November 7, 2013 

MEMORANDUM TO: Gregory Suber, Chief 

Low-Level Waste Branch 

Environmental Protection 

 and Performance Assessment Directorate 

Division of Waste Management 

 and Environmental Protection 

THRU: Christepher A. McKenney, Chief /RA/ 

Performance Assessment Branch 

Environmental Protection 

 and Performance Assessment Directorate 

Division of Waste Management 

 and Environmental Protection 

FROM: Karen E. Pinkston, Systems Performance Analyst /RA/ 

Performance Assessment Branch 

Environmental Protection 

 and Performance Assessment Directorate 

Division of Waste Management 

 and Environmental Protection 

SUBJECT: TECHNICAL REVIEW: SOLUBILITY OF TECHNETIUM DIOXIDES IN 

REDUCING CEMENTITIOUS MATERIAL LEACHATES, A 

THERMODYNAMIC CALCULATION, DOCKET NO. PROJ0734 

On March 8, 2013, the U.S. Department of Energy (DOE) provided the subject report for review 

by U.S. Nuclear Regulatory Commission (NRC) pursuant to Section 3116(b) of the National 

Defense Authorization Act for Fiscal Year 2005. The subject report is available on NRC’s 

Agencywide Documents Access and Management System (ADAMS) at accession number 

ML13070A135. This report was reviewed in accordance with monitoring activities described in 

U.S. Nuclear Regulatory Commission Plan for Monitoring Disposal Actions Taken by the U.S. 

Department of Energy at the Savannah River Site Saltstone Disposal Facility in Accordance 

with the National Defense Authorization Act for Fiscal Year 2005, Revision 1 (ML13100A076). 

This technical review supports Monitoring Factors 5.02 Chemical Reduction of Tc by Saltstone

and 5.05 Potential for Short-Term Rinse-Release from Saltstone. 

CONTACT: K. Pinkston, FSME/DWMEP 

 (301) 415-3650
2 

The NRC staff’s conclusions based on its review of the subject report and associated references 

include: 

• The NRC staff agrees with the approach in Li and Kaplan (2013) of using measured values 

to support the input parameters used in a performance assessment along with using 

geochemical modeling to better understand the system and the potential uncertainty. 

• The NRC staff agrees with DOE’s conclusion that the release of technetium from saltstone 

under young and moderately aged reducing conditions is more realistically represented as 

being solubility controlled rather than controlled by sorption (Kd values). 

• The NRC staff agrees with DOE’s conclusion that the solubility of technetium (Tc) in the 

saltstone leachates appears to be controlled by a TcO2·xH2O phase, which is likely to be 

TcO2 1.6H2O. 

• The measured values for Tc solubility show variability between the two different studies 

(i.e., a two order of magnitude difference) not fully explained by the difference in pH in the 

two studies. 

• The determination of the solubility of Tc in moderately-aged cement based only on the 

Estes et al. (2012) data without consideration of the Cantrell and Williams (2012) data does 

not capture the uncertainty in this parameter and may be non-conservative. However, this 

potential non-conservatism may be counteracted by the use of data from experiments that 

had a higher pH, and consequently a higher solubility, than is expected for moderately-aged 

cement. 

• It is important for the uncertainty in the solubility of Tc to be captured, especially for 

moderately-aged cementitious materials since this may be one of the most risk-significant 

parameters in the model. 

• It is only appropriate to exclude the early stage cement from the performance assessment 

modeling if this assumption does not lead to an underestimation of the dose. 

• Neglecting the potential for leaching of the phases responsible for the reducing capacity of 

blast furnace slag may be non-conservative, based on observations of reduced sulfur 

species in leachates from saltstone simulant experiments. The slag is likely to contain 

several different reduced sulfur species that have different solubilities. 

Additional information is still needed to close Monitoring Factors 5.02 and 5.05. The NRC staff 

will continue to monitor information provided by DOE on those factors as it is developed. 
G. Suber 3 

For Monitoring Factor 5.02, the additional information needed includes: 

• The fraction, if any, of the Tc in saltstone that remains oxidized initially, 

• Additional model support on the ability of saltstone to maintain Tc in the reduced form over 

time, 

• An estimate of the potential impact of leaching of reduced sulfur phases on the evolution of 

saltstone reducing capacity, and a list of the dominant reduced sulfur species expected to 

be in the slag, 

• A performance assessment model that adequately considers the uncertainty in the Tc 

solubility, or additional experimental data to reduce this uncertainty. 

For Monitoring Factor 5.05, the additional information needed includes: 

• Experimental measurements of the early rinse-release of Tc and an analysis that this 

fraction released would not significantly affect the projected dose. Alternately, an analysis 

based on existing measurements that demonstrates that an early rinse release would not 

significantly affect projected peak doses. The analysis should consider the fraction, if any, 

of Tc that is either not fully incorporated into saltstone or not fully reduced by saltstone as 

well as potential effects of high ionic strength. 

The NRC staff expects to receive the Special Analysis (SA) for the Saltstone Disposal Facility 

(SDF) in the near future. As part of the NRC’s review of the SA, the NRC staff will continue to 

review these areas. The NRC staff will also review any additional that is related to these areas 

(e.g., information on the form of reduced sulfur in cementitious materials provided as part of the 

H-Tank Farm review). 

The staff’s technical review summary is enclosed for your use. 

Enclosure: 

Technical Review: Solubility of Technetium 

Dioxides in Reducing Cementitious Material 

Leachates, a Thermodynamic Calculation 
G. Suber 3 

For Monitoring Factor 5.02, the additional information needed includes: 

• The fraction, if any, of the Tc in saltstone that remains oxidized initially, 

• Additional model support on the ability of saltstone to maintain Tc in the reduced form over 

time, 

• An estimate of the potential impact of leaching of reduced sulfur phases on the evolution of 

saltstone reducing capacity, and a list of the dominant reduced sulfur species expected to 

be in the slag, 

• A performance assessment model that adequately considers the uncertainty in the Tc 

solubility, or additional experimental data to reduce this uncertainty. 

For Monitoring Factor 5.05, the additional information needed includes: 

• Experimental measurements of the early rinse-release of Tc and an analysis that this 

fraction released would not significantly affect the projected dose. Alternately, an analysis 

based on existing measurements that demonstrates that an early rinse release would not 

significantly affect projected peak doses. The analysis should consider the fraction, if any, 

of Tc that is either not fully incorporated into saltstone or not fully reduced by saltstone as 

well as potential effects of high ionic strength. 

The NRC staff expects to receive the Special Analysis (SA) for the Saltstone Disposal Facility 

(SDF) in the near future. As part of the NRC’s review of the SA, the NRC staff will continue to 

review these areas. The NRC staff will also review any additional that is related to these areas 

(e.g., information on the form of reduced sulfur in cementitious materials provided as part of the 

H-Tank Farm review). 

The staff’s technical review summary is enclosed for your use. 

Enclosure: 

Technical Review: Solubility of Technetium 

Dioxides in Reducing Cementitious Material 

Leachates, a Thermodynamic Calculation 

DISTRIBUTION (w/enclosure): GAlexander ARidge HArlt HFelsher 

ML13304B159 

OFC DWMEP:TR DWMEP:LA DWMEP:PM DWMEP:BC 

NAME KPinkston TMoon NDevaser CMcKenney

DATE 10/31/13 11/04/13 11/06/13 11/07/13 

OFFICIAL RECORD COPY
Enclosure 

Technical Review: Solubility of Technetium Dioxides in Reducing Cementitious Material 

Leachates, a Thermodynamic Calculation 

November 2013 

Reviewers: K. Pinkston, A. Ridge, and G. Alexander 

Document: SRNL-STI-2012-00769, Rev. 1, D. Li and D.I. Kaplan, Solubility of 

Technetium Dioxides in Reducing Cementitious Material 

Leachates: A Thermodynamic Calculation. Savannah River 

National Laboratory. February 1, 2013. ML13070A135. 

Other Key References: SRNL-STI-2012-00596, Rev. 0, S.L. Estes, D.I. Kaplan, and B.A. 

Powell, Technetium Sorption by Cementitious Materials Under 

Reducing Conditions. Savannah River National Laboratory. 

September 2012. ML12345A134. 

PNNL-21723, K.J. Cantrell and B.D. Williams, Equilibrium 

Solubility Model for Technetium Release from Saltstone Based on 

Anoxic Single-Pass Flow through Experiments. Pacific Northwest 

National Laboratory. September 2012. ML12345A267. 

Background

Saltstone is made by mixing liquid salt solution with cement, fly ash, and blast furnace slag. 

The blast furnace slag is included to create a reducing environment in order to decrease the 

mobility of redox sensitive radionuclides, such as technetium (Tc). Sulfide and ferrous iron 

provide most of the reducing capacity in the slag, but the water in contact with saltstone will be 

influenced mainly by the sulfur chemistry because iron has a low solubility at high pH values 

(Kaplan, 2008 [SRNS-STI-2008-00045]). The fly ash and Portland cement add additional 

reducing capacity (Roberts and Kaplan, 2009 [SRNL-STI-2009-00637]), which could contribute 

to the reduction and retention of Tc. The initial pH of saltstone is high (~12.5 to 13), though as 

water leaches through the saltstone, the pH value will decrease. Similarly, as water leaches 

through the saltstone and the saltstone is exposed to oxygen, the saltstone will become more 

oxidized and the Eh will increase. 

In the U.S. Department of Energy (DOE) 2009 Saltstone Performance Assessment (PA) 

(SRR-CWDA-2009-00017), Tc-99 was the largest contributor to the predicted dose. The 

solubility of Tc and its release from the saltstone wasteform into the environment is strongly 

influenced by Eh. Tc is relatively insoluble and immobile under chemically reducing conditions, 

but is mobile under oxidizing conditions. The solubility of Tc is also pH dependent. Technetium 

is less soluble at more neutral pH values than at the high pH values found in fresh cementitious 

materials. 

The technical review of “Solubility of Technetium Dioxides in Reducing Cementitious Material 

Leachates, a Thermodynamic Calculation” (Li and Kaplan, 2013) and associated references by 

the U.S. Nuclear Regulatory Commission (NRC) staff supports Monitoring Factors 5.02 

“Chemical Reduction of Tc by Saltstone” and 5.05 “Potential for Short-Term Rinse-Release from 

Saltstone” in the 2013 NRC monitoring plan for the Saltstone Disposal Facility (NRC, 2013d). In 

the 2013 monitoring plan, the NRC staff noted that the studies that had been done to 
2 

demonstrate Tc retention in saltstone included experimental artifacts that made it difficult to 

interpret the results. As described under Monitoring Factor 5.02, the NRC staff concluded that 

the NRC needed to determine that DOE has robust model support for the chemical reduction of 

Tc(VII) to Tc(IV) and that this reduced state is maintained under field conditions. Additionally, 

under Monitoring Factor 5.05, the NRC staff concluded that additional support was needed to 

demonstrate that the short-term rinse-release from saltstone during the first few pore volumes 

will not significantly affect peak doses. The subject report and its associated references provide 

additional information both on the chemical reduction of Tc by saltstone and the release of Tc, 

including the initial release of Tc during the first few pore volumes. 

Summaries of Technical Reports

Summaries of the key documents reviewed by the NRC staff are provided in the following 

sections. 

Li and Kaplan 2013 (SRNL-STI-2012-00769)

The objective of the subject report was to generate best estimates for technetium geochemical 

input values to support transport modeling by comparing thermodynamic modeling results and 

recent laboratory measurements. The thermodynamic modeling was performed using the 

Geochemist’s Workbench React module (Bethke, 2010). In this modeling, the authors 

calculated the speciation and solubility of TcO2·1.6H2O, TcO2·2H2O, and TcO2(c) in reducing 

saltstone leachates, and Portland cement leachates with the presence of pyrrhotite. The 

pyrrhotite was included as a surrogate to simulate the reductant present in blast furnace slag. 

The results of these calculations were compared to similar calculations performed using 

different databases (i.e., Denham and Millings 2012 [SRNL-STI-2012-00404], Wang et al 2009, 

and Cantrell and Williams 2012). The solubility values calculated using the databases from Li 

and Kaplan (2013) were similar to those calculated using the databases described in Denham 

and Millings (2012) and Wang et al. (2009). The values calculated in Cantrell and Williams 

(2012) were higher by a factor of 2 to 5. Overall, the calculated solubility of each TcO2·xH2O 

species varied by less than an order of magnitude (for a given pH value) regardless of which 

database was used. 

Based on the research described in this paper, the authors concluded that the solubility of 

TcO2 xH2O, likely TcO2 1.6H2O, controlled the Tc concentration in saltstone leachates as long 

as the Eh was less than -0.38 V. As the Eh increased above -0.35, TcO4

-

 became the dominant 

species and the solubility of Tc increased significantly. As part of this research, the authors 

developed guidelines for the selection of parameters related to the release of Tc from saltstone 

for use in PA modeling (Table 1). 
3 

Table 1: DOE Guidelines for Selection of Parameters in PA Modeling of Saltstone and 

Saltstone Disposal Structures* 

Parameter 

Value Selected Basis 

Reduced

Young Cement 

1st Stage (pH~12) 

6x10-7 mol/L 

solubility 

Average of experimentally derived values in 

Cantrell and Williams (2012) and Estes et al. (2012)

Moderately-Aged Cement 

2nd Stage (pH~10.5) 

1x10-8 mol/L 

solubility Experimentally derived value in Estes et al (2012) 

Aged Cement 

3rd Stage (pH~5.5) 0.5 mL/g Kd

Based on the assumption that the aged cement 

becomes oxidized 

Oxidized

Young Cement 

1st Stage (pH~12) 0.8 mL/g Kd Kaplan (2010) [SRNL-STI-2009-00473] 

Moderately-Aged Cement 

2nd Stage (pH~10.5) 0.8 mL/g Kd Kaplan (2010) 

Aged Cement 

3rd Stage (pH~5.5) 0.5 mL/g Kd Kaplan (2010) 

* Based on information in Table 11 in Li and Kaplan [2013] 

Prior to the subject article being published, it was reviewed by an expert panel and was revised 

to address the reviewers’ comments. The panel recommended that the authors check the 

validity of Geochemist’s Workbench modeling for saltstone leachates with high ionic strength 

and check the consistency between the report and previous reports on this topic. The reviewers 

also requested additional guidance for modelers on how to account for uncertainty associated 

with the recommended solubility and Kd values. The authors addressed the topic of high initial 

ionic strength by stating that the high ionic strength environment will last for only a very short 

period, so it is expected that the error associated with activity coefficients during this time will 

have little effect on predicted long-term geochemistry. The subject report indicated that DOE 

should model uncertainty in the cement geochemistry constants with the equations provided by 

Almond et al (2012, SRNL-STI-2011-00672) (i.e., a lognormal distribution with a range of 

0.25*Kd to 1.75*Kd). 

Estes et al. 2012 (SRNL-STI-2012-00596)

The objective of the study described in Estes et al. (2012) was to measure Tc-99 sorption to 

cementitious materials under reducing conditions. In this study, the sorption of Tc onto four 

different cementitious materials was measured over 319 days. The four cementitious materials 

were a 50 year old aged cement sample, simulated Vault 2 concrete, simulated saltstone, and a 

material containing 90% slag and 10% Portland cement. The sorption experiments were 

conducted under an atmosphere of 0.1% H2(g)/99.9% N2(g) with a palladium catalyst to 

maintain anoxic conditions. Prior to the experiment, the samples were not stored in an inert 

environment, so some amount of oxidation might have occurred during that time. After the 319-

day sorption step, oxic conditions were introduced and desorption was measured for a period of 

20 days. 
4 

The aqueous Tc concentration, calculated Kd value, and Eh and pH values were reported over 

the time of the sorption experiment. The pH values were high and reducing conditions were 

observed over the time of the sorption experiment for the simulated saltstone sample. Little or 

no loss of Tc was observed in the no solids control, so the reducing atmosphere did not appear 

to be contributing to Tc precipitation or sorption due to the chemical reduction of Tc. 

The aqueous concentrations of Tc were similar for the simulated saltstone samples independent 

of the amount of Tc added to the system, so the aqueous Tc concentrations appeared to be 

controlled by solubility rather than sorption. The aqueous concentration of Tc converged at a 

value of 10-8 M, and the aqueous Tc measured at end of the sorption experiment was found to 

be almost all Tc(VII). Geochemical modeling was performed using Geochemist Workbench to 

simulate the experimental conditions in this study. The modeled solubility values are 

comparable to the measured values, which also supported the aqueous concentrations of Tc 

being controlled by solubility, not sorption. 

During the desorption experiments, the introduction of oxygen resulted in the Eh value 

increasing and the remobilization of the Tc. The authors concluded that the Kd model would be 

appropriate for modeling oxidized systems where Tc(VII) is expected to be the dominant form of 

Tc. 

Cantrell and Williams 2012 (PNNL-21723) 

The goal of the research described in Cantrell and Williams (2012) was to develop a 

mechanistically-based conceptual model for the release of technetium from saltstone. An 

additional goal was to establish the impact of oxidation on the leachability of technetium from 

saltstone. 

In this research, the authors measured the leaching from simulated saltstone using a singlepass flow-through system under anoxic conditions. In the experimental set-up, the researchers 

wanted to minimize oxygen contamination without using hydrogen because it could act as a 

reductant. To accomplish this, a large saltstone-to-leachant ratio was used to minimize the 

impact of trace amounts of oxygen in the system. They also used heavy walled Teflon reactors 

and placed the reactors in a controlled atmospheric environment maintained at low oxygen 

concentrations to minimize oxygen contamination. The simulated saltstone was cured for 

30 days before being crushed and sieved. The saltstone was crushed and sieved in an 

environment that was exposed to air, so this step was done quickly to minimize oxidation. 

The concentration of Tc measured in the leaching experiments was initially approximately 

300 μg/L, and this concentration decreased with time. The Eh, pH, and the concentrations of 

sulfide and other anions and cations were also measured. The Eh conditions were reducing 

over the course of the experiments and the pH was in the range of 12.5-13. The results of 

leaching experiments were compared to thermodynamic modeling performed using the 

Geochemist’s Workbench software to evaluate the potential solubility controlling phases. The 

authors concluded based on the geochemical modeling that the Tc release from saltstone under 

anoxic conditions is controlled by TcO2 xH2O. They also concluded that the likely TcO2 xH2O 

species is TcO2 1.6H2O. 
5 

The reactors were sparged with pure oxygen after 84 days. One of the reactors was exposed 

for 30 minutes on one day, while the other reactor was exposed for 30 minutes on 3 consecutive 

days. The exposure of the saltstone to multiple large amounts of oxygen resulted in the 

formation of soluble TcO4

-

 and its subsequent release. The researchers also concluded that the 

reducing capacity of saltstone could overcome an exposure that occurred over a short time by 

reducing the Tc back to TcO2 xH2O. 

Technical Exchanges

On April 30, 2013, NRC staff held a teleconference with DOE to discuss the Li and Kaplan 

(2013) report (NRC 2013b). DOE staff provided additional information on the basis for the 

recommended solubility and Kd values for Tc, the reasons for modeling the reducing capacity as 

pyrrhotite, the uncertainty in measured Eh values, and the basis for differences between 

solubility measurements in Estes et al. (2012) and Cantrell and Williams (2012). 

During this teleconference, DOE described the basis for the values recommended in Table 11 of 

Li and Kaplan (2013) for the use in PA modeling. They explained that the values selected were 

based on experimental values. The geochemical modeling was performed to support their 

conclusions, but was not directly used in generating the recommended solubility and Kd values. 

DOE also provided additional details on the way that Eh was modeled in the geochemical 

modeling and the basis for using pyrrhotite to represent the reducing capacity provided by sulfur 

species in the saltstone. It was discussed that blast furnace slag is likely to contain several 

different reduced sulfur species. DOE stated that the use of pyrrhotite in their geochemical 

model is a proxy to account for the reducing capacity. 

The DOE and NRC staffs also discussed the uncertainty around the measured Eh value and the 

sensitivity of the form of Tc to the Eh. NRC staff noted that the measured Eh values reported for 

saltstone are similar to the value reported by Li and Kaplan at which the solubility of Tc 

increases dramatically (-0.35 V). DOE described the difficulties in measuring Eh and stated that 

an experimental measurement of -0.35 V is not meaningfully different than one of -0.38 (i.e., the 

value assumed in Li and Kaplan). However, DOE also stated that there is experimental 

evidence (e.g., Cantrell and Williams, 2012) that shows that saltstone contains a sufficiently 

reducing environment for Tc to be in its reduced and less mobile form. 

DOE also provided their hypothesis on the basis for the difference between the solubility 

measurements reported in Estes et al. (2012) and Cantrell and Williams (2012) (i.e., 9x10-9 M at 

pH 11.9 in Estes et al and 1.5x10-6 M at pH 12.66 in Cantrell and Williams). The environment 

used in the Estes et al. experiments contained 0.1% H2(g) and was more reducing. Additionally, 

the contact time was longer in the Estes et al. experiments than in the Cantrell and Williams 

experiments. 

A follow up conversation was held during the June 2013 onsite observation (NRC, 2013d). 

During this discussion, DOE provided additional information on the basis for the solubility value 

recommended for Tc for moderately-aged reduced cementitious materials, the potential effect of 

the initial high ionic strength in the saltstone pore solution on Tc solubility, the basis for which 

species is expected to control solubility, and how DOE intends to use the information in Li and 

Kaplan (2013) to support a PA. 
6 

The DOE staff stated that the recommended value for Tc for moderately-aged reduced 

cementitious materials was based on an experimental value provided in Estes et al. (2012) 

which was measured at a pH value comparable to the pH of moderately-aged reduced 

cementitious materials. 

The DOE staff described the conditions under which the Debye-Hückel equation used in the 

thermodynamic modeling to calculate activity coefficients based on the ionic strength is 

applicable. The DOE staff stated that this equation is valid for ionic strengths up to 1 M. The 

ionic strength of the pore solution in saltstone is expected to exceed this molarity during the first 

few pore volumes. The DOE staff also noted that the leaching experiments performed included 

the early time when the ionic strength is high, so these experiments provide information on the 

potential for a higher solubility at that time. 

The DOE staff explained that the sulfur was modeled as one species (pyrrhotite), but in reality 

there is an assemblage of sulfur-bearing species. Additionally, over time as water passes 

through saltstone, the sulfur will become pyrite, which is a more soluble species than pyrrhotite. 

DOE is developing a new Special Analysis (SA) that incorporates recently available technical 

data and modeling lessons learned from other activities to resolve uncertainties and address 

issues identified in the NRC TER (NRC, 2012). The DOE staff described the proposed SA 

approach for modeling the release of Tc. They stated that the saltstone was initially modeled as 

being reduced and moderately-aged (2nd stage) and having a Tc solubility limit of 1x10-8 mol/L. 

At later times, the Tc was modeled as having a Kd value of 0.5 mL/g, which represents oxidized 

aged cement (3rd stage). Oxidized moderately-aged saltstone was not explicitly included in the 

model because the Kd value for this (0.8 mL/g) is similar to the Kd value for oxidized aged 

cement and the use of the lower Kd value for oxidized aged saltstone is conservative. The NRC 

staff stated that it is only appropriate to exclude the young cement (1st stage) from the model if 

the properties of the young and moderately-aged saltstone are similar and if this simplification 

does not lead to an underestimation of the dose. 

The NRC staff asked if the model used in the SA accounted for the possibility that the reduced 

sulfur is present in a more soluble form and is leached out. The DOE staff stated that in the 

model, the reductant is only consumed by oxygen and the model does not include any leaching 

of the sulfur reductant. 

In response to an NRC question, the DOE staff stated that the Tc solubility limit would not be 

modeled probabilistically in the new SA, but sensitivity cases were run in which the solubility 

limit was increased and decreased by an order of magnitude. 
7 

NRC Evaluation

The subject report (Li and Kaplan, 2013) and associated references provide useful information 

to support NRC monitoring regarding Monitoring Factors 5.02 “Chemical Reduction of Tc by 

Saltstone” and 5.05 “Potential for Short-Term Rinse-Release from Saltstone” (NRC, 2013d). In 

particular, these reports provide useful information on the ability of saltstone to reduce Tc 

initially and the potential for a higher initial release from saltstone. These reports also provide 

useful model support for parameters used to model the release of Tc from the saltstone 

wasteform in a performance assessment of the Saltstone Disposal Facility (SDF). 

The NRC staff finds that the use of geochemical modeling to better understand the system and 

the potential uncertainty is also useful. The NRC staff agrees with DOE’s use of measured 

values to support the development of input parameters, especially given the considerable 

uncertainty in the geochemical modeling and the thermodynamic data. It is important for the 

experimental measurements to be done in an environment that is representative of the real 

conditions. In Estes et al. (2012), a small amount of H2(g) was used along with a palladium 

catalyst to remove oxygen from the system, which is not consistent with the actual system. 

Although a no-solids control in the Estes et al. (2012) experiments did not show reduction of Tc, 

indicating the H2(g) was not causing Tc reduction, the NRC staff has previously expressed 

concern that other DOE reports (Kaplan et al., 2011 [SRNL-STI-2010-00668]) have shown that 

even trace quantities of oxygen (e.g., 30 to 60 ppm) can cause measurable Tc oxidation in 

saltstone samples (NRC, 2013d). In response to the concern that the methods used by 

Estes et al. (2012) do not adequately represent field-emplaced conditions, which may include 

trace quantities of oxygen, DOE responded that the complete lack of oxygen in the system used 

by Estes et al. is expected to be representative of the conditions inside a block of saltstone, 

where DOE expects that even trace quantities of oxygen would be scavenged by the reducing 

capacity of the saltstone wasteform. 

Another aspect of adequately representing field conditions is to mimic the chemical composition 

of the as-emplaced saltstone. As-emplaced saltstone contains admixtures which could affect 

the release of key radionuclides. The experiments that have been conducted to support the 

solubility of technetium do not include all of the chemicals that have been used in the production 

of saltstone. Waste release experiments conducted on actual saltstone samples would provide 

important additional insight. As a follow-up action of the June 2013 onsite observation visit, the 

NRC staff requested information on all admixtures that are currently in use at the SDF 

(NRC, 2013c). 

Research by Lukens et al. (2005, PIT-MISC-0117) showed that a fraction of the Tc in saltstone 

simulant samples remained oxidized even under reducing conditions. Although it does not 

appear to the NRC staff that this persistent oxidized fraction of Tc will be modeled explicitly in 

DOE’s waste release model in the SA, the potential release of this more mobile Tc fraction may 

be captured implicitly. The solubility study conducted by Cantrell and Williams (2012) is based 

on saltstone simulant samples that would include this persistent oxidized fraction of Tc. The 

observed release of Tc under reducing conditions would include at least a portion of the 

oxidized technetium and would therefore be included in DOE’s waste release models. 

However, the Tc that was released in the Cantrell and Williams (2012) experiments may not 

include the entire oxidized fraction because in these experiments the leachate flowed around 

particles of crushed saltstone. In the real system, there may be flow through the saltstone 
8 

matrix itself that could result in the water contacting and releasing a higher fraction of the 

oxidized Tc. 

The NRC staff agrees with the conclusion that the release of Tc is likely to be controlled by 

solubility rather than sorption under reducing conditions. Understanding the solubility of Tc in 

saltstone and the factors that influence its solubility is important to understanding the potential 

dose from saltstone because the measured Eh values are very similar to those at which 

oxidation and mobilization of Tc is observed to occur. The leaching experiments described in 

the subject report and its references provide some evidence that the saltstone contains 

conditions that are sufficiently reducing to reduce Tc and decrease its mobility. Additional 

information is still needed regarding: (i) what fraction, if any, of the Tc in saltstone remains 

oxidized and (ii) the ability of saltstone to maintain the Tc in reduced and less mobile form. 

The NRC staff understands the modeling approach of using pyrrhotite as a surrogate in 

geochemical modeling to represent the reducing capacity of saltstone. However, it is important 

to understand the particular form that the sulfur is in to understand the potential for saltstone to 

maintain a reducing environment for a long period of time. If the sulfur is in a more soluble form, 

the sulfur may be leached out of the saltstone, which would decrease the amount of reducing 

capacity remaining in the saltstone. Recent research by Cantrell and Williams (2012) indicates 

that the use of pyrrhotite may non-conservatively underrepresent the solubility of the reduced 

sulfur phase(s) present in blast furnace slag. The authors observed reduced sulfur species 

present in the leachates from saltstone simulant experiments. DOE indicated that in its current 

special analysis, DOE assumes that the consumption of the reducing capacity is due only to the 

reaction with oxygen. If the reduced sulfur phase or phases in the blast furnace slag are 

sufficiently soluble that leaching of the reducing agent could remove reducing capacity at a rate 

that is significant compared to the rate at which the reducing capacity is removed through 

oxidation, DOE’s geochemical modeling of saltstone would overestimate the longevity of 

reducing conditions. Additional support is needed for the assumption that the reducing capacity 

of saltstone is not significantly affected by leaching of the reducing phases. 

The value recommended for the solubility of Tc in young reduced cement was based on the 

averaging of experimentally determined values from Cantrell and Williams, 2012 (1.5x10-6 M) 

and Estes et al., 2012 (9x10-9 M). The pH in the Cantrell and Williams (2012) experiments were 

in the pH range of 12.6 to 12.9 and did not show a trend with time. The pH in the Estes et al. 

(2012) experiments also did not show a trend with time and remained at a value of 

approximately 11.9. Based on the results of the thermodynamic calculations, the solubility of Tc 

is expected to decrease with pH, so it is expected that the solubility would be higher in the 

Cantrell and Williams (2012) experiments than in the Estes et al. (2012) experiments. However, 

the results of these experiments differs over two orders of magnitude, which is a larger 

difference than the models predict. For example, in Table 7 in Li and Kaplan (2013), there is 

approximately one order of magnitude difference between the solubility at pH 11.8 and 

pH 12.66. In addition to the pH and the hypotheses provided by DOE (i.e., the presence of 

hydrogen gas and the longer contact time in the experiments by Estes et al. led to lower 

measured solubility), the difference in observed solubility values in the Cantrell and Williams 

(2012) and Estes et al. (2012) reports may also be due to sample preparation (i.e., grinding 

under atmospheric conditions) in the Cantrell and Williams (2012) study. Although the 

differences in the solubility values between these two studies may be due to the aforementioned 

experimental artifacts, there is insufficient information in these reports for NRC staff to conclude 
9 

that the observed value of 1x10-6 mol/L in the Cantrell and Williams (2012) study is not 

representative of field conditions. Because of the large difference between the solubility 

measurements, there is significant uncertainty in the solubility value. 

Additionally, it appears that the very early releases observed by Cantrell and Williams (2012, 

Figure 3.1) and in Estes et al. (2012, Figure 3) were higher than the values on which the 

recommended value was based (i.e., the measured values at early times were a factor of 

approximately 2-4 times higher than the values used as the basis for the recommended values). 

The exclusion of the very early release measurements adds to the uncertainty in the 

recommended value. 

During the onsite observation, DOE indicated the value used for the solubility of Tc in 

moderately-aged cement was based on an experimental value in Estes et al. (2012) measured 

at a pH value that was comparable to the pH value in moderately-aged cement (i.e., ~10.5). 

However, according to the data for TR547 (i.e., the simulated saltstone sample that had a 

formulation comparable to that used for actual saltstone) in Figure 6 in Estes et al. (2012), the 

pH remained much higher (i.e., in the range of 11.5-12) than the pH of moderately-aged cement 

for the duration of that experiment. Based on this difference in pH values, it is not clear to the 

NRC staff that this data is applicable for moderately-aged conditions. However, geochemical 

modeling suggests that the use of solubility data taken at pH 11.5-12 would conservatively 

overrepresent the Tc solubility limit at pH 10.5. It would be useful for this measurement to be 

made under conditions which are comparable to those seen for moderately-aged cements to 

verify the results of the geochemical modeling. Additionally, the pH value in the Estes et al 

(2012) experiments is not significantly different than the pH value in the Cantrell and Williams 

(2012) experiments, so it is not clear why the data from Cantrell and Williams (2012) is not 

being considered in determining the solubility of Tc in moderately-aged cementitious materials. 

Because the Cantrell and Williams (2012) data were approximately two orders of magnitude 

higher than the data from Estes et al. (2012), not considering the data from Cantrell and 

Williams (2012) seems to underestimate the uncertainty in the solubility and may be nonconservative. 

It is difficult to evaluate the appropriateness of particular parameter values and method used to 

model the release of Tc from the saltstone wasteform without the context of the complete SA 

model. However, the assumption that the increased solubility observed for the young cement 

(1st stage) can be ignored and that the solubility can be modeled using the lower (2nd stage) 

value appears to be non-conservative. In the DOE 2009 PA (SRR-CWDA-2009-00017), a 

similar modeling assumption was made. However, in that PA, the Kd values for the 2nd stage 

cement were generally the same as the 1st stage or were lower (i.e., the moderate-age 

conditions would result in more Tc release). It is only appropriate to exclude the young cement 

(1st stage) from the model if the properties of the young and moderately-aged saltstone are 

similar or if this simplification does not lead to an underestimation of the dose. 

The NRC staff does not find the method proposed in Li and Kaplan (2013) to model uncertainty 

in the geochemical parameters in the cementitious materials to be appropriate. The 

recommended uncertainty distribution was based on uncertainty in subsurface Kd values and is 

not relevant to the uncertainty in solubility and sorption in the wasteform. The uncertainty in the 

subsurface Kd values stems from variability in subsurface soils, while the uncertainty in the 

wasteform is likely to be due to variability in the grout formulation and the curing conditions, as 
10 

well as variability in the grout components. During the technical exchanges, the DOE staff 

stated that sensitivity cases would be run in which the solubility (1x10-8 mol/L) is increased and 

decreased by an order of magnitude. Given that the range of measured values for the solubility 

is over two orders of magnitude (i.e., 1x10-8 to 1x10-6 mol/L), increasing and decreasing the 

solubility by an order of magnitude (i.e., 1x10-9 to 1x10-7) may not be adequate to capture the 

uncertainty in this parameter. 

Follow-Up Actions

No new follow up actions were identified during this review. The NRC staff will continue 

monitoring these monitoring factors as described in the NRC monitoring plan (NRC, 2013d). 

Open Issues

No open issues were identified during this review. 

Conclusions

The NRC staff’s conclusions based on its review of Li and Kaplan (2013) and associated 

references include: 

• The NRC staff agrees with the approach in Li and Kaplan (2013) of using measured values 

to support the input parameters used in a performance assessment along with using 

geochemical modeling to better understand the system and the potential uncertainty. 

• The NRC staff agrees with DOE’s conclusion that the release of technetium from saltstone 

under young and moderately aged reducing conditions is more realistically represented as 

being solubility controlled rather than controlled by sorption (Kd values). 

• The NRC staff agrees with DOE’s conclusion that the solubility of technetium in the saltstone 

leachates appears to be controlled by a TcO2 xH2O phase, which is likely to be 

TcO2 1.6H2O. 

• The measured values for Tc solubility show variability between the two different studies (i.e., 

a two order of magnitude difference) that is not fully explained by the difference in pH in the 

two studies. 

• The determination of the solubility of Tc in moderately-aged cement based only on the Estes 

et al. (2012) data without consideration of the Cantrell and Williams (2012) data does not 

capture the uncertainty in this parameter and may be non-conservative. However, this 

potential non-conservatism may be counteracted by the use of data from experiments that 

had a higher pH, and consequently a higher solubility, than is expected for moderately-aged 

cement. 

• It is important for the uncertainty in the solubility of Tc to be captured, especially for 

moderately-aged cementitious materials since this may be one of the most risk-significant 

parameters in the model. 
11 

• It is only appropriate to exclude the early stage cement from the performance assessment 

modeling if this assumption does not lead to an underestimation of the dose. 

• Neglecting the potential for leaching of the phases responsible for the reducing capacity of 

blast furnace slag may be non-conservative, based on observations of reduced sulfur 

species in leachates from saltstone simulant experiments. The slag is likely to contain 

several different reduced sulfur species that have different solubilities. 

The research described in Li and Kaplan (2013) and associated references provides useful 

information to support NRC monitoring regarding Monitoring Factors 5.02 “Chemical Reduction 

of Tc by Saltstone” and 5.05 “Potential for Short-Term Rinse-Release from Saltstone”. 

Additional information is still needed to close Monitoring Factors 5.02 and 5.05. The NRC staff 

will continue to monitor information provided by DOE on those factors as it is developed. 

For Monitoring Factor 5.02, the additional information needed includes: 

• The fraction, if any, of the Tc in saltstone that remains oxidized initially, 

• Additional model support on the ability of saltstone to maintain Tc in the reduced form over 

time, 

• An estimate of the potential impact of leaching of reduced sulfur phases on the evolution of 

saltstone reducing capacity, and a list of the dominant reduced sulfur species expected to 

be in the slag, 

• A performance assessment model that adequately considers the uncertainty in the Tc 

solubility, or additional experimental data to reduce this uncertainty. 

For Monitoring Factor 5.05, the additional information needed includes: 

• Experimental measurements of the early rinse-release of Tc and an analysis that this 

fraction released would not significantly affect the projected dose. Alternately, an analysis 

based on existing measurements that demonstrates that an early rinse release would not 

significantly affect projected peak doses. The analysis should consider the fraction, if any, 

of Tc that is either not fully incorporated into saltstone or not fully reduced by saltstone as 

well as potential effects of high ionic strength. 

The NRC staff expects to receive the SA for the SDF in the near future. As part of the NRC’s 

review of the SA, the NRC staff will continue to review these areas. The NRC staff will also 

review any additional that is related to these areas (e.g., information on the form of reduced 

sulfur in cementitious materials provided as part of the H-Tank Farm review). 
12 

References

C. Bethke and S. Yeakel, The Geochemist’s Workbench, Release 8, Reference Manual. 

University of Illinois, Urbana, Illinois. 2010. 

PIT-MISC-0117, W.W. Lukens, J.I. Bucher, D.K. Shuh, and N.M. Edelstein, Evolution of 

Technetium Speciation in Reducing Grout. Environmental Science and Technology, Vol. 39. 

pp. 8,064–8,070. 2005. ML052720213. 

PNNL-21723, K.J. Cantrell and B.D. Williams, Equilibrium Solubility Model for Technetium 

Release from Saltstone Based on Anoxic Single-Pass Flow through Experiments. Pacific 

Northwest National Laboratory. September 2012. ML12345A267. 

SRNL-STI-2009-00473, D.I. Kaplan, Geochemical Data Package for Performance Assessment 

Calculations related to the Savannah River Site, Rev 0. Savannah River National Laboratory. 

March 15, 2010. ML113320386. 

SRNL-STI-2009-00637, K.A. Roberts, K.A. and D.I. Kaplan, Reduction Capacity of Saltstone 

and Saltstone Components. Savannah River National Laboratory, Aiken, SC. 

November 30, 2009. ML100550015. 

SRNL-STI-2010-00668, D.I. Kaplan, M.S. Lilley, P.M. Almond, and B.A. Powell, Long-term 

Technetium Interactions with Reducing Cement Materials. Rev 0. Savannah River National 

Laboratory. March 15, 2011. ML111290356. 

SRNL-STI-2011-00672, P.M. Almond, D.I. Kaplan, and E.P. Shine, Variability of Kd Values in 

Cementitious Materials and Sediments, Savannah River National Laboratory. January 2012. 

ML12185A073. 

SRNL-STI-2012-00404, M. Denham and M.R. Millings, Evolution of Chemical Conditions and 

Estimated Solubility Controls on Radionuclides in the Residual Waste Layer During PostClosure Aging of High-Level Waste Tanks. Savannah River National Laboratory. August 2012. 

ML13078A192. 

SRNL-STI-2012-00596, S.L. Estes, D.I. Kaplan, and B.A. Powell, Technetium Sorption by 

Cementitious Materials Under Reducing Conditions, Rev 0, Savannah River National 

Laboratory. September 2012. ML12345A134. 

SRNL-STI-2012-00769, D. Li and D.I. Kaplan, Solubility of Technetium Dioxides in Reducing 

Cementitious Material Leachates: A Thermodynamic Calculation, Rev. 1. Savannah River 

National Laboratory. February 1, 2013. ML13070A135. 

SRNS-STI-2008-00045, D.I. Kaplan, K.A. Roberts, J. Coates, M. Seigfried, and S. Serkiz, 

Saltstone and Concrete Interactions with Radionuclides: Sorption (Kd), Desorption, and 

Reduction Capacity Measurements. Savannah River National Laboratory. October 30, 2008. 

ML090150234. 
13 

SRR-CWDA-2009-00017, Savannah River Remediation (SRR), Performance Assessment for 

the Saltstone Disposal Facility at the Savannah River Site. Savannah River Site. Aiken, SC. 

October 2009. ML101590008. 

U.S. Nuclear Regulatory Commission (NRC), Technical Evaluation Report for the Revised 

Performance Assessment for the Saltstone Disposal Facility at the Savannah River Site, South 

Carolina. NRC. Washington, DC. April 2012. ML121020140. 

U.S. Nuclear Regulatory Commission (NRC), NRC Staff Questions on Technetium Solubility 

Research Related to Monitoring of the Saltstone Disposal Facility. NRC. Washington, DC. 

June 26, 2013a. ML13211A329. 

U.S. Nuclear Regulatory Commission (NRC), April 30, 2013, Summary of Telephone 

Conference Call to Discuss Technetium Solubility Research Related to Monitoring of the 

Saltstone Disposal Facility. NRC. Washington, DC. July 3, 2013b. ML13171A127. 

U.S. Nuclear Regulatory Commission (NRC), U.S. Nuclear Regulatory Commission 

June 26-27, 2013, Onsite Observation Visit Report for the Savannah River Site Saltstone 

Disposal Facility. NRC. Washington, DC. August 23, 2013c. ML13199A434. 

U.S. Nuclear Regulatory Commission (NRC), U.S. Nuclear Regulatory Commission Plan for 

Monitoring Disposal Actions Taken by the U.S. Department of Energy at the Savannah River 

Site Saltstone Disposal Facility in Accordance With the National Defense Authorization Act for 

Fiscal Year 2005, Rev. 1. NRC. Washington, DC. August 30, 2013d. ML13100A076. 

L. Wang, E. Martens, D. Jacques, P. De Cannière, J. Berry, and D. Mallants, Review of Sorption 

Values for the Cementitious Near Field of a Near Surface Radioactive Waste Disposal Facility: 

Project Near Surface Disposal of Category A Waste at Dressel, NIRAS-MP5-03 DATA-LF(NF), 

Version 1, 2009.