Appendix E

Long-Term Safety of the Direct Grout Option

Cook (1998) conducted an initial scoping performance assessment to compare the long-term safety of the direct grout option to the safety of the existing Z-Area saltstone disposal facility, as expressed in the performance assessment for the facility (Martin Marietta Energy Systems, Inc., et al., 1992; Westinghouse Savannah River Company, 1998a). The assessment was not done using the full performance assessment methodology developed by Westinghouse Savannah River Company (Martin Marietta Energy Systems, Inc., et al., 1992; Westinghouse Savannah River Company, 1998a). Instead, the assessment comprised the following three analyses:

  • a screening-level assessment1 was conducted of doses from the groundwater pathway,

  • a simplified groundwater dose analysis was undertaken, and

  • an analysis was conducted of doses to an inadvertent intruder.

The purpose of the screening-level assessment was to provide a conservative estimate of potential drinking water doses to an offsite individual. If for any radionuclide the calculated dose is significantly less than the acceptable dose constraint for this conservative analysis, that radionuclide was not considered further in the assessment. Based on this screening-level assessment, Cook (1998) identified that cesium-135 would need to be considered in a performance assessment of the direct grout option, whereas it had been screened from consideration in the saltstone performance assessment (Martin Marietta Energy Systems, Inc., et al., 1992; Westinghouse Savannah River Company, 1998a). The calculated dose for cesium-135 in this screening assessment was a factor of 30,000 higher than in the screening analysis using the reference salt supernate waste stream concentrations (see Table 7.1 in the main text).

1  

An assessment intended to remove radionuclides from further consideration, thus reducing the work load of the analyst.



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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE Appendix E Long-Term Safety of the Direct Grout Option Cook (1998) conducted an initial scoping performance assessment to compare the long-term safety of the direct grout option to the safety of the existing Z-Area saltstone disposal facility, as expressed in the performance assessment for the facility (Martin Marietta Energy Systems, Inc., et al., 1992; Westinghouse Savannah River Company, 1998a). The assessment was not done using the full performance assessment methodology developed by Westinghouse Savannah River Company (Martin Marietta Energy Systems, Inc., et al., 1992; Westinghouse Savannah River Company, 1998a). Instead, the assessment comprised the following three analyses: a screening-level assessment1 was conducted of doses from the groundwater pathway, a simplified groundwater dose analysis was undertaken, and an analysis was conducted of doses to an inadvertent intruder. The purpose of the screening-level assessment was to provide a conservative estimate of potential drinking water doses to an offsite individual. If for any radionuclide the calculated dose is significantly less than the acceptable dose constraint for this conservative analysis, that radionuclide was not considered further in the assessment. Based on this screening-level assessment, Cook (1998) identified that cesium-135 would need to be considered in a performance assessment of the direct grout option, whereas it had been screened from consideration in the saltstone performance assessment (Martin Marietta Energy Systems, Inc., et al., 1992; Westinghouse Savannah River Company, 1998a). The calculated dose for cesium-135 in this screening assessment was a factor of 30,000 higher than in the screening analysis using the reference salt supernate waste stream concentrations (see Table 7.1 in the main text). 1   An assessment intended to remove radionuclides from further consideration, thus reducing the work load of the analyst.

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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE A simplified groundwater transport analysis was undertaken using the PATHRAE computer code, a rather old code for evaluation of transport in groundwater and its environmental consequences. Several versions of the PATHRAE code exist (e.g., Rogers et al., 1985; Merrell, Rogers, and Bollenbacher, 1986; Rogers and Hung, 1987). Cook (1998) did not specify which version was used, nor how the simple model in PATHRAE was justified as an appropriate screening-level tool for the saltstone disposal facility. This assessment was conducted for the case of a degraded vault (Cook, 1998). Results of the assessment for three key radionuclides are shown in Table E.1, compared to results from the saltstone performance assessment model (Martin Marietta Energy Systems, Inc., et al., 1992; Westinghouse Savannah River Company, 1998a). The peak dose from cesium-135 corresponds to a dose from drinking water of 1.7 mrem yr-1. This value is quite close to the drinking water standard of 4 mrem yr-1 in DOE Order 435.1, but Cook (1998) argued that a detailed performance assessment would lead to lower doses from Cs-135 than were found in the PATHRAE analysis. TABLE E.1 Comparison of Results from the Saltstone Performance Assessment Model and the Simplified PATHRAE Analysis of Cook (1998) for the Direct Grout Waste Stream Radionuclide Saltstone Performance Assessment Time of Peak (year) Saltstone Performance Assessment Peak Concentration (pCi/L) PATHRAE Time of Peak (year) PATHRAE Peak Concentration (pCi/L) Selenium-79 150,000 4.4 20,000 73 Iodine-129 3,200 0.075 21,000 5.9 Cesium-135 negligible negligible 100,000 330 A full analysis of the intruder scenario was not conducted by Cook (1998). Instead, results were derived from existing results for the saltstone disposal facility, with revised radionuclide inventory values. Intrusion assessment results were not available for cesium-135 from the saltstone performance assessment (Martin Marietta Energy Systems, Inc., et al., 1992) because it had been screened out of the performance assessment for groundwater impacts. For the direct grout assessment, rather than recalculating intrusion scenario results for this radionuclide, results from the E-area vault performance assessment (Westinghouse Savannah River Company, 1998b) were used, and the results were corrected for differing vault volume and inventory.

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ALTERNATIVES FOR HIGH-LEVEL WASTE SALT PROCESSING AT THE SAVANNAH RIVER SITE REFERENCES CITED Cook, J.R. 1998. Effect of “Grout-it-all” on Saltstone Performance Assessment. Westinghouse Savannah River Company SRT-WED-98-0119, Rev. 2, Aiken, SC. Martin Marietta Energy Systems, Inc., EG&G Idaho, Inc., Westinghouse Company, and Westinghouse Savannah River Company. 1992 (December 18). Radiological Performance Assessment for the Z-Area saltstone disposal facility. Westinghouse Savannah River Company WSRC-RP-92-1360, Aiken, SC. Merrell, G.B., V.C. Rogers, and M.K. Bollenbacher. 1986. The PATHRAERAD Performance Assessment Code for the Land Disposal of Radioactive Wastes. Rogers and Associates Engineering Corporation RAE-8511-28, Salt Lake City, UT. Rogers, V.C., G.M. Sandquist, G.M. Merrell, and A. Sutherland. 1985. The PATHRAE-T Code for Analyzing Risks from radioactive Waste. Rogers and Associates Engineering Corporation RAE-8339/12-2, Salt Lake City, UT. Rogers, V.C., and C. Hung. 1987. PATHRAE-EPA: A Low-Level Radioactive Waste Environmental Transport and Risk Assessment Code. U.S. Environmental Protection Agency EPA 520/1-87-028, Washington, D.C. Westinghouse Savannah River Company. 1998a. Addendum to the Radio-logical Performance Assessment for the Z-Area Saltstone Disposal Facility. WSRC-RP-98-00156, Rev. 0, Aiken, SC. Westinghouse Savannah River Company. 1998b. Radiological Performance Assessment for the E-Area Vaults Disposal Facility. WSRC-RP-94-218, Rev 1, Aiken, SC.