DOE OSTI2012
Section 3116 of the Ronald W. Reagan National Defense Authorization Act for Fiscal Year 2005 (NDAA) requires the U.S. Department of Energy (DOE) to consult with the U.S. Nuclear Regulatory Commission (NRC) for certain non-high level waste determinations. The NDAA also requires NRC to monitor DOE's disposal actions related to those determinations. In Fiscal Year 2011, the NRC staff reviewed DOE performance assessments for tank closure at the F-Tank Farm (FTF) Facility and salt waste disposal at the Saltstone Disposal Facility (SDF) at the Savannah River Site (SRS) as part of consultation and monitoring, respectively. Differences in inventories, waste forms, and key barriers led to different areas of focus in the NRC reviews of these two activities at the SRS. Because of the key role of chemically reducing grouts in both applications, the evaluation of chemical barriers was significant to both reviews. However, radionuclide solubility in precipitated metal oxides is expected to play a significant role in FTF performance whereas release of several key radionuclides from the SDF is controlled by sorption or precipitation within the cementitious wasteform itself. Similarly, both reviews included an evaluation of physical barriers to flow, but differences in the physical configurations of the waste led to differences in the reviews. For example, NRC's review of the FTF focused on the modeled degradation of carbon steel tank liners while the staff's review of the SDF performance included a detailed evaluation of the physical degradation of the saltstone wasteform and infiltration-limiting closure cap. Because of the long time periods considered (i.e., tens of thousands of years), the NRC reviews of both facilities included detailed evaluation of the engineered chemical and physical barriers. The NRC staff reviews of residual waste disposal in the FTF and salt waste disposal in the SDF focused on physical barriers to flow and chemical barriers to radionuclide release from the waste. Because the waste inventory and concentration at both sites is sufficient to generate unacceptable doses to an off-site member of the public or inadvertent intruder in the absence of engineered barriers, the NRC staff review focused on the engineering features DOE plans to put in place to limit radionuclide release. At the FTF, DOE expects that peak doses are delayed beyond a 10,000 year performance period by a combination of (1) the flow-limiting effect of the steel tank liner and (2) chemical conditions created by the stabilizing grout overlying the waste that limit the solubility of key radionuclides for tens of thousands of years. At the SDF, DOE expects that flow will be significantly limited by water shedding along the closure cap lower drainage layer and that radionuclide release will be further limited by radionuclide precipitation or sorption within the high pH, chemically reducing conditions created within the saltstone waste form. Because the performance of both facilities depends on the performance of engineered barriers for thousands of years, the reviews included a detailed evaluation of the expected long-term behavior of these barriers. As previously discussed, NRC staff reviews of DOE waste determinations during consultation are designed to evaluate the three NDAA criteria, whereas the review of an updated PA during monitoring only addresses whether the NRC staff has reasonable assurance that the planned disposal action will meet the performance objectives of 10 CFR Part 61. The NRC staff review of the Waste Determination for the FTF did not include conclusions about whether the planned disposal of residual waste at the FTF would meet the NDAA criteria because of the substantial uncertainties in the degree of waste removal DOE would achieve and other technical uncertainties. The main product of the NRC staff review of the planned FTF disposal action is the recommendation that DOE should conduct waste release experiments to increase support for key modeling assumptions related to: (1) the evolution of pH and Eh in the grouted tank system over time; (2) identification of HRR association with solid phases comprising the residual wastes; and (3) expected solubility of HRRs under a range of environmental or service conditions that the residual wastes in the contaminated zone are expected to be exposed to over time. Implementation of this recommendation is deemed crucial for NRC staff to have reasonable assurance that the performance objectives in 10 CFR Part 61, Subpart C can be met. Given the risk-significance of Tank 18 to the overall PA and the short timeline for closure of this tank, the NRC staff recommended that DOE should initiate discussions with NRC staff regarding implementation of this recommendation for Tank 18 as soon as practical. The NRC staff also recommended that experiments to address this recommendation should be conducted prior to final closure of Tank 18. Results of the Tank 18 residual waste experiments, if conducted, will be evaluated by NRC staff to determine the need for additional data collection, experiments, and modeling for Tank 18, as well as other FTF tanks. Additional information regarding the NRC staff's recommendations in this area, including details on the suggested implementation of other recommendations will be provided in the NRC staff's plan for monitoring the FTF later in FY 2012, after DOE makes a final decision on the waste determination. The NRC staff's review of waste disposal at the SDF is ongoing. When complete, the SDF TER will indicate whether the NRC staff continues to have reasonable assurance that waste disposal at the SDF will meet the performance objectives of 10 CFR Part 61 (NDAA Criterion 3). The TER also will include risk insights that will form the basis of the NRC staff's revised monitoring plan for the SDF. The NRC staff will publish an updated monitoring plan for the SDF later in FY 2012. (authors)
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗