The State of Development of Waste Forms for Mixed Wastes U.S. Department of Energy's Office of Environmental Management (1999) / Chapter Skim
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6. Role of the Waste Form in Performance Assessment
6. Role of the Waste Form in Performance Assessment
Pages 83-96
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From page 83... ...
management process the where long-term stability and performance of the waste form is part of the formal evaluation of a disposal system. Ascertaining the role of the waste form in current PA methodology was necessary for the committee to evaluate the adequacy of available waste forms to meet present regulatory criteria.
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From page 84... ...
, that are present in mixed wasted Although substantial work is being done on developing probability based models for risk assessment of hazardous materials that consider variability and uncertainty, they generally do not extend over the long time periods required by the USNRC for its assessment of disposal facilities.3 Performance assessment encompasses the entire disposal facility as an integrated system. It Involves constructing a conceptual simplification of the system (conceptual model)
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From page 85... ...
The USNRC (1997) notes that the goal of the PA analysis is not to predict the future but rather to test the robustness of the disposal facility against a reasonable range of future scenarios.
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From page 86... ...
if it is incorporated in the facility design, out of the disposal facility to the surro~,nding geologic strata, and finally through the unsaturated and saturated geologic formations around the site to a critical group. The principal role of the waste form in this sequence is to limit the rate of leaching of the contaminants.
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From page 87... ...
Furthermore, there is a much higher degree of mixing in laboratory tests than in a disposal facility, which acts to further increase effective diffusion coefficients. However, it should also be noted that release rates measured finder laboratory conditions do not take into account heterogeneities that are likely to occur in actual waste forms produced by full-scale treatment and stabilization processes.
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From page 88... ...
Due to the lack of longterm performance data, the PAs developed for DOE LLW disposal sites give only short-term credit to waste form performance in the algorithms used to determine exposure (DOE, l99Sb) .6 This approach introduces a high degree of conservatism into PA exposure calculations by incorporating radionucTide release rates that are almost certainly greater than would actually occur in a disposal facility.
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From page 89... ...
In this context, about 95% of the waste was In grout form, about 4% was vitrified, and the remainder was macroencapsulated In polymer. These modeling studies tend to confirm that disposal options for EM's mixed waste are not 7 About 1,800 m3 of this waste could be disposed at the Envirocare MLLW facility in Utah, and all but 100 m3 of the waste met waste acceptance criteria for the Hanford LLW disposal facility.
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From page 90... ...
The study helped confirm that the fractional release rate, a parameter that is typically measured in waste form characterization tests, is a key parameter for assessing long-tenn performance of waste forms that contain both hazardous and radioactive wastes.
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From page 91... ...
The analysis used the approach for the PA documents written for LLW disposal facilities at Hanford, Idaho National Engineering and Environmental Laboratory, Nevada Test Site, Oak Ridge Reservation, and Savannah River Site. Volatile radionuclides were assumed to be transported to the soil surface by diffusion In the vapor phase, and then transported and dispersed in the atmosphere according to an analytical Gaussian dispersion model.
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From page 92... ...
Intrusion scenarios leading to exposure to radioactive wastes are notoriously difficult to quantify, because they depend on future activities under conditions that are not possible to foresee with any degree of certainty. Intrusion scenarios are generally assumed to be inadvertent since access to the waste disposal facility is unintentional and results from some other activity, such as construction, excavation, or drilling.
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From page 93... ...
An inadvertent intrusion at 100 years following closure was assumed for the post-closure drilling scenario. Due to its great depth, the inadvertent human intrusion scenarios for the Waste Isolation Pilot Plant (WIPP)
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From page 94... ...
At many sites, particularly arid sites, the inadvertent intrusion scenarios provide the most restrictive permissible waste concentrations for most radionuclides, because airborne and waterborne pathways are not significant (Waters, et al., ~ 9961. FINDINGS, DISCUSSION, AND RECOMMENDATIONS PA is required by the DOE and the USNRC to evaluate the safety of radioactive waste disposal systems for future generations and the environment.
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From page 95... ...
Current EPA regulations require that the waste form meet shortterm prescriptive criteria, such as the TCEP. USNRC regulations require PA evaluations that extend well beyond the expected useful life of any waste form.
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From page 96... ...
· OST should play a more significant role in promoting (funding) cooperation among investigators who are characterizing waste forms and those who are developing PA models.
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