 10,000 Years for the Mean Present-Day Infiltration Rate 
and Continuous Release 
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September 2003 5-19
Source: BSC 2003m, Figure VI.7. 
Figure 5-6. Distribution of the Relative Mass Fraction, XR, of the 6-nm 239PuO2 Colloid in the Fractures 
of the tsw39 Layer at 1,000 Years for the Mean Present-Day Infiltration Rate and 
Continuous Release 
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September 2003 5-20
Source: BSC 2003m, Figure VI.22. 
Figure 5-7. 
No. 8: Colloids 
Distribution of the Relative Mass Fraction, XR, of the 6-nm 239PuO2 Colloid in the Fractures 
Immediately above the Groundwater at 1,000 Years for the Mean Present-Day Infiltration 
Rate and Continuous Release 
Because of its larger size, the straining is more pronounced for the 450-nm colloid than in the 
6-nm colloid, and leads to higher concentrations at earlier times behind the interface. At the 
water table and at early times (less than 10,000 years), the maximum fracture relative mass 
fraction exceeds 1 (i.e., equals 1.50) because of straining (pore-size exclusion). 
5.5 UNCERTAINTIES AND SENSITIVITIES 
5.5.1 Uncertainties 
The uncertainties related to colloid transport modeling in the unsaturated zone are summarized in 
the following text. The approach is to either employ conservatism in treating parameters or 
processes whose uncertainty is unknown or not well characterized or to capture the uncertainty 
through a parameter value distribution. 
Colloid Retardation KineticsBasic knowledge about the kinetic clogging and declogging 
coefficients + and - is limited. Also, it is not known whether the applicability limits of the 
currently available kinetic models, developed from theoretical principles (Herzig et al. 1970) and 
tested in uniform sandy laboratory experiments (van de Weerd and Leijnse 1997) or small-scale 
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field tests (Harvey and Garabedian 1991), are valid under the unsaturated zone conditions at 
Yucca Mountain. Sensitivity analyses indicate an insensitivity of transport to the filtration 
parameters. In the TSPA-LA unsaturated zone transport model colloid retardation in fractures 
will be treated with uncertainty by sampling over a range of values based on C-Wells testing, see 
Section 5.7. In the TSPA-LA unsaturated zone transport model colloid matrix filtration 
parameters will be treated as fixed values but vary with geologic material (BSC 2003i, 
Section 4.2). 
Effects of Matrix Diffusion on Colloidal Transport SimulationA comparative analysis based 
on Case 2 (Section 5.4.3) was conducted and reported in Radionuclide Transport Models Under 
Ambient Conditions (CRWMS M&O 2000f, Section 6.17) using an earlier version of the 
unsaturated zone transport model. This analysis examined the transport of colloids with and 
without colloid matrix diffusion. The analysis indicates that diffusion is less significant in 
colloid transport than in solute transport, because (1) colloid diffusion is smaller than solute 
molecular diffusion because of the larger colloid size, and (2) size-exclusion effects at the 
interfaces of different geologic units further limit entry by diffusion into the matrix (especially 
for larger colloids). However, diffusion effects become increasingly important for a decreasing 
colloid size. Uncertainty in colloid matrix diffusion is treated by sampling the diffusion 
coefficient over a range of values in the TSPA-LA unsaturated zone transport model (BSC 
2003i, Section 4.2). 
AirWater InterfacesThe affinity of colloids for airwater interfaces can have a potential 
effect on their transport (CRWMS M&O 2000f, Section 6.15.8). These possible effects are not 
currently considered in either the radionuclide transport process model or the TSPA-LA particletracking 
model. Ignoring these effects results in increased predicted migration of colloids. 
5.5.2 Sensitivities 
The impact of these uncertainties is assessed by estimating (1) transport for a very wide range of 
the kinetic filtration parameters (clogging and declogging coefficients) and (2) the impact of 
filtration in the fractures (by considering minor fracture filling) (BSC 2003m, Section 8.3.2.4). 
Filtration KineticsEven though colloid retardation kinetics is uncertain, the simulations of 
Radionuclide Transport Models Under Ambient Conditions (BSC 2003m, Section 6.18) indicate 
that the transport is not significantly affected by varying the reverse or detachment filtration 
coefficient over the range from 0 to 100 percent. When there is no filtration in fractures, there is 
very limited retardation of the larger colloids (greater than 100 nm). However, filtration has a 
substantial impact on retardation of very small colloids (less than 6 nm). This is most likely a 
result of the larger colloids not being able to access the matrix where the filtration is assumed to 
occur. In this process model colloid retardation is not assumed to occur in the fractures. 
Colloid SizeColloid size is uncertain and variable and is an important parameter in describing 
the transport of colloids through the unsaturated zone. Travel times to the water table decrease 
with the colloid size: larger colloids show little retardation while very small ones are retarded 
significantly. Size exclusion (straining) at the interfaces of different hydrogeologic layers leads 
to colloid concentrations immediately above the interface that can be higher than that in the 
water released from the repository. This phenomenon is more pronounced in larger colloids. 
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Importance of FaultsThe potential points of release from the repository are uncertain and may 
vary with time; however, this does not impact the transport of colloids. Because of the 
importance of transport and flow within faults on the migration of dissolved radionuclides and 
colloids, the impact of releasing radionuclides directly into faults was assessed (BSC 2003m, 
Section 6.20). Despite the importance of faults (particularly the Drillhole Wash, the Pagany 
Wash, and the Sundance Faults) identified in the analyses of Radionuclide Transport Models 
Under Ambient Conditions (BSC 2003m, Section 6), eliminating potential sources from the 
immediate vicinity of the faults has a small (even negligible) effect on transport and arrivals at 
the water table. This is not to suggest that faults are unimportant but only that they continue to 
dominate whether radionuclides are directly released into them or not. 
Importance of Filtration in FracturesThe presence of fracture fill and the amount of filtration 
that may occur in fractures is uncertain and is evaluated for importance. The simulations of 
Radionuclide Transport Models Under Ambient Conditions (BSC 2003m, Section 6.18) indicate 
that when there is no fill material or filtration in the fractures, larger colloids (450 nm, 200 nm, 
and 100 nm) experience very limited retardation. Conversely, the retardation of the smallest 
colloid (6 nm) is very significant. When there is limited fill material and filtration in the 
fractures, the transport of the larger colloids is retarded because this filtration in the fractures 
becomes a dominant process retarding the larger colloids. There is not much impact on the 
smallest colloid particle because the small colloids can enter the matrix where substantial 
additional retardation takes place. 
5.6 MODEL CONFIDENCE BUILDING 
5.6.1 Model Validation 
The colloid transport model is a submodel of the radionuclide transport model. Many other 
validation studies of the transport of dissolved radionuclides documented in Radionuclide 
Transport Models Under Ambient Conditions (BSC 2003m) indicate that the radionuclide 
transport model is adequately validated for its intended use. In addition numerous other 
submodels and processes which may indirectly impact the transport of colloids are also 
adequately validated and documented in Radionuclide Transport Models Under Ambient 
Conditions (BSC 2003m). These other processes and submodels include (1) transport of 
nonsorbing and nondecaying radionuclides, (2) transport of sorbing radionuclides, (3) transport 
involving radioactive decay chains, (4) transport in a fracture-matrix system, (5) the matrix 
diffusion model, (6) the sorption model, and (7) the combined matrix/active fracture model. 
In general, five validation methods can be used. These are: 
1. Corroboration with experimental data 
2. Corroboration with data from the literature 
3. Corroboration with alternative mathematical models 
4. Independent technical review 
5. Publication in a refereed professional journal. 
The validation approach and criteria are discussed in Technical Work Plan for: Performance 
Assessment Unsaturated Zone (BSC 2002b, Section I-2-1-4), which indicates that the colloid 
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transport model was judged in the model validation status report (BSC 2001e, p. 28) to have 
been validated satisfactorily for its purposes in Unsaturated Zone Colloid Transport Model 
(CRWMS M&O 2000d). The validation was extended to the colloid transport model in the 
radionuclide transport model and further supported by some additional work described in 
Radionuclide Transport Models Under Ambient Conditions (BSC 2003m) and is summarized 
below. Despite this, there remains a lack of observational support for model validation by 
Methods 1 and 2 above. Methods 3, 4, and 5 are used. 
Validation through Scientific Acceptance (Method 4 and 5)Using T2R3D, Wu and Pruess 
(2000) studied the site-scale three-dimensional transport of radioactive solutes. Recently, 
Moridis et al. (2003) reported on a scientific study of site-scale three-dimensional transport of 
radioactive colloids in the unsaturated zone of Yucca Mountain using EOS9nT. Both studies are 
based on essentially the same radionuclide transport model as the one discussed here, but it 
involved a three-dimensional grid from the previous revision of the relevant model reports (BSC 
2003m; BSC 2001c). 
Additionally, EOS9nT is currently in use by Gesellschaft fr Anlagen und Reaktorsicherheit 
(GRS) mbH, a research and engineering organization owned by the Federal Government of 
Germany, the State of Bavaria, the state of North Rhine-Westphalia, the Technical Inspection 
Organization (TV) and the Germanischer Lloyd. GRS has a substantial involvement in 
radioactivity issues, focusing on studies of the effects of potential radioactivity releases. The 
work of GRS using the radionuclide transport model embodied in EOS9nT is documented in two 
GRS reports (Javeri 2001; 2002). 
Colloid and Colloid-Assisted Radionuclide Transport (Method 3)The transport of a colloid 
and a solute is described in this problem. The colloid filtration (attachment) is controlled by a 
kinetic process. Results of the numerical model are compared to its analytical solution. The 
solute is radioactive and is irreversibly sorbed onto the colloid; thus, it cannot enter the liquid 
phase and is transported only by the mobile colloids. All the radionuclides that enter the system 
are sorbed onto colloids. This being the case, the radionuclide decays uniformly, and its 
concentration is the same on all colloids. The radionuclide concentration on each colloid is 
controlled by radioactive decay, and by choosing an observation time equal to T1/2, its relative 
concentration is 0.5. 
Properties and parameters for this problem are listed in Table 7.2-5 of Radionuclide Transport 
Models Under Ambient Conditions (BSC 2003m), and the comparison of the analytical to the 
numerical solutions is listed in Table 7.2-6 of Radionuclide Transport Models Under Ambient 
Conditions (BSC 2003m). Because it involves transport of a kinetically filtering colloid, T2R3D 
(LBNL 1999b), cannot be used for this study. The numerical results were obtained with 
EOS9nT (LBNL 1999a), and show nearly identical agreement with the analytical solution, while 
the radionuclide relative concentration on the colloids is confirmed to be a uniform 0.5. 
5.6.2 Use of AnalogsNatural Analog Studies at Pea Blanca, Mexico 
From the 1980s, the Nopal I uranium deposit at Pea Blanca has been recognized as a natural 
analog for the potential repository at Yucca Mountain (CRWMS M&O 2000g, Section 6.5.2.1). 
Analysis of water samples from the Nopal I uranium deposits were all super-saturated with 
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respect to thorianite, which is attributed to the presence of colloidal thorium in the less than 
0.2 m fraction of the samples (CRWMS M&O 2000g, Section 6.5.2.15). Preliminary evidence 
suggests colloidal transport of thorium, protactinium, and possibly uranium (CRWMS M&O 
2000g, Section 6.5.2.1.6.2), but could not be verified without studies of concentration vs. size 
fraction on filtered or ultrafiltered samples. 
5.6.3 Alternative Conceptual Model 
FEHM, V2.20 (LANL 2003, STN: 10086-2.20-00), is used for the numerical analysis of this 
alternate conceptual model (CRWMS M&O 2000d). FEHM calculates hydrologic flow and the 
transport of radionuclides, the latter by a particle-tracking algorithm. The particle-tracking 
method in FEHM views the computational domain as an interconnected network of fluid storage 
volumes. The two steps in the particle-tracking approach for steady-state flow fields are: (1) to 
determine the time a particle spends in a cell and (2) to determine which cell the particle travels 
to next. The domain can consist of a single-continuum or dual-continua (e.g., fracture plus 
matrix) representation of the flow field. FEHM can be used to model transport breakthrough 
times in porous and fractured media. 
This implementation of FEHM examines a two-dimensional discrete fracture model (DFM), in 
which the fracture and matrix are discretely represented. This approach contains a parallel 
fracture that is more conservative but consistent with the dual-permeability approach used in the 
TSPA model. This model preceded the radionuclide transport dual continuum model. The 
currently accepted conceptual model for flow and transport is the dual continuum model. Even 
though FEHM is a continuum model, this application of FEHM is designed to simulate transport 
in a discrete fracture model, and it is useful to examine it as an alternate conceptual model. The 
discrete fracture conceptual model implemented in FEHM by itself is more conservative than the 
dual continuum model (CRWMS M&O 2000d). However, the FEHM implementation also 
considered retardation of colloids in the fractures based on C-Wells data as described below. As 
the results of the FEHM modeling will show, initial breakthrough times are delayed compared to 
the predictions with the radionuclide transport model, which does not consider retardation of 
colloids in the fractures. These differences provide insight into the behavior of the transport of 
colloids (particularly waste-form colloids) obtained from this alternative model documented in 
Unsaturated Zone Colloid Transport Model (CRWMS M&O 2000d). 
The objectives of this modeling include the following: 
1. Use of a process level model to evaluate the potential mechanisms for colloid transport 
at Yucca Mountain 
2. Provide ranges of parameters for significant colloid transport processes to performance 
assessment for the unsaturated zone 
3. Provide a basis for development of an abstracted model for use in performance 
assessment calculations. 
The details of the assumptions, conceptualization, implementation into a numerical model, data, 
validation, results, and analyses are presented in Unsaturated Zone Colloid Transport Model 
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(CRWMS M&O 2000d, Section 6). A summary to the information extracted from this reference 
is provided below. 
Stratigraphy, Numerical Grid, and Boundary ConditionsA simplified stratigraphy that 
contains three hydrogeologic units, the Topopah Spring welded (referred to in this report as 
TSw4), Calico Hills vitric (CH1v), and Calico Hills zeolitic (CH1z), was used in the numerical 
analysis. These units are equivalent to those considered in abstraction of transport properties for 
TSPA for the site recommendation (CRWMS M&O 2000h, Section 6.4). The hydraulic 
properties for these units were derived from DTN: LB970601233129.001 (the properties of 
TSw4 were assigned to the tsw34 model unit). The grid extends from 630 m to 1,150 m and 
models the half distance between fractures, which was 10 m (i.e., the grid is 5 m wide). This 
distance corresponds to the fracture spacing for the CH2v to CH5v and CH2z to CH5z units. 
The lower boundary was selected to be 100 m below the water table to isolate detailed flow 
conditions related to the boundary from the model domain of interest. The upper elevation 
boundary represents approximately the top of the TSw4. The fracture width is 1 mm based on 
estimates from the active fracture model developed in Particle Tracking Model and Abstraction 
of Transport Processes (CRWMS M&O 2000h, Section 6.2.1), which used data in 
DTN: LB990501233129.001 to derive the parameters. The vertical grid spacing is 2.5 m. The 
horizontal grid spacing is variable and extends from 1 mm to 80 cm. 
A flux of 5 mm/year (DTN: LB990801233129.003) averaged over the top surface of the model 
is injected into the fracture only at the top of the model. This allows the redistribution of flow to 
occur above the repository level (1,087 m) such that the flow field relevant to transport is parallel 
in the fracture and matrix units and is at steady state conditions for the transport simulations. In 
general, the flow through the system simulates fracture flow in the TSw4, the transition to 
predominately matrix flow in the CH1v, and matrix flow in the CH1z. The simulations are 
unsaturated and do not consider perched water systems. Release of actinides or colloids occurs 
at the repository level. This model examines important mechanisms for colloid transport that 
will need to be incorporated into a performance assessment calculation and is not intended to 
represent the site scale or what happens on the site scale. 
Pore Size ExclusionThe model considered pore size exclusion of colloids from the fractures to 
the matrix based on estimates of the pore size distributions for the three units considered in the 
model. These estimates were obtained from the moisture retention curves 
(DTN: GS980908312242.039; GS950608312231.008) judged to be representative of the site. 
The percent of colloids that could enter a unit was then determined based on the percent of pore 
sizes above and below the colloid size of interest (6 to 450 nm in this study) for the three units 
considered in the analysis. This approach results in, for example, in the TSw4, only five percent 
of the 450-nm colloids can enter the matrix versus 65 percent of the 6-nm colloids. This fact has 
a couple of implications, depending on the unit through which the colloids are transported. For 
the case in which fracture flow is dominant, large colloids can experience size exclusion from the 
matrix, which causes them to stay in the fracture, and smaller colloids can enter the matrix, 
which will increase their residence time in the system. In contrast, for the case in which matrix 
flow is dominant, large colloids that cannot enter the matrix are physically removed at the 
interface between the fracture and matrix or between different matrix units as a filter cake. 
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Source TermThe actinide source term was based on the 10,000-year radionuclide release at the 
edge of the engineered barrier system from an expected value run for the TSPA for the Viability 
Assessment (DOE 1998). Actinide release started at 1,000 years and was continuous until 
10,000 years, the duration of the simulated time. 
The transport of both natural colloids and waste form colloids were considered. The waste-form 
colloid concentration was held constant at 6  10-8 M, which is the high end of the concentration 
measurement from DTN: LL991109751021.094. 
Natural colloids are formed as the weathered by-products of the host material and are ubiquitous 
in nature. Multiple measurements have been done for groundwater samples at Yucca Mountain 
and other areas on the Nevada Test Site. The measured particle concentrations that are used vary 
between 1.05  106 and 2.72  1010 particles/mL, with the lowest being for water from well J-13 
and the highest for water from well U19 on Pahute Mesa. 
Conceptual Picture of the MatrixConceptually, the matrix in the unsaturated zone will have 
several different effects on colloid transport, depending on the unit being considered. In general, 
primary colloid removal in the matrix will be due to physical effects, such as small pore sizes 
and low volumetric water content. Chemical removal could occur onto mineral surfaces, but it is 
assumed that the far-field geochemistry will not vary significantly from current conditions, and 
therefore, chemical removal in the matrix is expected to be small. Specifically, the TSw4 has a 
very small pore-size distribution and is expected to limit the number of colloids that enter the 
matrix. Size exclusion will force the majority of the colloids to remain in the fractures in this 
unit, and the small percent of colloids that do enter the matrix will have a high probability of 
being retained due to filtration in the matrix and at the interfaces between units. For the CH1v, 
matrix flow is dominant, the pore sizes are large, and the majority of colloids will be able to 
enter the matrix due to imbibition. Physical removal of colloids in this unit is likely because the 
matrix is more tortuous than the fractures and variable pore sizes. The base unit in the model is 
the CH1z, which has a smaller pore-size distribution than the CH1v. This fact is important 
because it is expected that a significant quantity of colloids could be filtered at the interface 
between the two units. The CH1z will also exhibit size exclusion between the fracture and 
matrix unit for larger colloids. 
Colloid Transport in FracturesRetardation of colloids in the fractures was considered and 
was based on available data at the time from the C-Wells tracer experiments using microspheres. 
For flow in a fracture, an equilibrium relationship between the forward and reverse rate was used 
which is dependent on the half aperture of the fracture and the bulk density. For example, in the 
C-Wells analysis, the reverse rate is reported as a function of the fracture aperture (Reimus et al. 
1999). 
Based on the range of forward rates estimated from C-Wells for the Bullfrog and Prow Pass 
units, a range of forward rates that varied between 0.001 and 0.5 hr1 was considered (Reimus 
et al. 1999; DTN: LA9912PR831231.006). The reverse rates considered varied between 0.001 
and 4 hr1. Simulation results on the microsphere breakthrough from C-Wells completed after 
this analysis was reported indicate that the reverse rates multiplied by the fracture half aperture 
vary between 0.0001 to 3.33 hr1 (Reimus et al. 1999; DTN: LA9912PR831231.006). This 
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indicates that the reverse rate could be an order of magnitude smaller than considered in this 
analysis. Various combinations of forward and reverse rates were used in the analysis. 
Plutonium Sorption/Desorption onto Natural ColloidsA kinetic sorption model was used for 
the preliminary analysis of Pu(IV) and Pu(V) sorption and desorption onto colloidal matter. 
Preliminary curve fitting results of laboratory data on the sorption of plutonium onto clay 
colloids was used for numerical modeling (DTN: LA0003NL831352.001). Data were also 
available on the sorption of plutonium onto silica colloids. 
The results of the curve fitting yielded forward rates on the order of one. The reverse rates are 
around 1  10-5. Sorption is clearly very rapid, whereas desorption is very slow from the clay 
colloid material. 
ResultsThe results of the simulation of this model indicate initial breakthrough of about 1,500 
years for colloids of size 450 nm and 1,700 years for colloids of size 6 nm. These breakthroughs 
are significantly delayed compared to the results of the radionuclide transport model in Section 
5.4.3. 
A number of sensitivity studies were done with this model to evaluate the impact on release from 
(1) pore size exclusion from the matrix in fracture dominated flow, (2) colloid removal at 
matrix/fracture interface in matrix dominated flow, (3) source term (release rate), (4) colloid 
retardation, and (5) reversible sorption of radionuclides onto colloids. They are not discussed 
here as the focus is on the currently accepted model for colloid transport in the unsaturated zone. 
The conclusions of this alternate model indicate that waste-form colloids can be transported 
through the unsaturated zone. The size-exclusion mechanism that forces colloids to remain in 
the fractures enhances transport in rock units with fracture flow and retards transport in units 
where matrix flow dominates because there is no advective flux in the fracture to move the 
colloids. 
The FEHM modeling and analysis indicate that some colloids, particularly waste-form colloids 
or natural colloids with kinetically controlled actinide sorption/desorption, could enhance 
actinide transport. Colloid facilitated transport in the unsaturated zone is currently screened in 
the TSPA-LA. 
5.7 TSPA-LA IMPLEMENTATION OF UNSATURATED ZONE TRANSPORT 
The radionuclide transport model process model is intended to establish the technical basis for 
unsaturated zone transport that is carried through into the TSPA-LA model through an 
abstraction based on FEHM particle tracking. This section outlines some important differences 
and similarities between the radionuclide transport model process model and the TSPA-LA 
particle-tracking abstraction. The computer software code on which the unsaturated zone 
transport abstraction model is based is FEHM, Version 2.21 (STN: TBD). Details of the model, 
assumptions, development, input, and example results are provided in Particle Tracking Model 
and Abstraction of Transport Processes (BSC 2003i). The information presented here focuses 
on processes that uniquely affect the transport of colloids. 
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5.7.1 Background 
Particle-tracking transport models take a fundamentally different approach than the finite 
difference models (radionuclide transport model) in their solution of the advective-dispersion 
equations. The trajectory of individual molecules or packets of fluid containing molecules are 
tracked through the model domain. In a typical particle-tracking algorithm, a particle is sent to a 
new position assuming that the magnitude and direction of the velocity vector are constant 
during a time step. Dispersion is treated as a random process that diverts the particle a random 
distance from its dispersion-free, deterministic path. Linear equilibrium sorption can be handled 
through the use of a retardation factor to correct the magnitude of the particle velocity. Particle 
tracking solutions do not suffer from numerical dispersion compared to finite difference or finite 
element representations of the advection-dispersion equations 
In the TSPA-LA unsaturated zone transport model, a particle-tracking technique is employed 
that can be used for transient, multi-dimensional finite-difference or finite-element codes. The 
algorithm is designed for computing solute concentration fields quickly and easily with 
structured or unstructured numerical grids of arbitrary complexity. Both continuum and dualpermeability 
formulations can be simulated. This flexibility is accomplished by extending the 
cell-based strategy of Desbarats (1990) for mapping out the path of the particle. In this method, 
the calculation of an exact pathline is replaced with a cell-to-cell migration of the particle. The 
mass flux from cell to cell is used directly, and no velocity interpolations are required. Since 
numerical solutions for fluid flow are typically mass conservative (though not necessarily 
accurate) the particle-tracking method automatically conserves mass. 
As discussed in Section 5.4.1 of this report, radionuclides may have either reversible or 
irreversible interactions with colloids. The abstraction model for unsaturated zone transport 
treats colloid facilitated transport for both kinds of interactions. Most radionuclides are assumed 
to sorb onto colloids reversibly (that is, they have measurable desorption rates and can be 
entirely desorbed from the colloids) (BSC 2003f, Section 6.3.3.1). However, plutonium and 
americium can sorb either reversibly or irreversibly onto colloids with the relative percentages of 
these obtained from an analysis of waste-form degradation and waste package corrosion 
processes occurring in the repository near-field environment (BSC 2003f, Section 6.3.3.1). In 
general, the majority of plutonium and americium sorbed to colloids is irreversible, with a typical 
percentage being 90 to 99 percent irreversible and 1 to 10 percent reversible (BSC 2003f, Section 
6.3.3.2, p. 74). Irreversibly sorbed plutonium and americium are either embedded in the wasteform 
colloids (e.g., smecite colloids formed by degradation of high-level radioactive waste glass) 
or are so strongly sorbed onto colloids (e.g., iron oxyhydroxide colloids formed by corrosion of 
waste packages) that there is no detachment for typical transport time scales (thousands of years) 
through the unsaturated zone. By definition, the irreversibly sorbed plutonium and americium 
transport in a manner identical to the colloids onto which they are sorbed. Note that EBS 
Radionuclide Transport Abstraction (BSC 2001a), specifically Attachment II, discusses the 
mathematical implementation for irreversible sorption of plutonium and americium onto iron 
oxyhydroxide colloids in the engineered barrier system. 
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5.7.2 Comparison of Assumptions 
The assumptions taken to develop the unsaturated zone radionuclide transport abstraction model 
are outlined in detail along with their rationale as the first step toward developing the 
computational and mathematical models needed in radionuclide transport calculations for the 
TSPA model. A subset of those assumptions is listed here for the purpose of indicating any 
differences between the radionuclide transport model process model and the particle-tracking 
model to be used in the TSPA-LA. 
Assumption 1: The active fracture model appropriately accounts for reduced fracture/matrix 
interaction. The radionuclide transport model uses the active fracture model 
formulation. 
Assumption 2: The influence of matrix diffusion in a dual-permeability system can be handled 
with a sub-grid-block model consisting of parallel flow in a discrete fracture and 
connected matrix. The radionuclide transport model considers matrix diffusion 
for both dissolved species and colloids. The abstraction model treats aqueous 
radionuclide diffusion between fractures and the matrix, but conservatively 
neglects colloid fracture-matrix interaction through diffusion. 
Assumption 3: Radionuclide sorption can be approximated with a linear, equilibrium sorption 
model characterized by a single parameter, the distribution coefficient, Kd. 
Radionuclide transport model makes the same assumption. 
Assumption 4: Dispersion of both aqueous and colloidal species can be approximated as 
consisting only of longitudinal dispersion, characterized by a constant value of 
the dispersivity, l 
. Same assumption is made in the radionuclide transport 
modeling. 
Assumption 5: For the abstraction model, radionuclide mass sorbs reversibly to nondiffusing 
colloids with a constant equilibrium sorption parameter Kc = Ccoll/Cfluid, where 
Ccoll is the radionuclide concentration residing on the colloids (moles 
radionuclide on colloid per kilogram of fluid), and Cfluid is the corresponding 
concentration in the fluid phase (moles aqueous radionuclide per kilogram of 
fluid). The same assumption is made in the radionuclide transport modeling. 
Note that the radionuclide transport model does not address reversible colloidfacilitated 
radionuclide transport. It only considers irreversibly sorbed 
radionuclides on colloids. 
Assumption 6: In the abstraction model, two groupings of colloids are considered for 
radionuclides with irreversible attachment to colloids: irreversible fast 
colloids, which are assumed not to be affected by retardation in fractures and 
irreversible slow colloids which are affected by retardation in fractures. The 
fraction of colloids that is the former results from the C-Wells testing. The 
radionuclide transport model only considered retardation in fractures as a 
sensitivity case (see Section 5.5.2 in this report). 
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Assumption 7: In the abstraction model, colloids with irreversibly attached radionuclides 
undergo irreversible filtration in the matrix based on size exclusion at interfaces 
between different matrix units. Size exclusion filtration is also treated in the 
radionuclide transport model. 
Assumption 8: For the purposes of computing radionuclide transport, flow through the 
unsaturated zone can be approximated assuming that the system (rock mass 
and flow conditions) has not been influenced by repository waste heat effects. 
Durable changes to the rock mass hydrologic properties are also assumed to be 
negligible. The same assumption is made in the radionuclide transport 
modeling. 
Assumption 9: Climate changes can be considered in an approximate way by imparting an 
instantaneous jump from one steady state flow field to another, with a 
corresponding rise or fall in the water table representing the bottom of the 
unsaturated zone model. Shorter-term transients (wet and dry years, individual 
storm events, etc.) are assumed to be adequately captured with a model that 
assumes such transients can be averaged to obtain a long-term, effective steady 
state. The same assumption is made in the radionuclide transport modeling. 
In addition to the difference in modeling approaches (particle-tracking vs. finite difference), 
Assumption 6 is treated differently between the TSPA-LA unsaturated zone transport model and 
the radionuclide transport model. The implication of this assumption on colloid transport is 
important and is discussed here. Results from the radionuclide transport model, which ignores 
retardation of colloids in fractures, indicate very rapid colloid migration through the unsaturated 
zone (Figures 5-2 and 5-3). Results from the saturated zone transport model, which includes 
retardation of colloids in fractures based on the C-Wells testing, indicate significant attenuation 
of colloids and slow colloid migration through the unsaturated zone (see Section 6.4, Figures 6-5 
and 6-6). It is highly likely that the primary reason for this different behavior is in the different 
treatment for colloid retardation in fractures. The TSPA-LA model for unsaturated zone 
transport considers retardation of colloids in fractures based on the C-Wells testing. This results 
in significant retardation of colloids compared to the result of the radionuclide transport model. 
The application of colloid retardation data from testing done under saturated conditions to 
similar rock under unsaturated conditions can be justified. Literature suggests that colloid 
transport is more attenuated under unsaturated zone conditions (Wan and Wilson 1994). For 
instance, it is common practice for experimentalists studying unsaturated zone colloid transport 
to also run experiments under saturated conditions in the same or similar laboratory apparatuses; 
and in virtually all cases, there is greater attenuation under unsaturated zone conditions. There 
are several reasons why using the same distribution in the unsaturated zone as the saturated zone 
should be conservative: 
1. Experimental and field evidence indicates less colloid transport under unsaturated 
conditions compared to saturated conditions (mainly because of airwater interfaces) 
(Wan and Wilson 1994). 
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2. Slower flow under unsaturated conditions results in longer times for retardation to take 
place. 
3. Greater ionic strengths in unsaturated zone pore waters should tend to destabilize 
colloids resulting in greater retardation. 
4. All processes operating to retard colloids under saturated zone conditions are also 
operable under unsaturated zone conditions. 
5. Additional unsaturated zone processes that do not operate in the saturated zone can 
also retard colloids in the unsaturated zone, such as capture at the airwater interface 
and film straining (Wan and Tokunaga 1997). 
A possible but highly unlikely process that could result in less retardation in the unsaturated zone 
than saturated zone is associated with episodic flow. Episodic flow in the unsaturated zone could 
result in film flow in fractures where the airwater interfaces become mobile. Such events could 
enhance the detachment of colloids from surfaces because airwater interfaces will sweep 
surfaces. This could enhance colloid transport relative to the saturated zone. It seems very 
unlikely that film flow with a continuous airwater interface could be sustained over very large 
distances and for very long times in the unsaturated zone for a number of reasons: 
1. Any place that the film encounters a pinch-point in a fracture where the film 
thickness exceeds fracture aperture, the film will bridge the aperture and a 
discontinuity in the airwater interface will occur. 
2. Films also will flow only as long as the matrix is very close to full saturation. If this is 
not the case, capillary suction in the unsaturated matrix will draw the water out of the 
fractures and effectively stop the film from advancing. It would take a very long and 
sustained flow transient to effectively establish film flow for very long distances in the 
unsaturated zone. Once the film flow dies out, the water (and colloids) will either 
slow down dramatically or be imbibed into the matrix where other retarding processes 
occur. 
5.7.3 Example Results and Conclusions from TSPA-LA Unsaturated Zone Transport 
Model 
While results are presented in Particle Tracking Model and Abstraction of Transport Processes 
(BSC 2003i, Figures 6-16 and 6-17) for the breakthrough curves of 36 radionuclides under 
present-day and glacial infiltration, the contributions of dissolved vs. colloidal species are not 
broken out and presented. Only qualitative statements are made in Particle Tracking Model and 
Abstraction of Transport Processes (BSC 2003i) concerning the observed model behavior of 
colloid transport: 
1. The simulation results reveal that irreversible fast colloids (which are assumed not 
affected by matrix diffusion and retardation) have the shortest breakthrough times and 
the greatest breakthrough quantities. Within a time period of far less than 100 years, 
more than 50 percent of the irreversible fast colloids traveled through the UZ. 
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2. Irreversible slow colloids which undergo retardation moved slower than their 
corresponding fast colloids but faster than their corresponding dissolved species. The 
travel time of the irreversible slow colloids depends on their retardation factor. In 
TSPA-LA simulations, the retardation factors of the slow colloids will be sampled and 
its impact on system performance will be evaluated. 
3. With regard to colloid transport, the simulation results show that current 
conceptualizations suggest that colloids can play an important role in accelerating the 
transport of radionuclides in the UZ, especially the irreversible fast colloids. Of 
course, if the quantity of irreversible fast colloids is low, the impact on dose would not 
expected to be important. 
4. In TSPA-LA calculations, a conservative percentage of irreversible fast colloids will 
be selected to study its impact on dose. For irreversible slow colloids, the retardation 
factor will be sampled to investigate parameter uncertainty on system performance. 
5.8 SUMMARY OF UNSATURATED ZONE SCREENING ARGUMENTS 
Colloid transport through the unsaturated zone at Yucca Mountain is expected to be primarily 
through fractures because of pore-exclusion effects. The assumption made in the radionuclide 
transport model process that no colloid sorption will occur in fractures or along their walls is 
very conservative and contrary to evidence from the C-Wells test in the volcanics of the 
saturated zone (BSC 2003b). These tests indicate a very large degree of colloid sorption and 
result in significant delays on arrival times (BSC 2003a). This ignoring of colloid sorption in the 
fractures is largely responsible for the rapid travel times through the unsaturated zone predicted 
by the radionuclide transport model and shown in Figures 5-2 and 5-3. 
Because the radionuclide transport model does not consider retardation of colloids in the 
fractures of the unsaturated zone and because larger colloids (greater than 6 nm) have difficulty 
entering the matrix surrounding the fractures in the zeolitic portions of the TSw, colloids are 
predicted to travel quickly through the zeolitic regions of the unsaturated zone beneath the 
repository by this model. However, the radionuclide transport model does show that colloid 
transport is retarded at the interface of the zeolitic portion of the TSw and underlying CHn layers 
as well as at the unsaturated zone and underlying saturated zone interface. The results 
summarized in Section 5.4.3 show a significant build up of colloids at these interfaces. This is a 
result of the relatively small pore size and permeability of the underlying layers at each interface 
compared to that of the overlying layers. Because of this, colloids cannot be transported through 
the interface at the same rate at which they are transported to the interface. This filtering 
mechanism is important in retarding the migration of colloids through the unsaturated zone. 
The role of the airwater interface on enhancing or retarding colloid transport is determined by 
the affinity of colloids to the interface and the degree of saturation. Based on the current 
understanding, it is assumed that the tendency of the airwater interface will be to retard colloid 
transport. 
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6. COLLOID TRANSPORT IN THE SATURATED ZONE 
This section summarizes the current state of knowledge concerning the transport of colloids in 
the saturated zone and their release into the biosphere. The saturated zone is the fourth of the 
repository system subfeatures examined to determine the importance of colloids with respect to 
the postclosure performance of the Yucca Mountain repository, as shown in Figure 1-1. 
Modeling assumptions to be used in the TSPA-LA are discussed. The information presented in 
this section primarily draws on the discussion contained in the following YMP sources: 
 Site-Scale Saturated Zone Transport (BSC 2003a). 
 Saturated Zone Colloid Transport (BSC 2003b). 
This chapter is organized in the following way: Section 6.1 describes the relevant processes for 
describing colloid and colloid-facilitated transport in the saturated zone, and Sections 6.2 through 
6.7 describe the saturated zone TSPA model for radionuclides sorbed reversibly and irreversibly 
to colloids, data and validation. Unlike in Section 5 (the discussion of the unsaturated zone, 
there is no distinction between the process model and the TSPA model for the saturated zone. 
6.1 DESCRIPTION OF RELEVANT PROCESSES 
The base-case conceptual model for saturated zone transport begins in the neighborhood of the 
repository footprint at the water table and ends at the compliance boundary downstream from 
this footprint. As shown schematically in Figure 6-1, the flow path from the potential repository 
to the compliance boundary begins in the volcanic tuffs but ends in the alluvium with different 
transport processes operating in the volcanic tuffs and the alluvium. The compliance boundary 
corresponds to the far right edge of the figure and is located 18 km downgradient of the saturated 
zone under the repository. 
The colloid related aspects of this conceptualization are as follows: 
1. Radionuclides can undergo colloid-facilitated transport in the volcanics. Radionuclide 
affected colloids, which are transported to the saturated zone, may include the types 
shown in Figure 6-1 (i.e., natural colloids [typically clay or silica]), waste-form 
colloids resulting from degradation of spent fuel or glass, and iron oxyhydroxide 
colloids resulting from degradation of the waste container. The transport of colloids 
themselves in groundwater, as well as the mechanisms of sorption of radionuclides 
onto these colloid particles, is included in the transport model for the saturated zone. 
Mechanisms for transport of colloid particles in groundwater are considered in detail 
in Saturated Zone Colloid Transport (BSC 2003b). Movement of colloids occurs 
within fractures only. Due to the relatively large size of the colloids, matrix diffusion 
of these particles is negligible. Colloid attachment and detachment to fracture surfaces 
is modeled by first-order rate expressions. Radionuclides, which are reversibly 
adsorbed onto colloids, are modeled using the Kc model, which represents the 
equilibrium partitioning of radionuclides between the aqueous phase and the colloidal 
phase with the distribution coefficient Kc (CRWMS M&O 1997, Equation 8-10, 
p. 8-35). 
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Figure 6-1. Transport Processes Relevant to Colloid Transport in the Saturated Zone 
2. Radionuclides can undergo colloid-facilitated transport in alluvium. The conceptual 
model for colloid-facilitated transport in the alluvium is essentially the same as in 
fractured tuffs in that colloids are modeled as transported only by advective water (no 
diffusion into stagnant water or into grains). Colloid attachment and detachment onto 
alluvial surfaces are described by first-order rate expressions. Laboratory site-specific 
data for colloid transport in the alluvium along with literature data are used to obtain 
distributions and bounds for attachment and detachment rate constants (BSC 2003b). 
As in the case of colloid transport in fractured tuffs, it was shown (CRWMS M&O 
2001c) that the local equilibrium assumption is valid for all but the shortest transport 
times and is expected to overestimate transport mobility of radionuclides for those 
shortest transport times. 
6.2 MODEL ASSUMPTIONS 
In the unsaturated zone transport and TSPA model, most radionuclides are assumed to sorb onto 
colloids reversibly (that is, they have measurable desorption rates and can be entirely desorbed 
from colloids) (BSC 2003f, Section 6.3.3.1). However, plutonium and americium can sorb either 
reversibly or irreversibly onto colloids with the relative percentages of these obtained from an 
analysis of waste-form degradation and waste-package corrosion processes occurring in the 
repository near-field environment (BSC 2003f, Section 6.3.3.1). In general, the majority of the 
plutonium and americium sorbed to colloids is irreversibly sorbed, with a typical percentage 
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being 90 to 99 percent irreversible and 1 to 10 percent reversible (BSC 2003f, Section 6.3.3.2, 
p. 74). Irreversibly sorbed plutonium and americium are either embedded in waste-form colloids 
(e.g., smectite colloids formed by degradation of high-level radioactive waste glass) or are so 
strongly sorbed onto colloids (e.g., iron oxyhydroxide colloids formed by corrosion of waste 
packages) that there is no possibility of detachment for typical transport time scales (thousands 
of years) through the saturated zone. Note that the EBS Radionuclide Transport Abstraction 
(BSC 2001a), specifically Attachment II, discusses the mathematical implementation for 
irreversible sorption of plutonium and americium onto iron oxyhydroxide colloids in the 
engineered barrier system. 
By definition, the irreversibly sorbed plutonium and americium transport in a manner identical to 
the colloids onto which they are sorbed. Therefore, the colloid retardation factor uncertainty 
distributions developed in this analysis are necessary to describe the transport of the irreversibly 
sorbed radionuclides. Several field observations have suggested that a small percentage of 
colloids are transported with essentially no retardation in groundwater (Kersting et al. 1999, 
p. 56; Penrose et al. 1990, p. 228), whereas the majority undergoes either reversible or 
irreversible filtration, which can be described by a retardation factor, Rcol. The value of Rcol is 
dependent on several factors such as colloid size, colloid type, and geochemical conditions (e.g., 
pH, Eh, and ionic strength). These factors are folded into the distribution of Rcol that has been 
developed from field and experimental data collected under varying geochemical conditions with 
different colloid types and sizes. Attachment rate constants, katt, and detachment rate constants, 
kdet, of colloids to the rock matrix have been measured, and separate Rcol uncertainty distributions 
have been developed for the fractured volcanics and the alluvium. 
The attachment rate constant is also used to determine the fraction of the colloids that transport 
with no retardation. Specifically, the inverse of the attachment rate constant provides a measure 
of the reaction time scale. Colloids for which this reaction time scale is smaller than the travel 
time through the system will transport with no retardation. The following sections document the 
development of Rcol uncertainty distributions for the volcanics and the alluvium as well as the 
fraction of colloids that transport unretarded. 
6.3 DATA AND TESTING FOR DETERMINING COLLOID RETARDATION 
FACTORS FOR IRREVERSIBLY SORBED RADIONUCLIDES 
6.3.1 Data 
The base case flow model (DTN: LA0304TM831231.002) was used directly as an input to the 
base-case transport model. For most parameters, the base-case value was chosen to be the 
median of the uncertainty distribution, except for the sorption coefficient of dissolved 
radionuclides, which was assigned a value of 0 for the base case, which leads to faster transit 
times for the base case than the stochastic simulations. The flow model provides the numerical 
grid with geometric coefficients and the groundwater flow velocity field which are used by the 
transport model. 
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The input parameters, which are considered uncertain, for the base-case transport model 
(DTN: LA0304SK831231.001) are: 
1. Colloid retardation factor in volcanics for irreversible colloids; 
2. Groundwater concentration of colloids for calculating the relative concentration of 
radionuclide on colloids, needed for colloid facilitated reversible transport; 
3. Sorption coefficient onto colloids for calculating the relative concentration of 
radionuclide on colloids, needed for colloid facilitated reversible transport; 
4. Fraction of colloids transported unretarded for post processing the breakthrough 
curves generated by the transport model; and 
5. Colloid retardation factor in alluvium for irreversible colloids for calculating the 
colloid sorption coefficient. 
For each parameter, the rationale for the selection of the range of values, probability distribution, 
and expected value are presented summarized in the following sections. Details are given in 
Saturated Zone Flow and Transport Model Abstraction (BSC 2003c). 
6.3.2 Testing 
Colloid Transport in Fractured Tuff 6.3.2.1 
Test DescriptionColloid filtration rate constants and retardation factors for the fractured 
volcanics have been estimated in a number of laboratory and field experiments conducted for the 
YMP and the Underground Test Area project. All of the field measurements have involved 
fluorescent CML microspheres ranging in size from 280 to 830 nm in diameter. Laboratory 
fracture experiments have been conducted using silica, montmorillonite, and clinoptilolite 
colloids in addition to CML microspheres. In one study, silica colloid transport (~100-nm 
diameter) was compared directly with CML microsphere transport (330-nm diameter), and it was 
found that the microspheres transported conservatively relative to the silica colloids (Anghel 
2001, Chapter 6). This result suggests that colloid filtration and retardation parameters derived 
from CML microsphere responses in field tracer tests should be conservative if used to predict 
natural inorganic colloid transport in fractured systems. 
Colloid filtration and detachment rate constants have been derived from colloid responses in 
tracer tests by using the advection-dispersion equation with appropriate terms for a single 
reversible first-order reaction to account for mass transfer between mobile water and immobile 
surfaces (filtration and detachment) to fit the data. Details of the interpretation procedure for the 
tests are provided in Site-Scale Saturated Zone Transport (BSC 2003a). 
Test Results: Colloid Filtration Rate Constants for Fractured VolcanicsFiltration rate 
constants are obtained from interpretations of several field and laboratory tracer tests conducted 
in saturated fractured rocks as a function of the time to reach peak nonsorbing solute 
concentrations in the tests (Figure 6-2). The filtration rate constants reflect the fraction of 
colloids that were not filtered during the tests; that is, the rate constant is constrained primarily 
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by the magnitude of the early arrival of colloids. The figure also shows that, even though 
different sizes and types of colloids were used in the different tests, there is an apparent trend of 
decreasing filtration rate constant with residence time. The line drawn through the data (not a 
fit) has a slope of 1, which implies an inverse time dependence of the filtration rate constant. 
Source: BSC 2003b, Figure 1. DTNs: LA0007PR831231.001 (C-Wells BF data); LAPR831231AQ99.001 (C-Wells 
PP data); LA0302PR831352.002 (ER-20-6 well 3 data); LA0302PR831352.001 (ER-20-6 well 2 data); 
LA0302PR831231.003 (non-Q) (ER-20-6 well 3 solute data); LA0302PR831231.002 (non-Q) (ER-20-6 
well 2 solute data); LA0301PR831352.001 (silica and CML data); LA0301PR831361.003 (silica and 
montmorillonite data); LA0301PR831361.004 (montmorillonite and clinoptilolite data); 
LA0303PR831231.003 (analysis of C-Wells rate constants); LA0303PR831352.002 (analysis of ER-20-6 
rate constants); LA0303PR831352.003 (analysis of laboratory rate constants). 
Figure 6-2. CML Microsphere and Inorganic Colloid Filtration Rate Constants (1/hr.) as a Function of 
Time to Solute Peak Concentration in Several Field and Laboratory Tracer Tests in 
Saturated Fractured Media 
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The results suggest that some fraction of colloids may always transport through a fracture flow 
system regardless of the time or length scale of the observations. It is tempting to draw the 
conclusion that filtration rate constants will continually decrease with increasing time scales. 
However, it may be more appropriate to consider the possibility that, while the majority of 
colloids might be filtered quite quickly, the field tests show that there is a small fraction that are 
resistant to filtration and, therefore, capable of traveling large distances over long time periods. 
This statement implies that there may be a distribution of colloid filtration rate constants rather 
than a fixed rate constant that applies to all colloids. The appearance of a small fraction of 
colloids at about the same time as nonsorbing solutes in tracer tests, regardless of the overall 
time scale of the test, forces filtration rate constants to decrease with time when single rate 
constants are assumed to apply to all colloids. 
Ignoring the possible scale effect, a cumulative probability distribution function for the filtration 
rate constants is derived from the experimental results. 
Test Results: Colloid Retardation Factors for Fractured VolcanicsColloid retardation 
factors, Rcol, are then calculated from colloid filtration and detachment rate constants using the 
interpretive results from the tests (Figure 6-3). 
Source: BSC 2003b, Figure 3; Viswanathan 2003, p. 23. 
Figure 6-3. 
No. 8: Colloids 
Cumulative Probability Distribution of Log Colloid Retardation Factors and a Fit to the Data 
for Fractured Volcanics 
September 2003 6-6
Colloid Transport in Alluvial Material 6.3.2.2 
BackgroundColloid filtration rate constants and retardation factors for alluvial material have 
been estimated in a number of laboratory experiments for the YMP. With the absence of YMP 
field data for colloid filtration in the alluvium, data from Schijven et al. (1999, Table 3, p. 1107) 
were used to obtain field colloid filtration and detachment rates. These laboratory column 
experiments have been conducted using silica and natural colloids in addition to CML 
microspheres. As with the fractured volcanics, colloid filtration and detachment rate constants 
have been derived from colloid responses in tracer tests by using the advection-dispersion 
equation with appropriate terms for a single reversible first-order reaction to account for mass 
transfer between mobile water and immobile surfaces (filtration and detachment) to fit the data. 
Results: Colloid Filtration Rate ConstantsEven though different sizes and types of colloids 
were used in the different tests, there is an apparent trend of decreasing filtration rate constant 
with residence time. This trend is similar to the trend witnessed in the fractured volcanics. 
Results: Colloid Retardation FactorsAs with the fractured volcanics, colloid retardation 
factors, Rcol, can be calculated from colloid filtration and detachment rate constants. Using the 
interpretive results from all of the tests, a cumulative probability distribution for Rcol was 
generated (Figure 6-4). 
Source: BSC 2003b, Figure 5; Viswanathan 2003, p. 24. 
Figure 6-4. 
6.3.2.3 
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Fraction of Colloids Transporting With No Retardation 
Several field observations have suggested that a small percentage of colloids transport with 
essentially no retardation in groundwater, while the majority undergoes either reversible or 
irreversible filtration, both of which can be modeled by a retardation factor, Rcol. 
Cumulative Probability Distribution of Log Colloidal Retardation Factors and a Fit to the 
Data for Alluvium Material 
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The attachment rate constant is used to determine the fraction of the colloids that transport with 
no retardation. Specifically, colloids for which the ratio of the attachment rate to the travel time 
is greater than one will transport with no retardation. The details of determining the fraction are 
given in the Saturated Zone Colloid Transport (BSC 2003b). 
Other Issues Addressed through the Testing 6.3.2.4 
Validity of the Assumption That Colloid Filtration and Detachment Rates Are FastThe use 
of a retardation factor to describe colloid transport through the saturated zone implicitly assumes 
that colloid filtration and detachment rates are fast relative to groundwater travel times through 
the system. This assumption is shown to be valid for at least 94 percent of the colloid mass 
based on distributions of filtration rate constants from laboratory and field colloid transport 
experiments (BSC 2003b). 
Validity of Use of MicrospheresSaturated Zone Colloid Transport (BSC 2003b) documents 
laboratory experiments conducted to evaluate the applicability of CML microspheres as field-test 
surrogates for inorganic colloids in both saturated fractured media and saturated alluvium. The 
following results were obtained: 
 In fracture experiments, 330-nm diameter CML microspheres consistently experienced 
less filtration/attenuation than 100-nm silica colloids. 
 In alluvium-packed column experiments, natural colloids (most less than 100 nm) 
transported with slightly less filtration than 190-nm diameter CML microspheres and 
with considerably less filtration than 500-nm microspheres. These results suggest that 
(1) small (less than 200-nm diameter) CML microspheres should serve as reasonable 
surrogates for inorganic colloids in saturated alluvium, and (2) CML microspheres may 
actually serve as conservative colloid tracers in saturated fractured media (yielding 
transport parameter estimates that result in over prediction of inorganic colloid 
transport). 
Validity of the Local Equilibrium Assumption for Colloids That Are RetardedThe 
majority fraction of colloids is retarded by the Rcol distributions developed in this analysis. The 
validity of the local equilibrium assumption must be tested in order to justify the use of the Rcol 
distributions. To evaluate the validity of this assumption, a simple analysis can be performed 
with nondimensional Damkhler numbers presented in Saturated Zone Colloid Transport (BSC 
2003b). Bahr and Rubin (1987, p. 450) found that equilibrium was well approximated when the 
sum of the two Damkhler numbers is greater than 100 and reasonably well estimated when the 
sum is greater than 10. 
For a 100-year travel time, 96 percent of the colloids will have a Damkhler number greater than 
100. For a 1,000-year travel time, it will be over 98 percent; and for a 10-year travel time, it will 
be 94 percent. An analysis of the alluvium rate constants results in very similar results with an 
even greater percentage with Damkhler numbers greater than 100. Thus, the vast majority of 
colloids that are not part of the unretarded fraction will transport in accordance with the local 
equilibrium assumption. 
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6.4 MODEL FOR COLLOID-FACILITATED TRANSPORT FOR REVERSIBLY 
SORBED RADIONUCLIDES IN THE SATURATED ZONE 
6.4.1 Implementation 
Details of the transport model implementation are found in Site-Scale Saturated Zone Transport 
(BSC 2003a). The transport model takes the calibrated flow model (DTN: 
LA0304TM831231.002) described in detail in Site-Scale Saturated Zone Flow Model (BSC 
2003e) as the starting point. Using the steady-state velocity field supplied by the flow model the 
downstream radionuclide concentrations are computed. Other processes that impact the transport 
of radionuclides that are considered include advection, dispersion, diffusion, retardation, and 
colloid-facilitated transport. Input parameters to the transport model are: 
1. Radionuclide release locations 
2. Dispersivities in the volcanics 
3. Matrix porosity 
4. Matrix diffusion coefficient 
5. The sorption distribution coefficient Kd in the matrix 
6. Flowing interval porosity 
7. Flowing interval spacing 
8. Retardation factor in the flowing interval 
9. Effective porosity of the alluvium 
10. Dispersivities in the alluvium 
11. The Kd value in the alluvium. 
Colloid-Facilitated Transport of Reversibly Sorbed RadionuclidesRadionuclides cannot 
diffuse into the matrix while attached to colloids because the colloid particles themselves cannot 
diffuse into the matrix (BSC 2003b). Hence, the fraction of radionuclides that are attached 
irreversibly onto colloids does not experience any matrix diffusion and is transported at the same 
rate as the colloid particles. 
The fraction of colloids that are reversibly attached onto the colloids experiences a reduction in 
the diffusion process because this fraction can diffuse into the matrix only while unattached to 
the colloids. In the volcanics, this results in a reduction in the effective diffusion coefficient for 
the radionuclide as indicated in Equation 6-1 (BSC 2003c, Eq. 79) 
De adjusted 
e D 
c 
where 
( ) 1+ 
= 
k 2 
e Dadjusted is the effective diffusion coefficient of the radionuclide adjusted for the effect of 
reversible attachment to colloids, 
e D is the effective diffusion coefficient of the radionuclide in the matrix, 
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(Eq. 6-1) 
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kc is the distribution parameter expressing the relative amount of radionuclide residing on 
the colloids with respect to that in the aqueous phase, given by (BSC 2003c, 
Attachment II): 
(Eq. 6-2) c = C * kd c kc 
where 
new (Eq. 6-3) kd 
- 
c C is the colloid concentration in the groundwater, and kd -c is the sorption coefficient for 
the radionuclide onto the colloids. 
In the alluvium, diffusion from the fractures into the matrix is not significant since the matrix is 
readily available; however, the sorption coefficient for the radionuclide onto the rock surface is 
modified due to the competition with the colloids as follows (BSC 2003c, Eq. 81): 
koriginal 
d 
c 
where 
6.4.2 Model Results 
( ) + 
= 
1 k 
d k is the sorption coefficient for the radionuclide in the alluvium in the 
d koriginal is the sorption coefficient for the radionuclide in the alluvium in the absence of 
colloids and new 
presence of reversible attachment to colloids. 
Particle source locations were chosen to cover the anticipated repository footprint at the water 
table. Calculations were performed for an instantaneous release of particles at the source 
location (DTN: LA0304TM831231.002). Results presented in Figures 6-5 and 6-6 show 
breakthrough curves at the 18-km compliance boundary, calculated by starting 1,000 particles 
distributed over the repository footprint and outputting the cumulative number of particles 
crossing an eastwest vertical plane across the entire width and depth of the model. 
Calculations are presented that illustrate that the saturated-zone retards the migration of 
radionuclides that are bound to colloids. Given that the most deleterious colloid-related effect on 
saturated zone performance is likely to be the mobility of otherwise less mobile dissolved 
radionuclides via colloids, the focus is on that process. This analysis is restricted here to 
radionuclides irreversibly attached to colloids. Note that reversibly sorbed radionuclides would 
follow the Kc model described in Section 6.4. Furthermore, the small fraction that travels 
unretarded (see Section 6.3.2.3) would break through at the same time as the base case aqueous 
species. Therefore, the total breakthrough of a radionuclide irreversibly sorbed to the colloids 
would be a composite of the majority fraction colloid curve shown on the figures and the base 
case aqueous species that represent the small fraction of colloids that travel with no retardation. 
Figures 6-5 and 6-6 show the expected behavior of the majority fraction of colloidal-species 
transport compared to the base-case aqueous species transport. Figure 6-5 shows the 
6-10 September 2003 No. 8: Colloids
Revision 2 
breakthrough at the 18-km compliance boundary downstream from the point at which 
radionuclides first enter the saturated zone. First arrivals (the base case aqueous species and the 
small fraction of colloids that travel unretarded) are predicted to be at about 200 years. Most of 
the colloid inventory (about 80 percent) that arrives at the compliance boundary, arrives after 
10,000 years. Most of the aqueous species inventory (about 90 percent) arrives before 10,000 
years. The majority fraction of colloids experience a significant delay in travel time. This delay 
is also observed at the volcanic-alluvial interface (see Figure 6-6). However, note that if larger 
specific discharge values were assumed, as would be the case for a future, wetter climate, the 
entire breakthrough curve would shift to travel times of less than 10,000 years. The technical 
output of the saturated zone transport model consists of breakthrough curves and travel times that 
will be integrated into the saturated zone model abstractions (BSC 2003c). 
Source: BSC 2003a, Figure 6.7-5a. 
Figure 6-5. 
No. 8: Colloids 
Comparison of Breakthrough Curves for the Base Case and Radionuclides Irreversibly 
Attached to Colloids: 18-km Boundary 
September 2003 6-11
Revision 2 
Source: BSC 2003a, Figure 6.7-5b. 
Figure 6-6. 
No. 8: Colloids 
Comparison of Breakthrough Curves for the Base Case and Radionuclides Irreversibly 
Attached to Colloids: Volcanic/Alluvium Boundary (about 18 km) 
6.5 UNCERTAINTY AND SENSITIVITY 
6.5.1 Data and Testing Uncertainties 
Although results from field colloid transport tests and observations at several radionuclide 
affected field sites suggest it is reasonable to expect a small fraction of colloid mass to move 
unretarded through transport systems, there are uncertainties associated with the assumed 
partitioning of colloid mass in the saturated zone into fractions that travel unretarded and 
retarded. This partitioning is based largely on the assumption that the population of colloids 
having irreversibly-sorbed radionuclides entering the saturated zone has a cumulative 
distribution of filtration rate constants approximately equal to the cumulative distribution of 
filtration rate constants observed in laboratory and field saturated zone colloid transport 
experiments. Although the colloids generated in the engineered barrier system will have some 
characteristics different from the colloids used in the saturated zone transport experiments, the 
overall transport behavior of the colloids is not expected to differ significantly. 
Additional uncertainty is introduced as a result of extrapolating the cumulative distribution of 
observed filtration rate constants to very low probabilities for the purposes of estimating colloid 
mass fractions that move unretarded. In part to address these unquantifiable uncertainties, the 
distributions of filtration rate constants in the volcanics and alluvium are combined to obtain a 
single filtration-rate constant distribution to estimate colloid mass fractions that move unretarded 
through the entire saturated zone. 
September 2003 6-12
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Uncertainties are also associated with the retardation factors obtained for both the volcanics and 
alluvium: 
1. All the laboratory and field transport data used to derive the distributions were 
obtained at scales much smaller than the scales of interest for TSPA modeling. 
2. The retardation factors derived from many of the transport tests were based on fitting 
the low colloid concentrations in the tails of breakthrough curves. These tails were 
sometimes barely above detection or quantification limits, resulting in the fitted 
retardation with potentially considerable uncertainty. However, long tails are virtually 
always observed whenever analytical methods are sensitive enough to capture them. 
These uncertainties are addressed by: 
1. Assigning lower bounds for retardation factors to cumulative distributions in the 
volcanics and alluvium such that significant retardation of colloids always occurs, and 
2. Assuming that a small fraction of colloid mass moves unretarded through the transport 
system. 
The two sets of laboratory alluvium colloid transport experiments yielded significantly different 
results for both CML microsphere and inorganic colloid transport in alluvium (BSC 2003b). 
These disparate results, though possibly explainable by the presence of air bubbles in one set of 
experiments, can be interpreted as imparting potential uncertainty to colloid transport in 
alluvium. This uncertainty is preserved by including both sets of experimental results in the 
overall data set used to develop the cumulative distributions for colloid-filtration-rate constants 
and retardation factors in alluvium, resulting in the distributions of Figures 6-3 and 6-4. 
6.6 MODEL CONFIDENCE BUILDING 
6.6.1 Model Validation 
Recognizing that the site-scale saturated zone transport model is being used to perform 
probabilistic calculations in which parameter uncertainties are propagated through the model, the 
intent of validation was to confirm the appropriateness of the radionuclide parameters and 
processes included in the model. Confidence in the results of the model was built by a series of 
different approaches that included: (1) comparisons to analog sites, (2) model-data comparisons, 
(3) comparison with data published in refereed journals, and (4) comparison to an independent 
site-scale groundwater flow model for the Yucca Mountain saturated zone. 
The data used in the confidence building for the relevant transport parameters (e.g., sorption 
coefficient), submodel processes (e.g., advection, sorption), and site scale model processes 
(e.g., flow pathways, transit times) were based on laboratory testing, field tests, natural analog 
sites, and expert elicitations. The model was validated by post development comparison of 
model transit times with those inferred from carbon-14 data and qualitative comparison of flow 
paths predicted by the model and those inferred from the hydrochemistry data. 
September 2003 6-13 No. 8: Colloids
6.6.2 Alternate Models 
An alternative conceptual model was considered to evaluate colloid transport in the saturated 
zone transport. These evaluations are summarized in Table 6-1. 
Table 6-1. 
Alternative 
Conceptual 
Model 
Key 
Assumptions 
Locally 
varying 
transport 
properties 
Transport 
properties are 
strong 
functions of 
local water 
chemistry, rock 
mineralogy, 
and solute 
concentrations. 
The properties 
have to be 
calculated 
locally at each 
node along the 
travel path. 
Source: BSC 2003a, Table 6.4-1. 
6.7 SUMMARY OF SATURATED ZONE SCREENING ARGUMENTS 
Alternative Conceptual Models Considered 
Screening 
Assessment 
This 
alternative 
conceptual 
model is 
implicitly 
included in the 
saturated zone 
transport 
model through 
the range of 
uncertainty in 
the sorptioncoefficient 
values. 
Basis 
The approach taken in this model report is to use linear 
transport equations with transport parameters 
appearing in the equations being treated as effective 
stochastic variables appropriate for the model scale. 
This approach is consistent with the current level of 
available data. Transport parameters such as the Kd 
coefficients depend on type and concentration of the 
species, rock mineralogy, and groundwater pH and Eh. 
In the present model, broad distributions have been 
developed for effective Kds (BSC 2003a, Attachments 
I, II, and III). No credit is being taken for sorption onto 
zeolites, and the Kd distributions presented in 
Attachment I are based on silica surfaces. Given the 
current level of data availability, water pH data are 
being treated as spatially random along the transport 
path, and oxidizing conditions are taken as a 
conservative assumption, as justified in Section 5 of 
this report. 
To evaluate the effect of scale on the Kd distributions, 
calculations were performed to capture the effect of 
spatial variability of rock types and variability in water 
chemistries on effective Kd ((BSC 2003a, Attachment 
II). The effect of variability in water chemistry was 
captured in the input Kd distributions that were used for 
effective Kd calculations. It was observed that the 
effective Kd distributions calculated for a single 500-mby-
500-m grid block were narrower than the input Kd 
distributions. During performance assessment 
modeling studies, calculations will be made through 
multiple runs, each with a distinct Kd value sampled 
from the above-mentioned effective Kd distribution. 
This approach will be more approximate than 
assigning Kd values on a node basis, as the latter 
approach will lead to breakthrough behavior that can 
be described by an even narrower distribution than the 
effective Kd distribution. 
Field experiments using fluorescent CML microspheres and laboratory experiments using both 
natural colloids and the CML microspheres show that the longer the colloids travel through the 
fractured volcanics, the higher the probability that they will be reversibly or irreversibly filtered 
by the immobile rock phase. These results support the assumption that most colloids are filtered 
early on and that only a small percentage remain unretarded. 
No. 8: Colloids 
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September 2003 6-14
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Laboratory experiments for alluvial materials pertaining to Yucca Mountain using both natural 
colloids and CML microspheres suggest that colloid filtration decreases with time. Several of 
the field observations for alluvial materials have suggested that the majority of the colloids 
undergo reversible or irreversible filtration and only a small percentage of the colloids move 
unretarded. 
Sensitivity analyses performed comparing the travel time through the fracture volcanics in the 
saturated zone of aqueous radionuclides and of colloids with irreversibly attached radionuclides 
show that the colloid releases lag considerably behind the release of the aqueous species. The 
analyses suggest that, if radioactive colloids reach the saturated zone within the regulatory 
compliance period of 10,000 years, the saturated zone can significantly retard their release to the 
accessible environment. Some radioactive colloids could be released to the biosphere within 
10,000 years but that is likely to be a small fraction of the total radioactive colloid concentration 
entering the saturated zone. 
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7. SUMMARY 
The presence of colloids at the Yucca Mountain site raises the potential of accelerating the 
transport of radionuclides from the repository to a natural or man-made discharge point at the 
biosphere. There are several potential sources of colloids in groundwater (e.g., clay minerals, 
metal oxides, viruses, bacteria and humic macromolecules. It has been suggested that dissolved 
groundwater contaminants could have high affinity to sorption onto the surfaces of suspended 
colloids. As a result, colloids could possibly act as carriers for contaminants in natural 
subsurface environments and be significant contributors to the rate of migration of radionuclides. 
However, for colloids to provide a significant means for the transport of radionuclides from the 
waste form to the biosphere and, consequently, be a major contributor to dose, colloidal 
suspensions must: 
1. Contain high enough concentrations of colloids to carry a significant mass of 
radionuclides relative to the mass of dissolved radionuclides 
2. Be stable for distances on the order of tens of kilometers and for time periods of tens 
of thousands of years 
3. Not appreciably attach to immobile rock surfaces. 
If one or more of these conditions are not met, the importance of colloid-facilitated transport to 
system performance significantly decreases. 
In order to assess the overall significance of colloid transport to system performance, the 
formation, stability, transport and filtration of colloids were examined for the waste package, the 
invert, the unsaturated zone and the saturated zone. The most important phenomena affecting 
colloids in each of those subsystems were identified and examined to reach risk-informed 
screening arguments. Those screening arguments are summarized in the sections that follow. 
Their cumulative qualitative effect on the release of radioactive colloids to the biosphere was 
presented in Figure 1-2. This assessment leads to the conclusion that colloid-facilitated 
radionuclide transport is not as important to system performance as transport of radionuclides as 
dissolved species. 
7.1 COLLOID FORMATION, STABILITY, AND TRANSPORT IN REPOSITORY 
SUBSYSTEMS 
7.1.1 Waste Package 
Three different types of colloids are considered in the modeling of the radionuclide source term: 
1. Waste-form colloids resulting from corrosion of defense high-level radioactive waste 
glass are primarily smectite colloids with embedded discrete phases of plutonium and 
americium; 
2. Iron oxyhydroxide colloids resulting from the corrosion of waste package and metallic 
invert materials; and 
September 2003 7-1 No. 8: Colloids
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3. Natural colloids in groundwater and seepage water. 
Concerning the third type, humic substances are not considered to be sufficiently abundant in 
Yucca Mountain groundwater to be of significant impact to radionuclide transport. Also, 
microbes, due to their relatively large size (~1 m) within the colloid-size range, are highly 
susceptible to filtration. Therefore, natural colloids at the Yucca Mountain site are modeled as 
smectite colloids. 
True or intrinsic colloids were eliminated from consideration in TSPA analyses for two reasons. 
First, long-term corrosion testing with CSNF and DSNF under unsaturated, oxidizing conditions 
have produced very low colloid concentrations and small amounts of uranium within the colloid 
mass. Second, the same test indicates that meta-schoepite and UO2+x colloids readily dissolve 
upon the introduction of J-13 groundwater. 
Except for the amounts of plutonium and americium that can be (1) irreversibly embedded within 
defense high-level radioactive waste glass colloids, and (2) irreversibly sorbed to iron 
oxyhydroxide colloids, it is assumed that radioactive colloids are formed by the reversible 
sorption of dissolved radionuclides onto smectite colloids under equilibrium conditions and can 
be described by a distribution coefficient. The reversible sorption of plutonium, americium, 
protactinium, thorium, and cesium onto colloids is modeled using Kd values, the mass of 
dissolved radionuclides and the colloid concentration. 
The stability of colloid suspensions within the waste package and the drift will be highly 
dependent on the evolution of the physical and chemical conditions. For example, relative 
humidity decreases to low values shortly after emplacement of the waste containers. With time, 
as the containers cool down, the relative humidity increases asymptotically to 100 percent after 
about 2,000 years. At lower relative humidity, the groundwater entering the drift concentrates 
due to water evaporation and this, in turn, would lead to a high-ionic-strength solution. Colloid 
suspensions are not stable at ionic strengths above 0.05 M. Therefore, stable colloid suspensions 
during the first 2,000 to 3,000 years after repository closure are not likely. 
Only diffusive releases typically occur for the nominal case, and thus colloid transport is minor 
over the regulatory 10,000-year period. Advective releases from seeping water can potentially 
occur for containers disrupted by the seismic scenario or the volcanic scenario. 
The seepage-water dripping scenario is the most relevant in-package chemistry scenario with 
regard to colloid transport. For this scenario, reaction-path calculations indicate that the initial 
seepage water composition is rapidly overtaken by water-package interactions. Depending on 
the seepage flux the water can reach an ionic strength higher than 0.05 M, the lower 
ionic-strength limit for stable colloid suspension. Therefore, under the relevant in-package 
chemistry scenario, it is unlikely that colloid suspensions will be stable. Furthermore, once 
formed and stably suspended, colloids could be filtered within fractures in fuel pellets or trapped 
at the boundaries of disaggregating grains. Notwithstanding these potential colloid filtration 
processes that would reduce the amount of radioactive colloids released from the waste package, 
it has been assumed that the waste package does not provide a barrier to the release of 
radioactive colloids. 
September 2003 7-2 No. 8: Colloids
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7.1.2 Invert 
The drift invert beneath the waste packages, which is primarily composed of crushed tuff rocks, 
will likely be in intimate contact with the waste package. In the nominal scenario the invert 
would limit advective transport of radionuclides out of the engineered barrier by maintaining 
unsaturated conditions under the waste package. 
The invert is expected to act as a mixing cell. Any seepage water entering the invert would 
combine with the water flowing from a damaged waste package, possibly altering the chemistry 
of the water from the waste package. Any incoming water would usually be overwhelmed by the 
reaction with the invert material, and the resulting solution would usually be similar to J-13 
groundwater with a neutral to slightly basic pH. As a result, any iron oxyhydroxide colloids may 
become unstable. Also, the evaporation process may concentrate the solution, which may further 
increase the instability of colloids in the invert. 
The corrosion of invert support will generate iron and copper oxides, which are generally strong 
adsorbents for various radionuclides. Corrosion products would tend to be localized and 
relatively far apart from each other, with the possibility that seepage water from the waste 
package could completely miss corrosion products in the invert. There is a low probability that 
radionuclides will come into contact with corrosion products. Therefore, the sorption effect of 
these corrosion products is not considered in the current TSPA calculations. 
The extent of colloidal filtration within the invert is not expected to be significant due to the 
short filtration path. 
7.1.3 Unsaturated Zone 
Most colloid transport studies have been under saturated conditions; hence, much of the 
information related to colloid transport in unsaturated media has been inferred from such studies, 
augmented by information available from the limited experimental studies under unsaturated 
conditions. The current state of understanding regarding colloids in groundwater native to the 
Yucca Mountain site is that natural colloids are present in low concentrations and the 
preponderance of colloids affecting radionuclide transport will be waste-form colloids. 
Colloid transport through the unsaturated zone at Yucca Mountain is expected to be primarily 
through fractures because of pore-exclusion effects. The extremely small pores in some of the 
geologic formations within the unsaturated zone will likely provide a barrier to colloid transport. 
This is particularly important because large contrasts in porosity between adjacent formations 
have been shown to increase colloid filtration. 
The effect of the airwater interface on colloid transport is determined by the affinity of colloids 
to attach to the interface and the degree of saturation. Based on the current understanding, it is 
concluded that the attachment of colloids to the airwater interface will be to retard colloid 
transport. 
Pseudocolloids, with radionuclides either reversibly or irreversibly adsorbed, are the only 
radioactive colloids that are considered to be transported through the unsaturated zone below the 
September 2003 7-3 No. 8: Colloids
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repository, except for waste form colloids derived from defense high-level radioactive waste 
glass. 
7.1.4 Saturated Zone 
Colloids carrying radionuclides are assumed to enter the saturated zone due to downward travel 
through the fractures in the unsaturated zone directly below the repository foot print. Once they 
have entered the saturated zone, colloids will travel preferentially along horizontal paths until 
they reach the biosphere through a natural or man-made discharge point. 
Colloid filtration rate constants and retardation factors for colloid transport in fractured volcanics 
have been estimated from a number of laboratory and field experiments. These experiments 
suggest that the longer the colloids have to travel through the saturated medium, the higher the 
probability that they will be reversibly or irreversibly filtered by the immobile rock phase. These 
results support the assumption that most colloids are filtered early on and that only a small 
percentage remain unretarded. 
Colloid filtration rate constants and retardation factors for alluvial materials pertaining to Yucca 
Mountain have been estimated from laboratory tests. As with experiments in fractured 
volcanics, colloid-related parameters were obtained by fitting the solution of the advectiondispersion 
equation with a reversible linear model for mass transfer between the mobile and 
immobile rock to tracer tests. Several of the field observations have suggested that the majority 
of the colloids undergo reversible or irreversible filtration, captured with a retardation factor, and 
only a small percentage of the colloids move unretarded. 
Analyses performed to test the validity of the linear equilibrium-based adsorption model used to 
account for colloid retardation suggest that the equilibrium retardation factor for colloids is valid 
for the vast majority of colloids that get retarded. 
Sensitivity analyses performed comparing the travel time through the fractured volcanics in the 
saturated zone of dissolved radionuclides and of colloids with irreversibly attached radionuclides 
show that the colloid releases lag considerably behind the release of the dissolved species. The 
analyses show that, if radioactive colloids reach the saturated zone within the regulatory period 
of 10,000 years, the saturated zone can significantly retard their release to the biosphere. Some 
radioactive colloids could be released to the biosphere within 10,000 years but that is estimated 
to be a small fraction of the total radioactive colloid concentration entering the saturated zone. 
7.2 CONCLUDING REMARKS 
The conclusion of this technical basis document is that the contribution to system performance 
due to radionuclides carried by colloids from the waste form to the biosphere is insignificant. 
Some radionuclides (e.g., plutonium and americium) may reach the biosphere primarily in 
colloidal form, but in insignificant amounts compared to amounts of other radionuclides that 
arrive primarily as dissolved species, such as technetium, iodine, and neptunium. 
Different phenomena act upon colloids in their migration from the waste form to the biosphere. 
These phenomena lead to a continuous decrease in the concentration of colloids as they travel 
from one repository subsystem to the next. It has been shown that colloidal suspensions in the 
September 2003 7-4 No. 8: Colloids
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drift and the perturbed near-field environments are unlikely to be sufficiently stable due to 
thermal-chemical conditions, such as high temperature and high ionic strengths. Analyses of 
experimental data have shown that, under the perturbed conditions expected at the 
Yucca Mountain site, the probability that colloids will be present in high enough concentrations 
to effectively compete with the immobile rock for the adsorption of dissolved radionuclides is 
unlikely. 
In addition, large physical changes in the porosity of hydrologic units in the unsaturated zone 
below the repository, causing the flow to suddenly change from preferentially fracture flow to 
matrix flow, effectively retards or even prevents colloid transport. When such changes occur 
between hydrologic units, the interface between the units acts as a very effective colloid filter. 
The net effect of the changes in the porosity of hydrologic units in the unsaturated zone is 
considerable reduction in (1) colloid concentrations and (2) mass of radionuclides being carried 
by colloids; thus, the source of radioactive colloids into the saturated zone is reduced. 
Colloid-facilitated transport of radionuclides is further attenuated in saturated zone leading to the 
conclusion that colloid-facilitated transport of radionuclides is not a significant contributor to 
system performance. This low importance leads to the corollary that the uncertainties in 
colloid-transport-related phenomena and parameters are not important to the uncertainty in the 
system performance. Therefore, these phenomena and parameters would not appear as important 
in the uncertainty/sensitivity analyses based on total system performance. 
The discussion in this technical basis document comprises a risk-based screening argument for 
colloid-facilitated transport of radionuclides. However, it is not suggested that colloid-facilitated 
transport be excluded from the TSPA. Rather, it is concluded that on a risk basis, 
colloid-facilitated transport is not a significant contributor to estimating system performance, and 
therefore, issues associated with this transport mode are of relatively low importance. It is also 
concluded that the current modeling approach for colloid-facilitated transport of radionuclides in 
TSPA is adequate for its purpose. 
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8. REFERENCES 
8.1 CITED DOCUMENTS 
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BSC 2003l. Drift Scale THM Model. MDL-NBS-HS-000017 REV 00A. Las Vegas, Nevada: 
Bechtel SAIC Company. ACC: MOL.20030506.0239. 
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Nevada: Bechtel SAIC Company. ACC: MOL.20030910.0158. 
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at the Nevada Test Site (NTS). Submittal date: 03/06/2003. 
LA0302PR831352.002. Transport of CML Microspheres in Field Tracer Test at ER-20-6#3 Site 
at the Nevada Test Site (NTS). Submittal date: 03/06/2003. 
LA0002SK831352.003. Colloid Size Distribution and Total Colloid Concentration in INEEL 
Groundwater Samples. Submittal date: 02/25/2000. 
LA0002SK831352.004. Colloid Size Distribution and Total Colloid Concentration in 
Groundwaters from the Nuclear Test Site. Submittal date: 02/25/2000. 
LA0301PR831352.001. Breakthrough Curves of Iodide, CML Microspheres, and Silica Colloids 
in Saturated Fractured Cores from the Nevada Test Site. Submittal date: 01/23/2003. 
LA0301PR831361.003. Breakthrough Curves of Tritium, Plutonium, and Various Colloids in 
Saturated UE20C Fractured Cores from the Nevada Test Site. Submittal date: 01/22/2003. 
LA0301PR831361.004. Breakthrough Curves of Tritium, Plutonium, and Various Colloids in 
Saturated PM-1 and PM-2 Fractured Cores from the Nevada Test Site. Submittal date: 
01/22/2003. 
LA0302PR831231.002. Solute Data from ER-20-6#2 in the BULLION Forced-Gradient Field 
Tracer Test at the ER-20-6 Wells at NTS. Submittal date: 02/03/2003. 
LA0302PR831231.003. Solute Data from ER-20-6#3 in the BULLION Forced-Gradient Field 
Tracer Test at the ER-20-6 Wells at NTS. Submittal date: 02/03/2003. 
LA0302PR831352.001. Transport of CML Microspheres in Field Tracer Test at ER-20-6#2 Site 
at the Nevada Test Site (NTS). Submittal date: 03/06/2003. 
LA0302PR831352.002. Transport of CML Microspheres in Field Tracer Test at ER-20-6#3 Site 
at the Nevada Test Site (NTS). Submittal date: 03/06/2003. 
LA0303PR831231.003. Model Interpretations of C-Wells Field Tracer Transport Experiments. 
Submittal date: 03/31/2003. 
LA0303PR831352.002. Model Interpretations of ER-20-6 Field Tracer Transport Experiment. 
Submittal date: 03/31/2003. 
LA0303PR831352.003. Model Interpretations of NTS Fractured Core Colloid and Colloid- 
Facilitated Transport Experiments. Submittal date: 03/31/2003. 
LA0304SK831231.001. SZ Site-Scale Transport Model, FEHM Files for Base Case. Submittal 
date: 04/17/2003. 
September 2003 8-12 No. 8: Colloids
Revision 2 
LA0304TM831231.002. SZ Site-Scale Flow Model, FEHM Files for Base Case. Submittal date: 
04/14/2003. 
LA9912PR831231.006. Simulations of Microsphere Tailing in Bullfrog and Prow Pass Tests. 
Submittal date: 12/14/1999. 
LAPR831231AQ99.001. Prow Pass Reactive Tracer Test Field Data. Submittal date: 
02/10/1999. 
LB970601233129.001. The Site-Scale Unsaturated Zone Model of Yucca Mountain, Nevada for 
the Viability Assessment. Submittal date: 06/09/1997. 
LB990801233129.003. TSPA Grid Flow Simulations for AMR U0050, UZ Flow Models and 
Submodels (Flow Field #3). Submittal date: 11/29/1999. 
LB991220140160.017. Model Prediction of 3-D Colloid Transport, Present-Day Infiltration, #1 
Perched Water Model, Using EOS9nT Input and Output Files. AMR U0060, Radionuclide 
Transport Models Under Ambient Conditions. Submittal date: 03/11/2000. 
LL991109751021.094. Data Associated with the Detection and Measurement of Colloids in 
Scientific Notebook SN 1644. Submittal date: 01/10/2000. 
MO0002EBSDDC02.003. The Determination of Diffusion Coefficient of Invert Materials. 
Submittal date: 02/16/2000. 
MO0007MWDIDD31.001. Invert Diffusion Data. Submittal date: 07/06/2000. 
MO0212UCC034JC.002. Mass Transport of Solids in Effluent Solution During Miniature 
Waste Package Corrosion. Submittal date: 12/09/2002. 
MO0302UCC034JC.003. Graphical X-Ray Diffractometer Data and Mineral Analysis of 
Filtered Solids from Effluent Solution During Miniature Waste Package Corrosion.. Submittal 
date: 02/10/2003. 
MO0306ANLSF001.459. Colloids Generated from Irradiated N Reactor Fuel, Data Report. 
Submittal date: 06/04/2003. 
MO0308SPAPCESA.001. EBS P&CE Model THC Seepage Analysis Spreadsheets. Submittal 
date: 08/11/2003. 
September 2003 8-13 No. 8: Colloids
INTENTIONALLY LEFT BLANK 
8-14 No. 8: Colloids 
Revision 2 
September 2003
Revision 2 
APPENDIX A 
EXCLUSION OF ENTRAINED COLLOIDS 
IN THERMAL-CHEMICAL ALTERATION 
(RESPONSE TO ENFE 1.06, ENFE 4.04, AND GEN 1.01 (COMMENT 35)) 
September 2003 No. 8: Colloids
Revision 2 
Note Regarding the Status of Supporting Technical Information 
This document was prepared using the most current information available at the time of its development. This 
Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were 
prepared using preliminary or draft information reflect the status of the Yucca Mountain Projects scientific 
and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model 
Reports (AMRs) and other draft references whose contents may change with time. Information that evolves 
through subsequent revisions of the AMRs and other references will be reflected in the License Application 
(LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not 
routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to 
reflect changes in the supporting references prior to submittal of the LA. 
September 2003 No. 8: Colloids
Revision 2 
APPENDIX A 
EXCLUSION OF ENTRAINED COLLOIDS 
IN THERMAL-CHEMICAL ALTERATION 
(RESPONSE TO ENFE 1.06, ENFE 4.04, AND GEN 1.01 (COMMENT 35)) 
This appendix provides a response for Key Technical Issue (KTI) agreement Evolution of the 
Near-Field Environment (ENFE) 1.06, agreement ENFE 4.04, and general agreement 
(GEN) 1.01, comment 35. These KTI agreements relate to providing the technical basis for the 
screening of entrained colloids in its analysis of features, events, and processes (FEPs) and for 
new transport parameter distributions. 
A..1 KEY TECHNICAL ISSUE AGREEMENTS 
A.1.1 ENFE 1.06, ENFE 4.04, and GEN 1.01 (Comment 35) 
Agreements ENFE 1.06 and ENFE 4.04 were reached during the U.S. Nuclear Regulatory 
Commission (NRC)/U.S. Department of Energy (DOE) Technical Exchange and Management 
Meeting on Evolution of the Near-Field Environment held January 9 through 12, 2001, in 
Pleasanton, California. ENFE KTI subissues 1, 2, 3, and 4 were discussed at that meeting 
(Reamer 2001). ENFE 1.06 and ENFE 4.04 are identical agreements. 
Agreement GEN 1.01 was reached during the NRC/DOE Technical Exchange and Management 
Meeting on Range of Thermal Operating Temperatures held September 18 through 19, 2001. At 
that meeting, NRC provided an additional comment relating to ENFE 1.06 (GEN 1.01, 
comment 35), and DOE provided an initial response (Reamer and Gil 2001). 
The wording of these agreements and of DOEs initial response to the general agreement 
comment is as follows: 
ENFE 1.06 and ENFE 4.04 
Provide the technical basis for excluding entrained colloids in the analysis of 
FEP 2.2.10.06.00 (Thermo-Chemical Alteration) or an alternative FEP. The DOE 
will provide the technical basis for screening entrained colloids in the analysis of 
FEP 2.2.10.06.00 in a future revision of the Features, Events, and Processes in 
UZ Flow and Transport AMR (ANL-NBS-MD-000001), expected to be available 
in FY 02. 
GEN 1.01 (Comment 35) 
The SSPA recommends new values for EBS colloid transport parameters. If these 
are adopted by TSPA in the future, the technical basis for the new distributions 
will require close scrutiny. Relevant KTI agreements are RT 3.07, ENFE 4.03, 
ENFE 4.04, and ENFE 4.06. 
September 2003 A-1 No. 8: Colloids
Revision 2 
DOE Initial Response to GEN 1.01 (Comment 35) 
The new values for EBS colloidal transport parameters were designed to evaluate 
unquantified uncertainty for the SSPA. DOE understands that prior to any potential LA, 
a stronger technical basis must be provided for EBS colloidal transport parameter values 
carried forward to the base case analysis. 
A.1.2 Related Key Technical Issue Agreements 
ENFE 4.06 
Provide documentation to demonstrate suitability of the bounding values used for 
colloid transport through the perturbed near-field environment. For example, 
consider sensitivity analysis to investigate the effects of varying colloid sorption 
parameters (Kc) on repository performance. The DOE will evaluate the suitability 
of the colloid transport model under perturbed conditions as discussed in 
agreement 3 for this subissue. As part of this work, the DOE will consider 
sensitivity analyses to investigate the effects of varying colloid sorption 
parameters (Kc) on repository performance. The DOE will also provide the 
TSPA-SR (TDR-WIS-PA-000001) REV 00 ICN 01 in January 2001. The 
TSPA-SR includes sensitivity studies in the form of barrier degradation and 
parameter sensitivity analyses that investigate the effect of sorption and colloid 
parameters on repository performance. 
KTI agreements ENFE 4.03 and RT 3.07 have been grouped with KTI agreements ENFE 1.06 
and ENFE 4.04 through this technical basis document. Responses to KTI agreements 
ENFE 4.03 and RT 3.07 appear respectively in Appendix C and Appendix E of this technical 
basis document. 
A.2 RELEVANCE TO REPOSITORY PERFORMANCE 
Radioactive colloids, when entrained in groundwater flow, could provide a potentially faster 
means for the release of radionuclides to the accessible environment relative to the transport of 
dissolved radionuclides. For colloids to represent such a faster transport means, colloids must 
remain entrained in the flow (i.e., stable) for tens of kilometers and thousands of years. Effects 
of changes in the thermal and chemical environment on the formation and stability of colloids is 
addressed in Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: 
Abstraction and Summary (BSC 2003a). The near-field thermal-chemical analysis indicates only 
small changes in hydrologic properties and mineralogy as a result of these coupled processes 
(BSC 2003b, Section 6). Far-field changes are likewise expected to be small, including mineral 
precipitation/dissolution and alteration of minerals such as zeolites and clays. Therefore, this 
FEP is excluded from TSPA on the basis of low consequence. 
Therefore, excluding entrained colloids in the analysis of the thermal-chemical alteration FEP 
will not be significant to repository performance. The KTI agreements addressed in this 
appendix are related to the engineered and near-field environment as shown in Figure A-1. 
September 2003 A-2 No. 8: Colloids
Revision 2 
Components 
Figure A-1. Mapping of Colloid-Related Key Technical Issues Agreement to Repository System 
September 2003 A-3 No. 8: Colloids
Revision 2 
A.3 RESPONSE 
FEP 2.2.10.06.00 considers thermal-chemical alteration effects in the unsaturated zone, 
specifically solubility, speciation, phase changes and precipitation/dissolution. This FEP was 
revised and is now discussed as FEP 2.2.10.06.0A in Section 6.8.13 of Features, Events and 
Processes in UZ Flow and Transport (BSC 2003c). This FEP is conservatively ignored with 
respect to solubility reduction in the far-field; the effects of radionuclide precipitation are 
ignored in the far-field unsaturated zone transport problem for TSPA. The effects of colloid 
formation are accounted for in the colloid source term. Colloids are expected to form from the 
degradation of the HLW glass (with an insignificant contribution from SNF waste forms), 
engineered barrier system materials, and rock (see Section 3 of this technical basis document). 
Radionuclides associated with colloids are modeled as either irreversibly or reversibly attached 
to colloids (CRWMS M&O 2000a, Section 6; CRWMS M&O 2000b, Section 6). The near-field 
thermal-chemical analysis indicates only small changes in hydrologic properties and mineralogy 
as a result of these coupled processes (BSC 2003b, Section 6). Therefore, far-field changes are 
likewise expected to be small, including mineral precipitation/dissolution and alteration of 
minerals such as zeolites and clays. Therefore, this FEP is excluded from TSPA on the basis of 
low consequence. 
An issue regarding this FEP discussed at the NRC/DOE Technical Exchange and Management 
Meeting on Evolution of the Near-Field Environment held January 9 through 12, 2001, in 
Pleasanton, California (Reamer 2001) was the consideration of colloid entrainment due to 
boiling. At that time, DOE stated that, while colloid entrainment due to boiling had not been 
considered, this process was not expected to have a significant adverse impact on system 
performance. The specific statements regarding colloid entrainment due to boiling contained in 
the documented meeting summary are: 
1. Under NRC Comments Related to TSPA Treatment of Engineered Barrier System 
Chemical Environments, Total System Performance The NRC asked if the DOE 
considered colloid entrainment by vigorous water movement in the drift. The DOE 
answered that they had not considered this but would expect the effect on transport to 
be small due to (1) low flow rates in the drift and (2) the tendency of boiling-generated 
flow to be directed toward the source. 
2. Under NRC Comments on Colloidal Transport in the Unsaturated Zone (Technical 
Discussions  Subissue 4, Effects of Coupled Thermal-Hydrologic-Chemical 
Processes on Radionuclide Transport  the NRC asked whether the DOE had 
considered possible entrainment of colloids and particulates in convection/advection 
fluids during boiling. The DOE said that they had not, but that low fluid fluxes made 
it unlikely that this effect would be significant. 
As discussed below, boiling conditions may enhance the instability of colloid suspensions, rather 
than enhance colloid entrainment. Therefore, exclusion of colloid entrainment due to boiling is a 
conservative assumption. 
September 2003 A-4 No. 8: Colloids
Revision 2 
A revision to FEP 2.2.10.06.0A to include entrainment of colloids due to boiling is being 
addressed. As will be discussed in more detail in Section A.4, it is not believed that colloid 
entrainment due to thermal-chemical alteration will enhance radionuclide transport. 
GEN 1.01 (Comment 35) calls for new values of engineered barrier system colloid transport 
parameters. Within the engineered barrier system, the main transport mechanism for colloid 
transport is diffusion through the invert without colloid retardation (see Section 4 of this 
technical basis document). Therefore, the only relevant parameter for colloid transport through 
the engineered barrier system is the colloid diffusion coefficient. New values for the diffusivity 
of dissolved species through the invert are presented in Section 6.3.4.1 of EBS Radionuclide 
Transport Abstraction (BSC 2003d). Diffusivities for colloids are estimated from the dissolved 
species diffusivity values. These values for colloid diffusion coefficients through the invert are 
provided in Section A.4. 
The information in this report is responsive to agreements ENFE 1.06, ENFE 4.04, and GEN 
1.01 (Comment 35) made between the DOE and NRC. The report contains the information that 
DOE considers necessary for the NRC to review for closure of these agreements. 
A.4 BASIS FOR THE RESPONSE 
Section 6.8.13 (FEP 2.2.10.06.0A) of Features, Events and Processes in UZ Flow and Transport 
(BSC 2003c) presents the screening argument for excluding the effects of thermal-chemical 
alteration in the unsaturated zone (solubility, speciation, phase changes, and 
precipitation/dissolution). This FEP is conservatively excluded with respect to solubility 
reduction in the far field, and the effects of radionuclide precipitation are excluded in the 
unsaturated zone transport for TSPA. If solubility limits increase in the geosphere compared 
with the waste emplacement drift, there is no effect on transport because all available 
radionuclides from the source at the waste emplacement drift already exist as aqueous species. 
The thermal-chemical alteration effects on colloid formation have already been accounted for in 
the colloid source term (BSC 2003a). Therefore, any impact from thermal-chemical alteration 
on colloid entrainment will be insignificant with respect to total repository system performance. 
This screening argument is reasonable because in the rock mass around the repository the 
temperature may be relatively high compared to ambient host-rock temperature even at 
10,000 years. For example, in part (a) of Figure 3-2 of this technical basis document, it is shown 
that after 10,000 years the temperature is 45 to 50C, which is significantly higher than the 
initial temperature of around 25C. This higher temperature increases the instability of the 
colloids due to Brownian deposition, as discussed in Waste Form and In-Drift Colloids- 
Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003a). 
An issue regarding this FEP discussed at the NRC/DOE Technical Exchange and Management 
Meeting on Evolution of the Near-Field Environment held January 9 through 12, 2001, in 
Pleasanton, California (Reamer 2001) was the consideration of colloid entrainment due to 
boiling. As discussed herein, the effect of boiling on entrainment of colloids would be transient 
restricted to the first 200 years following waste emplacement and extending a few meters away 
from the drift. The issue raised at the aforementioned meeting was the possibility of 
boiling-induced detachment of colloids that are already attached to immobile rock. A revision of 
September 2003 A-5 No. 8: Colloids
this FEP to include this process is being addressed. As discussed below, the exclusion of colloid 
entrainment has no significant impact on total repository system performance. 
Boiling is the result of increased temperature and increased temperature contributes to the 
instability of colloid suspensions due to Brownian deposition. Also, boiling results in 
evaporation and this tends to increase the ionic strengths of colloid suspensions leading to 
instability as discussed in Waste Form and In-Drift Colloids-Associated Radionuclide 
Concentrations: Abstraction and Summary (BSC 2003a). In addition, the boiling front is a 
transient phenomenon, which is expected to happen only for a few hundred years over a scale of 
meters (BSC 2003b) (see Figures A-2 and A-3). Colloids suspended due to the boiling process, 
if there are any, will settle out after the physical condition becomes quiescent. Therefore, it is 
concluded that colloid entrainment as a result of boiling is not a plausible effect under expected 
repository conditions. Actually, boiling will be a contributor to the instability and settling of 
colloids out of suspension. Therefore, excluding colloid entrainment due to boiling from 
consideration in TSPA is a conservative assumption. 
In response to GEN 1.01 (Comment 35), the diffusion coefficient for dissolved species has been 
determined specifically for crushed tuff invert materials as a function of volumetric moisture 
content (m3 water/m3 bulk rock), using electric conductivity measurements (BSC 2003d). 
Figure A-4 presents the uncertainty in the statistical fit for the diffusion coefficient. 
Source: BSC 2003b. 
Figure A-2. 
No. 8: Colloids 
Thermal-Hydrologic Simulation (Tptpmn Model): Time Profiles of Modeled Temperatures 
and Liquid Saturations in Fractures and Matrix at Three Drift-Wall Locations and in the 
Waste Package 
September 2003 A-6 
Revision 2
Source: BSC 2003b. 
Figure A-3. Thermal-Hydrologic Simulation (Tptpmn Model): Contour Plot of Modeled Temperatures 
(C) and Liquid Saturations in Fractures at 600 Years (near Maximum DryoutMean 
Infiltration) 
No. 8: Colloids 
Revision 2 
September 2003 A-7
Revision 2 
Source: BSC 2003d, Figure 7. 
NOTE: The dashed lines correspond to two standard deviations above and below the statistical fit to the data. D is 
 . 
. 
Figure A-4. Uncertainty in the Statistical Fit for the Diffusion Coefficient 
The colloidal diffusion coefficient can be estimated from the following Stokes-Einstein 
relationship (BSC 2003d): 
ion 
ion Dcoll = D r
rcoll 
A.5 REFERENCES 
the diffusion coefficient of the crushed invert materials, D0 is the diffusion coefficient of the fully saturated 
crushed invert materials, and  is the percentage of saturation. 
(Eq. A-1) 
September 2003 No. 8: Colloids 
. .. 
. 
where Dcoll is the diffusion coefficient for a colloidal particle of radius rcoll and Dion is the 
diffusion coefficient of an ion of radius rion. Given a typical ion radius and colloidal particle 
radius of 0.1 and 1 nm, respectively, the diffusion coefficient of a colloidal particle is generally 
100 times smaller than that of a dissolved ion. 
BSC (Bechtel SAIC Company) 2003a. Waste Form and In-Drift Colloids-Associated 
Radionuclide Concentrations: Abstraction and Summary. MDL-EBS-PA-000004 REV 00. 
Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030626.0006. 
BSC 2003b. Drift-Scale Coupled Processes (DST and THC Seepage) Models. MDL-NBS-HS- 
000001 REV 02. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030804.0004. 
BSC 2003c. Features, Events and Processes in UZ Flow and Transport. ANL-NBS-MD- 
000001 REV 02B. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0198. 
A-8
Revision 2 
BSC 2003d. EBS Radionuclide Transport Abstraction. ANL-WIS-PA-000001 REV 01F. 
Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0199. 
CRWMS M&O (Civilian Radioactive Waste Management System Management and Operating 
Contractor) 2000a. Particle Tracking Model and Abstraction of Transport Processes. 
ANL-NBS-HS-000026 REV 00. Las Vegas, Nevada: CRWMS M&O. 
ACC: MOL.20000502.0237. 
CRWMS M&O 2000b. UZ Colloid Transport Model. ANL-NBS-HS-000028 REV 00. 
Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20000822.0005. 
Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy 
Technical Exchange and Management Meeting on Evolution of the Near-Field Environment 
(January 912, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), 
January 26, 2001, with enclosure. ACC: MOL.20010810.0033. 
Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and 
Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. 
Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. 
September 2003 A-9 No. 8: Colloids
INTENTIONALLY LEFT BLANK 
A-10 No. 8: Colloids 
Revision 2 
September 2003
SENSITIVITY ANALYSIS OF COLLOID TRANSPORT PARAMETERS 
(RESPONSE TO ENFE 4.06 AIN-1 AND GEN 1.01 (COMMENTS 35 AND 37)) 
No. 8: Colloids 
APPENDIX B 
Revision 2 
September 2003
Revision 2 
Note Regarding the Status of Supporting Technical Information 
This document was prepared using the most current information available at the time of its development. This 
Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were 
prepared using preliminary or draft information reflect the status of the Yucca Mountain Projects scientific 
and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model 
Reports (AMRs) and other draft references whose contents may change with time. Information that evolves 
through subsequent revisions of the AMRs and other references will be reflected in the License Application 
(LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not 
routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to 
reflect changes in the supporting references prior to submittal of the LA. 
September 2003 No. 8: Colloids
Revision 2 
APPENDIX B 
SENSITIVITY ANALYSIS OF COLLOID TRANSPORT PARAMETERS 
(RESPONSE TO ENFE 4.06 AIN-1 AND GEN 1.01 (COMMENTS 35 AND 37) 
This appendix provides a response for additional information needed (AIN) request for Key 
Technical Issue (KTI) agreements Evolution of the Near-Field Environment (ENFE) 4.06 and 
general agreement (GEN) (1.01), comments 35 and 37. These KTI agreements relate to 
providing more information regarding colloid transport parameters and their bounding values. 
B.1 KEY TECHNICAL ISSUE AGREEMENTS 
B.1.1 ENFE 4.06 AIN-1 and GEN 1.01 (Comments 35 and 37) 
Agreement ENFE 4.06 was reached during the U.S. Nuclear Regulatory Commission 
(NRC)/U.S. Department of Energy (DOE) Technical Exchange and Management Meeting on 
Evolution of the Near-Field Environment held January 9 through 12, 2001, in Pleasanton, 
California. ENFE KTI subissues 1, 2, 3, and 4 were discussed at that meeting (Reamer 2001). 
In evaluating the response to this agreement, NRC reviewed Total System Performance 
Assessment for the Site Recommendation (CRWMS M&O 2000), which contained sensitivity 
analyses of the effects of colloid parameters on repository performance. NRC staff noted that the 
analyses in the total system performance assessment (TSPA) for the site recommendation did not 
include values for parameters outside the adopted uncertainty range and, therefore, provided 
insufficient information regarding uncertainties in colloidal behavior (Schlueter 2002). They 
commented that Future sensitivity studies should address the substantial uncertainties regarding 
colloidal behavior during waste corrosion and radionuclide release. NRC requested specific 
additional information needed to complete KTI agreement ENFE 4.06, and added that 
information provided by DOE in response to ENFE Agreement 4.03 would also be used in the 
evaluation of ENFE 4.06. 
During the NRC-DOE technical exchange and management meeting on thermal operating 
temperatures, held September 18 through 19, 2001, the NRC provided additional comments that 
are related to ENFE 4.06 (Reamer and Gil 2001). These comments (GEN 1.01, comments 35 
and 37) related to colloidal transport parameters for the engineered barrier system and thermal 
chemical effects on transport in the unsaturated zone. DOE provided initial responses to these 
comments (Reamer and Gil 2001). 
The wording of these agreements and of DOEs initial response to the general agreement 
comments is as follows: 
ENFE 4.06 
Provide documentation to demonstrate suitability of the bounding values used for 
colloid transport through the perturbed near-field environment. For example, 
consider sensitivity analyses to investigate the effects of varying colloid sorption 
parameters (Kc) on repository performance. The DOE will evaluate the suitability 
of the colloid transport model under perturbed conditions as discussed in 
September 2003 B-1 No. 8: Colloids
Revision 2 
agreement #3 for this subissue. As part of this work, the DOE will consider 
sensitivity analyses to investigate the effects of varying colloid sorption 
parameters (Kc) on repository performance. The DOE will also provide the 
TSPA-SR (TDR-WIS-PA-000001) Rev 00 ICN 01 in January 2001. The 
TSPA-SR includes sensitivity studies in the form of barrier degradation and 
parameter sensitivity analyses that investigate the effect of sorption and colloid 
parameters on repository performance. 
ENFE 4.06-AIN 
Provide additional sensitivity analyses of colloid release and transport parameters 
as discussed in the agreement. Related issues are covered under TSPAI 
Agreements 3.17, 3.30, and 3.42. 
GEN 1.01 (Comment 35) 
The SSPA recommends new values for EBS colloid transport parameters. If these 
are adopted by TSPA in the future, the technical basis for the new distributions 
will require close scrutiny. Relevant KTI agreements are RT 3.07, ENFE 4.03, 
ENFE 4.04, and ENFE 4.06. 
DOE Initial Response to GEN 1.10 (Comment 35) 
The new values for EBS colloidal transport parameters were designed to evaluate 
unquantified uncertainty for the SSPA. DOE understands that prior to any 
potential LA, a stronger technical basis must be provided for EBS colloidal 
transport parameter values carried forward to the base case analysis. 
GEN 1.01 (Comment 37) 
The discussion of THC effects on UZ transport does not address chemical effects 
of the repository. This concern is related to KTI agreements ENFE 4.03 and 
ENFE 4.06, and TSPAI FEPs item J-8. 
DOE Initial Response to GEN 1.01 (Comment 37) 
DOE acknowledges this comment and notes that some limited studies were 
documented in Section 11.3.5.4.2 of SSPA Volume 1. Work is underway, 
consistent with the cited agreements, to study the effects of alkaline plumes 
generated by the cement-seepage interactions on rock properties (such as porosity 
and permeability) and thereby effects on radionuclide transport from the waste 
placement drifts, with preliminary results expected in FY03. 
September 2003 B-2 No. 8: Colloids
Revision 2 
B.1.2 Related Key Technical Issues 
ENFE 1.06 
Provide the technical basis for excluding entrained colloids in the analysis of 
FEP 2.2.10.06.00 (Thermo-Chemical Alteration) or an alternative FEP. The DOE 
will provide the technical basis for screening entrained colloids in the analysis of 
FEP 2.2.10.06.00 in a future revision of the Features, Events, and Processes in UZ 
Flow and Transport AMR (ANL-NBS-MD-000001), expected to be available in 
FY 02. 
GEN 1.01 
For NRC comments 3, 5, 8, 9, 10, 12, 13, 15, 16, 18, 21, 24, 27, 36, 37, 41, 42, 
45, 46, 50, 56, 64, 69, 75, 78, 81, 82, 83, 93, 95, 96, 97, 98, 102, 103, 104, 106, 
109, 110, 111, 113, 116, 118, 119, 120, 122, 123, 124, and 126, DOE will address 
the concern in the documentation for the specific KTI agreement identified in the 
DOE response (Attachment 2). The schedule and document source will be the 
same as the specific KTI agreement. 
NRC Comment 37 
The discussion of THC effects on UZ transport does not address chemical effects 
of the repository. This concern is related to KTI agreements ENFE.4.03 and 
ENFE.4.06, and TSPAI FEPs item J-8. 
DOE response to Comment 37 
DOE acknowledges this comment and notes that some limited studies were 
documented in Section 11.3.5.4.2 of SSPA Volume 1. Work is underway, 
consistent with the cited agreements, to study the effects of alkaline plumes 
generated by the cement-seepage interactions on rock properties (such as porosity 
and permeability) and thereby effects on radionuclide transport from the waste 
placement drifts, with preliminary results expected in FY03.1, 2 
TSPAI 2.02 
Provide the technical basis for the screening argument, as summarized in 
Attachment 2, for the highlighted FEPs. The technical basis will be provided in 
the referenced FEPs AMR and will be provided to the NRC in FY2003. 
2
1 The information presented in this document does not, at this time, represent a commitment to perform additional 
work. DOE is currently considering and scoping the appropriateness of a lower-temperature operating mode for 
potential LA, should the site be approved. 
Should the site be approved, DOE will, as appropriate, re-evaluate the impact of a lower-temperature operating 
mode upon existing KTIs, which were established on the basis of the higher-temperature operating mode. 
September 2003 B-3 No. 8: Colloids
Revision 2 
B.2 RELEVANCE TO REPOSITORY PERFORMANCE 
Colloid concentrations released from the engineered barrier system will travel through the nearfield 
region of the unsaturated zone. The perturbed thermal and chemical conditions in the nearfield 
region could affect the stability of the colloid suspension and therefore the concentration of 
colloids available to carry radionuclides through the unperturbed region of the unsaturated zone. 
This KTI agreement is related to the near-field environment as shown in Figure B-1. The 
technical basis for the response to this KTI agreement appears in Section 5 of this technical basis 
document. 
B.3 RESPONSE 
Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and 
Summary (BSC 2003a) presented and summarized the results of experiments related to colloid 
formation during waste corrosion and release of radioactive colloids from the waste package. 
Values of colloid parameters, including the sorption coefficient, are provided in Section B.4. 
Uncertainty in colloid distribution coefficient, Kc, has been captured by sampling both colloid 
concentration and the sorption coefficient, Kd, and multiplying the sampled values. 
As discussed in Sections 3.1.1, 3.2.5, 4.1.3, and Appendix C of this technical basis document, the 
thermal and chemical perturbations in the near-field during much of the 10,000-year regulatory 
period are likely to destabilize colloid suspensions by increasing Brownian motion and particle 
collision caused by higher temperature and increasing ionic strength of solution. Therefore, 
parameter ranges provided in Waste Form and In-Drift Colloids-Associated Radionuclide 
Concentrations: Abstraction and Summary (BSC 2003a) are still suitable for the perturbed 
near-field environment. In addition, any perturbed colloid concentration inside the drift will 
ultimately attenuate to the background concentration in the Yucca Mountain groundwater as the 
colloidal solution percolates through the invert and the unsaturated zone into the saturated zone, 
and the water chemistry converges to the ambient groundwater. 
For small Kd values, the colloid concentrations in the Yucca Mountain environment are not high 
enough to compete with dissolved species. The distribution coefficient (Kc), a product of the 
sorption coefficient Kd and colloid concentration, is calculated based on the distributions of both 
Kd values and colloid concentrations. The colloid concentration used in the TSPA for the license 
application (LA) is sampled over the range of 0.001 to 200 ppm, as compared to the single mean 
value of 3  10-2 ppm used in the TSPA for the site recommendation (BSC 2001, p. 60). The 
wide ranges of both colloid concentrations and Kd values constructed for TSPA-LA are expected 
to capture all potential uncertainties associated with Kc. 
New values for the diffusivity of dissolved species through the invert documented in 
Section 6.3.4.1 of EBS Radionuclide Transport Abstraction (BSC 2003b) and the related 
sensitivity analysis are summarized in Section 4 of this technical basis document. From the 
dissolved species diffusivity values, diffusivities for colloids are estimated. 
The information in this report is responsive to agreements ENFE 4.06 AIN-1 and GEN 1.01 
(comments 35 and 37) made between the DOE and NRC. The report contains the information 
that DOE considers necessary for the NRC to review for closure of these agreements. 
September 2003 B-4 No. 8: Colloids
Revision 2 
Components 
Figure B-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System 
September 2003 B-5 No. 8: Colloids
Revision 2 
B.4 BASIS FOR THE RESPONSE 
Estimation of typical colloid concentrations in the engineered barrier system and unsaturated 
zone are discussed in Waste Form and In-Drift Colloids-Associated Radionuclide 
Concentrations: Abstraction and Summary (BSC 2003a) and summarized below: 
 The static-saturated tests at Argonne National Laboratory indicated that colloids 
developed and increased in concentration with time, up to the point where the colloid 
concentration reached a maximum value and then the colloidal suspension became less 
stable. This is due to colloid agglomeration and settling because of high ionic strengths 
(BSC 2003a). The maximum colloid concentration from experimental degradation of 
the high-level radioactive waste glass is estimated to be 5 ppm (BSC 2003a, Figure 3). 
 The concentration range of iron oxyhydroxide colloids generated from the defense 
high-level radioactive waste glass values are estimated based on experiments performed 
at the University of Nevada, Las Vegas. In these experiments, scaled-down miniature 
waste packages were exposed to J-13 groundwater in either a bath-tub mode or a 
flow-through mode. The cumulative results have yielded average concentrations of 
colloidal size materials in the range of 20 mg/L within the initial four weeks of the 
experiments (BSC 2003a, p. 56). 
 The range of colloid concentration in seepage/groundwater was derived based on 
literature data and groundwater sampling from the Yucca Mountain area and Idaho 
National Engineering and Environmental Laboratory (BSC 2003a, Figures 11 and 12). 
Practically no colloids were detected for ionic strengths above 0.05 M. The upper limit 
of colloid concentration is about 200 ppm, with average values of approximately 
0.1 ppm. 
Features, Events and Processes in UZ Flow and Transport (BSC 2003c, Section 6.8.13) presents 
the screening argument for excluding coupled thermal-hydrologic-chemical effects on 
radionuclide transport properties and colloids in the unsaturated zone (solubility, speciation, 
phase changes, and precipitation/dissolution). This FEP is conservatively ignored with respect to 
solubility reduction in the far field, and the effects of radionuclide precipitation are ignored in the 
unsaturated zone transport for TSPA. If solubility limits increase in the geosphere compared 
with the waste emplacement drift, there is no effect on transport because all available 
radionuclides from the source at the waste emplacement drift are already aqueous species. The 
thermal-chemical effects on colloid formation have already been accounted for in the colloid 
source term (BSC 2003a). Therefore, any impact from thermal-chemical alteration on colloid 
entrainment will be insignificant with respect to total repository system performance. 
This screening argument is reasonable because in the rock mass around the repository the 
temperature may be relatively high even at 10,000 years. For example, in Drift-Scale Coupled 
Processes (DST and THC Seepage) Models (BSC 2003d, Section 6.5.2) it is shown that after 
10,000 years, the temperature is 45 to 50C, which is significantly higher than the initial 
temperature of around 25C. This higher temperature tends to decrease the stability of a 
colloidal suspension, as discussed in Waste Form and In-Drift Colloids-Associated Radionuclide 
Concentrations: Abstraction and Summary (BSC 2003a). 
September 2003 B-6 No. 8: Colloids
Revision 2 
The thermal-hydrologic-chemical model simulations have been performed for both the base case 
and the extended case and for both Tptpmn and Tptpll geologic formations (BSC 2003d). The 
extended case includes the major solid phases (minerals and glass) encountered in geologic units 
at Yucca Mountain, together with a range of possible reaction product minerals, CO2 gas, and the 
aqueous species necessary to include these solid phases and the pore-water composition within 
the thermal-hydrologic-chemical model. The base case is a subset of the extended case 
excluding aluminum silicate minerals, which form or dissolve much less easily than minerals 
such as calcite or gypsum. The results for the Tptpmn rock unit are shown in Figures B-2 and 
B-3. The pH in fracture waters varies within 7.4 to 8.6 but still remains alkaline over the 10,000- 
year regulatory time period (Figure B-2), and thus its effect on colloid stability is expected to be 
negligible. The concentrations of soluble components (e.g., Na and Cl) and therefore the ionic 
strength in the waters are predicted to increase over the time period of thermal event 
(Figure B-4), which will reduce colloid stability. The thermal-hydrologic-chemical effect may 
also change the transport properties of rock. However, Figure B-3 shows that this change is small 
and can be negligible for colloid transport. 
In addition to colloid concentration, which is determined by colloid stability and filtration, the 
partitioning coefficient of radionuclides between colloids and solution (denoted as Kc) is another 
factor controlling colloid-facilitated transport. Kc is the product of Kd and colloid concentration. 
The updated Kd values are tabulated in Table B-1. These Kd values are used to calculate Kc 
together with the sampled colloid concentrations (Figure B-5). Note that in the TSPA-LA the 
same concentration distribution of groundwater (i.e., smectite) colloids is used for the engineered 
barrier system, the unsaturated zone, and the saturated zone, except that in the engineered barrier 
system the concentration changes due to a potential shift in colloid stability is explicitly taken 
into account. The colloid concentration used in the TSPA-LA is sampled over 0.001 to 200 ppm, 
as compared to the single mean value of 3  10-2 ppm used in the TSPA-SR (BSC 2001, p. 60). 
These wide ranges of both colloid concentrations and Kd values constructed for TSPA-LA are 
expected to capture all potential uncertainties associated with Kc 
In response to the GEN 1.01 (comment 35) agreement, data on diffusion coefficients have been 
provided (BSC 2003b). The remainder of this section provides the technical basis in response to 
this agreement. 
The diffusion coefficient for dissolved species has been determined specifically for crushed tuff 
invert materials as a function of volumetric moisture content (m3 water/m3 bulk rock), using 
electric conductivity measurements (BSC 2003b). 
Figure B-6 presents the uncertainty in the statistical fit for the diffusion coefficient. 
September 2003 B-7 No. 8: Colloids
Source: BSC 2003d. 
Figure B-2. Thermal-Hydrologic-Chemical Simulation (Tptpmn Model): Time Profiles of the Modeled 
pH of Fracture Water at Three Drift-Wall Locations under Heating (Heat) and Nonheating 
(Ambient) Conditions for the Extended (E) and Base-Case (B) Geochemical Systems 
No. 8: Colloids 
Revision 2 
September 2003 B-8
Source: BSC 2003d. 
NOTE: Decrease in porosity is primarily due to the precipitation of calcite and amorphous silica. 
Figure B-3. 
No. 8: Colloids 
Thermal-Hydrological-Chemical Model Simulation (Tptpmn Model): Contour Plot of 
Modeled Fracture Porosity Change at 10,000 Years for (a) Base-Case and (b) Extended 
Geochemical Systems 
B-9 
Revision 2 
September 2003
Source: BSC 2003d. 
Figure B-4. Thermal-Hydrologic-Chemical Simulation (Tptpmn Model): Time Profiles of Modeled 
Total Aqueous Sodium Concentrations in Fracture Water at Three Drift-Wall Locations 
under Heating (Heat) and Nonheating (Ambient) Conditions for Extended and Base-Case 
Geochemical Systems 
No. 8: Colloids 
Revision 2 
September 2003 B-10
Table B-1. Kd Values (mL/g) Used for Reversible Radionuclide Sorption on Colloids in Total System 
Performance Assessment-License Application Calculations 
Radionuclide 
Pu 
Am, Th, Pa 
Cs 
Source: BSC 2003a, Table 10. 
NOTE: The Kd values for Tc and I are very low and not listed here. 
No. 8: Colloids 
Colloid 
Iron Oxyhydroxide 
Smectite 
Iron Oxyhydroxide 
Smectite 
Iron Oxyhydroxide 
Smectite 
Kd Value Range 
(mL/g) 
104 to 106 
103 to 106 
105 to 107 
104 to 107 
101 to 103 
102 to 104 
B-11 
Kd Value Intervals 
(mL/g) 
<1 x 104 
1 x 104 to 5 x 104 
5 x 104 to 1 x 105 
1 x 105 to 5 x 105 
5 x 105 to 1 x 106 
> 1 x 106 
< 1 x 103 
1 x 103 to 5 x 103 
5 x 103 to 1 x 104 
1 x 104 to 5 x 104 
5 x 104 to 1 x 105 
1 x 105 to 5 x 105 
5 x 105 to 1 x 106 
> 1 x 106 
< 1 x 105 
1 x 105 to 5 x 105 
5 x 105 to 1 x 106 
1 x 106 to 5 x 106 
5 x 106 to 1 x 107 
> 1 x 107 
< 1 x 104 
1 x 104 to 5 x 104 
5 x 104 to 1 x 105 
1 x 105 to 5 x 105 
5 x 105 to 1 x 106 
1 x 106 to 5 x 106 
5 x 106 to 1 x 107 
> 1 x 107 
< 1 x 101 
1 x 101 to 5 x 101 
5 x 101 to 1 x 102 
1 x 102 to 5 x 102 
5 x 102 to 1 x 103 
> 1 x 103 
< 1 x 102 
1 x 102 to 5 x 102 
5 x 102 to 1 x 103 
1 x 103 to 5 x 103 
5 x 103 to 1 x 104 
> 1 x 104 
Revision 2 
Kd Value Interval 
Probabilities 
0
0.15 
0.2 
0.5 
0.15 
0
0
0.04 
0.08 
0.25 
0.2 
0.35 
0.08 
0
0
0.15 
0.2 
0.55 
0.1 
0
0
0.07 
0.1 
0.23 
0.2 
0.32 
0.08 
0
0
0.13 
0.22 
0.55 
0.1 
0
0
0.2 
0.25 
0.5 
0.05 
0 
September 2003
Revision 2 
Source: BSC 2003e. 
Figure B-5. Cumulative Distribution Function of Uncertainty in Groundwater Colloid Concentrations 
Source: BSC 2003b, Figure 7. 
NOTE: The dashed lines correspond to two standard deviations above and below the statistical fit to the data. 
Figure B-6. Uncertainty in the Statistical Fit for the Diffusion Coefficient, D, of Solute as a Function of 
Log Percent Moisture, , where D0 is the Solute Diffusion Coefficient in Bulk Water 
September 2003 B-12 No. 8: Colloids
Revision 2 
The colloidal diffusion coefficient can be estimated from the following Stokes-Einstein 
relationship (BSC 2003b): 
ion (Eq. B-1) D D ion coll 
r
rcoll 
B.5 REFERENCES 
MOL.20020506.0917. 
= 
Company. ACC: DOC.20030626.0006. 
. .. 
. 
where Dcoll is the diffusion coefficient for a colloidal particle of radius rcoll and Dion is the 
diffusion coefficient of a ion of radius rion. Given a typical ion radius and colloidal particle 
radius of 0.1 and 1 nm, respectively, the diffusion coefficient of a colloidal particle is generally 
100 times smaller than that of a dissolved ion. 
BSC (Bechtel SAIC Company) 2001. Uncertainty Distribution for Stochastic Parameters. ANLNBS-
MD-000011 REV 00 ICN 01. Las Vegas, Nevada: Bechtel SAIC Company. ACC: 
BSC 2003a. Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: 
Abstraction and Summary. MDL-EBS-PA-000004 REV 00. Las Vegas, Nevada: Bechtel SAIC 
BSC 2003b. EBS Radionuclide Transport Abstraction. ANL-WIS-PA-000001 REV 01F. 
Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0199. 
BSC 2003c. Features, Events and Processes in UZ Flow and Transport. ANL-NBS-MD- 
000001 REV 02B. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0198. 
BSC 2003d. Drift-Scale Coupled Processes (DST and THC Seepage) Models. MDL-NBS-HS- 
000001 REV 02. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030804.0004. 
BSC 2003e. SZ Flow and Transport Model Abstraction. MDL-NBS-HS-000021 REV 00A. 
Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030612.0138. 
CRWMS M&O (Civilian Radioactive Waste Management System Management and Operating 
Contractor) 2000. Total System Performance Assessment for the Site Recommendation. 
TDR-WIS-PA-000001 REV 00 ICN 01. Las Vegas, Nevada: CRWMS M&O. 
ACC: MOL.20001220.0045. 
Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy 
Technical Exchange and Management Meeting on Evolution of the Near-Field Environment 
(January 9.12, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), 
January 26, 2001, with enclosure. ACC: MOL.20010810.0033. 
Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and 
Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. 
Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. 
 . 
. 
B-13 September 2003 No. 8: Colloids
Schlueter, J. 2002. Evolution of the Near-Field Environment Key Technical Issue Agreements 
Letter from J. Schlueter (NRC) to S. Brocoum (DOE/YMSCO), February 14, 2002, 0225021620, 
with enclosure. ACC: MOL.20020607.0086. 
No. 8: Colloids 
Revision 2 
September 2003 B-14
SCREENING OUT COUPLED THERMAL-HYDROLOGIC-CHEMICAL EFFECTS 
(RESPONSE TO ENFE 4.03 AND GEN 1.01 (COMMENTS 35 AND 37)) 
No. 8: Colloids 
APPENDIX C 
Revision 2 
September 2003
Revision 2 
Note Regarding the Status of Supporting Technical Information 
This document was prepared using the most current information available at the time of its development. This 
Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were 
prepared using preliminary or draft information reflect the status of the Yucca Mountain Projects scientific 
and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model 
Reports (AMRs) and other draft references whose contents may change with time. Information that evolves 
through subsequent revisions of the AMRs and other references will be reflected in the License Application 
(LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not 
routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to 
reflect changes in the supporting references prior to submittal of the LA. 
September 2003 No. 8: Colloids
Revision 2 
APPENDIX C 
SCREENING OUT COUPLED THERMAL-HYDROLOGIC-CHEMICAL EFFECTS 
(RESPONSE TO ENFE 4.03 AND GEN 1.01 (COMMENTS 35 AND 37)) 
This appendix provides a response for Key Technical Issue (KTI) agreement Evolution of the 
Near-Field Environment (ENFE) 4.03 and general agreement (GEN) 1.01, comments 35 and 37. 
These KTI agreements relate to providing the technical basis for the screening of 
thermal-hydrologic-chemical effects on colloids and colloid transport. 
C.1 KEY TECHNICAL ISSUE AGREEMENTS 
C.1.1 ENFE 4.03 and GEN 1.01 (Comment 35 and 37) 
Agreement ENFE 4.03 was reached during the U.S. Nuclear Regulatory Commission 
(NRC)/U.S. Department of Energy (DOE) Technical Exchange and Management Meeting on 
Evolution of the Near-Field Environment held January 9 through 12, 2001, in Pleasanton, 
California. ENFE KTI subissues 1, 2, 3, and 4 were discussed at that meeting (Reamer 2001). 
Agreement GEN 1.01 was reached during the NRC/DOE Technical Exchange and Management 
Meeting on Range of Thermal Operating Temperatures held September 18 through 19, 2001. At 
that meeting, NRC provided additional comments relating to ENFE 4.03 (GEN 1.01, 
comments 35 and 37), and DOE provided an initial response (Reamer and Gil 2001). 
The wording of these agreements and of DOEs initial response to the general agreement 
comments is as follows: 
ENFE 4.03 
Provide the technical basis for screening out coupled THC effects on radionuclide 
transport properties and colloids. The DOE will provide the technical basis for 
screening out coupled THC effects on radionuclide transport properties and 
colloids in a new AMR or in a revision to an existing AMR, expected to be 
available in FY 02. 
GEN 1.01 (Comment 35) 
The SSPA recommends new values for EBS colloid transport parameters. If these 
are adopted by TSPA in the future, the technical basis for the new distributions 
will require close scrutiny. Relevant KTI agreements are RT 3.07, ENFE 4.03, 
ENFE 4.04, and ENFE 4.06. 
DOE Initial Response to GEN 1.01 (Comment 35) 
The new values for EBS colloidal transport parameters were designed to evaluate 
unquantified uncertainty for the SSPA. DOE understands that prior to any 
potential LA, a stronger technical basis must be provided for EBS colloidal 
transport parameter values carried forward to the base case analysis. 
September 2003 C-1 No. 8: Colloids
Revision 2 
GEN 1.01 (Comment 37) 
The discussion of THC effects on UZ transport does not address chemical effects 
of the repository. This concern is related to KTI agreements ENFE 4.03 and 
ENFE 4.06, and TSPAI FEPs item J-8. 
DOE Initial Response to GEN 1.01 (Comment 37) 
DOE acknowledges this comment and notes that some limited studies were 
documented in Section 11.3.5.4.2 of SSPA Volume 1. Work is underway, 
consistent with the cited agreements, to study the effects of alkaline plumes 
generated by the cement-seepage interactions on rock properties (such as porosity 
and permeability) and thereby effects on radionuclide transport from the waste 
placement drifts, with preliminary results expected in FY03. 
C.1.2 Related Key Technical Issues 
GENERAL 1.01: For NRC comments 3, 5, 8, 9, 10, 12, 13, 15, 16, 18, 21, 24, 27, 36, 37, 41, 
42, 45, 46, 50, 56, 64, 69, 75, 78, 81, 82, 83, 93, 95, 96, 97, 98, 102, 103, 104, 106, 109, 110, 
111, 113, 116, 118, 119, 120, 122, 123, 124, and 126, DOE will address the concern in the 
documentation for the specific KTI agreement identified in the DOE response (Attachment 2). 
The schedule and document source will be the same as the specific KTI agreement. 
TSPAI 2.02: Provide the technical basis for the screening argument, as summarized in 
Attachment 2. See Comment #3, 4, 11, 12, 19 (Parts 1, 2, and 6), 25, 26, 29, 34, 35, 36, 37, 38, 
39, 42, 43, 44, 48, 49, 51, 54, 55, 56, 57, 59, 60, 61, 62, 63, 64, 65, 66, 68, 69, 70, 78, 79, J-1, 
J-2, J-3, J-4, J-7, J-8, J-9, J-10, J-11, J-12, J-13, J-14, J-15, J-17, J-20, J-21, J-22, J-23, J-24, J-25, 
J-26, and J-27. 
NRC Comment J-7 on FEP #2.2.08.01.00 (Groundwater chemistry/composition in UZ and SZ). 
NRC/DOE Agreed Path Forward: This issue is addressed by existing agreements between DOE 
and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, RT Subissue 1 
Agreement 5, and RT subissue 2 Agreement 10). Features, Events, and Processes in UZ Flow 
and Transport, ANL-NBS-MD-000001 will be revised upon completion of this work. 
NRC Comment J-8 on FEP #2.2.08.02.00 (Radionuclide transport occurs in a carrier plume in 
geosphere). NRC/DOE Agreed Path Forward: This issue is addressed by existing agreements 
between DOE and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, 
and RT Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, 
ANL-NBS-MD-000001 will be revised upon completion of this work. 
NRC Comment J-10 on FEP #2.2.08.06.00 (Complexation in geosphere). NRC/DOE Agreed 
Path Forward: This issue is addressed by existing agreements between DOE and NRC (ENFE 
Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT Subissue 1 
Agreement 5). Features, Events, and Processes in UZ Flow and Transport, ANL-NBS-MD- 
000001 will be revised upon completion of this work. 
September 2003 C-2 No. 8: Colloids
Revision 2 
NRC Comment J-11 on FEP #2.2.08.07.00 (Radionuclide solubility limits in the geosphere). 
NRC/DOE Agreed Path Forward: This issue is addressed by existing agreements between DOE 
and NRC (ENFE Subissue 4 Agreement 3). Features, Events, and Processes in UZ Flow and 
Transport, ANL-NBS-MD-000001 will be revised upon completion of this work. 
NRC Comment J-12 on FEP #2.2.10.01.00 (Repository-induced thermal effects in geosphere). 
NRC/DOE Agreed Path Forward: This issue is addressed by existing agreements between DOE 
and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT 
Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, 
ANL-NBS-MD-000001 will be revised upon completion of this work. 
NRC Comment J-14 on FEP #2.2.10.07.00 (Thermo-chemical alteration of the Calico Hills unit). 
NRC/DOE Agreed Path Forward: This issue is addressed by existing agreements between DOE 
and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT 
Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, 
ANL-NBS-MD-000001 will be revised upon completion of this work. DOE also stated that 
alteration of vitric rock has not been addressed and will need to be included in the overall 
thermal-hydrological-chemical analyses. 
NRC Comment J-15 on FEP #2.2.10.09.00 (Thermal-chemical alteration of the Topopah basal 
vitrophyre). NRC/DOE Agreed Path Forward: This issue is addressed by existing agreements 
between DOE and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, 
and RT Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, 
ANL-NBS-MD-000001 will be revised upon completion of this work. 
NRC Comment J-21 on FEP #2.2.11.02.00 (Gas Pressure Effects). NRC/DOE Agreed Path 
Forward: This issue is addressed by existing agreements between DOE and NRC (ENFE 
Subissue 1 Agreements 5 and 7, and ENFE Subissue 4 Agreement 3). Features, Events, and 
Processes in UZ Flow and Transport, ANL-NBS-MD-000001 will be revised upon completion of 
this work. 
NRC Comment J-22 on FEP #1.2.04.02.00 (Igneous activity causes changes to rock properties). 
NRC/DOE Agreed Path Forward: This issue is addressed by existing agreements between DOE 
and NRC (ENFE Subissue 1 Agreement 4, ENFE Subissue 4 Agreements 3 and 4, and RT 
Subissue 1 Agreement 5). Features, Events, and Processes in UZ Flow and Transport, 
ANL-NBS-MD-000001 will be revised upon completion of this work. 
C.2 RELEVANCE TO REPOSITORY PERFORMANCE 
Radionuclide transport as either dissolved species or as colloids represents the means for the 
release of radionuclides from the repository system to the biosphere. The time and amount of 
radionuclide release to the biosphere will depend on the transport properties of the dissolved 
radionuclides and of radionuclide-laden colloids. The transport properties may be dependent on 
coupled thermal-hydrologic-chemical effects. 
The technical basis for the response for this KTI is presented in Sections 3, 4, and 5 of this 
technical basis document. This KTI is related to the engineered and near-field environment as 
shown in Figure C-1. 
September 2003 C-3 No. 8: Colloids
Revision 2 
Figure C-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System 
Components 
September 2003 C-4 No. 8: Colloids
Revision 2 
C.3 RESPONSE 
Thermal effects may affect radionuclide transport directly by causing changes in radionuclide 
speciation and solubility in the unsaturated zone or indirectly by causing changes in the host rock 
mineralogy that affect the flow path. Relevant processes include volume effects associated with 
silica phase changes, precipitation and dissolution of fracture-filling minerals (including silica 
and calcite), and alteration of zeolites and other minerals to clays. The effects of colloid 
formation are accounted for in the colloid source term. Colloids are expected to be formed from 
the degradation of the high-level radioactive waste and spent nuclear fuel waste forms, 
engineered barrier system materials, and rock. Radionuclides associated with colloids are 
assumed to be either irreversibly or reversibly attached to colloids (CRWMS M&O 2000a, 
Section 6; CRWMS M&O 2000b, Section 6). The near-field thermal-chemical analysis indicates 
only small changes in hydrologic properties and mineralogy as a result of these coupled 
processes (BSC 2003a, Section 6). Therefore, far-field changes are likewise expected to be 
small, including mineral precipitation/dissolution and alteration of minerals such as zeolites and 
clays. Therefore, coupled thermal-hydrologic-chemical effects on radionuclide transport 
properties and colloids are excluded from TSPA on the basis of low consequence. 
Cementitious materials will not be used in the emplacement drift ground support (BSC 2003b). 
Therefore, the effects of alkaline plumes generated by cement-seepage interactions on rock 
properties and radionuclide transport in the vicinity of the emplacement drift are no longer an 
issue that needs to be considered. 
Coupled thermal-hydrologic-chemical effects are likely to result in unstable colloid suspensions 
and, therefore, reduce the concentration of colloids in suspension. With increasing temperature 
and increasing ionic strength, both conditions expected as a result of thermal-hydrologicchemical 
effects, colloid suspensions become less stable. High temperatures and high ionic 
strengths are not favorable to colloid transport. Therefore, the screening out of coupled 
thermal-hydrologic-chemical effects on the transport of radioactive colloids in the TSPA is 
justified. 
New values for the diffusivity of dissolved species through the invert are presented in EBS 
Radionuclide Transport Abstraction (BSC 2003c, Section 6.3.4.1). From the dissolved species 
diffusivity values, diffusivities for colloids are estimated. 
The information in this report is responsive to agreements ENFE 4.03 and GEN 1.01 comments 
35 and 37 made between the DOE and NRC. The report contains the information that DOE 
considers necessary for NRC review for closure of these agreements. 
C.4 BASIS FOR THE RESPONSE 
Features, Events and Processes in UZ Flow and Transport (BSC 2003d, Section 6.8.7) presents 
the screening argument for excluding coupled thermal-hydrologic-chemical effects on 
radionuclide transport properties and colloids (FEP 2.2.10.06.0A; FEP 2.2.08.03.0B) in the 
unsaturated zone (solubility, speciation, phase changes, and precipitation/dissolution). This FEP 
is conservatively ignored with respect to solubility reduction in the far field, and the effects of 
radionuclide precipitation are ignored in the unsaturated zone transport for TSPA. If solubility 
September 2003 C-5 No. 8: Colloids
Revision 2 
limits increase in the geosphere compared with the waste emplacement drift, there is no effect on 
transport because all available radionuclides from the source at the waste emplacement drift are 
already aqueous species. The thermal-chemical effects on colloid formation have already been 
accounted for in the colloid source term (BSC 2003e). Therefore, any impact from 
thermal-chemical alteration on colloid entrainment will be insignificant with respect to total 
repository system performance. 
This screening argument is reasonable because in the rock mass around the repository the 
temperature may be relatively high even at 10,000 years. For example, in Section 6.5.2 in 
Drift-Scale Coupled Processes (DST and THC Seepage) Models (BSC 2003a) it is shown that 
after 10,000 years the temperature is 45 to 50C, which is significantly higher than the initial 
temperature of around 25C. This higher temperature tends to decrease the stability of a 
colloidal suspension, as discussed in Waste Form and In-Drift Colloids-Associated Radionuclide 
Concentrations: Abstraction and Summary (BSC 2003e). 
The diffusion coefficient for dissolved species has been determined specifically for crushed tuff 
invert materials as a function of volumetric moisture content (m3 water/m3 bulk rock), using 
electric conductivity measurements (BSC 2003c). 
The thermal-hydrologic-chemical model simulations have been performed for both the base case 
and the extended case and for both Tptpmn and Tptpll geologic formations (BSC 2003a). The 
extended case includes the major solid phases (minerals and glass) encountered in geologic units 
at Yucca Mountain, together with a range of possible reaction product minerals, CO2 gas, and the 
aqueous species necessary to include these solid phases and the pore-water composition within 
the thermal-hydrologic-chemical model. The base case is a subset of the extended case 
excluding aluminum silicate minerals, which form or dissolve much less easily than minerals 
such as calcite or gypsum. The results for the Tptpmn rock unit are shown in Figures C-2 
and C-3. The pH in fracture waters varies within 7.4 to 8.6 but still remain within a neutral to 
mildly basic range over the 10,000-year regulatory time period (Figure C-2), and thus its effect 
on colloid stability is expected to be negligible. The concentrations of soluble components 
(e.g., Na and Cl) and therefore the ionic strength in the waters are predicted to increase over the 
time period of thermal event (Figure C-4), which will reduce colloid stability. The THC effect 
may also change the transport properties of rock. However, Figure C-3 shows that this change is 
small and can be negligible for colloid transport. 
Figure C-5 presents the uncertainty in the statistical fit for the diffusion coefficient. 
September 2003 C-6 No. 8: Colloids
Source: BSC 2003a. 
Figure C-2. Thermal-Hydrologic-Chemical Simulation (Tptpmn Model): Time Profiles of the Modeled 
pH of Fracture Water at Three Drift-Wall Locations under Heating (Heat) and Nonheating 
(Ambient) Conditions for the Extended (E) and Base-Case (B) Geochemical Systems 
No. 8: Colloids 
Revision 2 
September 2003 C-7
Revision 2 
Source: BSC 2003a. 
NOTE: Decrease in porosity is primarily due to the precipitation of calcite and amorphous silica. 
Figure C-3. Thermal-Hydrological-Chemical Model Simulation (Tptpmn Model): Contour Plot of Modeled 
Fracture Porosity Change at 10,000 Years for (a) Base-Case and (b) Extended 
Geochemical Systems 
September 2003 C-8 No. 8: Colloids
Revision 2 
Source: BSC 2003a. 
Figure C-4. Thermal-Hydrologic-Chemical Simulation (Tptpmn Model): Time Profiles of Modeled Total 
Aqueous Sodium Concentrations in Fracture Water at Three Drift-Wall Locations under 
Heating (Heat) and Nonheating (Ambient) Conditions for Extended (E) and Base-Case (B) 
Geochemical Systems 
September 2003 C-9 No. 8: Colloids
Source: BSC 2003c, Figure 7. 
NOTE: The dashed lines correspond to two standard deviations above and below the statistical fit to the data. D is 
. .. 
. 
where Dcoll is the diffusion coefficient for a colloidal particle of radius rcoll and Dion is the 
diffusion coefficient of a ion of radius rion. Given a typical ion radius and colloidal particle 
radius of 0.1 and 1 nm, respectively, the diffusion coefficient of a colloidal particle is generally 
100 times smaller than that of a dissolved ion. 
BSC (Bechtel SAIC Company) 2003a. Drift-Scale Coupled Processes (DST and THC Seepage) 
Models. MDL-NBS-HS-000001 REV 02. Las Vegas, Nevada: Bechtel SAIC Company. 
BSC 2003b. Repository Design Project, Repository/PA IED Emplacement Drift Committed 
Materials. 800-IED-EBS0-0030-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. 
the diffusion coefficient of the crushed invert materials, D0 is the diffusion coefficient of the fully saturated 
crushed invert materials, and  is the percentage of saturation. 
No. 8: Colloids 
(Eq. C-1) 
September 2003 
Figure C-5. Uncertainty in the Statistical Fit for the Diffusion Coefficient 
The colloidal diffusion coefficient can be estimated from the following Stokes-Einstein 
relationship (BSC 2003c): 
ion 
ion Dcoll = D r
rcoll 
C.5 REFERENCES 
ACC: DOC.20030804.0004. 
ACC: ENG.20030311.0021. 
C-10 
Revision 2 
 . 
.
Revision 2 
BSC 2003c. EBS Radionuclide Transport Abstraction. ANL-WIS-PA-000001 REV 01F. 
Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0199. 
BSC 2003d. Features, Events and Processes in UZ Flow and Transport. ANL-NBS-MD- 
000001 REV 02B. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0198. 
BSC 2003e. Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: 
Abstraction and Summary. MDL-EBS-PA-000004 REV 00. Las Vegas, Nevada: Bechtel SAIC 
Company. ACC: DOC.20030626.0006. 
CRWMS M&O (Civilian Radioactive Waste Management System Management and Operating 
Contractor) 2000a. Particle Tracking Model and Abstraction of Transport Processes. 
ANL-NBS-HS-000026 REV 00. Las Vegas, Nevada: CRWMS M&O. 
ACC: MOL.20000502.0237. 
CRWMS M&O 2000b. UZ Colloid Transport Model. ANL-NBS-HS-000028 REV 00. 
Las Vegas, Nevada: CRWMS M&O. ACC: MOL.20000822.0005. 
Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy 
Technical Exchange and Management Meeting on Evolution of the Near-Field Environment 
(January 912, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), 
January 26, 2001, with enclosure. ACC: MOL.20010810.0033. 
Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and 
Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. 
Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. 
September 2003 C-11 No. 8: Colloids
INTENTIONALLY LEFT BLANK 
C-12 No. 8: Colloids 
Revision 2 
September 2003
APPENDIX D 
CONTRASTING COLLOID CONCENTRATIONS 
IN THE ENGINEERED BARRIER SYSTEM AND SATURATED ZONE 
(RESPONSE TO TSPAI 3.30 AND GEN 1.01 (COMMENTS 43 AND 46)) 
No. 8: Colloids 
Revision 2 
September 2003
Revision 2 
Note Regarding the Status of Supporting Technical Information 
This document was prepared using the most current information available at the time of its development. This 
Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were 
prepared using preliminary or draft information reflect the status of the Yucca Mountain Projects scientific 
and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model 
Reports (AMRs) and other draft references whose contents may change with time. Information that evolves 
through subsequent revisions of the AMRs and other references will be reflected in the License Application 
(LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not 
routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to 
reflect changes in the supporting references prior to submittal of the LA. 
September 2003 No. 8: Colloids
Revision 2 
APPENDIX D 
CONTRASTING COLLOID CONCENTRATIONS 
IN THE ENGINEERED BARRIER SYSTEM AND SATURATED ZONE 
(RESPONSE TO TSPAI 3.30 AND GEN 1.01 (COMMENTS 43 AND 46)) 
This appendix provides a response for Key Technical Issue (KTI) agreement Total System 
Performance Assessment and Integration (TSPAI) 3.30 and general agreement (GEN) 1.01, 
comments 43 and 46. These agreements relate to providing the technical basis for concentrations 
of colloids available for reversible attachment. 
D.1 KEY TECHNICAL ISSUE AGREEMENTS 
D.1.1 TSPAI 3.30 and GEN 1.01 (Comments 43 and 46) 
Agreement TSPAI 3.30 was reached during the NRC/DOE technical exchange and management 
meeting on total system performance assessment and integration held August 6 through 10, 
2001, in Las Vegas, Nevada. TSPAI KTI subissues 1, 2, 3, and 4 were discussed at that meeting 
(Reamer 2001). 
Agreement GEN 1.01 was reached during the NRC/DOE technical exchange and management 
meeting on range of thermal operating temperatures held September 18 through 19, 2001. At 
that meeting, NRC provided additional comments (GEN 1.01, comments 43 and 46) relating to 
TSPAI 3.30, and DOE provided an initial response to those comments (Reamer and Gil 2001). 
The wording of these agreements and of DOEs initial response to the general agreement 
comments is as follows: 
TSPAI 3.30 
Provide the technical basis for the contrasting concentrations of colloids available 
for reversible attachment in the engineered barrier system and the saturated zone. 
Sensitivity analyses planned in response to RT Agreement 3.07 should address the 
effect of colloid concentration on Kc. Update, as necessary, the Kc parameter as 
new data become available from the Yucca Mountain region  
c 
DOE will provide the technical basis for the contrasting concentrations of colloids 
available for reversible attachment in the engineered barrier system and the 
saturated zone. The sensitivity analyses planned in response to RT 
Agreement 3.07 will address the effect of colloid concentration on the K 
parameter. The technical basis will be documented in the Waste Form Colloid 
Associated Concentration Limits: Abstractions and Summary (ANL-WIS-MD- 
000012) in FY 2003. The Kc parameter will be updated as new data become 
available from the Yucca Mountain region in the Uncertainty Distribution for 
Stochastic Parameters AMR (ANL-NBS-MD-000011) in FY 2003. 
September 2003 D-1 No. 8: Colloids
Revision 2 
GEN 1.01 (Comment 43) 
The SSPA presents a new distribution for retardation of colloids with 
irreversibly-attached radionuclides. The distribution takes into account new 
site-specific alluvium data. However, any future use of this distribution in TSPA 
will require comparison with results of field and laboratory tests. This concern is 
indirectly related to agreement TSPAI 3.30. 
DOE Initial Response to GEN 1.01 (Comment 43)1 
DOE acknowledges that any future use of this distribution in TSPA will require 
comparison with results of field and laboratory tests 1,2. This concern is indirectly 
related to KTI agreements RT 3.07 and RT 3.08. Laboratory testing of 
microsphere and silica colloid retardation in alluvium-packed columns is in 
progress. Microspheres will be used as colloid tracers in ATC cross-hole tracer 
testing. 
GEN 1.01 (Comment 46) 
The analysis of sensitivity to increased uncertainty in the reversible colloid 
parameter Kc (Section 12.5.2.4) yielded somewhat longer transport times in the 
saturated zone. This analysis does not illustrate the effect of possibly 
underestimating Kc, because it is not clear that the mean value of Kc is 
significantly different from the base case. This concern is related to agreements 
RT 3.07 and TSPAI 3.30. 
DOE Initial Response to GEN 1.01 (Comment 46) 
This issue will be handled as part of agreements RT 3.07 and TSPAI 3.30. 
D.1.2 Related Key Technical Issue Agreements 
None. 
D.2 RELEVANCE TO REPOSITORY PERFORMANCE 
Radionuclide transport as either dissolved species or colloids represents the means for the release 
of radionuclides from the repository to the biosphere. The concentration of colloids released 
from the engineered barrier system provides a source term for colloid transport within the 
unsaturated zone. In turn, the colloid concentration reaching the water table provides the source 
term for colloid transport in the saturated zone. The colloid concentration (mc) and the 
distribution coefficient (Kd) are two important parameters controlling colloid-facilitated 
radionuclide transport because Kc, the ratio of radionuclide mass in colloids to the concentration 
in solution, is defined as the product of Kd and mc. 
1 [F]ield and laboratory tests 1, 2 refers to test conducted using CML microspheres as colloid surrogates. These 
tests will be discussed in detail in Appendix M (Response to KTI Agreement RT 3.08) of the saturated zone 
technical basis document. 
September 2003 D-2 No. 8: Colloids
Revision 2 
The technical basis for the response for this KTI is presented in Sections 3, 4, 5 and 6 of this 
technical basis document. This KTI is related to the engineered and near-field environment, the 
unsaturated and the saturated zones as shown in Figure D-1. 
D.3 RESPONSE 
D.3.1 Response to Key Technical Issue Agreement TSPAI 3.30 
c 
In contrast to the constraints on colloid concentrations used in the total system performance 
assessment (TSPA) for the site recommendation to represent reversible radionuclide attachment, 
the engineered barrier system and the saturated zone colloid models use the same distribution of 
colloidal concentration for natural system colloids. This distribution has been revised since the 
TSPA for the site recommendation with additional data that raise the upper limits of colloid 
concentration by about two orders of magnitude over the previous values considered. This 
translates to about a four-order-of-magnitude increase for the saturated zone colloid treatment 
from the TSPA for the site recommendation that used only the mean value of colloid 
concentration. This change in colloid concentration is reflected directly in changes to the K 
parameter used in the saturated zone colloid treatment. Such increased concentration limits for 
colloids that can reversibly sorb radionuclides in the saturated zone provides a higher level of 
confidence that the evaluation of saturated zone radionuclide transport by reversible colloids is 
bounding. This increased level of consistency for colloid concentrations in the saturated zone 
does not mean that the treatment is identical to that for the engineered barrier system. The 
differences and the respective technical bases for the colloid concentrations in the engineered 
barrier system and saturated zone are discussed below. 
Within the engineered barrier system, the concentration of colloids available for reversible 
sorption, mc, is evaluated for the effects of temperature and the chemical environment. Such 
effects are not expected within the saturated zone, and therefore the colloid concentration is 
unaffected by them. In the saturated zone, colloid concentration is affected by 
attachment/detachment of the colloids to the immobile rock surfaces. 
It is expected that under the high-temperature and high ionic strengths conditions within the 
engineered barrier system, particularly in the first 2,000 to 3,000 years after closure, the 
concentration of colloids will not be sufficiently high to effectively compete with the immobile 
rock surfaces for the reversible attachment of dissolved radionuclides. Colloids leaving the 
engineered barrier system will travel through the unsaturated zone to reach the saturated zone. 
Within the unsaturated zone, colloid transport is expected to be primarily through fractures, and 
only those colloids with characteristic dimension considerably smaller than the film coating the 
fracture surfaces are expected to be transported. Colloids will be subjected to retardation effects, 
thus increasing their transport time through the unsaturated zone. Larger colloids are expected to 
be subjected to filtration by film straining. As colloids travel through the unsaturated zone 
within the Topopah Spring Tuff, they will reach the Calico Hills Formation, which has a 
significantly different porosity. The interface between the Topopah Spring and the Calico Hills 
units presents a barrier to colloid transport. At that interface colloids may move laterally until 
they find another fracture system to continue traveling vertically towards the saturated zone. 
Therefore, the concentration of colloids reaching the saturated zone is expected to be 
significantly lower than that within the engineered barrier system. Because the only direct 
September 2003 D-3 No. 8: Colloids
Revision 2 
source of colloids in the saturated zone is the natural system materials, they are the focus of the 
reversibly sorbing colloids in the treatment for this subsystem. 
Figure D-1. Mapping of Colloid-Related Key Technical Issues Agreement to Repository System 
Components 
No. 8: Colloids September 2003 D-4
Revision 2 
D.3.2 Response to Agreement GEN 1.01 (Comments 43 and 46) 
c), a product of sorption coefficient Kd and colloid concentration, is sampled based 
The saturated zone transport simulations of radionuclides that are irreversibly attached to 
colloids are conducted for radioisotopes of plutonium and americium. The retardation of 
colloids with irreversibly attached radionuclides is a kinetically controlled process, which 
approaches equilibrium behavior for long transport times. For transport of colloids through the 
saturated zone, equilibrium behavior is nearly achieved. However, nonequilibrium behavior 
results in unimpeded migration of some of the colloids. Consequently, a small fraction of these 
colloids are transported through the saturated zone with no retardation; whereas the larger 
fraction is delayed by a retardation factor. The saturated zone transport simulations of 
radionuclides that are reversibly attached to colloids are conducted for radioisotopes of 
plutonium, americium, thorium, protactinium, and cesium. In the simulations, the distribution 
coefficient (K 
on the distributions of both Kd values and colloid concentrations. The simulations show a 
significant retardation of radionuclide transport in the saturated zone as compared to the case 
where radionuclides irreversibly sorbed on colloids. 
The information in this report is responsive to agreements TSPAI 3.30 and GEN 1.01 comments 
43 and 46 made between the DOE and NRC. The report contains the information that DOE 
considers necessary for the NRC to review for closure of these agreements. 
D.4 BASIS FOR THE RESPONSE 
D.4.1 Response to Key Technical Issue Agreement TSPAI 3.30 
Reversible Colloid Concentrations in the Engineered Barrier SystemThe technical basis for 
selection of typical colloid concentrations in the engineered barrier system and the saturated zone 
are discussed in Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: 
Abstraction and Summary (BSC 2003a) and summarized below: 
 The static-saturated tests for Defense High-Level Radioactive Waste Glass at Argonne 
National Laboratory indicated that colloids developed and increased in concentration 
with time, up to the point where the colloid concentration reached a maximum value and 
then the colloidal suspension became less stable because of high ionic strengths resulting 
in agglomeration and settling (BSC 2003a). The maximum colloid concentration from 
the experimental degradation of high-level radioactive waste glass is estimated to be 
5 ppm (BSC 2003a, Figure 3). 
 The concentration range of iron oxyhydroxide colloids generated from the defense 
high-level radioactive waste glass was estimated based on experiments performed at the 
University of Nevada at Las Vegas and on professional judgment. In these experiments, 
scaled-down miniature waste packages were exposed to J-13 groundwater in either a 
bath-tub mode or a flow-through mode. The cumulative results have yielded average 
concentrations of colloidal size materials in the range of 20 mg/L within the initial 
four weeks of the experiments (BSC 2003a, p. 56). 
September 2003 D-5 No. 8: Colloids
Revision 2 
 The range of colloid concentration in seepage/groundwater was derived based on 
literature data and groundwater sampling from the Yucca Mountain area and Idaho 
National Engineering and Environmental Laboratory (BSC 2003a, Figures 11 and 12). 
Practically no colloids were detected for the ionic strength above 0.05 M. The upper 
limit of colloid concentration is about 200 ppm, with average values of approximately 
0.1 ppm (Figure D-2). 
 Results from the unsaturated testing of commercial spent nuclear fuel and defense spent 
nuclear fuel at Argonne National Laboratory indicated the formation of alteration 
products containing very low concentrations of uranium-based colloids and dissolution 
of uranium-based colloids in a few months (BSC 2003a, Section 6.3.1.2). 
For purposes of this discussion, colloid concentration within the engineered barrier system will 
consist of colloid concentrations within the waste package (Section 3 of this technical basis 
document) and the invert (Section 4 of this technical basis document). 
Source: BSC 2003a, Figure 12. 
Figure D-2. 
No. 8: Colloids 
Cumulative Distribution Function Showing the Probability of Occurrence of Colloid 
Concentration Levels (ppm or mg/L) in Groundwater Samples in the Yucca Mountain 
September 2003 
Area and Idaho National Engineering and Environmental Laboratory 
Within the waste package, the concentration of three different types of colloids available for 
reversible sorption are estimated (see Figure 3-11 of this technical basis document): 
(1) reversible defense high-level radioactive waste glass colloids (also referred to as waste form 
colloids, (2) reversible iron oxyhydroxide colloids, and (3) reversible groundwater colloids. In 
general, the concentration of each of these three types of colloids are calculated at each time step 
D-6
Revision 2 
of the simulation in TSPA. At each time step, (1) the colloid mass available for reversible 
sorption is estimated as a function of ionic strength, (2) the stability of colloids as a function of 
ionic strength and pH is determined, and (3) the sorption of available mass of the dissolved 
inventory of cesium, americium, plutonium, protactinium, and thorium onto colloids is 
calculated. Colloids are assumed to move unretarded from the waste package to the invert. 
Colloids are not assumed to form within the invert. First, the invert is divided into two pore 
spaces (see Section 4.3.2 of this technical basis document). As colloids are released from the 
waste package into the invert, the released colloids are divided into two fractions (see Figure 4-2 
of this technical basis document): (1) the inter-granular pore space fraction and (2) the 
intragranular pore space fraction. No mass transfer of colloids is assumed to occur within these 
two pore spaces. The distribution of colloids between the two pore spaces depends on the 
particular scenario (nominal scenario, seismic scenario, or igneous scenario) being simulated. 
Colloids entering the intra-granular pore space are transported by diffusion as defined by the 
chemistry (ionic strength and pH) of the tuff matrix. The colloids entering the inter-granular 
pore space are transported by advection and diffusion as defined by the chemistry of the 
fractures. The stability of the colloids is reevaluated using the same ionic strength and pH 
relationships for the three types of reversible colloids shown in Figure 3-11 of this technical 
basis document. 
The cumulative distribution function of colloid concentration for natural groundwater colloids is 
shown in Figure D-2. In the calculations summarized above, this cumulative distribution 
function is sampled and modified based on the chemical conditions within the waste package and 
the invert. 
d and the colloid concentration, values of Kc are calculated as 
In the TSPA, the transport model for the engineered barrier system explicitly accounts for the 
partitioning coefficient (Kd) and the colloid concentration. Updated Kd values are tabulated in 
Table D-1. From the values of K 
the product of Kd and colloid concentration. 
The Kc approach is used for both the unsaturated zone and saturated zone colloid transport. The 
same concentration distribution of smectite colloids in Figure D-2 is used for the engineered 
barrier system, unsaturated zone, and saturated zone colloid models. Note that in the engineered 
barrier system this distribution of smectite colloid concentration is reevaluated using the pH and 
ionic strength values, while for the unsaturated zone and saturated zone colloid transport models 
the distribution is assumed to be unaffected by temperature or chemical effects. 
As discussed in Section 3.2.5 of this technical basis document, the thermal and chemical 
perturbations within the engineered barrier system are likely to destabilize colloid suspensions by 
increasing Brownian motion and particle collision due to the elevated temperature and increasing 
ionic strength of the solution. Therefore, the bounding values summarized above are still 
suitable for the engineered barrier system. In addition, any perturbed colloid concentration 
inside the drift will ultimately attenuate to the background concentration in the Yucca Mountain 
groundwater, as the colloidal solution percolates through the invert and the unsaturated zone into 
the saturated zone, and the water chemistry converges to the ambient groundwater. As a result, 
the total system performance becomes insensitive to the near-field thermal and chemical 
perturbations. 
September 2003 D-7 No. 8: Colloids
Table D-1. Kd Values (mL/g) Used for Reversible Radionuclide Sorption on Colloids in Total System 
Performance Assessment-License Application Calculations 
Radionuclide 
Pu 
Am, Th, Pa 
Cs 
Source: BSC 2003a, Table 10. 
NOTE: The Kd values for Tc and I are very low and not listed here. 
No. 8: Colloids 
Colloid 
Iron Oxyhydroxide 
Smectite 
Iron Oxyhydroxide 
Smectite 
Iron Oxyhydroxide 
Smectite 
Kd Value Intervals 
(mL/g) 
Kd Value Range 
(mL/g) 
104 to 106 
<1 x 104 
1 x 104 to 5 x 104 
5 x 104 to 1 x 105 
1 x 105 to 5 x 105 
5 x 105 to 1 x 106 
> 1 x 106 
103 to 106 
< 1 x 103 
1 x 103 to 5 x 103 
5 x 103 to 1 x 104 
1 x 104 to 5 x 104 
5 x 104 to 1 x 105 
1 x 105 to 5 x 105 
5 x 105 to 1 x 106 
> 1 x 106 
105 to 107 
< 1 x 105 
1 x 105 to 5 x 105 
5 x 105 to 1 x 106 
1 x 106 to 5 x 106 
5 x 106 to 1 x 107 
> 1 x 107 
104 to 107 
< 1 x 104 
1 x 104 to 5 x 104 
5 x 104 to 1 x 105 
1 x 105 to 5 x 105 
5 x 105 to 1 x 106 
1 x 106 to 5 x 106 
5 x 106 to 1 x 107 
> 1 x 107 
101 to 103 
< 1 x 101 
1 x 101 to 5 x 101 
5 x 101 to 1 x 102 
1 x 102 to 5 x 102 
5 x 102 to 1 x 103 
> 1 x 103 
102 to 104 
< 1 x 102 
1 x 102 to 5 x 102 
5 x 102 to 1 x 103 
1 x 103 to 5 x 103 
5 x 103 to 1 x 104 
> 1 x 104 
D-8 
Revision 2 
Kd Value Interval 
Probabilities 
0
0.15 
0.2 
0.5 
0.15 
0
0
0.04 
0.08 
0.25 
0.2 
0.35 
0.08 
0
0
0.15 
0.2 
0.55 
0.1 
0
0
0.07 
0.1 
0.23 
0.2 
0.32 
0.08 
0
0
0.13 
0.22 
0.55 
0.1 
0
0
0.2 
0.25 
0.5 
0.05 
0 
September 2003
Revision 2 
Concentration of Reversible Colloids in the Saturated ZoneRadionuclides that are reversibly 
sorbed onto colloids are modeled to be temporarily attached to the surface of colloids. Thus, 
these radionuclides are available for dissolution in the aqueous phase and their transport 
characteristics are a combination of the transport characteristics of solute and colloids. The 
saturated zone transport simulations of radionuclides that are reversibly attached to colloids are 
conducted for radioisotopes of plutonium, americium, thorium, protactinium, and cesium 
(BSC 2003b). (Note that a major fraction of plutonium and americium is transported as 
irreversibly sorbed onto colloids.) 
The distribution coefficient (Kc), a product of sorption coefficient Kd and colloid concentration, 
represents the equilibrium partitioning of radionuclides between the aqueous phase and the 
colloidal phase. The sorption coefficient distributions are listed in Table D-1. The distribution 
of colloid concentration is shown in Figure D-3. The same Kc applies to transport of a 
radionuclide in both the volcanic units and the alluvium. The simulation results are shown in 
Figure D-4. 
Source: BSC 2003b. 
Figure D-3. Cumulative Distribution Function of Uncertainty in Groundwater Colloid Concentrations 
September 2003 D-9 No. 8: Colloids
Revision 2 
Source: BSC 2003b. 
Figure D-4. Mass Breakthrough Curves (Upper) and Median Transport Times (Lower) for Plutonium on 
September 2003 
Reversible Colloids at 18-km Distance 
D.4.2 Response to Agreement GEN 1.01 (Comments 43 and 46) 
Transport of Irreversibly Sorbed Radionuclides in Saturated ZoneFor the TSPA for the 
license application, colloid-facilitated transport of radionuclides in the saturated zone is 
simulated to occur by two basic modes (BSC 2003b). In the first mode, radionuclides that are 
irreversibly attached to colloids are transported at the same rate as the colloids, which are 
themselves retarded by interaction with the aquifer material. In the second mode, radionuclides 
that are reversibly attached to colloids are in equilibrium with the aqueous phase and the aquifer 
D-10 No. 8: Colloids
Revision 2 
material. In this mode of transport, the effective retardation of these radionuclides during 
transport in the saturated zone is dependent on the sorption coefficient of the radionuclide onto 
colloids, the concentration of colloids, and the sorption coefficient of the radionuclide onto the 
aquifer material. 
The saturated zone transport simulations of radionuclides that are irreversibly attached to 
colloids are conducted for radioisotopes of plutonium and americium (BSC 2003b). The 
retardation of colloids with irreversibly attached radionuclides is a kinetically controlled process, 
which approaches equilibrium behavior for long transport times. For transport of colloids 
through the saturated zone, equilibrium behavior is nearly achieved. However, nonequilibrium 
behavior results in unimpeded migration of some of the colloids. Consequently, a small fraction 
of these colloids is transported through the saturated zone with no retardation; whereas the larger 
fraction is delayed by a retardation factor. The processes important to the transport of 
irreversible colloids in the volcanic units of the saturated zone are as follows: advection and 
dispersion of colloids in the fracture water, exclusion of the colloids from the matrix waters, and 
chemical filtration or adsorption of the colloids onto the fracture surfaces. The processes 
modeled for irreversible colloids in the alluvium are the same as those modeled for irreversible 
colloids in the volcanic units, with the exception of matrix exclusion, because the alluvium is 
modeled as a single porous medium. 
Figure D-5 shows the cumulative distribution function used for retardation factors for the 
saturated zone transport abstraction model. The simulation results for plutonium and americium 
are shown in Figure D-6. 
Source: BSC 2003b. 
Figure D-5. 
No. 8: Colloids 
Cumulative Distribution Function Used for Retardation Factors for the Saturated Zone 
Transport Abstraction Model 
September 2003 D-11
Revision 2 
Source: BSC 2003b. 
Figure D-6. 
No. 8: Colloids 
Mass Breakthrough Curves (Upper) and Median Transport Times (Lower) for Plutonium 
and Americium on Irreversible Colloids at 18-km Distance 
D.5 REFERENCES 
BSC (Bechtel SAIC Company) 2003a. Waste Form and In-Drift Colloids-Associated 
Radionuclide Concentrations: Abstraction and Summary. MDL-EBS-PA-000004 REV 00. 
Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030626.0006. 
BSC 2003b. Saturated Zone Flow and Transport Model Abstraction. MDL-NBS-HS-000021 
REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030612.0138. 
September 2003 D-12
Revision 2 
Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy 
Technical Exchange and Management Meeting on Total System Performance Assessment and 
Integration (August 6 through 10, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum 
(DOE/YMSCO), August 23, 2001, with enclosure. ACC: MOL.20011029.0281. 
Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and 
Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. 
Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. 
September 2003 D-13 No. 8: Colloids
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D-14 No. 8: Colloids 
Revision 2 
September 2003
APPENDIX E 
SENSITIVITY STUDIES TO TEST IMPORTANCE 
OF COLLOID TRANSPORT PARAMETERS AND MODELS 
(RESPONSE TO RT 3.07 AND GEN 1.01 (COMMENTS 35, 43, AND 46)) 
No. 8: Colloids 
Revision 2 
September 2003
Revision 2 
Note Regarding the Status of Supporting Technical Information 
This document was prepared using the most current information available at the time of its development. This 
Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were 
prepared using preliminary or draft information reflect the status of the Yucca Mountain Projects scientific 
and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model 
Reports (AMRs) and other draft references whose contents may change with time. Information that evolves 
through subsequent revisions of the AMRs and other references will be reflected in the License Application 
(LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not 
routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to 
reflect changes in the supporting references prior to submittal of the LA. 
September 2003 No. 8: Colloids
Revision 2 
APPENDIX E 
SENSITIVITY STUDIES TO TEST IMPORTANCE 
OF COLLOID TRANSPORT PARAMETERS AND MODELS 
(RESPONSE TO RT 3.07 AND GEN 1.01 (COMMENTS 35, 43, AND 46)) 
This appendix provides a response for Key Technical Issue (KTI) agreement Radionuclide 
Transport (RT) 3.07 and general agreement (GEN) 1.01, comments 35, 43 and 46. These KTI 
agreements relate to providing sensitivity studies for colloid transport parameters and models. 
E.1 KEY TECHNICAL ISSUE AGREEMENTS 
E.1.1 RT 3.07 and GEN 1.01 (Comments 35, 43, and 46) 
Agreement RT 3.07 was reached during the U.S. Nuclear Regulatory Commission (NRC)/U.S. 
Department of Energy (DOE) Technical Exchange and Management Meeting on Radionuclide 
Transport held December 5 through 7, 2000, in Berkeley, California. RT KTI subissues 1, 2, 
and 3 were discussed at that meeting (Reamer and Williams 2000). 
Agreement GEN 1.01 was reached during the NRC/DOE Technical Exchange and Management 
Meeting on Range of Thermal Operating Temperatures, held September 18 through 19, 2001. At 
that meeting, NRC provided additional comments (GEN 1.01, comments 35, 43, and 46) relating 
to RT 3.07, and DOE provided an initial response to those comments (Reamer and Gil 2001). 
The wording of these agreements and of DOEs initial response to the general agreement 
comments is as follows: 
RT 3.07
Provide sensitivity studies to test the importance of colloid transport parameters 
and models to performance for UZ and SZ. Consider techniques to test colloid 
transport in the Alcove 8/Niche 3 test (for example, microspheres). DOE will 
perform sensitivity studies as the basis for consideration of the importance of 
colloid transport parameters and models to performance for the unsaturated and 
saturated zones and will document the results in updates to appropriate AMRs, 
and in the TSPA-LA document, all to be available in FY 2003. DOE will 
evaluate techniques to test colloidal transport in Alcove 8/Niche 3 and provide a 
response to the NRC in February 2001. 
GEN 1.01 (Comment 35) 
The SSPA recommends new values for EBS colloid transport parameters. If these 
are adopted by TSPA in the future, the technical basis for the new distributions 
will require close scrutiny. Relevant KTI agreements are RT 3.07, ENFE 4.03, 
ENFE 4.04, and ENFE 4.06. 
September 2003 E-1 No. 8: Colloids
Revision 2 
DOE Initial Response to GEN 1.01 (Comment 35) 
The new values for EBS colloidal transport parameters were designed to evaluate 
unquantified uncertainty for the SSPA. DOE understands that prior to any 
potential LA, a stronger technical basis must be provided for EBS colloidal 
transport parameter values carried forward to the base case analysis. 
GEN 1.01 (Comment 43) 
The SSPA presents a new distribution for retardation of colloids with 
irreversibly-attached radionuclides. The distribution takes into account new 
site-specific alluvium data. However, any future use of this distribution in TSPA 
will require comparison with results of field and laboratory tests. This concern is 
indirectly related to agreement TSPAI 3.30. 
DOE Initial Response to GEN 1.01 (Comment 43)1 
DOE acknowledges that any future use of this distribution in TSPA will require 
comparison with results of field and laboratory tests 1, 2. This concern is 
indirectly related to KTI agreements RT 3.07 and RT 3.08. Laboratory testing of 
microsphere and silica colloid retardation in alluvium-packed columns is in 
progress. Microspheres will be used as colloid tracers in ATC cross-hole tracer 
testing. 
GEN 1.01 (Comment 46) 
The analysis of sensitivity to increased uncertainty in the reversible colloid 
parameter Kc (Section 12.5.2.4) yielded somewhat longer transport times in the 
SZ. This analysis does not illustrate the effect of possibly underestimating Kc, 
because it is not clear that the mean value of Kc is significantly different from the 
base case. This concern is related to agreements RT 3.07 and TSPAI 3.30. 
DOE Initial Response to GEN 1.01 (Comment 46) 
This issue will be handled as part of agreements RT 3.07 and TSPAI 3.30. 
E.1.2 Related Key Technical Issue Agreements 
None. 
E.2 RELEVANCE TO REPOSITORY PERFORMANCE 
Transport parameters and models for colloid transport in the unsaturated zone and saturated zone 
will determine (1) the ability of colloids to move through the unsaturated zone and saturated 
zone and into the accessible environment, and (2) the ability of the unsaturated zone and 
1 [F]ield and laboratory tests 1, 2 refers to test conducted using CML microspheres as colloid surrogates. These 
tests will be discussed in detail in Appendix M (Response to KTI Agreement RT 3.08) of the saturated zone 
technical basis document. 
September 2003 E-2 No. 8: Colloids
Revision 2 
saturated zone to effectively filter colloids or retard colloid transport. The interplay among 
several key colloid transport parameters and mechanisms will determine the significance of 
colloid transport through the unsaturated zone and saturated zone. Based on several pertinent 
studies, it has been concluded that significant filtration and retardation of colloids within both the 
unsaturated zone and saturated zone will take place. The technical basis for the response for this 
KTI agreement is presented in Sections 3, 4, 5 and 6 of this technical basis document. This KTI 
agreement is related to the engineered and near-field environment, the unsaturated and the 
saturated zones as shown in Figure E-1. 
E.3 RESPONSE 
There are a number of factors associated with the transport of colloids through the unsaturated 
zone and saturated zone. These include: (1) where (into fractures or into the matrix) colloids are 
released from the repository, (2) the degree of colloid resuspension or declogging, (3) colloid 
retardation factors, (4) colloid reversible sorption, (5) the degree of matrix diffusion, (6) the 
application of extrapolating saturated zone determined colloid parameters to the unsaturated 
zone, (7) colloid particle size, (8) the specific discharge rate, (9) the use of linear equilibrium 
isotherms to describe the sorption of radionuclides onto colloids, and (10) the use of linear 
kinetics models to describe the retardation of colloids. Sensitivity analyses have been done that 
address many of these uncertainties. Details of the analyses can be found in Radionuclide 
Transport Models under Ambient Conditions (BSC 2003a, Section 6). The impact of these 
uncertainties on colloid transport varies from negligible (items 1 and 2) to important (items 7 
and 8). The uncertainties are dealt with in two ways: 1) distributions describing the parameter 
uncertainty are established and propagated through the modeling systems leading to a quantified 
uncertainty in the results and 2) choosing a conservative conceptual model (one which results in 
a larger release) when two or more conceptual models are potentially possible. 
The information in this report is responsive to agreements RT 3.07 and GEN 1.01 comments 
35, 43, and 46 made between the DOE and NRC. The report contains the information that DOE 
considers necessary for NRC review for closure of these agreements. 
E.4 BASIS FOR THE RESPONSE 
Sensitivity analyses for colloid transport in the unsaturated zone have been performed. 
Four colloid transport simulations using TOUGH2 V1.11MEOS9NTV1.0 (Module 
EOS9nTV1.0) were conducted, referred herein as Cases 1, 2, 3, and 4. Details and presentation 
of the results for these four cases appear in Radionuclide Transport Models under Ambient 
Conditions (BSC 2003a, Section 6.18, Attachment VI). It is useful to define two terms for the 
purpose of facilitating the analysis of the results. These are (1) the relative mass fraction of a 
species in the fractures or matrix, (XR), defined as the ratio of the mass fraction in the liquid in a 
fracture or matrix to the mass fraction of that species in liquid released from the repository, and 
(2) the relative filtered concentration, (FR), defined as the ratio of the concentration of species 
sorbed or filtered onto a fracture or the matrix to the concentration of that species released in the 
liquid from the repository. 
September 2003 E-3 No. 8: Colloids
Revision 2 
Figure E-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System 
Components 
September 2003 E-4 No. 8: Colloids
Revision 2 
The simulation for Case 1 assumes that no declogging occurs once colloids are attached to the 
matrix. Two observations appear particularly important. 
1. The fast breakthrough of the larger colloids (characterized by a rapid rise of the 
breakthrough curve), the rapidity of which is about the same for all the larger colloids. 
2. Contrary to expectations, the smallest colloid (6 nm) exhibits the slowest 
breakthrough. It is noteworthy that this colloid never reaches the 0.1 fraction level, 
indicating that it is sufficiently small to enter the matrix or become sorbed to the 
fracture walls. This behavior results from a combination of the following factors: 
a. The larger transport velocity of larger colloids, which, by virtue of their size, can 
only move in the center of pores/fractures where velocities are larger than the 
average water velocity. 
b. The inability of larger colloids to penetrate into the matrix from the fractures 
because of size exclusion. Thus, the colloid mass in the fractures is not reduced 
through colloidal diffusion and/or hydrodynamic dispersion, and practically all of 
it moves exclusively in the fractures. 
The simulation for Case 2 assumes that there is a strong declogging factor. The difference in 
breakthrough curves between Case 1 and Case 2 is very small for all colloids, and is most 
prominent for 6 nm colloids. The relative insensitivity to the clogging model in the matrix 
indicates the dominant role of the fractures in the three-dimensional site scale system, with the 
matrix appearing to have a negligible contribution. 
A comparative analysis based on Case 2 (Section 5.4.3) was conducted and reported in 
Radionuclide Transport Models Under Ambient Conditions (CRWMS M&O 2000, Section 6.17) 
using an earlier version of the unsaturated zone transport model. This analysis examined the 
transport of colloids with and without colloid matrix diffusion. The analysis concludes that 
diffusion is less significant in colloid transport than in solute transport, because (1) colloid 
diffusion is smaller than solute molecular diffusion because of the larger colloid size, and (2) 
size-exclusion effects at the interfaces of different geologic units further limit entry by diffusion 
into the matrix (especially for larger colloids). However, diffusion effects become increasingly 
important for a decreasing colloid size. Uncertainty in colloid matrix diffusion is treated by 
sampling the diffusion coefficient over a range of values in the total system performance 
assessment for the license application unsaturated zone transport model (BSC 2003b, 
Section 4.2). 
The simulation for Case 3 shows that the change in the magnitude of the reverse kinetic filtration 
coefficient - (as a fraction of +) has a very small effect on colloid transport, attesting to the fact 
that the role of advection through fracture flow (the same in Cases 1 through 3) is by far the 
dominant mechanism of colloid transport. The effect of a change in the reverse filtration 
coefficient on transport is apparent only in the case of the 6 nm colloid (which can enter the 
pores, and thus be subject to filtration). The insensitivity of transport of the larger colloids to the 
sorption model in the matrix indicates that the matrix has practically no participation in the 
September 2003 E-5 No. 8: Colloids
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retardation, which is attributed to straining at matrix/fracture interfaces. The pattern that 
emerges is the same as the one discussed in Case 1. 
In Case 4, assigning a porosity of 1 percent to the fractures, and making the fracture filtration 
properties equal to those of the matrix (assumed to be those of Case 2), is equivalent to creating a 
system of minor partial fracture filling. The effect of limited diffusion on transport (because of 
pore-size exclusion and filtration) becomes more obvious in this case. While the more-freely 
diffusing 6 nm colloids exhibit a behavior similar to the one in Cases 1 through 4, the effect on 
the larger colloids is more dramatic. The occurrence of even a minor fracture filling retards the 
colloid transport. 
The potential points of release from the repository are uncertain and may vary with time; 
however, this does not impact the transport of colloids. Because of the importance of transport 
and flow within faults on the migration of dissolved radionuclides and colloids, the impact of 
releasing radionuclides directly into faults was assessed (BSC 2003a, Section 6.20). Despite the 
importance of faults (particularly the Drill Hole Wash, the Pagany Wash, and the Sundance 
faults) identified in the analyses of Radionuclide Transport Models Under Ambient Conditions 
(BSC 2003a, Section 6), eliminating potential sources from the immediate vicinity of the faults 
has a small (even negligible) effect on transport and arrivals at the water table. This is not to 
suggest that faults are unimportant but only that they continue to dominate whether radionuclides 
are directly released into them or not. 
Sensitivity Analysis for Colloid Transport in the Saturated ZoneUncertainties associated 
with model parameters are explicitly captured by using probability distributions of model 
parameter values. These uncertainties are propagated through the model to the output 
breakthrough curves. The results are presented in Saturated Zone Colloid Transport 
(BSC 2003c, Section 7.3). Radionuclide transit times are most sensitive to groundwater specific 
discharge. This is because increasing the specific discharge not only increases the advective 
velocity but also reduces the time available for matrix diffusion to be effective. In assessing the 
sensitivity of breakthrough times to the specific discharge through the model, permeabilities of 
the various units are scaled along with the specific discharge to preserve the model calibration. 
Other parameters of importance to the breakthrough times are the retardation factor for 
irreversible colloids in the alluvium, matrix diffusion, the sorption coefficient in the matrix as 
well as the alluvium, the effective fracture porosity in the volcanics, the effective porosity in the 
alluvium, and the sorption coefficients for reversible colloids. 
Quantification of the sensitivity of the model output breakthrough curves for various 
radionuclides of concern to parameter uncertainties are further evaluated in SZ Flow and 
Transport Model Abstraction (BSC 2003d). There are uncertainties associated with scaling 
parameter values from the scale of measurements to the scale of interest. Much of the data used 
for deriving parameter values in this report is from laboratory or field experiments conducted on 
spatial and temporal scales much smaller than those expected to occur in site-scale saturated 
zone model. 
The sensitivity of the output breakthrough curves to each of the uncertain input parameters is 
summarized as follows. 
September 2003 E-6 No. 8: Colloids
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Specific Discharge: The results show that the output is sensitive to this parameter. The upper 
limit of specific discharge, 6 m/year, with extremely fast fluid flow (such as would be expected 
for the unlikely case of a high permeability channel going continuously over the distance of 
18 kilometers in a highly faulted region), results in breakthrough as fast as 50 years. The lower 
limit of specific discharge, 0.02 m/year, leads to breakthrough times greater than 10,000 years. 
Colloid Retardation Factor in Alluvium for Irreversible Colloids: Colloid retardation is not 
modeled in the base case. Compared to that, both the low and high limiting values of the colloid 
retardation factor for irreversibly sorbed radionuclides lead to significant retardation, resulting 
from retardation of the colloids themselves. The time for the 50 percent breakthrough for the 
lower limit of the retardation factor (R = 7.9) is 5,041 years, and that for the upper limit of the 
retardation factor (R = 5188) exceeds 10,000 years. 
Reversible Sorption onto Colloids in the Alluvium: The range of values for the modified sorption 
coefficient used in this case is the same range of values as the original sorption coefficient in the 
alluvium. Thus, the results for the lower limiting case are identical to the base case, and those 
for the upper limiting case show no breakthrough at all within 10,000 years. 
Colloid Retardation Factor in Volcanics for Irreversible Colloids: The colloids are modeled as 
having no diffusion into the volcanic matrix, and the retardation arises from the reversible 
filtration of the colloids by attachment onto the surfaces of the fractures. The range of 
uncertainty in the retardation factor of 6 to 794 translates into an output uncertainty range of 
1,420 to more than 10,000 years for the breakthrough time for 50 percent of the colloids. 
Reversible Sorption onto Colloids in the Volcanics: The modified sorption coefficient for this 
case has the same range of values as the original sorption coefficient in alluvium. In this case, 
the diffusion coefficient is also modified for the upper limit. Thus, the results for the lower 
limiting case are identical to the base case, and those for the upper limiting case show no 
breakthrough at all within 10,000 years. A small fraction of colloids traveled unretarded. 
Alcove 8/Niche 3 TestThe matrix diffusion model is an important component of the 
unsaturated zone transport model. This model is corroborated with data from the 
Alcove 8/Niche 3 fault test (BSC 2003a, Section 7.4). The test was carried out in the upper 
lithophysal and middle nonlithophysal subunits in the Yucca Mountain unsaturated zone. These 
geological subunits correspond to model layers TSw33 and TSw34, respectively, in the site-scale 
model for the Yucca Mountain unsaturated zone. The TSw33 has some lithophysal cavities that 
may intersect fractures. Liquid water with and without tracers was released at the floor of an 
alcove along the fault (about 5 m long) within TSw33. Seepage from the fault into a niche and 
tracer concentrations of seeping liquid as a function of time were monitored. The niche is 
located within TSw34, about 20 m below the floor of the alcove; the interface between TSw33 
and TSw34 is about 15 m below the floor of the alcove. A water-pressure head of 2 cm was 
applied at an infiltration plot along the fault at the alcove. The plot consists of four trenches that 
have different infiltration rates as a result of subsurface heterogeneity along the fault. Seepage 
from the fault into the niche was measured during the test, with a number of trays used to cover 
the areas where seepage might occur. Seepage was found to be highly spatially variable. 
Several boreholes were installed around the niche. Water arrival times at these boreholes were 
monitored through electrical resistivity probes. After 209 days, two tracers with different 
September 2003 E-7 No. 8: Colloids
Revision 2 
molecular diffusion coefficients (Br and pentafluorobenzoic acid, or PFBA) were introduced into 
infiltrating water at the infiltration plot. Tracer concentrations in three of the trays (at the niche) 
capturing seeping water from the fault were measured. For technical reasons, seepage rates 
corresponding to these three trays were not measured during the period of tracer concentration 
measurement. In this study, a flux-averaged breakthrough curve (concentration as a function of 
time) from these trays was used to represent the average breakthrough curve for all trays at the 
niche where seepage was captured. A constant flux value for each of the three trays was used for 
calculating the flux-averaged breakthrough curve. The constant flux values for the three trays 
were determined as the averaged value over 56 days before tracers were introduced. This 
flux-averaged breakthrough curve is comparable to the simulation results. 
Use of CML MicrospheresAll of the field measurements have involved fluorescent 
carboxylate-modified polystyrene latex (CML) microspheres ranging in size from 250 to 500 nm 
diameter. Laboratory fracture experiments have been conducted using silica, montmorillonite, 
and clinoptilolite colloids in addition to CML microspheres. In one study, silica colloids 
(approximately 100 nm diameter) were compared directly with CML microspheres transport 
(330 nm diameter), and it was found that the microspheres transported conservatively relative to 
silica colloids. This result suggests that colloid filtration and retardation parameters derived 
from CML microsphere responses in field tracer tests should be conservative if used to predict 
natural inorganic colloid transport in fracture systems. See detailed discussions in Saturated 
Zone Colloid Transport (BSC 2003c). 
E.5 REFERENCES 
BSC (Bechtel SAIC Company) 2003a. Radionuclide Transport Models under Ambient 
Conditions. MDL-NBS-HS-000008 REV 01A. Las Vegas, Nevada: Bechtel SAIC Company. 
ACC: MOL.20030828.0326. 
BSC 2003b. Particle Tracking Model and Abstraction of Transport Processes. MDL-NBS-HS- 
000020 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030611.0040. 
BSC 2003c. Saturated Zone Colloid Transport. ANL-NBS-HS-000031 REV 01A. Las Vegas, 
Nevada: Bechtel SAIC Company. ACC: MOL.20030602.0288. 
BSC 2003d. SZ Flow and Transport Model Abstraction. MDL-NBS-HS-000021 REV 01. Las 
Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030916.0008. 
CRWMS M&O (Civilian Radioactive Waste Management System Management and Operating 
Contractor) 2000. Radionuclide Transport Models Under Ambient Conditions. MDL-NBS-HS- 
000008 REV 00. Las Vegas, Nevada: CRWMS M&O. ACC: MOL.19990721.0529. 
Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and 
Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. 
Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. 
September 2003 E-8 No. 8: Colloids
Revision 2 
Reamer, C.W. and Williams, D.R. 2000. Summary Highlights of NRC/DOE Technical 
Exchange and Management Meeting on Radionuclide Transport. Meeting held December 5-7, 
2000, Berkeley, California. Washington, D.C.: U.S. Nuclear Regulatory Commission. 
ACC: MOL.20010117.0063. 
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E-10 No. 8: Colloids 
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September 2003
APPENDIX F 
TRANSPORT OF DISSOLVED AND COLLOIDAL RADIONUCLIDES 
THROUGH INVERT 
(RESPONSE TO TSPAI 3.17 AND GEN 1.01 (COMMENTS 36 AND 38)) 
No. 8: Colloids 
Revision 2 
September 2003
Revision 2 
Note Regarding the Status of Supporting Technical Information 
This document was prepared using the most current information available at the time of its development. This 
Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were 
prepared using preliminary or draft information reflect the status of the Yucca Mountain Projects scientific 
and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model 
Reports (AMRs) and other draft references whose contents may change with time. Information that evolves 
through subsequent revisions of the AMRs and other references will be reflected in the License Application 
(LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not 
routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to 
reflect changes in the supporting references prior to submittal of the LA. 
September 2003 No. 8: Colloids
Revision 2 
APPENDIX F 
TRANSPORT OF DISSOLVED AND COLLOIDAL RADIONUCLIDES 
THROUGH INVERT 
(RESPONSE TO TSPAI 3.17 AND GEN 1.01 (COMMENTS 36 AND 38)) 
This appendix provides a response for Key Technical Issue (KTI) agreement Total System 
Performance Assessment and Integration (TSPAI) 3.17 and general agreement (GEN) 1.01, 
comments 36 and 38. These KTI agreements relate to providing an uncertainty analysis of the 
colloid transport diffusion coefficient. 
F.1 KEY TECHNICAL ISSUE AGREEMENTS 
F.1.1 TSPAI 3.17 and GEN 1.01 (Comments 36 and 38) 
Agreement TSPAI 3.17 was reached during the U.S. Nuclear Regulatory Commission 
(NRC)/U.S. Department of Energy (DOE) Technical Exchange and Management Meeting on 
Total System Performance Assessment and Integration held August 6 through 10, 2001, in 
Las Vegas, Nevada. TSPAI KTI subissues 1, 2, 3, and 4 were discussed at that meeting 
(Reamer 2001). 
Agreement GEN 1.01 was reached during the NRC/DOE Technical Exchange and Management 
Meeting on Range of Thermal Operating Temperatures held September 18 through 19, 2001. At 
that meeting, NRC provided additional comments (GEN 1.01, comments 36 and 38) relating to 
TSPAI 3.17, and DOE provided an initial response to those comments (Reamer and Gil 2001). 
The wording of these agreements and of DOEs initial response to the general agreement 
comments is as follows: 
TSPAI 3.171 
Provide an uncertainty analysis of the diffusion coefficient governing transport of 
dissolved and colloidal radionuclides through the invert. The analysis should 
include uncertainty in the modeled invert saturation (ENG4.4.1). DOE will 
provide an uncertainty analysis of the diffusion coefficient governing transport of 
dissolved and colloidal radionuclides through the invert. The analysis will 
include uncertainty in the modeled invert saturation. The uncertainty analysis will 
be documented in the EBS Radionuclide Transport Abstraction AMR (ANL-WISPA-
000001) expected to be available to NRC in FY 2003. 
GEN 1.01 (Comment 36) 
The discussion of uncertainty in the saturation level of the invert does not 
consider the possibility of higher saturation. This comment is related to KTI 
agreement TSPAI 3.17. 
1 ENG4.4.1 in this agreement refers to NRC integrated subissue ENG 4 (NRC 2002, Table 1.1-2). 
September 2003 F-1 No. 8: Colloids
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DOE Initial Response to GEN 1.01 (Comment 36) 
Studies with the MSTH model, as reported in Chapter 5 of the SSPA Volume 1, 
investigated the sensitivity of invert liquid saturation to a variety of repository 
parameters. These parameters included bulk permeability, host-rock thermal 
conductivity, lithophysal porosity, and invert thermal conductivity. Predicted 
liquid saturation remained within a narrow range, between four percent and 
10 percent, for all parameter variations. In addition, the diffusive breakthrough 
time for the invert is already relatively rapid, so any increase in saturation levels 
is expected to have a negligible impact. 
DOE will provide an uncertainty analysis of diffusion in the invert. This analysis 
will include uncertainty in invert saturation per KTI agreements TEF 2.05 and 
TSPAI 3.17. 
GEN 1.01 (Comment 38) 
The effect of the drift shadow assumption on invert transport needs to be 
evaluated. Also, as mentioned in the chapter, any adoption of a drift shadow 
model will require additional justification. This concern may be related to 
agreement TSPAI 3.17. 
DOE Initial Response to GEN 1.01 (Comment 38) 
The invert transport abstraction does not incorporate any direct assumptions 
related to a drift shadow effect. The hydrologic inputs to the invert transport 
calculation come primarily from the MSTH model that tracks water and gas 
within the near-field rock and the drift. The specific inputs from the MSTH 
model to invert transport are the temperature of the invert and liquid saturation of 
the invert. DOE acknowledges that models carried forward to support a potential 
license application will be qualified and documented, and may require 
supplemental justification or analysis. 
Also, see response to comment 29. 
F.1.2 Related Key Technical Issue Agreements 
GEN 1.01 (comment 36), TSPAI 3.30, and RT 3.07. 
F.2 RELEVANCE TO REPOSITORY PERFORMANCE 
Radionuclide transport through the invert is expected to be primarily dominated by diffusion of 
both dissolved and colloidal species. One of the key parameters is the diffusion coefficient for 
both types of species. The diffusion coefficient for dissolved species is a function of degree of 
saturation, with higher values at the higher moisture contents. The diffusion coefficient for 
colloids is estimated from diffusion coefficients for dissolved species multiplied by the ratio of 
ionic radius to colloid radius. 
September 2003 F-2 No. 8: Colloids
Revision 2 
The technical basis for the response for this KTI agreement is presented in Section 4 of this 
technical basis document. This KTI agreement is related to the invert as shown in Figure F-1. 
Figure F-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System 
Components 
No. 8: Colloids September 2003 F-3
Revision 2 
F.3 RESPONSE 
Using electric conductivity measurements, the diffusion coefficient for dissolved species has 
been measured for crushed tuff invert material as a function of volumetric moisture content and 
presented in EBS Radionuclide Transport Abstraction (BSC 2003). From these values, diffusion 
coefficients for colloids can be estimated using the ratio of ionic size (molecular size) to colloid 
size. These measurements show that for typical moisture and saturation levels expected in the 
invert, diffusion of dissolved species would take more than 10,000 years, and much longer for 
colloids. 
Uncertainty of the dissolved species diffusivity is discussed in EBS Radionuclide Transport 
Abstraction (BSC 2003, Section 6.3.4.1). 
As discussed in EBS Radionuclide Transport Abstraction (BSC 2003, Section 7.1.2), the invert 
diffusion coefficient has a low impact on performance because the response of the unsaturated 
zone generally dominates the response of the invert. The unsaturated zone is dominant because 
the invert is filled with crushed tuff that is similar to the host rock in the unsaturated zone and 
because the thickness of the invert is quite small in relation to the travel distance in the 
unsaturated zone. 
The information in this report is responsive to agreements TSPAI 3.17 and GEN 1.01 comments 
36 and 38 made between the DOE and NRC. The report contains the information that DOE 
considers necessary for the NRC to review for closure of these agreements. 
F.4 BASIS FOR THE RESPONSE 
The diffusion coefficient for dissolved species has been determined specifically for crushed tuff 
invert materials as a function of volumetric moisture content (m3 water/m3 bulk rock), using 
electric conductivity measurements (BSC 2003). Figure F-2 presents the uncertainty in the 
statistical fit for the diffusion coefficient. 
For the nominal scenario evaluated out to 10,000 years, releases only occur because a small 
fraction of the waste packages have small manufacturing defects. Hence, all the releases occur 
via diffusive pathways through the waste package and invert. In this scenario, the only source of 
water is vapor from evaporation and condensation. Based on corrosion analysis, the drip shield 
will remain intact for the regulatory period and, thus, any seepage water that enters the drifts will 
contact the drip shield and be diverted away from the package. This diversion creates a drip 
shadow zone underneath the waste package. Therefore, no advective transport will occur 
through the waste package or invert even in seepage environments. Within this drip shadow, all 
radionuclides will be transported via diffusion through the intragranular pore space. Diffusive 
transport through the intergranular pore space will be insignificant since water saturations in the 
intergranular pore space will be near zero. Hence, transport of colloidal material through the 
invert will likely be negligible, since the diffusion coefficient for colloids is at least 100 times 
lower than for dissolved species based on the relative size of colloids and dissolved species. Any 
mixing that occurs between waters in this shadow will be by the slow process of diffusion of 
anions and cations. The water chemistry in this shadow will remain similar to matrix water as 
ions migrate and react with the water constituents and minerals in the matrix. Thus, the 
chemistry for evaluating the stability of the colloids corresponds to the matrix water chemistry. 
September 2003 F-4 No. 8: Colloids
Revision 2 
Source: BSC 2003, Figure 7. 
NOTE: The dashed lines correspond to two standard deviations above and below the statistical fit to the data. D is 
 . 
. 
. .. 
. 
where Dcoll is the diffusion coefficient for a colloidal particle of radius rcoll and Dion is the 
diffusion coefficient of a ion of radius rion. Given a typical ion radius and colloidal particle 
radius of 0.1 and 1 nm, respectively, the diffusion coefficient of a colloidal particle is generally 
100 times smaller than that of a dissolved ion. Thus, it takes at least 10,000 years for colloids to 
diffuse through the invert. 
the diffusion coefficient of the crushed invert materials, D0 is the diffusion coefficient of the fully saturated 
crushed invert materials, and  is the percentage of saturation. 
(Eq. F-1) 
(Eq. F-2) 
September 2003 No. 8: Colloids 
Figure F-2. Uncertainty in the Statistical Fit for the Diffusion Coefficient 
The lack of diffusion can be shown as follows. The characteristic time (T) for dissolved 
radionuclides to diffuse through the invert of thickness L can be calculated by: 
D L / 2 T = 
where D is the diffusion coefficient. With L = 0.8 m, the calculated T values are listed in the 
fourth column of Table F-1. As shown in Table F-1, it will take 100 to 1.65  106 years for a 
dissolved species to diffuse through the invert, depending on the saturation degree. 
The colloidal diffusion coefficient can be estimated from the following Stokes-Einstein 
relationship (BSC 2003): 
ion 
ion Dcoll = D r
rcoll 
F-5
As discussed in EBS Radionuclide Transport Abstraction (BSC 2003, Section 7.1.2), the invert 
diffusion coefficient has a low impact on performance because the response of the unsaturated 
zone generally dominates the response of the invert. The unsaturated zone is dominant because 
the invert is filled with crushed tuff that is similar to the host rock in the unsaturated zone and 
because the thickness of the invert is quite small in relation to the travel distance in the 
unsaturated zone.
Diffusion Coefficient of and Time of Diffusion through Crushed Tuff Invert Materials Table F-1. 
Volumetric Moisture 
Content 
(%) 
32.13 
18.15 
9.26 
8.29 
7.54 
7.36 
7.22 
7.03 
6.97 
6.89 
6.84 
6.75 
6.63 
6.63 
6.23 
6.11 
6.00 
5.55 
5.46 
5.41 
4.45 
3.64 
0.29 
0.20 
Source: BSC 2003, Table 43. DTN: MO0002EBSDDC02.003 
NOTE: a Saturation degree = volumetric moisture/porosity. The porosity of crushed tuff invert material is assumed 
b 
to be 0.545 (BSC 2003, Table 1). 
Time for diffusion is calculated using Equation F-1 with L = 0.8 m. 
Diffusion Coefficient 
(cm2/s) 
2.02E-06 
5.40E-07 
4.05E-08 
2.24E-09 
6.81E-09 
6.21E-09 
4.38E-09 
6.75E-09 
7.45E-09 
6.73E-09 
2.19E-09 
5.42E-09 
4.39E-09 
3.76E-09 
3.40E-09 
1.55E-09 
3.43E-09 
2.04E-09 
2.04E-09 
9.97E-10 
6.19E-10 
5.00E-10 
1.24E-10 
1.25E-10 
Saturation Degreea 
(%) 
58.95 
33.30 
16.99 
15.21 
13.83 
13.50 
13.25 
12.90 
12.79 
12.64 
12.55 
12.39 
12.17 
12.17 
11.43 
11.21 
11.01 
10.18 
10.02 
9.93 
8.17 
6.68 
0.53 
0.37 
Time for Diffusion 
through Invertb 
(year) 
1.02E+02 
3.81E+02 
5.09E+03 
9.20E+04 
3.02E+04 
3.32E+04 
4.70E+04 
3.05E+04 
2.77E+04 
3.06E+04 
9.41E+04 
3.80E+04 
4.69E+04 
5.48E+04 
6.06E+04 
1.33E+05 
6.01E+04 
1.01E+05 
1.01E+05 
2.07E+05 
3.33E+05 
4.12E+05 
1.66E+06 
1.65E+06 
No. 8: Colloids F-6 
Revision 2 
September 2003
Revision 2 
F.5 REFERENCES 
F.5.1 Documents Cited 
BSC (Bechtel SAIC Company) 2003. EBS Radionuclide Transport Abstraction. ANL-WIS-PA- 
000001 REV 01F. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030922.0199. 
NRC (U.S. Nuclear Regulatory Commission) 2002. Integrated Issue Resolution Status Report. 
NUREG-1762. Washington, D.C.: U.S. Nuclear Regulatory Commission, Office of Nuclear 
Material Safety and Safeguards. TIC: 253064. 
Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy 
Technical Exchange and Management Meeting on Total System Performance Assessment and 
Integration (August 6 through 10, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum 
(DOE/YMSCO), August 23, 2001, with enclosure. ACC: MOL.20011029.0281. 
Reamer, C.W. and Gil, A.V. 2001. Summary Highlights of NRC/DOE Technical Exchange and 
Management Meeting of Range on Thermal Operating Temperatures, September 18-19, 2001. 
Washington, D.C.: U.S. Nuclear Regulatory Commission. ACC: MOL.20020107.0162. 
F.5.2 Data, Listed by Data Tracking Number 
MO0002EBSDDC02.003. The Determination of Diffusion Coefficient of Invert Materials. 
Submittal date: 02/16/2000. 
September 2003 F-7 No. 8: Colloids
INTENTIONALLY LEFT BLANK 
F-8 No. 8: Colloids 
Revision 2 
September 2003
SCREENING CRITERIA FOR ATTACHMENT OF RADIONUCLIDES TO COLLOIDS 
(RESPONSE TO RT 1.03 AIN-1, ENFE 3.05 AIN-1, AND ENFE 4.05 AIN-1) 
No. 8: Colloids 
APPENDIX G 
Revision 2 
September 2003
Revision 2 
Note Regarding the Status of Supporting Technical Information 
This document was prepared using the most current information available at the time of its development. This 
Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were 
prepared using preliminary or draft information reflect the status of the Yucca Mountain Projects scientific 
and design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model 
Reports (AMRs) and other draft references whose contents may change with time. Information that evolves 
through subsequent revisions of the AMRs and other references will be reflected in the License Application 
(LA) as the approved analyses of record at the time of LA submittal. Consequently, the Project will not 
routinely update either this Technical Basis Document or its Key Technical Issue Agreement appendices to 
reflect changes in the supporting references prior to submittal of the LA. 
September 2003 No. 8: Colloids
Revision 2 
APPENDIX G 
SCREENING CRITERIA FOR ATTACHMENT OF RADIONUCLIDES TO COLLOIDS 
(RESPONSE TO RT 1.03 AIN-1, ENFE 3.05 AIN-1, AND ENFE 4.05 AIN-1) 
This appendix provides a response for additional information need (AIN) requests for Key 
Technical Issue (KTI) agreements Radionuclide Transport (RT) 1.03, Evolution of the 
Near-Field Environment (ENFE) 3.05 and ENFE 4.05. These KTI agreements relate to 
providing a technical basis for selection of radionuclides that are transported via colloids in 
Performance Assessment analyses. 
G.1 KEY TECHNICAL ISSUE AGREEMENTS 
G.1.1 RT 1.03 AIN-1, ENFE 3.05 AIN-1, and ENFE 4.05 AIN-1 
Agreement RT 1.03 was reached during the U.S. Nuclear Regulatory Commission (NRC)/U.S. 
Department of Energy (DOE) Technical Exchange and Management Meeting on Radionuclide 
Transport held December 5 through 7, 2000, in Berkeley, California. RT KTI subissues 1, 2, 
and 3 were discussed at that meeting (Reamer and Williams 2000). 
NRC used four DOE documents in its review of the response to KTI agreement RT 1.03: 
Inventory Abstraction (CRWMS M&O 2000a); Waste Form Colloid-Associated Concentrations 
Limits: Abstraction and Summary (CRWMS M&O 2001); Total System Performance 
Assessment (TSPA) Model for Site Recommendation (CRWMS M&O 2000b); and Total System 
Performance Assessment for the Site Recommendation (CRWMS M&O 2000c). Additional 
information was requested by the NRC after the staffs review of these reports was completed, 
resulting in RT 1.03 AIN-1 (Reamer 2002). 
Agreements ENFE 3.05 and 4.05 were reached during the NRC/DOE Technical Exchange and 
Management Meeting on Evolution of the Near-Field Environment held January 9 through 12, 
2001, in Pleasanton, California. ENFE KTI subissues 1, 2, 3, and 4 were discussed at that 
meeting (Reamer 2001). 
NRC used three DOE documents in its review of the response to KTI agreement 
ENFE 3.05: Total System Performance Assessment (TSPA) Model for Site Recommendation 
(CRWMS M&O 2000b); Total System Performance Assessment for the Site Recommendation 
(CRWMS M&O 2000c); and Waste Form Colloid-Associated Concentrations 
Limits: Abstraction and Summary (CRWMS M&O 2000d). Additional information was 
requested by the NRC after their review of these reports was completed, resulting in ENFE 3.05 
AIN-1 (Schlueter 2002). 
NRC used four DOE documents in its review of the response to KTI agreement ENFE 4.05: 
Inventory Abstraction (CRWMS M&O 2000a); Waste Form Colloid-Associated Concentrations 
Limits: Abstraction and Summary (CRWMS M&O 2000d); TSPA for the Site Recommendation 
(CRWMS M&O 2000c); and TSPA Model for Site Recommendation (CRWMS M&O 2000b). 
Additional information was requested by the NRC after their review of these reports was 
completed, resulting in ENFE 4.05 AIN-1 (Schlueter 2002). 
September 2003 G-1 No. 8: Colloids
Revision 2 
The wording of these agreements is as follows: 
RT 1.03
Provide the screening criteria for the radionuclides selected for PA. Provide the 
technical basis for selection of the radionuclides that are transported via colloids 
in the TSPA. The screening criteria for radionuclides selected for TSPA are 
contained in the AMR Inventory Abstraction. DOE is documenting identification 
of radionuclides transported via colloids for TSPA in the AMR Waste Form 
Colloid-Associated Concentration Limits: Abstraction and Summary, in the 
TSPA-SR Technical Report, and in the TSPA-SR Model Document. These 
documents will be available to the NRC in January 2001. 
RT 1.03 AIN-1 
Provide clarification and justification of radionuclides for which reversible and 
irreversible colloidal transport is modeled. 
ENFE 3.05 
Provide the technical basis for selection of radionuclides that are released via 
reversible and irreversible attachment to colloids for different waste forms in the 
TSPA. The technical basis for the selection of radionuclides released via 
reversible and irreversible attachments to colloids for different waste forms is 
provided in Section 3.5.6.1 of the Total System Performance Assessment (TSPA) 
Model for Site Recommendation (MDL-WIS-PA-000002) Rev 00. This 
document will be provided to the NRC in January 2001. 
ENFE 3.05 AIN-1 
1. Provide clarification and justification of radionuclides for which reversible 
and irreversible colloidal release is modeled. 
2. Provide a stronger technical basis that release by irreversible attachment can 
be neglected for spent nuclear fuel. 
ENFE 4.05 
Provide the screening criteria for the radionuclides selected for PA. Provide the 
technical basis for selection of radionuclides that are transported via colloids in 
the TSPA. The screening criteria for radionuclides selected for TSPA are 
contained in the AMR Inventory Abstraction (ANL-WIS-MD-000006) Rev 00, 
ICN 01. The DOE is documenting identification of radionuclides transported via 
colloids for TSPA in the AMR Colloid-Associated Concentration Limits: 
Abstraction and Summary (ANL-WIS-MD-000012) Rev 0, in the Total System 
Performance Assessment for the Site Recommendation (TDR-WIS-PA-000001) 
Rev 00 ICN 01, and in the Total System Performance Assessment (TSPA) Model 
September 2003 G-2 No. 8: Colloids
Revision 2 
for Site Recommendation (MDL-WIS-PA-000002) Rev 00. These documents 
will be available to the NRC in January 2001. 
ENFE 4.05 AIN-1 
1. Provide clarification and justification of radionuclides for which reversible 
and irreversible colloidal release is modeled. 
G.1.2 Related Key Technical Issue Agreements 
None. 
G.2 RELEVANCE TO REPOSITORY PERFORMANCE 
Determining which radionuclides can reversibly and irreversibly attach to colloids is an 
important consideration in establishing the relative importance of colloid-facilitated radionuclide 
transport. Once the concentration of colloids and the available surface area per unit volume is 
determined, the mechanism for the attachment of dissolved radionuclides to colloids will 
establish the mass of radionuclides that can be transported via colloids. 
The technical basis for the response for this KTI agreement is presented in Section 3 of this 
technical basis document. This KTI agreement is related to the waste package as shown in 
Figure G-1. 
G.3 RESPONSE 
Section 6.3.3.1 of Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: 
Abstraction and Summary (BSC 2003) provides detailed discussion of the screening criteria for 
radionuclides undergoing reversible sorption and Section 6.3.3.2 for radionuclides undergoing 
irreversible sorption. 
The rationale for selecting radionuclides was based on a combination of long half-life, relatively 
strong sorption characteristics, relatively large abundance during the initial 10,000 years in the 
repository, and observed field and laboratory behavior. The sorption of Pu, Am, Th, Cs, and Pa, 
are modeled using a linear isotherm model. Kd values for uptake of cesium (Cs), thorium (Th), 
protactinium (Pa), plutonium (Pu), and americium (Am) by colloids are provided in Waste Form 
and In-Drift Colloids-Associated Radionuclide Concentrations: Abstraction and Summary 
(BSC 2003, Table 10). These radionuclides were modeled because they are considered to be 
important to dose. 
A major fraction of Pu and Am (greater than 90 percent) are assumed to attach to smectite 
colloids irreversibly. Evidence from sorption experiments with Pu and Am with colloidal 
hematite and goethite show that the rates of desorption (backward rate) of Pu and Am are 
significantly slower than the rates of sorption (forward rate). More importantly, over a 
significant time period (up to 150 days in some experiments), the extent of desorption is 
considerably less than the extent of sorption. Pu and Am are considered so strongly sorbed to 
colloids that, for practical purposes, they can be considered irreversibly sorbed and are modeled 
in this manner within the engineered barrier system. 
September 2003 G-3 No. 8: Colloids
Revision 2 
Figure G-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System 
Components 
September 2003 G-4 No. 8: Colloids
Revision 2 
Recent long-term corrosion tests with commercial and DOE spent nuclear fuel have 
demonstrated alteration products containing low concentrations of uranium-based colloids (and 
low concentrations of actinides), dissolution of the uranium-based colloids over a short time 
duration (less than several months), and sorption dominating the behavior of the actinides 
(particularly onto the vessels metal surfaces). 
The information in this report is responsive to agreements RT 1.03 AIN-1, ENFE 3.05 AIN-1, 
and ENFE 4.05 AIN-1 made between the DOE and NRC. The report contains the information 
that DOE considers necessary for NRC review for closure of these agreements. 
G.4 BASIS FOR THE RESPONSE 
G.4.1 Screening of Radionuclides for Reversible and Irreversible Attachment to Colloids 
Section 6.3.3.1 of Waste Form and In-Drift Colloids-Associated Radionuclide Concentrations: 
Abstraction and Summary (BSC 2003) provides detailed discussion of the screening criteria for 
radionuclides undergoing reversible sorption and Section 6.3.3.2 for radionuclides undergoing 
irreversible sorption. 
G.4.1.1 Reversible Sorption 
Transport of radionuclides on colloids is potentially important for radioisotopes that 1) have low 
solubility; 2) have long half-lives, 3) can be entrained in waste-form colloids, or can be sorbed 
onto waste-form colloids, engineered barrier materials colloids, or geologic barrier materials 
colloids; 4) represent a major portion of the inventory in terms of total activity; and/or 5) can 
contribute significantly to radioactivity during the 10,000-year regulatory period. Considering 
these five criteria as part of radionuclide screening in Section 6.3.3.1, Waste Form and In-Drift 
Colloids-Associated Radionuclide Concentrations: Abstraction and Summary (BSC 2003) 
evaluated eight radionuclides for reversible sorption onto to colloids: Pu, Am, Th, Cs, Pa, Np, 
U, and Sr. Below is the rationale used in that report to include or exclude these radionuclides 
from the model for reversible attachment to colloids: 
 PlutoniumA large quantity of Pu will exist in the repository. Pu is sparingly soluble 
but sorbs strongly to oxide mineral surfaces (generally less strongly to silicates). Pu is 
observed to sorb strongly to soil minerals, and laboratory investigations have shown that 
it sorbs readily to colloids as well. 
 AmericiumAm will be a significant contributor to radioactivity during the 
10,000 years. As is Pu, Am is sparingly soluble but strongly sorbs to mineral surfaces, 
including colloids. Laboratory investigations have shown that it sorbs strongly to 
colloids. 
 Cesium135Cs has a long half-life and can attach strongly to certain sheet silicates 
(including clays) by means of ion exchange. For this reason, 135Cs has been observed to 
sorb to soil minerals, and it could potentially form pseudocolloids particularly with 
groundwater and defense high-level radioactive waste glass-derived clay colloids. 
September 2003 G-5 No. 8: Colloids
Revision 2 
 ProtactiniumPa will be a significant contributor to radioactivity during the first 
10,000 years. Because of this, and the fact that relatively little is known of the colloid 
behavior of Pa, it was included in this analysis. 
 ThoriumTh will be a significant contributor to radioactivity during the first 
10,000 years. Because of this, and the fact that there is evidence that Th sorbs strongly 
to oxides, it was included in this analysis. There is relatively little known of the 
colloid-related behavior of Th. 
 NeptuniumBecause Np will be the most significant contributor to radioactivity beyond 
the first 10,000 years, it was considered for inclusion in this analysis. Np is more 
soluble under anticipated repository conditions than many of the other important 
radionuclides, and it sorbs considerably less strongly than, for example, Pu and Am. It 
would appear then that the mobility of Np is influenced mostly by its solubility. For 
these reasons, and to simplify the modeling, Np was not included in the 
reversible-sorption portion of the colloid-associated radionuclide transport analysis. 
 UraniumU will be by far the most abundant radioactive element in the repository and 
primarily for this reason was considered for the analysis. U is more soluble under 
anticipated repository conditions than many of the other important radionuclides, and it 
sorbs considerably less strongly than, for example, Pu and Am. As with Np, the 
mobility of U is influenced mostly by its solubility. Field observations at U deposits and 
mine sites have indicated that little or no colloid U transport occurs. For these reasons, 
and to simplify modeling, U was not included in the reversible-sorption of the 
colloid-associated radionuclide transport analysis. 
 StrontiumSr is expected to be a potentially significant contributor to dose but because 
of its very short half-life was not considered further in this analysis. 
G.4.1.2 Irreversible Sorption 
2 
Defense high-level radioactive waste glass degradation experiments show that plutonium is 
probably irreversibly attached to smectite colloids generated during the experiments. Further, 
evidence from sorption experiments with Pu and Am (Lu et al. 2000) with colloidal hematite and 
goethite show that the rates of desorption (backward rate) of Pu and Am are significantly slower 
than the rates of sorption (forward rate). More importantly, over a significant time period (up to 
150 days in some experiments), the extent of desorption is considerably less than the extent of 
sorption. Pu and Am are considered so strongly sorbed to colloids that, in essence, they can 
almost be considered irreversibly sorbed and are modeled in this manner within the engineered 
barrier system. Pu transport velocities in soils reflect the fact that Pu binds strongly to soils, 
leaving very little, if any, soluble Pu available for groundwater transport or plant uptake. 
Coughtrey et al. (1985) estimated exchangeable Pu to be less than one percent. At Rocky Flats, 
soil Pu is largely bound to soil metal hydroxides. Litaor and Ibrahim (1996) used 0.01M CaCl 
as an extractant and measured Pu in Rocky Flats soil to be 0.04 to 0.08 percent exchangeable. 
Bunzl et al. (1995) measured exchangeable 239Pu and 240Pu (0.5 to 1 percent) and 241Am (1.5 to 
15 percent) from fallout-contaminated soils in Germany using 1M C2H7NO2 (ammonium acetate 
NH4C2H3O2) as the extractant. Laboratory experiments of Pu sorption onto iron oxides have 
September 2003 G-6 No. 8: Colloids
Revision 2 
shown that only approximately one percent of the initially sorbed Pu can be desorbed into 
solution, even after months of time have elapsed (Lu et al. 2000), which is broadly consistent 
with field observations. For these reasons, Pu and Am are modeled as irreversibly attaching to 
colloids. No other radionuclides are considered to be irreversibly attached to colloids. 
In order to accommodate these observations in the colloid abstraction, the TSPA-LA model 
calculates irreversible and reversible sorption of Pu and Am onto corrosion colloids as functions 
of specific surface area (SA), site density (NA), mass of corrosion colloids, dissolved 
concentration of Pu and Am, target-flux out ratio, and other parameters (such as flow and 
diffusion rate) internal to the TSPA-LA model. This is done in such a way that a majority (90 to 
99 percent) of Pu and Am are irreversibly sorbed. The reversibly sorbed portion is determined 
according to an equilibrium Kd value model. 
G.4.2 Colloid Formation from Commercial and Defense Spent Nuclear Fuel 
Long-term corrosion testing of commercial spent nuclear fuel and defense spent nuclear fuel 
under unsaturated, oxidizing conditions has been performed to examine the release of 
radionuclides and, specifically for the purpose of this report, the release in the form of colloids. 
Testing was designed to simulate a variety of Yucca Mountain repository relevant 
water-exposure conditions for several spent nuclear fuels with a range of fuel burnups and 
compositions. Results from the unsaturated testing of commercial spent nuclear fuel and defense 
spent nuclear fuel indicated spallation of alteration products containing low concentrations of 
uranium-based colloids (and low concentrations of actinides), dissolution of the uranium-based 
colloids over a short time duration (less than several months), and sorption dominating the 
behavior of the actinides (onto clay colloids or onto metal surfaces). 
Assessment of the importance of potential colloid formation from commercial spent nuclear fuel 
was based on four major observations: (1) very low colloid concentrations were observed in the 
commercial spent nuclear fuel degradation tests, at least an order of magnitude less than 
concentrations observed in the defense high-level radioactive waste glass degradation tests 
(based on dynamic light scattering measurements) (Mertz et al. 2003); (2) the fraction of 
uranium in the colloid mass was uniformly low in the commercial spent nuclear fuel tests, the 
only deviation from this occurring immediately following one of two changes in vessel 
configuration in which the uranium fraction increased but rapidly decreased to the approximate 
level of earlier values (Mertz et al. 2003); (3) suspensions of meta-schoepite and UO2+x colloids 
in J-13 groundwater appear to exhibit decreasing stability in short-term saturated (with respect to 
the uranium phases) tests; their stability in unsaturated solutions has not been tested (Mertz 
et al. 2003); and (4) field studies at uranium-bearing deposits indicate generally that under 
oxidizing conditions at near-neutral pH, colloid particles contain little uranium and there is little 
sorption of uranium complexes to colloids. 
One of the reasons hypothesized for the low colloid release in the commercial spent nuclear fuel 
tests was the test configuration in which the Zircaloy-4 support for the fuel fragments had 
7-micron holes. However, the results from the unirradiated UO2 tests also show few colloids 
after the formation of alteration products (Wronkiewicz et al. 1997). The unsaturated tests on 
unirradiated UO2 had a test configuration with large 2 to 3 mm holes at the holder base allowing 
for the spallation of UO2+x particulate during initial corrosion; however, the formation of a dense 
September 2003 G-7 No. 8: Colloids
Revision 2 
mat of alteration products during the UO2 corrosion apparently reduced particulate release by 
trapping particulates in the altered products (Wronkiewicz et al. 1997). A similar mechanism 
whereby the alteration products minimize particulate release may be applicable to the 
commercial spent nuclear fuel unsaturated tests. 
The concentration of released particulates or colloids from the commercial spent nuclear fuel 
tests is very low except during test conditions corresponding to disruptive events, such as 
movement of the fuel from one retainer to another (Mertz et al. 2003). In that case, colloid and 
particulate concentrations increased temporarily but returned to very low concentrations after the 
disruption (Mertz et al. 2003). While this indicates that disruptive events may contribute to the 
release of particulates and colloids from commercial spent nuclear fuel, it also indicates that the 
longevity of the colloids in the leachate is short. 
Of the different types of fuel in the defense spent nuclear fuel inventory, metallic uranium fuel 
comprises approximately 85 percent (by weight) of that inventory and thus was selected for 
corrosion testing. An irradiated uranium metal fuel from the N-Reactor at Hanford was tested in 
an experimental setup similar to that used at Argonne National Laboratory for testing 
commercial spent nuclear fuel. Additional details on this testing can be found elsewhere 
(DTN: MO0306ANLSF001.459). Corrosion testing of metallic uranium samples resulted in 
rapid oxidation (within a few months) of the uranium primarily to an oxide sludge consisting of 
UO2 and higher oxides of uranium (DTN: MO0306ANLSF001.459). Although the uranium fuel 
disintegrated rapidly, corrosion testing was continued to determine the effect of groundwater 
leaching on the fuel sludge. Results from the corrosion tests showed that the composition of the 
defense spent nuclear fuel colloids evolve over time from an initially UO2-rich population, to a 
mixed colloid population containing UO2 and higher oxides of U as well as smectite clays, to a 
population that appears to be dominated by U-containing smectite clays. 
After approximately one year of testing on the defense spent nuclear fuel, the total quantity of 
uranium in the sludge represented approximately 99 weight percent of the original uranium fuel 
sample, while that in the colloid size group corresponded to 0.002 to 0.006 weight percent of the 
original uranium fuel sample. The quantity of uranium in the fraction attached to the stainless 
steel vessel was 0.1 to 0.3 weight percent of the original fuel sample 
(DTN: MO0306ANLSF001.459). The attached material is measured by washing the stainless 
steel vessel in HNO3; the attached material includes sorbed solutes, sorbed colloids, and 
precipitates (DTN: MO0306ANLSF001.459). The disposition of Pu during defense spent 
nuclear fuel corrosion showed that Pu is associated predominantly with the colloidal, particulate, 
and sorbed size fractions. The 239Pu/ 238U ratios in the colloid fraction are significantly larger 
than those in the other fractions (sorbed, particulate, and dissolved) and is the only fraction that 
showed enrichment of Pu in comparison to that in the fuel prior to corrosion 
(DTN: MO0306ANLSF001.459). Results from the testing support a model of defense spent 
nuclear fuel in which Pu is significantly adsorbed to the surface of colloids (such as corrosion 
products of the waste package or groundwater clays) but does not occur as an embedded 
radionuclide in waste form colloids (DTN: MO0306ANLSF001.459). 
The results of the tests conducted for commercial spent nuclear fuel and defense spent nuclear 
fuel along with the corroborative field information indicate that the impact of colloids from the 
degradation of these waste forms is not going be significant compared to the potential impact of 
September 2003 G-8 No. 8: Colloids
Revision 2 
colloids from defense high-level radioactive waste glass and from pseudocolloids. Therefore, it 
is reasonable to exclude true colloids from the TSPA modeling. 
G.4.3 Response to Additional Information Needed Requests 
In response to the AINs listed in Section G.1.1, clarification and justification of radionuclides for 
which reversible and irreversible colloidal release is modeled has been provided. In addition, 
recent tests in commercial spent nuclear fuel and defense spent nuclear fuel indicate that colloids 
forming from the degradation of those waste forms will not be significant; thus, providing a 
stronger technical basis that colloid-associated radionuclides release can be neglected for 
commercial spent nuclear fuel and defense spent nuclear fuel. 
G.5 REFERENCES 
G.5.1 Documents Cited 
BSC (Bechtel SAIC Company) 2003. Waste Form and In-Drift Colloids-Associated 
Radionuclide Concentrations: Abstraction and Summary. (MDL-EBS-PA-000004 REV 00. 
Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20030626.0006. 
Bunzl, K.; Flessa, H.; Kracke, W.; and Schimmack, W. 1995. Association of Fallout 239+240Pu 
and 241Am with Various Soil Components in Successive Layers of a Grassland Soil. 
Environmental Science & Technology, 29, (10), 2513-2518. Washington, D.C.: American 
Chemical Society. TIC: 234160. 
Coughtrey, P.J.; Jackson, D.; Jones, C.H.; Kane, P.; and Thorne, M.C. 1985. Radionuclide 
Distribution and Transport in Terrestrial and Aquatic Ecosystems, A Compendium of Data. 
Volume 6. Rotterdam, The Netherlands: A.A. Balkema. TIC: 240299. 
CRWMS M&O (Civilian Radioactive Waste Management System Management and Operating 
Contractor) 2000a. Inventory Abstraction. ANL-WIS-MD-000006 REV 00 ICN 01. Las Vegas, 
Nevada: CRWMS M&O. ACC: MOL.20001130.0002. 
CRWMS M&O 2000b. Total System Performance Assessment (TSPA) Model for Site 
Recommendation. MDL-WIS-PA-000002 REV 00. Las Vegas, Nevada: CRWMS M&O. 
ACC: MOL.20001226.0003. 
CRWMS M&O 2000c. Total System Performance Assessment for the Site Recommendation. 
TDR-WIS-PA-000001 REV 00 ICN 01. Las Vegas, Nevada: CRWMS M&O. 
ACC: MOL.20001220.0045. 
CRWMS M&O 2000d. Waste Form Colloid-Associated Concentrations Limits: Abstraction 
and Summary. ANL-WIS-MD-000012 REV 00. Las Vegas, Nevada: CRWMS M&O. 
ACC: MOL.20000525.0397. 
CRWMS M&O 2001. Waste Form Colloid-Associated Concentrations Limits: Abstraction and 
Summary. ANL-WIS-MD-000012 REV 00 ICN 01. Las Vegas, Nevada: CRWMS M&O. 
ACC: MOL.20010130.0002. 
September 2003 G-9 No. 8: Colloids
Revision 2 
Litaor, M.I. and Ibrahim, S.A. 1996. Plutonium Association with Selected Solid Phases in Soils 
of Rocky Flats, Colorado, Using Sequential Extraction Technique. Journal of Environmental 
Quality, 25, (5), 1144-1151. Madison, Wisconsin: American Society of Agronomy. 
TIC: 252783. 
Lu, N.; Conca, J.; Parker, G.R.; Leonard, P.A.; Moore, B.; Strietelmeier, B.; and Triay, I.R. 
2000. Adsorption of Actinides Onto Colloids as a Function of Time, Temperature, Ionic 
Strength, and Colloid Concentration, Waste Form Colloids Report for Yucca Mountain Program 
(Colloid Data Summary from 1999 to 2000 Research). LA-UR-00-5121. Los Alamos, 
New Mexico: Los Alamos National Laboratory. TIC: 249708. 
Mertz, C.J.; Finch, R.J.; Fortner, J.A.; Jerden, J.L., Jr.; Yifen, T.; Cunnane, J.C.; and Finn, P.A. 
2003. Characterization of Colloids Generated from Commercial Spent Nuclear Fuel Corrosion. 
Activity Number: PAWTP30A. Argonne, Illinois: Argonne National Laboratory. 
ACC: MOL.20030422.0337. 
Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy 
Technical Exchange and Management Meeting on Evolution of the Near-Field Environment 
(January 912, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), 
January 26, 2001, with enclosure. ACC: MOL.20010810.0033. 
Reamer, C.W., 2002. Radionuclide Transport Key Technical Issue Agreements. Letter 
C.W. Reamer (NRC) to S. Brocoum (DOE/YMSCO), February 6, 2002, with enclosure. 
ACC: MOL.20020920.0318. 
Reamer, C.W. and Williams, D.R. 2000. Summary Highlights of NRC/DOE Technical 
Exchange and Management Meeting on Radionuclide Transport. Meeting held December 5-7, 
2000, Berkeley, California. Washington, D.C.: U.S. Nuclear Regulatory Commission. 
ACC: MOL.20010117.0063. 
Schlueter, J. 2002. Evolution of the Near-Field Environment Key Technical Issue 
Agreements. Letter from J. Schlueter (NRC) to S. Brocoum (DOE/YMSCO), February 14, 
2002, with enclosure. ACC: MOL.20020607.0086. 
Wronkiewicz, D.J.; Buck, E.C.; and Bates, J.K. 1997. Grain Boundary Corrosion and 
Alteration Phase Formation During the Oxidative Dissolution of UO2 Pellets. Scientific Basis 
for Nuclear Waste Management XX, Symposium held December 2-6, 1996, Boston, 
Massachusetts. Gray, W.J. and Triay, I.R., eds. 465, 519-526. Pittsburgh, Pennsylvania: 
Materials Research Society. TIC: 238884. 
G.5.2 Data, Listed by Data Tracking Number 
MO0306ANLSF001.459. Colloids Generated from Irradiated N Reactor Fuel, Data Report. 
Submittal date: 06/04/2003. 
September 2003 G-10 No. 8: Colloids
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APPENDIX H 
CHANGES IN COLLOID CONCENTRATIONS 
DUE TO SHIFTS IN pH AND IONIC STRENGTH 
(RESPONSE TO TSPAI 3.42) 
September 2003 No. 8: Colloids
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Note Regarding the Status of Supporting Technical Information 
This document was prepared using the most current information available at the time of its development. This 
Technical Basis Document and its appendices providing Key Technical Issue Agreement responses that were 
prepared using preliminary or draft information reflect the status of the Yucca Mountain Projects scientific and 
design basis at the time of submittal. In some cases this involved the use of draft Analysis and Model Reports 
(AMRs) and other draft references whose contents may change with time. Information that evolves through 
subsequent revisions of the AMRs and other references will be reflected in the License Application (LA) as the 
approved analyses of record at the time of LA submittal. Consequently, the Project will not routinely update either 
this Technical Basis Document or its Key Technical Issue Agreement appendices to reflect changes in the 
supporting references prior to submittal of the LA. 
September 2003 No. 8: Colloids
Revision 2 
APPENDIX H 
CHANGES IN COLLOID CONCENTRATIONS 
DUE TO SHIFTS IN pH AND IONIC STRENGTH 
(RESPONSE TO TSPAI 3.42) 
This appendix provides a response for Key Technical Issue (KTI) agreement Total System 
Performance Assessment and Integration (TSPAI) 3.42. This KTI agreement relates to providing 
a sensitivity analysis on changes in colloid concentration due to changes in pH and ionic 
strength. 
H.1 KEY TECHNICAL ISSUE AGREEMENT 
H.1.1 TSPAI 3.42 
Agreement TSPAI 3.42 was reached during the U.S. Nuclear Regulatory Commission 
(NRC)/U.S. Department of Energy (DOE) technical exchange and management meeting on total 
system performance assessment and integration held August 6 through 10, 2001, in Las Vegas, 
Nevada. TSPAI KTI subissues 1, 2, 3, and 4 were discussed at that meeting (Reamer 2001). 
The wording of this agreement is as follows: 
TSPAI 3.42 
DOE should provide a sensitivity analysis on the potentially abrupt changes in 
colloid concentrations due to shifts in modeled pH and ionic strength across 
uncertain stability boundaries. This analysis may be combined with plans to 
address ENFE Agreement 4.06 and RT Agreement 3.07. 
DOE will complete sensitivity analyses to investigate the effects of varying 
colloid concentration due to shifts in model predicted pH and ionic strength across 
uncertain stability boundaries. These analyses will be documented in TSPA for 
any potential license application expected to be available to NRC in FY 2003. 
H.1.2 Related Key Technical Issue Agreement 
ENFE 4.06 and RT 3.07. 
H.2 RELEVANCE TO REPOSITORY PERFORMANCE 
The stability of colloidal suspensions impacts the significance of colloid-facilitated radionuclide 
transport. The pHionic strength relationship is one of several phenomena that impact the 
concentration of colloids in suspension. In addition, temperature and relative humidity also have 
an impact on the stability of colloid suspensions. 
The technical basis for the response for this KTI agreement is presented in Sections 4, 5, and 6 of 
this technical basis document. This KTI agreement is related to the invert, saturated zone, and 
unsaturated zone as shown in Figure H-1. 
September 2003 H-1 No. 8: Colloids
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Figure H-1. Mapping of Colloid-Related Key Technical Issue Agreements to Repository System 
Components 
September 2003 H-2 No. 8: Colloids
Revision 2 
H.3 RESPONSE 
The sensitivity analysis required by this KTI agreement was not performed. However, as 
discussed below, performance of the repository system is not believed to be sensitive to abrupt 
changes across the pH-ionic strength stability boundaries. 
Only during the first 300 years following waste package breach can pH-ionic strength values for 
both smectite and iron-oxyhydroxide colloids fall within the range for stable colloid suspensions 
(BSC 2003). After 300 years, all possible pH-ionic strength combination of values would be 
within the range for which the colloid suspensions are unstable (BSC 2003). As shown in 
Section H.4, even during the first 300 years of the postclosure period only a very small number 
(less than 6 percent) of pH-ionic strength values will fall within the stable colloid suspension 
region. 
In addition, within the drift environment, temperature will be high during the early postclosure 
period (e.g., the first 1,000 years) as shown in part (a) of Figure 3-2 of this technical basis 
document. These conditions will contribute to making colloid suspensions unstable. Therefore, 
even if the pH-ionic strength values were to be favorable to stable colloids, the high temperature 
will destabilize the colloid suspensions. 
Based on these considerations, the total repository system performance will not be impacted by 
abrupt changes across pH-ionic strength stability boundaries. Therefore, a detailed sensitivity 
analysis as stated in this KTI agreement is not necessary. 
The information in this report is responsive to agreement TSPAI 3.42 made between the DOE 
and NRC. The report contains the information that DOE considers necessary for NRC review 
for closure of this agreement. 
H.4 BASIS FOR THE RESPONSE 
Ranges of sampled values of ionic strength and pH for the co-disposal waste package with 
N-Reactor fuel and no-drip conditions are shown in Table H-1 (BSC 2003). It is shown in the 
table that for the first 300 years after the waste package breaches there is a small chance (less 
than 6 percent) of colloids being stable within the waste package.1 Note that the analysis in In- 
Package Chemistry Abstraction (BSC 2003) assumes no water evaporation during waste package 
degradation. As discussed in Section 3.1 (Figures 3-2 and 3-3), radiation heat-induced water 
evaporation will likely concentrate any solution available inside the drift and lead to a high ionic 
strength environment (greater than 0.05 M, a threshold for colloid instability) environment for a 
significant portion of the regulatory time period. The evaporation effect will further reduce the 
chance for the formation of a stable colloid suspension with the drift. In summary, the 
probability of forming a stable colloid suspension is low (less than 5 percent), and so is the 
chance for the potentially abrupt changes in colloid concentration due to modeled pH and ionic 
strength across uncertain stability boundaries. This means that changes in pH and ionic 
1 Similar data for a CSNF package under no-drip conditions indicate that there is a similarly small chance of 
sampling a pH-ionic strength combination that would yield a stable colloid suspension persisting over the entire 
10,000-year regulatory period, not just the first 300 years. These data were not presented since it has been 
concluded that CSNF colloid contribution is negligible. 
September 2003 H-3 No. 8: Colloids
strengths across uncertain stability boundaries will have little, if any, impact on colloid 
concentrations. 
0 to 55 
55 to 300
N-Reactor Fuel, No-Drip 
Ionic Strength Range 
Time Period (yr) (M) 
4x10-6 to 0.85 
0.01 to 0.85 
0.05 to 0.85 300 to 10,000 
Table H-1. Ranges of Sampled Values of Ionic Strength and pH for Co-Disposal Packages with 
pH Range 
H-4 
4.1 to 9.6 
4.1 to 9.6 
4.1 to 9.6 
Probability 
Stable 
0.06 
0.05 
0.0 
Unstable 
0.94 
0.95 
1.00 
September 2003 
Source: BSC 2003. 
Within the drift environment, temperature will be high during the early postclosure period (e.g., 
the first 1,000 years) as shown in part (a) of Figure 3-2 of this technical basis document. The 
high temperature will make colloid suspensions unstable. Therefore, even if the pH-ionic 
strength relationship was to be favorable towards stable colloids (first 300 years of the 
postclosure period), the high temperature will destabilize the colloid suspensions. 
H.5 REFERENCES 
BSC (Bechtel SAIC Company) 2003. In-Package Chemistry Abstraction. ANL-EBS-MD- 
00037 REV 01D. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20030617.0024. 
Reamer, C.W. 2001. U.S. Nuclear Regulatory Commission/U.S. Department of Energy 
Technical Exchange and Management Meeting on Total System Performance Assessment and 
Integration (August 6 through 10, 2001). Letter from C.W. Reamer (NRC) to S. Brocoum 
(DOE/YMSCO), August 23, 2001, with enclosure. ACC: MOL.20011029.0281. 
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