June 4, 2015 

MEMORANDUM TO: Gregory F. Suber, Chief 

 Low-Level Waste Branch 

 Division of Decommissioning, Uranium Recovery 

 and Waste Programs 

 Office of Nuclear Material Safety 

 and Safeguards 

THRU: Christepher A. McKenney, Chief /RA/

 Performance Assessment Branch 

 Division of Decommissioning, Uranium Recovery 

 and Waste Programs 

 Office of Nuclear Material Safety 

 and Safeguards 

FROM: A. Christianne Ridge, Sr. Systems Performance Analyst /RA/

 Performance Assessment Branch 

 Division of Decommissioning, Uranium Recovery 

 and Waste Programs 

 Office of Nuclear Material Safety 

 and Safeguards 

SUBJECT: TECHNICAL REVIEW: OXIDATION OF REDUCING 

CEMENTITIOUS WASTE FORMS, DOCKET NO. PROJ0734 

The U.S. Nuclear Regulatory Commission (NRC) staff performed this review pursuant to 

Section 3116(b) of the National Defense Authorization Act for Fiscal Year (FY) 2005. This 

report was performed 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 (2013 monitoring plan) 

(ML13100A076). This technical review is related to Monitoring Factors (MF) 5.01 “Radionuclide 

Release from Field-Emplaced Saltstone,” MF 5.02 “Chemical Reduction of Tc by Saltstone,” MF 

5.03, “Reducing Capacity of Saltstone,” and MF 5.05 “Potential for Short-Term Rinse-Release 

from Saltstone.” 

CONTACT: A. Christianne Ridge, NMSS 

 (301) 415-567
G. Suber -2- 

NRC staff made the following conclusions based on this review: 

• A significant amount of chemical reduction of technetium (Tc) from Tc(VII) to Tc(IV) 

appears to occur within the first week of curing and additional chemical reduction may 

continue for months. However, no study has shown complete reduction of Tc(VII) in 

saltstone under conditions that are consistent with field-emplaced saltstone. 

• Different methods of quantifying reducing capacity yield different measured reducing 

capacities of saltstone and its components. The cerium (Ce) method (also called the 

Angus-Glasser method) may overestimate the amount of reducing capacity available to 

reduce Tc in saltstone. 

• It is unclear which components of saltstone dry premix or salt solution are responsible 

for the measured reducing capacity of saltstone. The identity of the components 

contributing reducing capacity is important to long-term predictions of the ability of 

saltstone to chemically reduce contaminants. 

• There is evidence for gas-phase transport of oxygen in laboratory samples of a reducing 

grout exposed to moist soil. 

• It is unclear how much reducing capacity is consumed during field-scale mixing, 

pumping, and pouring. Measurements of reducing capacity or studies designed to 

evaluate the progress of an oxidizing front in cored samples of field-emplaced saltstone 

may address this issue. 

• Independent experiments suggest Tc oxidation is not well correlated to consumption of 

reducing capacity in reducing cementitious wasteforms. 

• Short-term rinse-release may be a concern if significant saltstone oxidation occurs from 

gas-phase transport of oxygen into saltstone fractures prior to water infiltration to 

saltstone. Release of Tc that remains oxidized in saltstone (a “persistent oxidized 

fraction”) also could contribute to this short-term release, if a fraction of Tc remains 

oxidized. 

• There is evidence that once chromium (Cr) is oxidized in saltstone, it is not re-reduced 

by residual reducing capacity in saltstone if oxygen is present. The applicability of this 

result to Tc re-reduction is unclear. However, because Tc(IV) appears to be more 

readily oxidized in saltstone than Cr(IV), the result suggests if low levels of oxygen can 

keep Cr oxidized in areas of saltstone with residual reducing capacity, low levels of 

oxygen also could keep Tc oxidized in areas of saltstone with residual reducing capacity. 

The NRC staff will continue to monitor information provided by the U.S. Department of Energy 

(DOE) as it is developed. As described in the NRC 2013 monitoring plan, additional information 

is needed to close Monitoring Factors (MF) 5.01, 5.02, 5.03, and 5.05. The NRC staff updated 

those information needs based on the information in this review. 
G. Suber -3- 

For MF 5.01, “Radionuclide Release from Field-Emplaced Saltstone,” the additional information 

needed includes the following: 

• Measurements of radionuclide release from core samples of field-emplaced saltstone; 

and 

• An estimate of the oxidation of Tc in saltstone that could occur if oxygen preferentially 

reacts with Tc before consuming saltstone reducing capacity. This estimate may be 

affected by the degree of fracturing expected in field-emplaced saltstone as a function of 

time. 

For MF 5.02, “Chemical Reduction of Tc by Saltstone,” the additional information needed 

includes the following: 

• Experimental evidence supporting the degree of re-reduction of oxidized Tc that DOE 

predicts in mathematical models of Tc transport in saltstone; or 

• Further analyses to combine DOE sensitivity analyses showing the effects of nondepleting sources of oxygen and fractions of Tc entering the saturated zone when 

oxidized (i.e., not undergoing re-reduction). 

For MF 5.03, “Reducing Capacity of Saltstone,” the additional information needed includes the 

following: 

• Information to support the identity of the components of saltstone dry premix or salt 

solution that supply the measured reducing capacity in saltstone, the ability of these 

components to reduce Tc in saltstone under field conditions, and the expected evolution 

of the reducing capacity over time. 

For MF 5.05, “Potential for Short-Term Rinse-Release from Saltstone,” the additional 

information needed includes the following: 

• Experimental measurements of rinse-release of risk-significant radionuclides from 

saltstone and an analysis of the effect of the release on the projected dose; and 

• Analyses of the potential effect on dose of the fraction, if any, of Tc that is likely to be (1) 

rinsed from the surface of saltstone, (2) oxidized before advective flow of water through 

saltstone begins, or (3) not chemically reduced by saltstone. The analysis should also 

address potential effects of high ionic strength. 

The NRC staff is reviewing the U.S. Department of Energy (DOE) “FY2014 Special Analysis for 

the Saltstone Disposal Facility at the Savannah River Site” (ML14316A586). As part of the 

NRC’s review, the NRC staff will continue to review these areas. 

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

Enclosure: 

Technical Review 
G. Suber -3- 

• An estimate of the oxidation of Tc in saltstone that could occur if oxygen preferentially 

reacts with Tc before consuming saltstone reducing capacity. This estimate may be 

affected by the degree of fracturing expected in field-emplaced saltstone as a function of 

time. 

For MF 5.02, “Chemical Reduction of Tc by Saltstone,” the additional information needed 

includes the following: 

• Experimental evidence supporting the degree of re-reduction of oxidized Tc that DOE 

predicts in mathematical models of Tc transport in saltstone; OR 

• Further analyses to combine the effects of DOE sensitivity analyses showing the results 

of non-depleting sources of oxygen and certain fractions of Tc entering the saturated 

zone when oxidized (i.e., not undergoing re-reduction). 

For MF 5.03, “Reducing Capacity of Saltstone,” the additional information needed includes the 

following: 

• Information to support the identity of the components of saltstone dry premix or salt 

solution that supply the measured reducing capacity in saltstone, the ability of these 

components to reduce Tc in saltstone under field conditions, and the expected evolution 

of the reducing capacity over time. 

For MF 5.05, “Potential for Short-Term Rinse-Release from Saltstone,” the additional 

information needed includes the following: 

• Experimental measurements of rinse-release of risk-significant radionuclides from 

saltstone and an analysis of the effect of the release on the projected dose; and 

• Analyses of the potential effect on dose of the fraction, if any, of Tc that is likely to be (1) 

rinsed from the surface of saltstone, (2) oxidized before advective flow of water through 

saltstone begins, or (3) not chemically reduced by saltstone. The analysis should also 

address potential effects of high ionic strength. 

The NRC staff is reviewing the U.S. Department of Energy (DOE) “FY2014 Special Analysis for 

the Saltstone Disposal Facility at the Savannah River Site” (ML14316A586). As part of the 

NRC’s review, the NRC staff will continue to review these areas. 

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

Enclosure: 

Technical Review 

DISTRIBUTION: 

GAlexander KPinkston HArlt HFelsher TBrimfield 

ML15098A031 

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

NAME ARidge TMoon HFelsher CMcKenney 

DATE 4/20/15 4/29/15 4/30/15 6/4/15 

OFFICIAL RECORD COPY 
Technical Review: Oxidation of Technetium in Reducing Cementitious Material 

June 2015 

Reviewers: A. Ridge, G. Alexander, K. Pinkston, H. Arlt 

Documents Reviewed: 

Almond, P.M., Stefanko, D.B., and C.A. Langton, Effect of Oxidation on Chromium Leaching 

and Redox Capacity of Slag-containing Waste Forms. Cementitious Barriers Partnership. 

March, 2013. CBP-TR-2013-02. ML15105A170. 

Almond, P.M., and Kaplan, D.I., Distribution Coefficients (Kd) Generated from a Core Sample 

Collected from the Saltstone Disposal Facility. Savannah River National Laboratory. Aiken, SC. 

April 30, 2011. SRNL-STI-2010-00667. ML113320284. 

Cantrell, K.J. 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. PNNL-21723. ML12345A267. 

Langton, C.A. and P.M. Almond, Cast Stone Oxidation Front Evaluation: Preliminary Results for 

Samples Exposed to Moist Air. Savannah River National Laboratory. Aiken, SC. November 

2013. SRNL-STI-2013-00541. ML15106A902. 

Langton, C.A., Tc Oxidation in Slag-based Sodium Salt Waste Forms Exposed to Water and 

Moist Hanford Soil. Cementitious Barriers Partnership. August 2014. CBP-TR-2014-05. 

ML15105A171. 

Kaplan, D.I., M.S. Lilley, P.M. Almond, and B.A. Powell, Longterm Technetium Interactions with 

Reducing Cementitious Materials. Savannah River National Laboratory. Aiken, SC. March 15, 

2011. SRNL-STI-2010-00668. ML111290356. 

Pabalan, R.T., Alexander, G.W., and D.J. Waiting, Experimental Study of Contaminant Release 

from Reducing Grout. Center for Nuclear Waste Regulatory Analyses, San Antonio, TX. 

March 29, 2012. ML112089A319.

Painter, S.L., and Pabalan, R.T. Estimated Longevity of Reducing Environments in Grouted 

Systems for Radioactive Waste Disposal. Center for Nuclear Waste Regulatory Analyses, San 

Antonio, TX. 2009. ML101160513. 

Um,W., Jung, H.B., Wang, G., Westsik, J.H. Jr., and Peterson, R.A., Characterization of 

Technetium Speciation in Cast Stone. Pacific Northwest National Laboratory. November 2013. 

PNNL-22977. ML15105A154. 

Witmer, M. and Powell, B.A., Determination of Constituent Concentrations in Field Lysimeter 

Effluents, FY13 Final Report, Task 2: Lysimeter Leachate Chemistry. Savannah River 

Remediation, LLC. Aiken, SC. September 2013. SRR-CWDA-2013-00121. ML15105A159. 
- 2 - 

Background

Saltstone is made by mixing radioactive liquid salt solution with cement, fly ash, and Blast 

Furnace Slag (BFS). The BFS is included to create a reducing environment to decrease the 

mobility of redox sensitive radionuclides, such as technetium (Tc). In the U.S. Department of 

Energy (DOE) “FY2014 Special Analysis for the Saltstone Disposal Facility at the Savannah 

River Site” (Fiscal Year (FY) 2014 Special Analysis) (ML14316A586), Tc-99 was the largest 

contributor to the projected dose within 50,000 years of site closure1

. In accordance with 

NUREG-1854, “NRC Staff Guidance for Activities Related to U.S. Department of Energy Waste 

Determinations”, the time for which the 0.25 mSv/yr (25 mrem/yr) dose limit in §61.41 must be 

met is generally2 10,000 years. However, as described in the NRC Request for Additional 

Information (RAI) regarding the FY 2013 Special Analysis for the Saltstone Facility (NRC, 

2014a) (ML14148A153), the U.S. Nuclear Regulatory Commission (NRC) staff questions many 

of the DOE assumptions regarding the projected timing of the peak dose from Tc-99. 

The solubility of Tc and its release from saltstone into the environment is strongly influenced by 

Eh. Tc is relatively insoluble and immobile in its reduced form (i.e., Tc(IV)) but mobile in its 

oxidized form (i.e., Tc(VII)). This behavior is key to projecting the long-term ability of saltstone 

to immobilize Tc, because the Eh of water in contact with saltstone will rise as oxygen consumes 

the reactive fraction of chemically reducing phases in the saltstone. The solubility of Tc is also 

pH dependent. Technetium is less soluble at the pH characteristic of middle-aged cementitious 

material (approximately 10.5) than at the high pH values found in fresh cementitious materials 

(approximately 12.5) (NRC, 2014b) (ML13304B159). 

Different studies have come to different conclusions about how BFS contributes to the reducing 

capacity of saltstone. Um et al. (2013) (PNNL-22977) concluded both reduced sulfur and 

reduced iron species likely contribute to the reduction of Tc in samples of a reducing 

cementitious wasteform. Kaplan et al. (2008) (SRNS-STI-2008-00045) characterized ferrous 

iron (i.e., Fe(II)) as the most significant reducing agent contributed by BFS and sulfide as a 

precipitating agent that limits the solubility of several species, including Tc(IV). Arai and Powell 

(2014) (SRRA042328SR) also suggested that the reducing capacity contributed by BFS is 

attributable to ferrous iron species because they found that the sulfur in BFS oxidized to sulfate 

(SO4

2-) within hours of contacting a simulated salt solution. However, Cantrell and Williams 

(2012) (PNNL-21723) suggest it is unclear whether ferrous iron contributes significantly to the 

reduction of Tc because ferrous iron has a low solubility at high pH values. 

Some authors indicate fly ash should contribute additional reducing capacity to saltstone 

(Roberts and Kaplan, 2009 [SRNL-STI-2009-00637]). However, the reducing capacity 

attributable to fly ash depends on whether a Ce-based method3

 or a Cr-based method is used 

to measure the reducing capacity. The less acidic Cr-based method showed no measureable 

reducing capacity from fly ash (PNNL-22977). 

 

1

 I-129 caused the largest projected peak dose within 10,000 years of site closure. 2

 NUREG-1854 also indicates that assessments beyond 10,000 years may be necessary in some cases to 

demonstrate that the disposal of certain types of waste does not result in high impacts to future generations. 3

 Also referred to as the Angus-Glasser method.
- 3 - 

This technical review by the NRC staff is related to Monitoring Factors 5.01 “Radionuclide 

Release from Field-Emplaced Saltstone,” MF 5.02 “Chemical Reduction of Tc by Saltstone,” MF 

5.03 “Reducing Capacity of Saltstone,” and MF 5.05 “Potential for Short-Term Rinse-Release 

from Saltstone” in the 2013 NRC monitoring plan for the Saltstone Disposal Facility (NRC, 2013) 

(ML13100A076). In the 2013 monitoring plan, the NRC staff noted that the studies that had 

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

difficult to interpret the results. As described in the 2013 NRC monitoring plan under Monitoring 

Factor 5.01, the NRC staff concluded additional information is needed to show radionuclide 

release rates used in the DOE Performance Assessment (and subsequent Special Analyses) 

are reliable. Under Monitoring Factor 5.02, the NRC staff concluded that additional information 

is needed to support the modeled extent of Tc(VII) reduction to Tc(IV) by saltstone and the 

projected maintenance of Tc as Tc(IV) in saltstone. Under Monitoring Factor 5.03, the NRC 

staff concluded that additional information is needed to support the initial reducing capacity of 

saltstone and the expected evolution of redox conditions over time. Under Monitoring Factor 

5.05, the NRC staff concluded that additional support was needed to support the assumption 

that short-term rinse release of radionuclides from saltstone seen in laboratory experiments will 

not significantly affect projected peak doses from groundwater pathways at the SDF. 

The key references reviewed in this report provide additional information both on the chemical 

reduction of Tc by saltstone and the oxidation of the saltstone waste form. 

Summaries of Technical Reports

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

sections. 

Arai and Powell (2014) (SRRA042328SR)

This study used X-Ray Absorption Spectroscopy (XAS) to evaluate the speciation of Tc, sulfur, 

and iron in laboratory-prepared simulated saltstone samples. The study also included XAS 

measurements of speciation of sulfur species in saltstone leachate and BFS, as well as leaching 

tests of iron and sulfur from laboratory-prepared simulated saltstone samples. Available results 

include leaching and XAS results of simulated saltstone samples cured for 29 days in the 

presence of atmospheric oxygen. The study is ongoing and results from simulated saltstone 

samples cured for longer times are anticipated in the future. 

In samples cured for 29 days, the researchers found that Tc existed in significant fractions of 

both Tc(IV) and Tc(VII). Specifically, using X-ray absorption near edge spectroscopy (XANES) 

measurements in the layer closest to the exposed surface, researchers found 38 percent Tc(IV) 

and 62 percent Tc(VII). In a layer further from the surface, researchers found 41 percent Tc(IV) 

and 59 percent Tc(VII). However, the authors noted that the relative amount of oxidized Tc may 

have been affected by the high concentrations of Tc used in the study to accommodate the XAS 

detection limit (i.e., over 200 times the concentration expected in saltstone). 

XANES measurements of the oxidation state of sulfur in BFS showed that the sulfur in BFS was 

oxidized to SO4

2- rapidly (i.e., within hours) after exposure to the simulated salt solution. 

Um et al. (2013) (PNNL-22977)
- 4 - 

This study evaluated the reducing capacity and oxidation of laboratory-prepared samples of 

simulated Cast Stone, as well as Tc speciation in and release from Cast Stone under a variety 

of leaching conditions. Cast Stone is a cementitious waste form made of Portland cement, BFS, 

and fly ash, with a composition very similar to saltstone. The dry premix for Cast Stone has a 

cement:slag:fly ash ratio of 8:47:45, whereas Saltstone has a cement:slag:fly ash ratio of 

10:45:45. 

To evaluate the main contributors to the reducing capacity, the researchers evaluated each dry 

component of the Cast Stone with two different measurement methods. The cerium (i.e., 

Ce(IV)) method uses a very acidic solution (i.e. pH of approximately 1 to 2) whereas the 

chromium (i.e., Cr(VI)) method uses a pH near 7 (i.e., approximately neutral). For the slags and 

fly ash samples (Table 1), the Ce(IV) method resulted in significantly greater measured reducing 

capacities than the Cr(VI) method. The authors attributed this difference to the difference in pH 

between the two measurement methods. Because the Ce(IV) method uses a lower pH, it 

typically measures larger reducing capacity than the Cr(VI) method because solids dissolve 

completely. In contrast, the Cr(VI) method typically does not completely dissolve solids and so 

may measure only the reducing capacity available on surfaces. 

In addition, there is evidence that Cr(III) may react with Fe(III) to form a layer of Cr-Fe 

hydroxides that passivate the iron surfaces and prevent some of the reducing capacity of the 

ferrous iron from reacting. Despite this general trend, for the Portland cement sample the Cr(VI) 

method resulted in the greater measured reducing capacity than the Ce(IV) method. The 

authors suggested that the reducing capacity measured by the Cr(VI) method may have been 

overestimated because the Cr precipitated as Cr-carbonate phases. 

Despite the differences in the reducing capacities of the dry components measured with the two 

different methods (Table 1), the reducing capacities of Cast Stone samples were similar when 

measured with the two different methods. In addition, the measured reducing capacities of the 

simulated Cast Stone samples were higher than would be predicted if the dry components were 

the only source of reducing capacity in the Cast Stone. The authors suggest both results are 

explained by reducing capacity contributed by dissolved NO2

-

, which contributes similarly to the 

reducing capacity measured by either method. However, the authors stated they did not expect 

the reducing capacity attributable to NO2

- to effectively reduce Tc in Cast Stone because the 

reduction potential of the NO2

-

/ NO3

-

redox couple (approximately 0.01 millivolt) at the high pH of 

a cementitious wasteform is weak compared to the reduction potential of the Tc(IV) / Tc(VII) 

redox couple at high pH (approximately -0.36 millivolt). 
- 5 - 

Table 1. Adapted from Um et al (2013) (PNNL-22977). Reducing capacity of dry components 

of Cast Stone determined with two different measurement 

methods.

Almond et al. (2013) (CBP-TR-2013-02)

This study was designed to test a method of analyzing contaminant leaching and redox capacity 

of reducing grout as a function of the distance from an exposed surface of a chemically reducing 

cementitious wasteform. The study demonstrates a method of “depth-discrete” sub-sampling, 

which was used in subsequent studies. The term “depth-discrete” subsampling refers to slicing 

the sample into disks parallel to the exposed surface, and then studying each slice. 

The dry premix consisted of 10 percent Type I/II Portland cement, 45 percent Holcim BFS, and 

45 percent Class F fly ash. The dry premix was blended with a simulated salt solution at liquid 

to dry premix ratio of 0.60. After 21 days of curing in the laboratory, the researchers moved the 

samples to an enclosure in the field and exposed one surface of the samples to air for either 

118 or 302 days. 

After the air exposure, the authors took depth-discrete subsamples and crushed each one 

separately in contact with air. They split each subsample in half to measure leachable Cr in one 

half and residual reducing capacity in the other. Samples were leached with deionized, de-aired 

water for either 18 ± 2 hours or 28 days. 

The researchers found that the longer leaching time did not change the results. Leachable 

chromium was detected at greater depths in the sample exposed for 302 days than it was in the 

sample exposed to air for 118 days (at least 50 millimeters (mm) (2.0 inches) compared to at 

least 20 mm (0.79 inches)). They also found that the reducing capacity measured with the 

Ce(IV) method4

 was not a good predictor of the leachable Cr in each section. Once Cr was 

oxidized to Cr(VI), the authors found that it remained oxidized in regions of the samples where 

oxygen was present, even if residual reducing capacity was measured in those regions. 

 

4

 The authors referred to it as the Angus-Glasser method. 

Dry Ingredient 

Reducing Capacity 

Ce(IV) Method 

(meq/g) 

Cr(VI) Method 

(meq/g) 

Northwest Slag 0.793 0.078 

Southeast Slag 0.800 0.226 

Northwest Fly Ash 0.060 0.007 

Southeast Fly Ash 0.288 0.000 

Portland Cement (Type I/II) 0.042 0.349 
- 6 - 

Tc leached from depth-discrete

subsamples (mass percent)

Langton (2014a) (CBP-TR-2014-05)

This study examined Tc leaching from two laboratory-prepared chemically reducing grouts 

exposed to moist soil or deionized water. The grout samples were poured into sealed cylindrical 

containers with a height of 90 mm (3.5 inches) and a diameter of 35 mm (1.4 inches). The 

samples were isolated from air in the containers and cured for 113 days5

 at ambient 

temperature (about 22 C) and 65 percent to 75 percent relative humidity. After curing, the 

samples were removed from the sealed containers and coated with epoxy to isolate them from 

oxygen, and a fresh surface was cut to allow exposure from one end of the cylinder. The 

samples were exposed to either moist soil or deionized water for 154 days. After the exposure 

period, leaching was evaluated as a function of depth from the surface with depth-discrete 

subsampling. The subsamples were then each ground before leaching for 48 ± 2 hours in 

deionized, de-aerated water. 

The authors found that soluble Tc was leached from all of the depth-discrete subsamples from 

both samples (i.e., the sample exposed to deionized water and the sample exposed to moist 

soil). Although all depths had some fraction of leachable Tc, there was a front that separated 

areas of relatively more and less leachable Tc (Figure 1). The authors interpreted the 

30 

25 

Exposed in 

unsaturated 

20 Hanford 

 sediment 

15

 Exposed in 

10 deionized water 

5 

0 

0 20 40 60 80 

Distance from cut "fresh" surface (mm) 

Figure 1. Adapted from Langton (2014a) (CBP-TR-2014-0005). Tc leached from samples of 

Cast Stone exposed to moist soil and deionized water. 

 

5

 The text of CBP-TR-2014-05 states samples were cured for 103 days; however Table 5 of CBP-TR2014-05 states samples were cured for 113 days. The value from the table, 113 days, was used because 

it is consistent with the total sample age and days of exposure listed in Table 5 of CBP-TR-2014-05. 
- 7 - 

leachability of Tc along the entire length of the samples to suggest that oxygen had penetrated 

to parts of the entire length of both samples. The authors acknowledged that the small 

leachable fraction found throughout the sample could be an experimental artifact, because 

depth-discrete samples were crushed in contact with air prior to leaching. However, the authors 

also observed that, based on previous experiments (Langton et al., 2012), they would expect 

less than 5 percent of the Tc to oxidize during sample handling, as compared to more than 10 

percent of Tc that could be leached in all of the depth-discrete subsamples.

The authors found that the front of increased Tc leachability extended further into the sample 

exposed to moist soil than it extended into the sample exposed to deionized water. The authors 

concluded from this observation that the rate of oxygen migration into the sample exposed to 

soil was faster than the rate of migration into the sample exposed to water. The authors 

concluded that this observation was consistent with the more rapid transport of ions through a 

gas phase as compared to a liquid phase. 

Langton and Almond (2013) (SRNL-STI-2013-00541)

This experiment evaluated the oxidation of Cr and Tc in laboratory-prepared samples of Cast 

Stone, which has a formula similar to saltstone. Three samples are discussed in this summary, 

all of which were isolated from oxygen in sealed containers during curing at ambient 

temperatures (i.e., approximately 22 °C (72 °F)). The sealed containers were placed in a 

secondary container with moist towels to increase humidity (precise humidity values were not 

given). Of these three samples, one was spiked with Cr and two were spiked with Tc. The Cr 

spiked sample was cured for 63 days and exposed to moist air and Savannah River Site (SRS) 

ambient outdoor temperature for 68 days. One Tc-spiked sample was cured for 48 days and 

then exposed to moist air in a laboratory for 50 days. The other Tc-spiked sample was cured for 

10 days and immediately sampled, without intentional air exposure. 

For each of the three samples, the researchers cast the samples, and after curing, cut a fresh 

surface for exposure. The researchers then cut depth-discrete subsamples. In each 

subsample, they evaluated the leachability of Cr and Tc, as well as the reducing capacity as 

measured by the Ce(IV) method6

. Oxidation of Cr and Tc was surmised based on their 

leachability. The researchers observed that the front of increased Tc leachability extended to a 

greater depth than the front of increased Cr leachability. In addition, in the area of increased Tc 

leachability, Tc was more leachable than Cr was. From these results (Table 2), the researchers 

concluded that Tc was more readily oxidized in the reducing grout samples than Cr was. 

One of the key conclusions of the study was that the residual reducing capacity in a depthdiscrete subsample was not a good predictor of whether Tc or Cr at that depth was chemically 

reduced. That is, the authors found a poor correlation between Tc leachability in the depthdiscrete subsamples and the residual reducing capacity in the subsamples as measured by the 

Ce(IV) method. The authors suggested the correlation might be poor because oxygen entering 

 

6

 The authors referred to it as the Angus-Glasser method. 
- 8 - 

Table 2. From Langton and Almond (2013) (SRNL-STI-2013-00541). Fraction of constituents 

leached from depth-discrete samples of Cast Stone as a function of depth.

Cast Stone Sample Distance from 

Surface 

(mm) 

Weight Percent 

Leached 

Na - NO3 Tc-99 Cr

Cr500-1 

0 98 52 -- 0.5

1 111 79 -- 0.6

3 97 84 -- <0.01

Tc2 

Cured for 48 days 

and exposed for 50 

0 90 75 40 --

10 101 84 43 --

16 94 89 15 --

Tc 4-1 

Cured 10 days, 

No intentional 

exposure to air 

1 - 85 

Not 

measured 

Not 

measured 

5 Not 

measured 

the samples reacted preferentially with the Tc and Cr before consuming the residual reducing 

capacity of the wasteform. In addition, the researchers found that oxidized Tc and Cr co-existed 

with residual reducing capacity in areas of the samples to which oxygen had penetrated. The 

researchers suggested that this finding showed that residual reducing capacity is not effective in 

re-reducing oxidized Cr or Tc if oxygen is present. 

The authors also observed that in the sample cured for 10 days and not intentionally exposed to 

air, no Tc oxidation front was observed, but 5 percent of the Tc could be leached at each depth 

(Figure 2). The researchers concluded from this observation that 5 percent of the Tc remained 

oxidized after 10 days of curing, and suggested that additional testing at longer curing times 

would be needed to determine whether 100 percent of the Tc(VII) in Cast Stone could be 

reduced with longer curing times. However, the authors acknowledged that the oxidized fraction 

could be an experimental artifact because each depth-discrete subsample was crushed in 

contact with air prior to leaching. 

Painter and Pabalan (2009)

This study used a numerical model to estimate oxidation front movement by aqueous diffusion 

in near-surface slag-bearing grouted systems. The researchers did not include movement of 

dissolved oxygen by water advection through the intact matrix in the mathematical model; 

however, they did consider dissolved oxygen in water entering fractures. They also considered 

gas-phase oxygen entering unsaturated fractures in the grout. The researchers evaluated grout 

oxidation for a range of hydrological conditions, fracturing scenarios, and grout parameter 

values, including reducing capacity. The numerical model is based on the assumption that 

there is no damage to the grout matrix (i.e., no change in parameters affecting oxygen diffusion) 

behind the oxidation front. 
- 9 - 

 0.0 10 20 30 40 50 60 70 80 90 

Distance from Top of Sample (mm)

Figure 2. Adapted from Langton and Almond (2013) (SRNL-STI-2013-00541). Comparison of 

Tc-99 fraction leached as a function of distance from the top surface for a 10 day old and a 98 

day old (exposed to air) Cast Stone sample. 

The researchers also evaluated the effects of fracture saturation. As shown in Figure 3, certain 

assumed fracture characteristics allowed slight depletion of oxygen along the length of the 

fracture. The authors determined this effect was caused when the amount of water in the 

fracture, in combination with the size of the fracture aperture, was sufficient to limit diffusion of 

oxygen from the gas phase, but not sufficient to comparably increase advection of water 

through the fracture. However, the authors noted that in each case, the oxygen concentrations 

in the fractures returned to, or very close to, the steady-state concentration in the surrounding 

soil. The authors concluded this result occurs because oxygen transport in the fracture is fast 

compared to the diffusive loss of oxygen to the porous grout matrix. One of the central 

conclusions of the paper is that for a range of fracture characteristics, grout oxidation is limited 

by the diffusion rate in the grout. In addition, based on the approach to steady-state conditions 

shown in Figure 3, the authors observed that for the long time frames of interest in performance 

assessment, gas-phase diffusion and liquid-phase advection in the fractures can resupply the 

fractures with oxygen from the surrounding soil. 

The authors also suggested that because diffusion in the pore space of intact grout material is 

expected to be the rate-limiting process for grout oxidation, detailed process-level models 

coupling fracture transport processes with matrix diffusion and chemical reactions are not 

needed to adequately project grout oxidation. Instead, fractures can represent internal Percent Tc Leached

50 

70 

60 

40 

30 

20 

10 

Cured 48 days, exposed to air 50 days 

Cured 10 days, no air exposure 
- 10 - 

Figure 3 Adapted from Painter and Pabalan (2009). Fracture aperture, saturation, and 

porosity are as follows for each case: (a) 0.3 mm, 1.0, 0.15; (b) 0.1 mm, 1.0, 0.28; (c) 0.1 mm, 

1.0, 0.43; (d) 0.1 mm, 1.0, 0.57; (e) 0.1 mm, 1.0, 0.87; (f) 10 mm, 0.25, 0.31; (g) 10 mm, 0.25, 

0.58, (h) 10 mm, 0.25, 0.80. 

5 years 

10 years 

100 years 
- 11 - 

boundary conditions with specified oxygen concentrations in simpler models that couple oxygen 

diffusion and grout oxidation reactions. 

For the base case, the study assumed a lower reducing capacity than DOE used to model 

saltstone in the FY 2014 Special Analysis (i.e., 0.0498 milliequivalents electrons (meq e-

) / g 

grout in the Painter and Pabalan study as compared to 0.607 meq e- / g saltstone). The grout 

modeled in the Painter and Pabalan study also had a lower porosity than saltstone (i.e., 0.46 as 

compared to 0.60 in saltstone). Therefore, the progress of the oxidation front predicted with the 

numerical model is not directly applicable to saltstone. However, the authors concluded that, for 

a range of parameters relevant to saltstone waste disposal, the complex numerical model could 

be adequately represented with the following analytical equation: 

 ߦሺݐሻ ൌ ඨ2

ݐ଴ܥФ௘ܦ

௨௧௚௥௢ߩ଴ܴ

where 

ξ(t) = position of the oxidation front measured from the fracture wall (m) 

De = effective oxygen diffusion coefficient in the matrix (m2 / yr) 

Ф = matrix porosity (dimensionless) 

C0 = oxidant (oxygen) concentration at interface of fracture and matrix (meq e-

 / g grout) 

R0 = initial reducing capacity of the grout (meq e-

 / g grout) 

ρgrout = bulk density of grout (g / m3

) 

Thus, although Painter and Pabalan used assumptions about the grout properties that differ 

from saltstone, using this equation, their results can be adjusted for different parameter values, 

as discussed further as part of the NRC evaluation. 

Pabalan et al. (2012)

The study included two types of experiments, column tests performed with samples of crushed 

simulated saltstone, and static leaching experiments performed with intact laboratory-formed 

simulated saltstone samples. In the column experiments, the researchers studied the release of 

Tc, selenium, neptunium, uranium, nitrite, and nitrate with oxygen-bearing simulated SRS 

groundwater under atmospheric conditions. The researchers also studied the evolution of pH 

and Eh of the eluent. 

The column experiments were characterized by a pronounced initial Tc release for the first 

several pore volumes (10 in column 1, 14 in column 2), followed by a more gradual release (to 

51 pore volumes in column 1 and 26 in column 2), finally ending with increased releases. The 

researchers concluded that the Tc that was released in the first several pore volumes was likely 

to be attributable to Tc that was not effectively immobilized in the reducing grout or Tc that was 

reoxidized during the crushing and sieving of the grout material. The researchers concluded 

that the increased release rate after 26 or 51 pore volumes was most likely to be due to Tc 

oxidation, because it followed a significant increase in the Eh in the eluent. The researchers 

attributed the delay between the Eh increase and the increase in the rate of Tc release to slow 

oxidation of Tc at depth within the grout particles, which in turn is likely controlled by oxygen 
- 12 - 

diffusion into the particles. The authors noted that the behavior also could be explained by the 

formation of a low solubility metastable Tc(IV) phase that is resistant to oxidative remobilization. 

Almond and Kaplan (2011) (SRNL-STI-2010-00667)

This study examined sorption coefficients (Kd) of several elements, including Tc, in a saltstone 

sample cored from field-emplaced saltstone (Saltstone Disposal Structure (SDS) 4 Cell E). 

Although the researchers reported Kd values, they were based on measurements of release of 

Tc that was initially present in the field-emplaced samples, and therefore likely reflected 

mechanisms other than sorption to surface sites alone. 

The authors ground the sample in a sub-oxic glove bag filled with nitrogen and leached 

radionuclides from the sample with deionized water equilibrated with portlandite. Samples were 

leached for 20 days. Both oxic and anoxic tests were run. In the anoxic tests, the leach 

solution was purged with nitrogen. The authors noted that the samples were olive green when 

freshly cored, but that they had a grey outer layer after storage, before the leaching tests began. 

The authors concluded this color change indicated that some oxidation had occurred during 

storage, although the PVC tubes the samples were stored in had been filled with nitrogen. The 

glovebag maintained a reduced oxygen environment of 30-60 parts per million (ppm) oxygen 

during the course of the experiment. The authors concluded that the measured Kd value of 

139 mL/g likely reflected some oxidation of the sample. If the dissolved Tc was interpreted as a 

solubility limit instead of a Kd value, the measured solubility would be 2 x 10-10 moles / liter. A 

sample exposed to a solution continuously bubbled with air had a measured solubility of 

approximately 2 x 10-9 moles / liter. These values differed from the results of other authors, as 

discussed in the evaluation section. 

Witmer and Powell (2013) (SRR-CWDA-2013-00121)

Witmer and Powell (SRR-CWDA-2013-00121) studied 3.2-cm (1.25-inch) diameter, 1.3-cm thick 

(0.5 inch-thick) pucks of laboratory-made simulated saltstone. Sample construction is 

discussed by Roberts et al. (2012) (SRNL-STI-2012-00603). The samples were placed in field 

lysimeters exposed to natural precipitation at SRS. The pucks were placed in the lysimeters 

approximately 2 months before precipitation was allowed into the lysimeters. Four samples 

were exposed: two that contained Tc but no BFS, and two that contained Tc and BFS. After the 

first three quarters, the lysimeters containing Tc and no slag released approximately 50 to 60 

percent of the Tc-99. For the two lysimeters containing Tc and slag, one released 27 percent 

within the first three quarters and the second released approximately 11 percent within the first 

two quarters. 

Cantrell and Williams (2012) (PNNL-21723)

Cantrell and Williams (2012) measured Tc leaching from simulated saltstone using a singlepass flow-through system under anoxic conditions. The Eh, pH, and the concentrations of 

sulfide and other ions were also measured. The researchers used heavy-walled Teflon reactors 

operated in a low-oxygen atmosphere to minimize oxygen contamination without using 

hydrogen gas, which appeared to reduce Tc in other experiments (Kaplan et al., 2011 
- 13 - 

(SRNL-STI-2010-00668)). They also used a large saltstone-to-leachant ratio to minimize the 

impact of trace amounts of oxygen. The simulated saltstone was cured for 30 days before 

being crushed and sieved. The samples were exposed to oxygen while being crushed and 

sieved, so this step was done quickly to minimize oxidation. 

The concentration of Tc measured in the leachate began at approximately 300 μg/L 

(3 x 10-6 moles / liter) and decreased with time. 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 that the Tc released under anoxic conditions was controlled by an oxide of the form 

TcO2·xH2O, which the authors expected was likely to be TcO2·1.6H2O. 

Two reactors were sparged with pure oxygen after 84 days. Reactor 3 was exposed for 30 

minutes on one day, while Reactor 2 was exposed for 30 minutes on 3 consecutive days. 

Exposure to pure oxygen resulted in increased Tc release in both cases. The sample that was 

exposed to oxygen three times showed a much greater release of Tc than the reactor that was 

only exposed to oxygen once. That is, the Tc release from Reactor 2 increased from 17 

micrograms / liter to 112 micrograms / liter less than a day after the three exposures. The Tc 

release from Reactor 3 increased from 15.2 micrograms / liter to 24.3 micrograms / liter after the 

single oxygen exposure. 

The researchers did not sample Reactor 2 after the 112 micrograms / liter Tc release was 

observed. However, the release from Reactor 3 decreased gradually in the subsequent weeks 

without oxygen exposure (i.e., from 24.3 micrograms / liter shortly after oxygen exposure to 13.6 

micrograms / liter within 15 days without oxygen exposure). Based on this result, the 

researchers 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. 

Kaplan et al., 2011 (SRNL-STI-2010-00668)

This study includes a literature review as well as original experiments. The most significant 

papers from the literature review of Kaplan et al. are included in this review. The results of the 

original experiments are ambiguous because the experiments were conducted in a controlled 

anoxic atmosphere containing 2 percent hydrogen gas. The researchers found that Tc(VII) was 

reduced to Tc(IV) even in control samples that did not contain BFS and concluded the Tc was 

chemically reduced by the hydrogen gas. 

Even with the reducing atmosphere (i.e., <0.5 ppm oxygen, Eh of -585 millivolt, 2 percent 

hydrogen gas, pH 11.66), however, sorption was not immediate. Sorption coefficients increased 

logarithmically until the measurements were stopped at 56 days, with little sorption occurring in 

the first eight days. Steady state had not yet been achieved when the measurements were 

stopped.

NRC Evaluation

Tc in salt waste, before incorporation into a saltstone waste form, is oxidized (i.e., Tc(VII)). The 

DOE conceptual model of the oxidation state of Tc in saltstone is as follows: 
- 14 - 

1. Tc begins to be chemically reduced to Tc(IV) as saltstone is produced, and the 

fraction of Tc(IV) gradually increases as saltstone cures; 

2. Tc oxidizes and becomes mobile when oxygen in infiltrating water consumes the 

saltstone’s reducing capacity; and 

3. Tc(VII) is re-reduced to Tc(IV) if it moves to an area in saltstone that still has 

reducing capacity. 

The NRC staff has Monitoring Factors related to each step of that conceptual model. 

Specifically, NRC staff has the following questions: 

1. Is Tc ever completely reduced in saltstone, or does a fraction remain oxidized? 

2. Does the Ce(IV) method of measuring reducing capacity of saltstone provide an 

appropriate measurement of the capacity of saltstone to reduce Tc(VII) to Tc(IV)? 

a. What components of saltstone supply the measured reducing capacity, 

and are these components capable of chemically reducing Tc(VII) to 

Tc(IV) under field conditions? 

b. Is there an unreactive fraction of saltstone reducing capacity (e.g., 

passivated minerals that do not contribute to effective reducing capacity 

in field-emplaced saltstone)? 

3. Have all of the relevant mechanisms of oxygen ingress into saltstone been 

identified? 

a. Is water infiltration the only relevant mechanism for oxygen infiltration into 

saltstone, or is oxygen in soil gas migrating into unsaturated fractures 

also important to saltstone oxidation? 

b. How much of the saltstone reducing capacity is consumed by contact with 

and entrainment of air during mixing, pumping, and pouring of saltstone 

under field conditions? 

4. Does oxygen entering saltstone react preferentially with Tc before consuming the 

reducing capacity of saltstone? 

5. How effectively can Tc(VII) be re-reduced if it moves into an area of saltstone 

with residual reducing capacity? 

Each of these issues is discussed below. 

Question 1 – Is Tc ever completely reduced in saltstone, or does a fraction remain oxidized?

NRC staff has not found sufficient evidence to make a conclusion about whether Tc is 

completely reduced by saltstone. An ongoing DOE experiment (SRRA042328SR) showed that 

a significant fraction of Tc remained oxidized in simulated saltstone samples after 29 days of 

curing; however, (1) additional chemical reduction may take place with longer curing times and 

(2) the high concentrations of Tc used in the experiment make the applicability to emplaced 

saltstone uncertain. Langton and Almond (2013) (SRNL-STI-2013-00541) observed that 95 

percent of the Tc in Cast Stone, a cementitious waste form similar to saltstone, was reduced 

after 10 days of curing, but suggested that longer curing times should be tested to determine 

whether complete reduction is achieved. Pabalan et al. (2013) noted one possible explanation 

for the initial pulse release of Tc observed in their column experiment was that Tc was not 

completely reduced by the simulated saltstone sample. However, the research teams of 
- 15 - 

Langton and Almond (2013) and Pabalan et al. (2013) both acknowledged that the observation 

of oxidized Tc may have been an experimental artifact, because both teams prepared samples 

for measurement in contact with air. 

Sample preparation in air was avoided by Lukens et al. (2005), who cured grout samples in 

cuvettes analyzed with x-ray absorption fine structure (EXAFS) spectroscopy and XANES. The 

researchers used EXAFS to study the structure of precipitated Tc in a reducing grout and 

XANES to study the oxidation state of Tc. The researchers measured approximately 3 percent 

of Tc as Tc(VII) after 30 months of curing in low-permeability cuvettes, which the researchers 

referred to as “impermeable”. Although the thickness of the “impermeable” cuvette walls was 

not given, for reasonable assumptions of the thickness of the cuvette walls (e.g., 1 mm) and at 

the oxygen diffusion coefficient the researchers provided for the impermeable cuvette walls, 

(i.e., diffusion coefficient of oxygen = 2.3 x 10-9 cm2

/s), the oxidized Tc(VII) can be explained by 

oxygen diffusion through the cuvette wall. Thus in each of these studies it is unclear if the 

observed oxidized fraction of Tc represented a small fraction of Tc that could remain oxidized in 

field-emplaced saltstone or if it was due to an experimental artifact. 

Almond and Kaplan (2011) (SRNL-STI-2010-00667) observed Tc solubility from an SDS 4 core 

sample under suboxic and oxic (i.e., atmospheric) conditions. The suboxic sample was kept in 

a glove bag with oxygen concentrations of 30 to 60 ppm, and had a measured solubility of 

approximately 2 x 10-10 mole / liter. The sample measured under oxic conditions was exposed 

to a solution continuously bubbled with air, and had a measured solubility of approximately 

2 x 10-9 mole / liter. If these results are representative of as-emplaced saltstone, the study 

would provide additional confidence in the ability of saltstone to reduce Tc. 

As discussed in the February 2015 Onsite Observation Report (ML15041A562), DOE 

characterized this sample as a high quality sample because it was cored from field-emplaced 

saltstone. However, the values reported by Almond and Kaplan (2011) were unexpected. The 

relatively small (i.e., single order of magnitude) difference in Tc solubility between reducing and 

oxidizing conditions is not consistent with the current understanding of the redox sensitivity of Tc 

solubility. Furthermore, Estes et al., (2012) and Lilley (2010) both observed Tc solubilities of 

approximately 1 x 10-8 mole / liter under reducing conditions, which were higher than the 

solubility value reported by Almond and Kaplan (2011) for oxidizing conditions. Additionally, 

both Estes et al., (2012) and Lilley (2010) used hydrogen gas to eliminate oxygen in their 

reducing experiments, so their results should reflect a best-case-scenario for reducing 

conditions. 

The reasons for the unexpected results are not clear. One possibility is that there is a difference 

between field-emplaced and laboratory-prepared samples that is not yet understood, and is not 

characterized by the redox conditions alone. Another possibility is that there was an 

unexplained experimental artifact in the solubility study of Almond and Kaplan (2011). For 

example, Almond and Kaplan (2010) observed the formation of calcium carbonate in an 

oxidizing sample. Presumably, calcium carbonate could have precipitated without observation 

in the reducing sample. For any samples that included calcium carbonate precipitation, the 

release of Tc into solution may have been limited. It is not clear if this mechanism is 

representative of field conditions, or whether it is responsible for the differences between Tc 

solubility values observed by Almond and Kaplan (2011) and those observed by other authors. 
- 16 - 

The Almond and Kaplan (2011) solubility measurements could significantly affect long-term 

projections of doses from the SDF. Additional information (e.g., confirmatory testing on future 

core samples) would provide additional confidence in the ability of saltstone to completely 

reduce Tc. 

The question of whether Tc is completely reduced by saltstone grout is relevant to MF 5.05, 

“Potential for Short-Term Rinse-Release from Saltstone,” because any oxidized Tc present 

when water initially infiltrates to the saltstone waste form could be released more rapidly than Tc 

release limited by the annual oxidation of saltstone (e.g., as seen in the early pulse release 

observed by Pabalan et al. (2012)). 

Question 2 – Does the Ce(IV) method of measuring reducing capacity of saltstone provide an 

appropriate measurement of the capacity of saltstone to reduce Tc(VII) to Tc(IV)? 

The uncertainty in the chemical species that contribute reducing capacity to saltstone is related 

to safety in three ways: 

1. The identity of the species that supply the chemical reducing capacity affects the 

persistence of the reducing capacity because different chemical species leach from 

saltstone at different rates; 

2. The identity of the species that contribute reducing capacity is needed to correctly 

interpret the measured reducing capacity with different measurement methods, 

which yield different results; and 

3. The identity of the species that contribute reducing capacity is needed to determine 

what fraction of the measured reducing capacity will be able to reduce Tc(VII) to 

Tc(IV) under the conditions of emplace saltstone. 

The relative importance of reduced sulfur and iron species to the measured reducing capacity is 

unclear. While a recent DOE study (SRRA042328SR) supports the conclusion that Fe(II) in 

BFS contributes more to the reducing capacity of saltstone than reduced sulfur species, another 

study (PNNL-21723) questions the contribution of Fe(II) to the reducing capacity of emplaced 

saltstone. Additional studies (PNNL-22977; CBP-RP-2010-013-01) support the conclusion that 

one of the primary contributors to the measured reducing capacity is not in the dry premix, but in 

the decontaminated salt waste itself. Specifically, the studies suggest that one of the main 

contributors to the measured reducing capacity by either the Ce or Cr method is nitrate in the 

treated liquid salt waste. However, as previously discussed, because of the relative Eh values 

of the NO2

-

/ NO3

-

 and Tc(IV) / Tc(VII) redox couples, the reducing capacity attributable to nitrate

is not expected to effectively reduce Tc(VII) to Tc(IV). 

In a Request for Additional Information (RAI) for the FY 2013 Special Analysis (ML14148A153), 

the NRC staff expressed concern that if the reducing capacity is primarily attributable to reduced 

sulfur species, sulfur leaching should be represented in DOE’s evaluation case because it would 

represent a mechanism for the loss of reducing capacity from saltstone independent of oxygen 

ingress. In the reply to the RAI (ML15020A655), DOE cited recent research results 

(SRRA042328SR) that indicate that Fe(II) may contribute more to the reducing capacity of 
- 17 - 

saltstone than reduced sulfur species. However, the ability of Fe(II) species to reduce Tc(VII) to 

Tc(IV) under the conditions of field-emplaced saltstone are unclear because saltstone pore 

water has a higher pH than either of the methods used to measure reducing capacity, and Fe(II) 

has a low solubility at high pH. 

In addition, in the same RAI (ML14148A153), the NRC staff expressed concern that the Ce(IV) 

method, which is performed at a low pH (i.e., pH 1 to 2) with sulfuric acid, may overestimate the 

reducing capacity. As discussed in PNNL-22977, the Ce(IV) method measures nearly all of the 

reducing capacity of the solid sample because most of the solids dissolve in the strong acid. 

The NRC staff expressed concern that reducing capacity measured with this method may not be 

applicable to saltstone pore solutions, which have a much higher pH (e.g., 10.5 to 12.5). 

Specifically, the NRC staff expressed concern that if a passivation layer forms on the BFS, 

some of the reducing capacity measured with the Ce(IV) method would not be available to 

reduce Tc(VII) in saltstone pore water. 

Given these sources of uncertainty, it is unclear to the NRC staff whether the reducing capacity 

of saltstone measured by the Ce(IV) method, which DOE uses as a parameter in the FY 2014 

Special Analysis, is a good representation of the capacity of saltstone to chemically reduce 

Tc(VII) to Tc(IV). 

Question 3 – Have all of the relevant mechanisms for oxygen ingress into saltstone been 

identified?

In the DOE conceptual model, the only source of oxygen modeled as contacting saltstone is 

oxygen dissolved in groundwater. DOE excludes soil gas infiltrating into saltstone as a potential 

source of oxygen in its base-case analysis because DOE has concluded the saltstone matrix 

will remain saturated. In the NRC RAI on the DOE FY 2013 Special Analysis, the NRC staff 

questioned this conclusion, explaining that if saltstone contains unsaturated fractures, oxygen 

could be introduced into those fractures in the gas phase. The additional source of oxygen is 

relevant to safety for two reasons: 

1. The amount of oxygen infiltrating into saltstone is related to projected Tc release in 

DOE’s mathematical model; and 

2. Short-term rinse release, as covered in MF 5.05, could occur if grout oxidation 

occurs prior to infiltrating water contacting saltstone, which can only occur if there is 

another source of oxygen in addition to infiltrating water. 

If significant oxidation occurs prior to water interacting with saltstone, there is the potential for a 

short-term pulse release in the first pore volume released, when the Tc that has oxidized prior to 

water infiltrating to saltstone is released. This potential for short-term release is not captured in 

the DOE evaluation case, because the model does not include a mechanism for saltstone to 

oxidize prior to advective flow of water through saltstone. 

The NRC staff has previously expressed concern about the possibility of the ingress of oxygen 

in soil gas into fractures in saltstone in review documents (NRC, 2010a; NRC 2012b; NRC 

2012). Although fractures were not specifically mentioned in a recent study by Langton (2014a) 
- 18 - 

(CBP-TR-2014-05), the study found that the rate of oxygen migration into samples of Cast 

Stone exposed to soil was faster than the rate of migration into the sample exposed to water. 

The authors concluded that this observation was consistent with the more rapid transport of ions 

through a gas phase as compared to a liquid phase. 

Similarly, the results of the numerical model developed by Painter and Pabalan (2009) 

demonstrate the importance of gas-phase transport of oxygen in saltstone fractures. As 

previously discussed, the authors modeled a cementitious material with a significantly lower 

reducing capacity than saltstone, in addition to differences in density, porosity, and diffusivity. 

Updating Equation (1), taken from the Painter and Pabalan (2009) study, with parameters more 

characteristic of saltstone, the equation shows slightly slower oxidation than predicted by 

Painter and Pabalan (Figure 4). However, the oxidation rates were similar enough that the main 

conclusion of the Painter and Pabalan study that oxygen from soil gas in unsaturated fractures 

is a non-negligible source of oxidation is expected to be relevant. Figure 4 is intended to show 

the relationship between the projections of Painter and Pabalan (2009) and what the projections 

would be if updated with parameters more relevant to saltstone. NRC staff does not regard 

Figure 4 as a likely projection of saltstone oxidation because there are several complexities the 

analytical equation developed by Painter and Pabalan (2009) does not address. These 

complexities include the following issues: 

1. There was assumed to be no damage to the wasteform behind the oxidation front, 

which is likely not realistic. Assuming damage behind the front would cause an 

increase in the projected extent of oxidation (i.e., the square root of time relationship 

would be closer to linear). 

2. Using the value of 0.607 meq e-

 / g for the reducing capacity of saltstone assumes all 

of the measured reducing capacity in saltstone is capable of reducing Tc (i.e., it is 

due to chemical components that will not be easily leached and have a redox couple 

Eh below that of Tc(IV) / Tc(VII)). 

3. Using the value of 0.607 meq e-

 / g saltstone for the reducing capacity of saltstone 

assumes all of the measured reducing capacity in saltstone is in a physical form 

available to reduce Tc (i.e., not obstructed with a passivating layer). 

4. Using the value of 0.607 meq e-

 / g saltstone assumes the amount of oxidation that 

occurs in the field during field-scale mixing, pumping, and pouring of saltstone in 

contact with air is adequately represented by the amount of oxidation that occurred 

during laboratory-scale mixing and pouring of saltstone samples. 
- 19 - 

Figure 4. Oxidation projections based on the model of Painter and Pabalan (2009) and 

predictions modified for saltstone parameters, including a reducing capacity of 0.607 meq e-

 / g 

saltstone, porosity of 0.58, density of 1010 kg per m3

, and effective diffusion coefficient of 

3.15 x 10-5 m2

 / year.

The NRC staff is not aware of studies specifically designed to quantify oxidation that occurs as 

saltstone is mixed, pumped, and poured in contact with air. DOE has plans to measure Tc 

leaching from cored samples of field-emplaced saltstone. Comparing leaching from fieldemplaced saltstone and laboratory-created samples may capture some effects of oxidation 

during field emplacement if the cores are partially oxidized. However, reducing capacity is more 

likely to affect the duration of reducing conditions than it is to affect Tc leaching from areas that 

remain reducing. Because the oxidation state of a redox couple depends on the Eh of solution, 

and not the reducing capacity, Tc leaching is not necessarily a good indication of whether some, 

but not all, of the reducing capacity in a region of saltstone has been consumed. Thus it is not 

clear whether measurements of Tc leaching from reduced regions of saltstone will adequately 

support conclusions about how much reducing capacity is consumed by mixing, pumping, and 

pouring saltstone in contact with air. Because the reducing capacity is a chemical barrier to 

oxygen ingress, studies that evaluate the progress of an oxidation front could provide 

information about the effective reducing capacity of field-emplaced samples. Measurements of 

reducing capacity also could be useful, if they address the uncertainties related to reducing 

capacity discussed in this review. 

In addition to the limitations of the projections of Painter and Pabalan (2009) discussed above, a 

more fundamental limitation exists if Tc leaching is not well correlated to residual reducing 

capacity. This question is discussed further in the evaluation of Question 4. 

In the FY 2014 Special Analysis, DOE addressed the possibility of additional sources of oxygen 

in saltstone with a sensitivity analysis that modeled non-depleting sources of oxygen comprising 

5, 10, 15, and 20 percent of the saltstone volume. Unlike fractures, the sources of oxygen were 

placed randomly in the saltstone and were disconnected from each other by reduced regions of 

saltstone until approximately 10,000 years after closure, when oxygen diffusion from the 

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

0 10000 20000 30000

Oxidation Front Movement (m)

Time (yr)

Painter and Pabalan 2009

Saltstone Parameters
- 20 - 

randomly-placed sources began to form connected pathways for oxidized Tc out of saltstone. 

Although DOE addressed the possibility of oxidized Tc not being re-reduced in saltstone with an 

additional sensitivity analysis that modeled Tc exiting saltstone as soon as it is oxidized, that 

sensitivity analysis did not include the possible effects of oxygen in fractures as was captured by 

the sensitivity analysis with non-depleting oxygen sources. NRC staff is evaluating the meaning 

of the results of the two separate sensitivity analyses as informed by one another. 

Question 4 – Does oxygen entering saltstone react preferentially with Tc before consuming the 

reducing capacity of saltstone?

It is not clear that oxygen migrating into the saltstone grout will preferentially consume the 

reducing capacity of saltstone before oxidizing Tc-99. Research from both the Savannah River 

National Laboratory (SRNL) (SRNL-STI-2013-00541) and the Cementitious Barriers Partnership 

(CBP) (CBP-TR-2013-002) indicated that residual reducing capacity was not correlated to Tc 

mobility in chemically reducing cementitious wasteforms. One explanation for the lack of 

correlation could be that oxygen preferentially reacts with the Tc wherever the oxygen can 

ingress into saltstone, even in the presence of residual reducing capacity. 

In February 2013, NRC and DOE staffs had a technical exchange (ML14057A578) to discuss 

those studies, among other topics. DOE questioned whether research done on a static system 

(in CBP-TR-2013-002), rather than flow-through system, would accurately predict behavior in 

the saltstone system. NRC staff questioned whether the flow through saltstone was expected to 

be slow enough that the difference between the static system used in the CBP experiment and 

the slow flow expected in field-emplaced saltstone is significant. 

In Witmer and Powell (2013) (SRR-CWDA-2013-00121), comparison of laboratory-prepared 

simulated saltstone samples made with and without BFS and exposed to the atmosphere in field 

lysimeters showed the presence of slag reduced Tc-99 release by only a factor of 2 to 4. This 

field result is inconsistent with the results of a previous DOE study conducted with field 

lysimeters using larger samples (i.e., 55-gallon wasteforms) (Langton, 1988). That study 

demonstrated a much larger difference between the Tc retention in cementitious wasteforms 

with and without slag. Specifically, no Tc leaching was detectable in the slag-containing 

saltstone for the duration of the experiment (i.e., 2.5 years), whereas Tc leached out of the slagfree saltstone at the rate of nitrate loss. 

One possible explanation for the difference in the results between the two studies was that Tc 

release from the smaller samples was dominated by surface defects, whereas the larger sample 

maintained reducing conditions in a greater fraction of the sample because of its much smaller 

surface-area to volume ratio. In the more recent study (SRR-CWDA-2013-00121), the two 

lysimeters with Tc and slag released approximately 27 percent in 9 months and approximately 

11 percent in six months. If Tc is only released from the oxidized layer (i.e., neglecting 

solubility-controlled release from the reduced saltstone) an 11 percent release represents Tc 
- 21 - 

release from a 0.4 mm deep layer around the sample 7. A 28 percent release represents Tc 

release from a layer 1 mm deep. For comparison, the analytical approximation developed by 

Painter and Pabalan (Equation 1), when updated with parameters DOE uses for saltstone, 

would predict approximately 0.2 mm of oxidation in 0.75 years. One explanation for the 

apparent release of Tc from depths to which an oxidation front was not expected to penetrate is 

that the results may have been dominated by 1 mm of surface flaws. Surface effects on the 

leaching of non-redox sensitive species were detected up to 16 mm from the surface by Almond 

and Langton (2013) (SRNL-STI-2013-00541). Another possible explanation for the observed Tc 

release is that oxygen preferentially oxidized Tc in some areas before completely consuming 

reducing capacity in those areas. 

The DOE conceptual and mathematical models for Tc release assume Tc oxidation is governed 

by the progress of an oxidation front that consumes saltstone reducing capacity. Based on the 

studies described in this section, NRC staff questions the validity of the assumption that Tc 

oxidation is adequately predicted by the progress of this type of oxidation front. DOE has 

performed a sensitivity analysis related to non-depleting oxygen sources in saltstone that may 

provide insight into the potential effects of Tc oxidation in areas of saltstone with residual 

reducing capacity. That analysis was provided originally in the FY 2013 Special Analysis 

(ML14002A069) and discussed further in Section 5.6.7.4 of the FY 2014 Special Analysis. 

Although that sensitivity analysis bases Tc oxidation on the consumption of reducing capacity in 

saltstone, it is relevant to NRC questions about the correlation between Tc oxidation and 

progress of an oxidizing front because the non-depleting oxygen sources could be interpreted 

(non-mechanistically) as areas in which Tc is oxidized prior to being reached by an oxidation 

front moving through saltstone. 

In the NRC RAI on the FY 2013 Special Analysis, NRC staff commented that the results of the 

sensitivity analysis with non-depleting oxygen sources were difficult to interpret because Tc-99 

released from areas near the non-depleting oxygen sources was immediately re-reduced by 

surrounding material and the NRC staff does not find the assumed re-reduction to be 

adequately supported. As discussed in the following section, DOE has addressed NRC 

questions about the support for Tc re-reduction in saltstone with a sensitivity analysis. Thus one 

way to address NRC concerns about Tc oxidation in the presence of residual reducing capacity 

would be to combine DOE sensitivity analyses showing the effects of non-depleting sources of 

oxygen and fractions of Tc entering the saturated zone when oxidized (i.e., not undergoing rereduction). 

Question 5 – How effectively can Tc(VII) be re-reduced if it moves into an area of saltstone with 

residual reducing capacity?

In the final part of DOE’s near-field conceptual model, Tc(VII) that is oxidized at one location in 

saltstone is reduced and immobilized if it is transported into another part of the saltstone waste 

form that has remaining reducing capacity. The NRC staff does not have sufficient information 

 

7

 In a 3.2-cm diameter 1.3-cm high (1.25 inch-diameter by 0.25 inch high) sample, a 0.4 mm layer around 

the cylinder is 11 percent of the sample volume (sample dimensions from Roberts et al. (2012) (SRNLSTI-2012-00603)). 
- 22 - 

to conclude that this aspect of DOE’s conceptual model is adequately supported. As evidence 

for re-reduction, DOE relies on the results of Cantrell and Williams (2012) (PNNL-21723). 

Specifically, DOE cites the decrease in Tc releases from Reactor 3 from 24.3 microgram / liter 

shortly after oxygen exposure to 13.6 microgram / liter within 15 days without oxygen exposure. 

However, the decrease in Tc release by approximately 50 percent does not seem to support the 

complete re-reduction of Tc entering a reduced zone of saltstone as assumed in the FY 2014 

Special Analysis. 

In the FY 2014 Special Analysis, DOE addressed reconcentration with a sensitivity analysis in 

which a fraction of the oxidized Tc was moved directly from the wasteform to the saturated zone 

when it was oxidized. This sensitivity analysis showed that when Tc was assumed to be 

released from saltstone as soon as it is oxidized (i.e., it does not re-reduce), doses increased 

before 10,000 years but did not exceed 25 mrem / year. This value could change based on 

changes in assumptions (e.g., the rate of water infiltration through saltstone and Tc solubility). 

However, this sensitivity analysis is an effective way to address the effects of the assumed rereduction in the DOE evaluation case. Thus while the NRC staff does not find that DOE’s 

assumption about complete re-reduction of Tc entering a reduced zone of saltstone is 

adequately supported, the potential effects of the assumption could potentially be addressed 

through sensitivity analyses. 

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

NRC staff made the following conclusions based on this review: 

• A significant amount of chemical reduction of technetium (Tc) from Tc(VII) to Tc(IV) 

appears to occur within the first week of curing and additional chemical reduction may 

continue for months. However, no study has shown complete reduction of Tc(VII) in 

saltstone under conditions that are consistent with field-emplaced saltstone. 

• Different methods of quantifying reducing capacity yield different measured reducing 

capacities of saltstone and its components. The Ce method (also called the AngusGlasser method) may overestimate the amount of reducing capacity available to reduce 

Tc in saltstone. 

• It is unclear which components of saltstone dry premix or salt solution are responsible 

for the measured reducing capacity of saltstone. The identity of the components 

contributing reducing capacity is important to long-term predictions of the ability of 

saltstone to chemically reduce contaminants. 
- 23 - 

• There is evidence for gas-phase transport of oxygen in laboratory samples of a reducing 

grout exposed to moist soil. 

• It is unclear how much reducing capacity is consumed during field-scale mixing, 

pumping, and pouring. Measurements of reducing capacity or studies designed to 

evaluate the progress of an oxidizing front in cored samples of field-emplaced saltstone 

may address this issue. 

• Independent experiments suggest Tc oxidation is not well correlated to consumption of 

reducing capacity in reducing cementitious wasteforms. 

• Short-term rinse-release may be a concern if significant saltstone oxidation occurs from 

gas-phase transport of oxygen into saltstone fractures prior to water infiltration to 

saltstone. Release of Tc that remains oxidized in saltstone (a “persistent oxidized 

fraction”) also could contribute to this short-term release, if a fraction of Tc remains 

oxidized. 

• There is evidence that once Cr is oxidized in saltstone, it is not re-reduced by residual 

reducing capacity in saltstone if oxygen is present. The applicability of this result to Tc 

re-reduction is unclear. However, because Tc(IV) appears to be more readily oxidized in 

saltstone than Cr(IV), the result suggests if low levels of oxygen can keep Cr oxidized in 

areas of saltstone with residual reducing capacity, low levels of oxygen also could keep 

Tc oxidized in areas of saltstone with residual reducing capacity. 

The NRC staff will continue to monitor information provided by the U.S. Department of Energy 

(DOE) as it is developed. As described in the NRC 2013 monitoring plan, additional information 

is needed to close Monitoring Factors (MF) 5.01, 5.02, 5.03, and 5.05. The NRC staff updated 

those information needs based on the information in this review. 

For MF 5.01, “Radionuclide Release from Field-Emplaced Saltstone,” the additional information 

needed includes the following: 

• Measurements of radionuclide release from core samples of field-emplaced saltstone; 

and 

• An estimate of the oxidation of Tc in saltstone that could occur if oxygen preferentially 

reacts with Tc before consuming saltstone reducing capacity. This estimate may be 

affected by the degree of fracturing expected in field-emplaced saltstone as a function of 

time. 

For MF 5.02, “Chemical Reduction of Tc by Saltstone,” the additional information needed 

includes the following: 

• Experimental evidence supporting the degree of re-reduction of oxidized Tc that DOE 

predicts in mathematical models of Tc transport in saltstone; or 
- 24 - 

• Further analyses to combine DOE sensitivity analyses showing the effects of nondepleting sources of oxygen and fractions of Tc entering the saturated zone when 

oxidized (i.e., not undergoing re-reduction). 

For MF 5.03, “Reducing Capacity of Saltstone,” the additional information needed includes the 

following: 

• Information to support the identity of the components of saltstone dry premix or salt 

solution that supply the measured reducing capacity in saltstone, the ability of these 

components to reduce Tc in saltstone under field conditions, and the expected evolution 

of the reducing capacity over time. 

For MF 5.05, “Potential for Short-Term Rinse-Release from Saltstone,” the additional 

information needed includes the following: 

• Experimental measurements of rinse-release of risk-significant radionuclides from 

saltstone and an analysis of the effect of the release on the projected dose; and 

• Analyses of the potential effect on dose of the fraction, if any, of Tc that is likely to be (1) 

rinsed from the surface of saltstone, (2) oxidized before advective flow of water through 

saltstone begins, or (3) not chemically reduced by saltstone. The analysis should also 

address potential effects of high ionic strength. 

The NRC staff is reviewing the FY 2014 Special Analysis for the Saltstone Disposal Facility 

(ML15097A366). As part of the NRC’s review, the NRC staff will continue to review these 

areas. 
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