September 1, 2009 

MEMORANDUM TO: Gregory Suber, Chief 

 Low-Level Waste Branch 

 Environmental Protection 

 and Performance Assessment Directorate 

 Division of Waste Management 

 and Environmental Protection 

 Office of Federal and State Materials 

 and Environmental Management Programs 

THRU: Christepher McKenney, Chief /RA/ by K. Pinkston for 

 Performance Assessment Branch 

 Environmental Protection 

 and Performance Assessment Directorate 

 Division of Waste Management 

 and Environmental Protection 

 Office of Federal and State Materials 

 and Environmental Management Programs 

FROM: David Esh, Sr. Systems Performance Analyst /RA/ 

 Performance Assessment Branch 

 Environmental Protection 

 and Performance Assessment Directorate 

 Division of Waste Management 

 and Environmental Protection 

 Office of Federal and State Materials 

 and Environmental Management Programs 

SUBJECT: TECHNICAL REVIEW: “EVALUATION OF SULFATE ATTACK ON 

SALTSTONE VAULT CONCRETE AND SALTSTONE, PART I: FINAL 

REPORT”, “EVALUATION OF SULFATE ATTACK ON SALTSTONE 

VAULT CONCRETE AND SALTSTONE, PART II: TEST METHODS TO 

SUPPORT MOISTURE AND IONIC TRANSPORT MODELING USING 

THE STADIUM® CODE” 

On January 9, 2009, the U.S. Department of Energy, Savannah River Operations Office, 

provided the subject report for review by U.S. Nuclear Regulatory Commission (NRC) staff 

pursuant to Section 3116(b) of the Ronald W. Reagan National Defense Authorization Act for 

Fiscal Year 2005. The subject reports are available on NRC’s Agencywide Documents Access 

and Management System (ADAMS) at accession numbers ML090150306 and ML090150312. 

These reports were reviewed in accordance with monitoring activities described in “U.S. Nuclear 
2 

Regulatory Commission Plan for Monitoring the U.S. Department of Energy Salt Waste Disposal 

at the Savannah River Site in Accordance with the National Defense Authorization Act for Fiscal 

Year 2005” (ML070730363). The staff’s technical review summary is attached for your use. 

Docket No.: PROJ0734 

Enclosure: Technical Review Summary 
2 

Regulatory Commission Plan for Monitoring the U.S. Department of Energy Salt Waste Disposal 

at the Savannah River Site in Accordance with the National Defense Authorization Act for Fiscal 

Year 2005” (ML070730363). The staff’s technical review summary is attached for your use. 

Docket No.: PROJ0734 

Enclosure: Technical Review SummaryEnclosure: Technical Review Summary 

DISTRIBUTION: 

DBrown CGrossman KPinkston LHoward/CNWRA 

ML092300610 

OFC DWMEP DWMEP DWMEP:BC DWMEP 

NAME D.Esh AWalker-Smith KPinkston for C.McKenney DEsh 

DATE 8/25/09 8/27/09 9/1/09 9/1/09 

OFFICIAL RECORD COPY
Enclosure 

Technical Review Summary: “EVALUATION OF SULFATE ATTACK ON SALTSTONE VAULT 

CONCRETE AND SALTSTONE, PART I: FINAL REPORT”, “EVALUATION OF SULFATE 

ATTACK ON SALTSTONE VAULT CONCRETE AND SALTSTONE, PART II: TEST 

METHODS TO SUPPORT MOISTURE AND IONIC TRANSPORT MODELING USING THE 

STADIUM® CODE” 

Review Completed: June 24, 2009 

Reviewer(s): D. Esh 

Document(s): Langton, C. A., Evaluation of Sulfate Attack on Saltstone Vault 

Concrete and Saltstone, Part I: Final Report, SRNS-STI-2008-00050, 

Revision 0, SimCo Technologies Inc. for Savannah River National 

Laboratory, SRNS. August 2008, ADAMS accession no. ML090150306 

 Langton, C. A., Evaluation of Sulfate Attack on Saltstone Vault 

Concrete and Saltstone, Part II: Test Methods to Support Moisture and 

Ionic Transport Modeling Using the STADIUM® Code, SRNS-STI2008-00052, Revision 0, SimCo Technologies Inc. for Savannah River 

National Laboratory, SRNS. August 2008, ADAMS accession no. 

ML090150312 

Evaluation

The reports provide preliminary results to assess the effects of contacting saltstone vault 

concretes with highly alkaline solutions containing high concentrations of dissolved sulfate. 

Part I provides the description of and results from numerical simulations, whereas Part II 

provides a description of the test methods used to develop information to support the numerical 

modeling. Part I provides results for two surrogate concretes that were used in the preliminary 

durability analysis. The modeling provided key inputs for the current performance assessment 

(PA) analysis including but not limited to: 

• The relationship between the rate of advancement of the sulfate front and the rate of 

change of the concrete permeability and diffusivity. 

• The relationship between the sulfate ion concentration in the corrosive leachate and the 

rate of the sulfate front progression, and 

• An equation describing the change in hydraulic properties as a function of sulfate ion 

concentration in the corrosive leachate. 

The STADIUM® code and data from two surrogate concretes were used in the preliminary 

durability analysis. Validation of the results was completed for surrogate concretes. 

Measurements on SRS Vault 1/4 and Disposal Unit 2 concrete samples are being performed 

and the analysis will be revised when they are available. Modeling of and measurements on 

saltstone are also being performed. In addition, sulfate exposure tests of Vaults 1/4 and 

Disposal Unit 2 concrete samples to three alkaline, high sulfate leachates is underway to 

validate the predicted results. A revised report is expected to be available September 2009. 
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The saltstone wasteform is cast in concrete vaults. The vaults provide a barrier to movement of 

deleterious groundwater species into the saltstone wasteform and a barrier to migration of 

radionuclides in the saltstone wasteform to the environment. The studies were performed 

because the saltstone wasteform contains high concentrations of sulfate. Sulfate could diffuse 

into the vault concrete and deteriorate its performance. The measurements, analysis, and 

modeling were performed to estimate the degradation of the concrete over 10,000 years. The 

STADIUM® code is an ionic transport model that accounts for electrical coupling, chemical 

activity, and transport due to water gradients and temperature. The model also includes a 

chemical equilibrium module. 

Overall the analysis and supporting measurements represented a well-designed evaluation of 

the potential for sulfate attack using best available technology. Three cases are evaluated: 

case 1 – saltstone pore water (undiluted), case 2 – saltstone pore water diluted 10:1, and case 

3 – leached saltstone pore water at 10:1 dilution. Table 3-1 of the report provides previously 

measured saltstone pore solutions and Table 3-2 provides the values used in the simulations. 

The cases selected are reasonable, although, as noted in the report, the concentration of sulfate 

used is approximately 20% less than experimentally measured values. Experimental validation, 

when completed, will ensure the simulations are realistic. The U.S. Nuclear Regulatory 

Commission (NRC) staff will review the results of those experiments when they are available. 

For exposure to high sulfate concentrations (Case 1), the ettringite front was estimated to 

penetrate a 20 cm vault wall in about 5000 years. For cases 2 and 3 the ettringite front was 

estimated to penetrate approximately 40% and 20% of the vault thickness, respectively. The 

report discussed a number of uncertainties, such as the effect of micro cracks and macro 

cracks. Macro cracks (greater than 100 μm) in concrete have been previously observed to 

result in saturated diffusion coefficients corresponding to those reported for free water which 

represents about a factor of 50 increase. 

Section 7.0 of Part I of the report provides a description of how the results will be abstracted for 

use in the PA. NRC generally agrees with the outlined approach, although modifications may 

be necessary when the experimental measurements are completed. 

The NRC staff observes that the following areas may need to be considered by the Department 

of Energy (DOE) as part of model abstraction in the PA, depending on the degree to which the 

performance of saltstone grout, Vault 1/4 concrete, and Disposal Unit 2 is relied upon against 

degradation by sulfate attack: 1) consideration of fast pathways (e.g., cracks) for sulfate 

penetration and associated higher localized degradation, 2) new information on bulk properties 

of the vaults and saltstone grout, and 3) coupling of damage to reaction/transport. Each of 

these items is described below in more detail. NRC recognizes that the first two items would 

likely be addressed in the PA revision. Other questions were communicated to DOE during the 

March 25-26, 2009 onsite observation meeting. Those are listed in the “Teleconferences and 

Meetings” section of this report and are summarized below, as needed. 

This study used an idealized and simplified conceptual model with one-dimensional transport for 

front propagation. Vaults 1 and 4 are known to have a number of imperfections (cracks, 

pathways around pipes or other discrete features) that would likely provide faster diffusion rates 

of sulfate. The calculated penetrated (degraded) wall fraction as a function of time in the PA 

should account for diffusion into the vault concrete (perpendicular to what is currently evaluated) 

at these imperfections and other features. This approach would result in earlier penetration of 
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some fraction of the vault wall and formation of highly-permeable pathways consistent with the 

damaged state of the material. 

This study included measurements of the ionic diffusivity for small-scale, laboratory-prepared 

samples. The staff concludes that the bulk properties of the as-built materials may be 

significantly different due to scale, quality, or other issues. The sulfate attack and secondary 

phase formation processes occur at the pore and grain scale of the material. However, as 

discussed in these reports, the presence of macroscopic features can significantly influence the 

effective properties of the materials. The sulfate attack simulations should address uncertainty 

in the bulk properties of the as-built materials (compared to the laboratory measurements of 

surrogate samples used to support the analysis). At a minimum, the abstraction for the PA 

should include the diffusivity of the concrete as a variable. 

This study did not assign higher diffusivity to the damaged region of the concrete. As the 

concrete is damaged, the diffusivity of the damaged region could increase resulting in a higher 

rate of front propagation. This effect may not be resolvable in short duration experiments 

because the derivative of the rate of front propagation may not be observable on the scale of 

such experiments. It is recommended that additional simulations be performed where the 

diffusivity of the degraded region is increased appropriately, and that those simulations are 

compared to the current simulations for common sets of input. 

DOE should include uncertainty in the PA abstraction that is developed from this study. NRC 

staff will evaluate the revised saltstone PA when it is available. 

Teleconferences and Meetings

The subject report was discussed with representatives of the DOE and its contractors on 

March 25-26, 2009 at the onsite observation conducted at the Savannah River Site. A summary 

of the discussion is provided in the onsite observation report (ADAMS accession no. 

ML091320439). The follow-up actions as a result of the discussion were: 

1) Evaluate the sensitivity of grid spacing to predicted front propagation in the 

sulfate attack evaluation. 

2) Explain how spatial representation in the numerical experiments of sulfate attack 

will be translated into a PA model, since the geometries of the real system will be 

much more complex (e.g., a random collection of different size blocks determined 

by crack distributions) than those considered in the numerical experiments. 

3) Explain how cracks are incorporated into the sulfate attack representation in the 

PA model, since cracks could significantly impact the degradation assessment 

(page 15). Explain assumption i that the transport rate of sulfate ions through 

damaged concrete is not different from undamaged concrete (page 21). 

4) Clarify the conceptual model for sulfate attack. For example, does sulfate attack 

proceed along a front, or is it a generalized mechanism. 
4 

5) Clarify the conceptual model represented by case 2 (page 6). If the 

concentration was diluted by diffusion, then what is the fate of diffused species? 

If species are diffusing through the vault wall, then why isn’t the vault wall 

degraded. 

6) Explain why it is appropriate to neglect minor species (page A2-14). 

7) Justify the use of Berner’s approach for these materials and solutions 

(page A2-15). 

Open Issues

No open issues were identified as a result of the review, however as indicated above a number 

of follow-up actions were identified. 

Conclusion

The NRC staff has not identified open issues associated with the methods and data reported in 

this study at this time. However, DOE has several follow-up actions in response to staff 

questions. The staff plans to review any new information developed by DOE as part of these 

follow-up actions. 

This study contains data that will be relied upon in future performance assessment updates in 

support of the 2005 DOE waste determination for the Saltstone Disposal Facility. However, until 

such time as a PA update is completed and reviewed by NRC staff, the staff cannot fully assess 

the risk significance of the information. For this reason, all monitoring activities identified under 

Factors 1-3 of the NRC monitoring plan remain open at this time.