May 31, 2013 

MEMORANDUM TO: Janelle Jesse, Acting Chief 

Low-Level Waste Branch 

Environmental Protection 

 and Performance Assessment Directorate 

Division of Waste Management 

 and Environmental Protection 

Office of Federal and State Materials 

 and Environmental Management Programs 

THRU: Christepher McKenney, Chief /RA/ 

Performance Assessment Branch 

Environmental Protection 

 and Performance Assessment Directorate 

Division of Waste Management 

 and Environmental Protection 

Office of Federal and State Materials 

 and Environmental Management Programs 

FROM: George Alexander, Systems Performance Analyst /RA by C. Barr for/ 

Performance Assessment Branch 

Environmental Protection 

 and Performance Assessment Directorate 

Division of Waste Management 

 and Environmental Protection 

Office of Federal and State Materials 

 and Environmental Management Programs 

SUBJECT: TECHNICAL REVIEW: WASTE RELEASE AND SOLUBILTY 

RELATED DOCUMENTS PREPARED BY UNITED STATES 

DEPARTMENT OF ENERGY TO SUPPORT FINAL BASIS FOR 

SECTION 3116 DETERMINATION FOR THE F-AREA TANK FARM 

FACILITY AT SAVANNAH RIVER SITE. PROJECT NO. PROJ0734. 

The U.S. Nuclear Regulatory Commission (NRC) staff has performed a technical review of 

several documents prepared by the United States Department of Energy (DOE) that address 

NRC staff Technical Evaluation Report (TER) comments and recommendations related to waste 

release assumptions in the F-Area Tank Farm Facility (FTF) performance assessment (DOE, 

2010). This technical review activity supports Monitoring Factors 2.1, “Solubility Limiting 

Phases/Limits” and 2.2, “Chemical Transition Times” in NRC staff’s monitoring plan for the FArea Tank Farm facility (FTF) (NRC, 2012). 

CONTACT: George Alexander, FSME/DWMEP 

 (301) 415-6755 
J. Jessie 2 

In 2011, NRC issued a TER for the FTF with the primary recommendation for DOE to conduct 

waste release experiments to support several key modeling assumptions. Following issuance 

of NRC staff’s TER, DOE conducted a number of studies to provide additional support for 

several waste release assumptions. NRC staff has reviewed these additional studies and 

concludes, consistent with DOE plutonium solubility expert recommendations, that experimental 

verification of modeled solubilities of key radionuclides and solid phase analysis continues to be 

needed to provide support for key modeling assumptions in DOE’s FTF Performance 

Assessment (PA). 

Additionally, NRC staff has identified two follow-up actions for DOE to provide additional 

information related to waste release assumptions: 

• DOE should provide additional information to support assumptions regarding longevity of 

reducing conditions in the contaminated zone. Recent studies (Cantrell and Williams, 

2012) suggest that the reducing capacity of the tank grout could be depleted much 

earlier than assumed in the FTF PA (SRS-REG-2007-00002 Rev 1) and in more recent 

Pu solubility modeling performed for Tank 18 (Denham, 2012). Uncertainty in the 

normative mineralogy assumed in geochemical modeling should be considered under 

this action. 

• DOE should provide additional support for the assumption that the Eh of infiltrating water 

will remain below a critical threshold at which Pu solubility will increase to a risksignificant value (e.g., updated geochemical modeling indicates a dramatic increase in 

Pu solubility occurs at Eh greater than +0.45 V). Uncertainty in the critical threshold and 

the Eh of infiltrating groundwater should be considered under this action. 

Additional details related to NRC staff’s findings and conclusions are provided in the enclosure. 

Enclosure: Technical Review of Waste Release-Related Documents. 
J. Jessie 2 

In 2011, NRC issued a TER for the FTF with the primary recommendation for DOE to conduct 

waste release experiments to support several key modeling assumptions. Following issuance 

of NRC staff’s TER, DOE conducted a number of studies to provide additional support for 

several waste release assumptions. NRC staff has reviewed these additional studies and 

concludes, consistent with DOE plutonium solubility expert recommendations, that experimental 

verification of modeled solubilities of key radionuclides and solid phase analysis continues to be 

needed to provide support for key modeling assumptions in DOE’s FTF Performance 

Assessment (PA). 

Additionally, NRC staff has identified two follow-up actions for DOE to provide additional 

information related to waste release assumptions: 

• DOE should provide additional information to support assumptions regarding longevity of 

reducing conditions in the contaminated zone. Recent studies (Cantrell and Williams, 

2012) suggest that the reducing capacity of the tank grout could be depleted much 

earlier than assumed in the FTF PA (SRS-REG-2007-00002 Rev 1) and in more recent 

Pu solubility modeling performed for Tank 18 (Denham, 2012). Uncertainty in the 

normative mineralogy assumed in geochemical modeling should be considered under 

this action. 

• DOE should provide additional support for the assumption that the Eh of infiltrating water 

will remain below a critical threshold at which Pu solubility will increase to a risksignificant value (e.g., updated geochemical modeling indicates a dramatic increase in 

Pu solubility occurs at Eh greater than +0.45 V). Uncertainty in the critical threshold and 

the Eh of infiltrating groundwater should be considered under this action. 

Additional details related to NRC staff’s findings and conclusions are provided in the enclosure. 

Enclosure: Technical Review of Waste Release-Related Documents. 

DISTRIBUTION (w/ enclosure): 

CGrossman LParks MFuhrmann DPickett, CNWRA WIR Service List 

DOE SRS Email List 

ML12272A082 

OFC DWMEP DWMEP DWMEP DWMEP DWMEP 

NAME CBarr/GAlexander TMoon HFelsher CMcKenney CBarr/GAlexander

DATE 5/22/2013 5/22/2013 5/23/2013 5/302013 5/31/2013 

OFFICIAL RECORD COPY 
Enclosure 

Technical Review of Waste Release Documents Supporting FTF Closure 

Date: May 2013 

Reviewers: 

Mark Fuhrmann, NRC 

George Alexander, NRC 

Cynthia Barr, NRC 

Bobby Pabalan, Center for Nuclear Waste Regulatory Analyses (CNWRA) 

David Pickett, CNWRA 

 

Key Documents Reviewed: 

Cantrell, K., Clark, D.L., Janecky, D.R., Psaras, J., and Runde, W. Plutonium Solubility Peer 

Review Report. LA-UR-2012-00079. Prepared for DOE Savannah River Operations Office 

(DOE-SR), Los Alamos National Laboratory, New Mexico, December 2011. 

Denham, M. Evolution of Chemical Conditions and Estimated Plutonium Solubility in the 

Residual Waste Layer During Post-Closure Aging of Tank 18. SRNL-STI-2012-00087. 

Savannah River National Laboratory, Savannah River Nuclear Solutions, LLC, Aiken, SC 

29808, February, 2012. 

Hay, M.S., O’Rourke, P.E., and Ajo, H.M. Scanning Electron Microscopy and X-Ray Diffraction 

Analysis of Tank 18 Samples. SRNL-STI-2012-00123. March 2012. 

Hobbs, D.T. Form and Aging of Plutonium in Savannah River Site Waste Tank 18. SRNL-STI2012-00106. Savannah River National Laboratory, Savannah River Nuclear Solutions, LLC 

Aiken, SC 29808, February 2012. 

Millings, M., Noonkester, J., and Bagwell, L. Memo to K. Rosenberger and M. Layton. 

Summary Dissolved Oxygen in Water Table Wells at SRS. SRNL-L3200-2011-00011. 

Savannah River National Laboratory, Savannah River Nuclear Solutions, LLC, Aiken, SC 

29808, December 2011. 

Summaries of Technical Reports

A summary of the key documents reviewed by NRC staff are provided in the following sections. 

Cantrell et al., 2012

To obtain expert technical advice related to a key FTF Performance Assessment (PA) modeling 

assumption (plutonium solubility in Tank 18) DOE assembled a peer review group consisting of 

plutonium chemistry experts. A number of areas for improvement were identified in the report, 

including conceptual model issues, solubility assumptions, quality assurance, and 

communication.
2 

Findings and recommendations were made in 5 major areas. The recommendations of the peer 

review group are summarized below. 

A. Solubility Controlling Phase/Thermodynamic Database: In the PA, plutonium in residual tank 

waste is assumed to be co‐precipitated with iron minerals in Region II, while solubility in Region 

III is assumed to be controlled by PuOH4. These assumptions are not consistent with current 

scientific understanding or supported by experimental data. It is recommended that the 

contractor employ a more technically valid model using amorphous PuO2·xH2O (am) as the 

solubility controlling phase. Additionally, the thermodynamic database that was used was not 

appropriate. A properly vetted and internationally accepted database should be used to 

calculate plutonium solubility, such as the Nuclear Energy Agency (NEA) database. 

B. Experimental Verification: The plutonium and grout chemistry assumptions need to be 

validated and verified where feasible. Residual tank waste should be characterized using 

advanced spectroscopic techniques. Solubility and release studies should be conducted on the 

waste to provide further support of plutonium and grout chemistry assumptions used for 

modeling. 

C. Conceptual Models: Sorption of plutonium in soils should be modeled allowing for elevated 

pH of groundwater as a result of grout leaching. Important geochemical parameters should be 

evaluated, including the concentrations of oxygen and carbon dioxide. In addition, the 

conceptual model of degradation being controlled by the diffusion of carbon dioxide into the 

grout seems like a low probability scenario. Fracturing of the grout with preferential flow through 

the cracks would appear to be much more likely and should therefore be evaluated. 

D. Quality Assurance/Quality Control (QA/QC): The plutonium solubility peer review group 

stated that they found no evidence of a credible quality assurance program for geochemical 

modeling during their on-site review. A credible QA/QC program for geochemical modeling is 

needed to provide confidence in the PA model results. 

E. Communications: The reviewers found a lack of coordination and project integration that led 

to a lack of communication regarding important model assumptions and model approaches 

(e.g., plutonium chemists were not involved in the selection of thermodynamic data or peer 

reviews). 

Hobbs, 2012 and Hay et al., 2012

The report by Hobbs (SRNL-STI-2012-00106) incorporates results of the solid-phase tank waste 

analyses from Hay, et al. (2012) in SRNL-STI-2012-00123. Hobbs reviews the history of the 

use of Tank 18 and discusses the forms that plutonium could take in the waste solids as a result 

of the major elements in the waste and the treatments (primarily neutralization with NaOH) that 

it received. The author states that the oxidation state of plutonium, entering the tanks, should 

be plutonium (IV) and that most of the plutonium is precipitated as amorphous plutonium 

hydroxide, Pu(OH)4(am) or hydrous plutonium oxide, PuO2(am,hyd) and coprecipitated with iron and 

other major elements in the waste. The author also discusses that Tank 18 waste removal 

operations likely removed the more soluble salts and the plutonium associated with these salts. 

Recent Scanning Electron Microscopy (SEM) analyses disclosed small discrete plutonium 

particles, associated with particles of an iron oxide matrix (SRNL-STI-2012-00123). Hay et al. 
3 

(2012) discusses that these plutonium particles do not appear to account for all of the plutonium 

in the sample, which was determined by chemical analysis. Consequently, the authors 

conclude that the plutonium appears to be present as both discrete particles and distributed 

throughout the iron oxide matrix. 

In 2009, samples were collected from Tank 18 during the final heel removal. Of particular 

interest are the concentrations of plutonium in the liquid phase that was isolated from the 

samples. The concentrations of plutonium for the north and south hemisphere samples 

measured 1.3 x 10-8 and 2.6 x 10-8 M, respectively. The author states that these concentrations 

are well above the predicted solubilities for PuO2(am,hyd) and coprecipitated plutonium (IV). The 

high solubility values are attributed to possible presence of colloidal plutonium that passed 

through the 0.45 μm filter or the presence of a plutonium carbonate phase. The high pH of the 

waste and active venting of the tank to relieve hydrogen accumulation resulted in the possibility 

of carbonate accumulation in the waste. Hobbs (2012) states that carbonate is an effective 

complexing agent for plutonium (IV) and several studies have shown that plutonium carbonate 

solids as well as aqueous complexes could form, thereby increasing the solubility of plutonium. 

Recent X-Ray Diffraction (XRD) analysis is consistent with the presence of carbon dioxide in 

solution as Na4UO2(CO3)3 has been observed as a major crystalline component of the Tank 18 

solids (SRNL-STI-2012-00123). Hobbs (2012) states that a portion of the plutonium has likely 

converted to a hydroxy-carbonate complex, but that upon interaction with grout (i.e., an increase 

in pH and decrease in carbonate concentration) the plutonium carbonate phases are expected 

to transform back into PuO2(am, hyd). 

Hobbs (2012) discusses that as precipitated solids age, they undergo Ostwald ripening and 

become more crystalline. The author states that this has been shown to occur for plutonium 

precipitates with a related lowering of solubility by about two orders of magnitude. According to 

Hobbs (2012), the use of grout in the tank can be expected to convert PuO2(am, hyd) into a more 

crystalline PuO2 phase. The author goes on to state that it would be an extremely slow process 

and the amorphous forms of plutonium would not be expected to completely convert into 

crystalline form due to the continual presence of water, reactions with products from the 

radiolysis of water, and low temperatures. 

The Hobbs report concludes that there are three dominant forms of plutonium present in the 

Tank 18 residual waste: PuO2(am, hyd), plutonium coprecipitated primarily with iron oxides, and as 

PuO(CO3)(am, hyd). However, Hobbs recommends X-ray Adsorption Spectroscopy (XAS) should 

be used to assess the form of plutonium in the waste. Hobbs recommends that if plutonium 

carbonate is observed, experiments should be conducted to determine if these species are 

converted back into PuO2(am, hyd) upon contact with grout. 

Hobbs recommends that other tanks should be evaluated for specific conditions or operations 

that may alter the form of plutonium present in the residual waste. Depending on this history, 

characterization of the form of plutonium present in the waste solids would be appropriate. The 

report by Hay et al.(2012) noted that XAS could be used to determine plutonium oxidation state 

and local atomic coordination. The authors also discussed that leaching and solubility tests 

could provide information on plutonium speciation. 
4 

Millings et al. (2011)

In support of FTF PA geochemical models of infiltrating water, this study compiled available 

SRS data on dissolved oxygen concentrations at the water table. The dissolved oxygen in the 

water table wells at SRS typically ranged from 5.6 to 8.9 mg/L, which was "slightly less than the 

values expected for fully saturated fresh surface water (8 to 10 mg/L).” This study concluded 

that an assumption of equilibrium with atmospheric oxygen in DOE’s grout evolution models 

(e.g., SRNL-STI-2012-00087) “is reasonable, if not slightly conservative.” The authors also 

noted that because the FTF is in the vadose zone, the tanks will be exposed to a partial 

pressure of oxygen that is between atmospheric and the partial pressure in equilibrium with the 

groundwater dissolved oxygen concentrations. 

Denham (2012)

Denham (2012) provides updated modeling of Eh-pH transitions in aging grout and on the 

plutonium waste release model. The conceptual approach to contaminant release is consistent 

with geochemical modeling in the FTF PA. However, this modeling uses internationally 

accepted thermodynamic data, as recommended by the plutonium peer review group (LA-UR12-00079), includes an updated grout formula, and considers the results of solid phase analysis 

of Tank 18 heels by Hay et al. (2012). Denham estimates the solubility of plutonium that is 

assumed to be present as discrete PuO2(am, hyd) particles and coprecipitated with iron in the 

residual waste. The estimated solubilities for PuO2(am, hyd) and coprecipitated plutonium under 

the assumed chemical conditions are reproduced from Denham (2012) in Table 1. These 

plutonium solubilities are characterized in the report as bounding values. The lower solubility 

bound is captured by the apparent solubility of plutonium coprecipitated in the iron matrix. 

Denham also states that the upper solubility bound is represented by PuO2(am, hyd) in aqueous 

solution equilibrated with dissolved oxygen. A series of stability diagrams indicate that for Eh 

values less than about +0.45 V, plutonium solubility will be relatively low. However, plutonium 

solubility is highly sensitive to Eh and increases significantly above an Eh of +0.45 V. The 

author also discusses uncertainties associated with the grout simulations and plutonium 

solubility estimates. 

Table 1. Estimated Plutonium Solubilities (adapted from Denham, 2012) 

Chemical 

Condition 

PuO2(am, hyd)

(mol/L) Maxa 

PuO2(am, hyd) (mol/L) 

More Realisticb

Coprecipitated 

(mol/L) 

Reduced Region 

II 

3.2E-11 3.2E-11 3.0E-14 

Oxidized Region II 5.2E-8 3.2E-11 2.5E-13 

Oxidized Region 

III 

7.8E-8 3.2E-11 5.0E-15 

a. Eh values in equilibrium with dissolved oxygen 

b. More realistic Eh values 

Denham states that the largest uncertainties include the nature of the grout and the plutonium in 

the future. The geochemical modeling in this report assumes that the grout is in an intact 

porous medium and that the bounding effects of fast flow paths are addressed within the 

performance assessment. As previously discussed, the nature of plutonium when the tank liner 

fails is considered to be discrete PuO2(am, hyd) particles and plutonium coprecipitated with iron. 

The report also discusses other forms of plutonium that may exist in the tanks. Recent 
5 

characterization of Tank 18 residual waste samples indicated that a plutonium carbonate phase 

may be present, based on observed concentrations of plutonium in water separated from the 

Tank 18 waste samples and the identification of a uranyl carbonate phase by XRD. However, 

Denham argues that the form of the discrete plutonium particles will be PuO2(am, hyd) based on its 

thermodynamic stability, relative to plutonium carbonate phases. Denham also discusses that 

the solubility of PuO2(am, hyd) may decrease due to dehydration and/or crystallization, although the 

solubility is unlikely to reach that of crystalline PuO2. 

In SRNL-STI-2012-00087, Denham discusses additional uncertainties, including pH and grout 

mineralogy. For Reduced Region II, the solubility of plutonium was insensitive to pH above 

approximately a pH of 9. Depending on the assumed Eh for Oxidized Region II, plutonium 

solubility can be sensitive to pH. At an Eh of +0.24V, which Denham describes as the more 

realistic Eh, the solubility of plutonium is constant at 3.2E-11 mol/L. At an Eh of +0.56V, 

plutonium solubility increases by approximately 3 orders of magnitude to 5.2E-8 mol/L. The 

behavior of plutonium solubility in Oxidized Region III is similar to that of Oxidized Region II. At 

an Eh of +0.29V and +0.68V, the solubility of plutonium is 3.2E-11 mol/L and 7.8E-8 mol/L, 

respectively. The assumed mineralogy is noted as influencing the pH transition. 

According to this modeling, a major control on the pH transition from 11.1 to 9.2 is the amount of 

hydrous calcium silicates present. This in turn is influenced by the quantity of dissolved silica in 

the infiltrating water and the type of silica in the grout. This modeling also demonstrates that 

even though silica is important to pH, it has little impact on dissolved plutonium concentrations. 

Teleconference

On July 26, 2012, the NRC staff and representatives from DOE participated in a teleconference 

to discuss the staff's request for additional information pertaining to the evolution of solubility, in 

particular plutonium, of Tank 18 residual waste. This section provides an overview of the 

teleconference; the complete summary of the teleconference is available in Agencywide 

Documents Access and Management System (ADAMS) (ML12236A370).

NRC staff requested information regarding DOE plans for conducting (i) solubility and leach 

tests recommended by the Plutonium solubility peer review group (LA-UR-12-00079) and NRC 

in its Technical Evaluation Report (TER) (ADAMS accession number ML112371751), (ii) SRNL 

recommended solid phase analysis to determine the presence of plutonium (Pu) carbonate in 

the waste (SRNL-STI-2012-00106, SRNL-L3100-2012-00017), (iii) tests to determine the ability 

of plutonium carbonates to transform to less soluble forms upon contact with grout (SRNL-STI2012-00106), and (iv) other test related to waste release (e.g., chemical evolution of waste zone 

and resultant changes in solubility). DOE indicated that PA maintenance is required by DOE 

Order 435.1. FTF PA maintenance items take into consideration NRC TER recommendations. 

Due to limited resources, feedback is needed on prioritization of activities. In reference to 

previous NRC staff interest in obtaining tank waste samples for analysis, DOE also expressed 

an interest in leveraging NRC resources to perform experimental analyses. DOE noted that it 

has almost 1 kg of individual and composite samples from Tank 18 that could be used for 

experiments. 

Based on sensitivity of modeled plutonium solubility to Eh, NRC also requested clarification for 

the basis of the assumed Eh values. For example, according to model predictions in Denham 

(2012), the solubility of the assumed plutonium solid phase, PuO2(am,hyd), remains relatively low if 

the Eh is less than +0.45 V. The solubility was shown to increase by approximately three orders 
6 

of magnitude if the Eh of the water is greater than +0.45 V. For “realistic” plutonium solubility 

modeling, DOE interpolates between a series of SRS groundwater data points and a 

hypothetical cement at pH 12.5 (Figure 9 in SRNL-STI-2012-00087). This estimation results in 

Eh less than +0.45 V at pH 11.1 and 9.2, and relatively low PuO2(am,hyd) solubilities. DOE 

indicated that the assumption regarding the linearity of Eh as a function of pH is consistent with 

Krupka and Serne (NUREG/CR-6377) and that the extrapolation of Eh between cement and 

natural system groundwaters does not reflect any particular Eh couple. DOE stated that the Eh 

of groundwater is oftentimes found to be significantly lower than one would assume based on 

an assumption that the groundwater is in equilibrium with oxygen. This is due to the influence of 

mineral phases and Eh couples present in soils and cementitious materials. 

The mineral phases that are used to represent the grout (i.e., the normative grout mineralogy) in 

the geochemical model affect the timing and magnitude of the predicted chemical transitions. 

NRC staff requested information on the validation of the assumed normative grout mineralogy 

by comparison to actual SRS grout analyses. Denham (2012) discusses that the initial 

mineralogy was estimated from the proposed grout formulation and representative phases used 

in published cement simulations, since the actual mineralogy is unknown. In the 

teleconference, DOE stated that they have not attempted to validate the assumed normative 

mineralogy. 

NRC Evaluation

DOE assembled a plutonium solubility peer review group that addressed solubility and modeling 

assumptions in the FTF PA related to the residual plutonium in Tank 18. The peer review group 

provided several recommendations that are consistent with those suggested by NRC. The peer 

review group also raised additional concerns, such as the use of an outdated and inconsistent 

thermodynamic database, lack of quality assurance and control that called into question FTF PA 

results, and the lack of coordination and project integration across lines and contractors. In 

response to recommendations made by the NRC and the plutonium solubility peer review 

group, DOE has conducted spectroscopic analyses of residual waste samples from Tank 18 

and updated the geochemical modeling for Tank 18. 

The SEM and XRD analyses of Tank 18 residual waste by Hay et al. (2012) demonstrate the 

importance of characterizing residual waste. Information contained in Hay et al. (2012) and 

Hobbs (2012) indicates that the form of plutonium in the Tank 18 heels is inconsistent with the 

assumptions in the FTF PA. The FTF PA assumes that plutonium is initially coprecipitated with 

iron and constrained to very low aqueous solubility for tens of thousands of years. If this were 

the case, then plutonium should be dispersed within the precipitated iron oxide mass. However, 

the SEM analysis disclosed that a fraction of the plutonium was present as small discrete 

particles, suggesting that plutonium is present both as a unique plutonium phase and distributed 

within the iron matrix. Although the concentration of the crystalline plutonium phase(s) was 

below the XRD detection limit, a uranyl carbonate phase, Na4UO2(CO3)3, was detected. The 

presence of Na4UO2(CO3)3 suggests that plutonium in Tank 18 residual waste may have also 

been converted into a more soluble carbonate phase. 

Analytical results from residual waste liquid samples showed plutonium concentrations of 1.3 x 

10-8 and 2.6 x 10-8 M (Hobbs, 2012). These measured Pu concentrations in washed Tank 18 
7 

residuals are greater than initial solubilities assumed in DOE’s revision 1 PA1

 and comparable to 

what is described as “conservative” solubilities in DOE’s Tanks 18 and 19 Special Analysis or 

SA (SRR-CWDA-2010-00124). Simulations using “conservative solubilities” in DOE’s Tanks 18 

and 19 SA lead to risk-significant peak doses of around 2 and 5 mSv/yr (200 and 500 mrem/yr)2

. 

Additionally, the measured Pu concentrations could have been higher if dissolved Pu was not at 

equilibrium at the time of analysis. 

Hobbs (2012) discusses that any plutonium carbonate phases, which formed during active 

venting, are expected to transform back into PuO2(am, hyd) under grouted conditions. Denham 

(2012) provides thermodynamic evidence that at the time of tank closure, PuO2(am, hyd) will be the 

stable phase under the assumed conditions. The subsequent solubility calculations for 

plutonium are based on the assumption that PuO2(am, hyd), not plutonium carbonate species, are 

present. However, based on the potential risk significance of plutonium solubility, the 

thermodynamic potential may not be a sufficient basis. Although PuO2(am, hyd) may be 

thermodynamically favored, several factors may inhibit or preclude transformation of plutonium 

carbonate species to less soluble forms. The assumption that plutonium carbonate species will 

convert to PuO2(am, hyd) could be undermined in several scenarios, including (i) thermodynamic or 

kinetic limitations to PuO2(am, hyd) formation, or (ii) greater-than-assumed carbonate ion 

concentrations due to infiltrating water bypassing the grout or water table rise. Accordingly, 

NRC staff disagree that the values reported in Table 1 (Denham, 2012) for PuO2(am, hyd) and 

coprecipitated plutonium under Reduced Region II represent bounding solubilities for plutonium 

based on observation of higher aqueous phase Pu concentrations in washed Tank 18. 

Therefore, depending on the amount of plutonium that has reacted with carbonate, the 

assumption of plutonium transformation under grouted conditions may be risk significant. 

Monitoring factor 2.1 in NRC staff’s FTF monitoring plan (2013) addresses the transformation of 

plutonium carbonate species to PuO2(am, hyd).

Hobbs (2012) suggests that other methods (e.g., XAS) for solids analysis would provide 

valuable information on plutonium chemistry in the waste. Hobbs also recommends that if the 

presence of plutonium carbonate phase is confirmed, that experiments be conducted to 

determine if plutonium carbonates are transformed back into PuO2(am, hyd) upon contact with 

grout. NRC staff thinks that additional solid phase analysis and waste release experiments are 

necessary to define the speciation and forms of major radionuclides in the waste, which in turn 

are needed to better constrain estimates of radionuclide solubility and potential releases rates. 

These types of analyses should also be applied to the residual waste in other tanks that contain 

risk significant concentrations of radionuclides consistent with NRC staff recommendations in 

the FTF TER and monitoring plan (NRC, 2011; NRC, 2012). 

Denham (2012) addresses several recommendations made by the DOE plutonium solubility 

peer review group (Cantrell et al., 2011), groundwater data developed by Strom and Kaback 

(1992) and Millings et al. (2011) and analyses by Hay et al. (2012) and Hobbs (2012). In SRNLSTI-2012-00087, Denham states that the geochemical analyses assume that flow is through an 

intact grout matrix and that the impact of preferential flow paths is addressed in sensitivity 

analyses within the PA. The influence of fast flow paths is addressed in the PA as a low 

 

1

 The actual, dissolved plutonium concentrations measured in Tank 18 significantly exceed the modeled 

solubility of plutonium for Reduced and Oxidized Region II. The Pu solubility is assumed to be 4.1 x 10-12

M for Reduced Region II and 4.0 x 10-14 M for Oxidized Region II. Region II conditions are assumed to 

last from the time of closure until approximately 30,000 years in the future for Tank 18. 2

 Differences in peak dose are based on differences in assumed natural system Kd. 
8 

probability scenario; however, NRC staff agrees with the plutonium solubility peer review group 

comment that fracturing of the grout with preferential flow through the cracks is much more likely 

than flow through an intact grout matrix over very long time periods. The implementation of the 

NEA thermodynamic database in the geochemical modeling provides additional confidence in 

the model results. However, several key assumptions in Denham’s report need additional 

support. Monitoring factors 3.2 and 3.3 in NRC staff’s FTF monitoring plan (2013) address the 

likelihood and performance impact of preferential pathways. 

The updated geochemical modeling results (Denham, 2012) indicate that plutonium solubility is 

primarily influenced by Eh and pH to a lesser extent. The solubility of plutonium was predicted 

to increase by approximately three orders of magnitude for infiltrating water with an Eh greater 

than +0.45 V. Evaluation of whether the Eh of infiltrating water is above or below the critical 

threshold is important because Denham’s “realistic” Pu solubilities lead to significantly lower 

peak doses of limited concern. If “max” or “conservative”3

 Pu solubilities are used, then the 

peak dose is significantly higher (2 and 5 mSv/yr in SA sensitivity cases). There is significant 

uncertainty with respect to the Eh of infiltrating water and therefore, the likelihood that Eh will be 

above or below the critical threshold at which Pu solubility increases to risk-significant values. 

Given these uncertainties there may not be a good basis for labeling one Eh (or Pu solubility) 

“max”/”conservative” while another “realistic”. DOE uses data from Strom and Kaback (1992) to 

evaluate the likelihood and conservatism of Eh (and Pu solubilities) evaluated in the SA. 

Difficulty in selecting a “reasonable” range of assumed Eh and Pu solubility results from the 

following uncertainties: 

• Uncertainty in thermodynamic modeling calculations and the threshold Eh value of +0.45 

V where Pu solubility increases to risk-significant values 

• Uncertainty introduced by the small (eight) sample size of water table well data compiled 

by Strom and Kaback (1992) with one of eight measurements above the (uncertain) 

critical Eh threshold and other measurements potentially within the range of error 

• Uncertainty associated with the reported Eh measurements provided in Strom and 

Kaback (1992) 

• Uncertainty in the representativeness of the Eh data from Strom and Kaback (1992) for 

infiltrating groundwater 

As stated above, the measured water table well data are not necessarily representative of 

waters infiltrating the waste tanks. The Eh of the infiltrating water is a function of the DO and 

the mineral species interacting with the water. The Eh of the infiltrating water may be greater 

than that of the water table because of the different mineral phases present in the grout and 

vadose zone soils. In addition to the mineral phase differences, the DO is expected to be 

greater in the vadose zone, than in the water table. Millings et al. (2011) discussed that the 

tanks will be exposed to a partial pressure of oxygen between atmospheric oxygen and the 

partial pressure in equilibrium with the groundwater. For these reasons, NRC staff thinks 

additional information is needed to support the selection of Pu solubility based on the expected 

Eh and updated geochemical modeling. DOE should provide additional support for the 

assumption that the Eh of infiltrating water will remain below a critical threshold at which the 

solubility will increase to a risk-significant value considering the uncertainties listed above. 

 

3

 Instead of the description “max” used in Denham (2012), the Tanks 18 and 19 SA (SRR-CWDA-2010-

00124) uses the term “conservative Eh”. 
9 

The slow hydration kinetics of the grout and the use of equilibrium thermodynamic models 

contribute to model uncertainty. Denham (2012) discussed the uncertainty in the assumed 

silica phase, which could affect the timing of the pH transition. Although the release of 

plutonium in the current modeling by Denham does not show a significant sensitivity to pH 

changes, the transition of Region II to Region III could potentially be important, depending on 

future model assumptions. In addition to the uncertainty in the pH transition, recent research by 

Cantrell and Williams (2012) indicates that the use of pyrite or pyrrhotite to represent the 

reduced sulfur-bearing phase may lead to an overestimate of the Eh transition time. 

The transition from reduced to oxidized conditions is largely dependent on the reducing capacity 

of components within the blast furnace slag. In recent saltstone-related experiments, Cantrell 

and Williams (2012) observed reduced sulfur species in leachates. Although the experiments 

conducted by Cantrell and Williams were for a saltstone simulant, rather than a tank grout 

formulation, both materials use a similar blast furnace slag. These results suggest that a portion 

of the reducing capacity in the blast furnace slag used by DOE may be more soluble than 

assumed by the use of pyrite or pyrrhotite in geochemical modeling. Accordingly, the transition 

to oxidizing conditions could occur more quickly than assumed in the geochemical modeling by 

Denham if this sulfur phase(s) is leached from the system prior to reacting with oxygen. DOE 

should provide additional information regarding the potential early release of reduced sulfur 

species from the grout, which could be determined during the waste release experiments 

recommended by NRC staff and the plutonium solubility peer review group. 

Given the risk significance and uncertainty of the solubility of plutonium and consistent with 

previous NRC recommendations, as well as recommendations made by DOE's plutonium 

solubility peer review team (Cantrell et al., 2011) and other DOE experts, NRC continues to 

believe that experimental verification of modeled plutonium solubility under a range of chemical 

conditions potentially relevant to the contaminated zone should be undertaken as described in 

NRC staff’s monitoring plan (NRC, 2013) under monitoring factor 2.1. Tank wastes should be 

characterized using advanced spectroscopic techniques such as Scanning Electron Microscopy 

(SEM)/Energy Dispersive X-Ray Analysis (EDAX), X-Ray Diffraction (XRD), X-Ray Absorption 

Near Edge Structure (XANES) or Extended X-Ray Absorption Fine Structure (EXAFS). Leach 

tests and solubility tests should be conducted on the waste residuals and solid phase analysis 

should be repeated after the leach tests are completed. 

In addition to the experiments listed above, NRC finds that additional information is needed 

regarding (i) the assumed longevity of reducing conditions in the contaminated zone, and (ii) the 

assumption that the Eh of infiltrating groundwater remains below a critical threshold above 

which Pu solubility increases to risk-significant levels. These additional studies that are not 

specifically discussed in the FTF monitoring plan (NRC, 2013), but that are pertinent to closure 

of monitoring factors 2.1 and 2.2, are listed in the follow-up actions section. 

Follow-up Actions

Related to closure of monitoring factors 2.1 and 2.2 listed in NRC staff’s monitoring plan (NRC, 

2013), DOE should also perform the following actions: 

• DOE should provide additional information to support assumptions regarding longevity of 

reducing conditions in the contaminated zone. Recent studies (Cantrell and Williams, 

2012) suggest that the reducing capacity of the tank grout could be depleted much 
10 

earlier than assumed in the FTF PA (SRS-REG-2007-00002 Rev 1) and in more recent 

Pu solubility modeling performed for Tank 18 (Denham, 2012). Uncertainty in the 

normative mineralogy assumed in geochemical modeling should be considered under 

this action. 

• DOE should provide additional support for the assumption that the Eh of infiltrating water 

will remain below a critical threshold at which Pu solubility will increase to a risksignificant value (e.g., updated geochemical modeling indicates a dramatic increase in 

Pu solubility occurs at Eh greater than +0.45 V). Uncertainty in the critical threshold and 

the Eh of infiltrating groundwater should be considered under this action. 

Open Issues

There are currently no open issues. 

Conclusions

In 2011, NRC issued the FTF TER with the primary recommendation for DOE to conduct waste 

release experiments to increase support for several key modeling assumptions related to: (i) the 

evolution of pH and Eh in the grouted tank system over time; (ii) identification of HRR 

association with solid phases comprising the residual wastes; and (iii) solubility of HRRs under a 

range of expected conditions. NRC’s FTF monitoring plan (2013) provides additional details 

regarding these TER recommendations under monitoring factors 2.1 and 2.2. DOE has 

provided additional support for several waste release assumptions including convening a 

plutonium solubility peer review that provided expert technical advice related to residual 

plutonium in Tank 18, spectroscopic analyses of residual waste samples from Tank 18, and 

revising the geochemical modeling. However, these additional support activities have not 

obviated the need for experimental verification of modeled waste release. In fact, these 

activities have further illustrated the need for experimental support. Analyses of residual waste 

indicate that plutonium exists in the residual waste with a solubility that is potentially risksignificant and inconsistent with assumptions in the PA. In addition, geochemical modeling 

indicates that the predicted Eh threshold for increased plutonium solubility is within the range of 

observed Eh values. Given the risk significance of the solubility of plutonium and consistent 

with previous NRC recommendations, as well as recommendations made by DOE's plutonium 

solubility peer review group (Cantrell et al., 2011) and other DOE experts, NRC continues to 

believe that experimental verification of modeled plutonium solubility under a range of chemical 

conditions potentially relevant to the contaminated zone should be undertaken. 

NRC staff have identified two follow-up actions for DOE to provide additional information 

regarding the longevity of reducing conditions afforded by the tank grout considering uncertainty 

in the normative mineralogy and additional support for the assumption that the Eh of infiltrating 

water will remain below a critical threshold (e.g., +0.45 V identified in Denham (2012)) 

considering uncertainties in geochemical modeling and infiltrating groundwater Eh. 

 
11 

References

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DOE Savannah River Operations Office (DOE-SR), Los Alamos National Laboratory, New 

Mexico, December 2011. 

Cantrell, K.J. and Williams, B.D. Equilibrium Solubility Model for Technetium Release from 

Saltstone Based on Anoxic Single-Pass Flow through Experiments. PNNL-21723. Pacific 

Northwest National Laboratory, Richland, Washington, September, 2012. 

Denham, M. Evolution of Chemical Conditions and Estimated Plutonium Solubility in the 

Residual Waste Layer During Post-Closure Aging of Tank 18. SRNL-STI-2012-00087. 

Savannah River National Laboratory, Savannah River Nuclear Solutions, LLC, Aiken, SC 

29808, February, 2012. 

Hay, M.S., O’Rourke, P.E., and Ajo, H.M. Scanning Electron Microscopy and X-Ray Diffraction 

Analysis of Tank 18 Samples. SRNL-STI-2012-00123. March 2012. 

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