The U.S. is currently re-evaluating the policy on high-level nuclear waste (HLW) management. As part of the Fuel Cycle Research and Development (FCRD) program supported by the U.S. DOE Office of Nuclear Energy, the Used Fuel Disposition (UFD) campaign has been studying generic disposal system environment (GDSE) concepts to support the development of a long-term strategy for geologic disposal of high-level nuclear waste including commercial used nuclear reactor fuels. The GDSE study focuses on the comparative analysis of different GDSE options, and a salt repository is one of the options currently under study.
The immediate goal of the generic salt repository study is to develop the necessary modeling tools to evaluate and improve understanding on the repository system response and processes relevant to long-term HLW disposal in salt. This initial phase of study considers, where applicable, representative geologic settings and features adopted from the literature data for salt repository sites. The conceptual model and scenario for radionuclide release and transport from a salt repository was developed utilizing the literature data. The current version of the salt GDSE model consists of four major model components: source-term, near-field, far-field, and biosphere. The model was developed for a probabilistic analysis framework.
To assist in more meaningful comparative studies for different GDSE options for their geologic features and characteristics, it is necessary to employ uniform modeling assumptions and approaches, to the extent applicable, for the analysis of different GDSE options. For this effort, a reference source-term model was developed for use in all GDSE option analysis. In addition, the IAEA BIOMASS Example Reference Biosphere 1B (ERB 1B) dose model was adopted for the reference biosphere model for the GDSE analysis. The reference source-term model has incorporated: 1) inventory for different waste types (commercial used nuclear reactor fuels, existing high-level waste, and hypothetical reprocessing waste of commercial used fuels); 2) waste package configurations; 3) thermal heat output as a function of time for each waste-type waste package; 4) waste package temperature as a function of time; 5) solubility of key radio-elements as a function of temperature; and (6) fractional waste form degradation rate for commercial used reactor fuel matrix and glass waste form.
This paper presents an initial version of the salt GDSE performance assessment model and discusses the preliminary analysis results, emphasizing key attributes of a salt repository that are potentially important to the long-term safe disposal of HLW. It also discusses the preliminary results on the repository response to the effects of different waste types (commercial used reactor fuels, existing HLW, and reprocessing HLW), and radionuclide release scenarios (undisturbed and human intrusion). In addition, the paper elaborates on the identified knowledge gaps and path forwards for future R&D efforts to advance understanding of salt repository system performance for HLW disposal.