Postclosure Analysis of the Range of Design Thermal Loadings

This report presents a two-phased approach to develop and analyze a “thermal envelope” to represent the postclosure response of the repository to the anticipated range of repository design thermal loadings. In Phase 1 an estimated limiting waste stream (ELWS) is identified and analyzed to determine the extremes of average and local thermal loading conditions. The coldest thermal loading condition is represented by an emplacement drift loaded exclusively with high-level radioactive waste (HLW) and/or defense spent nuclear fuel (DSNF). The hottest thermal loading condition is a local average identified within a likely ELWS loading sequence. Phase 2 of this study analyzes the postclosure geomechanical, geochemical, and hydrogeologic responses of the repository host rock to these hottest and coldest thermal loading conditions.
The actual waste stream that will be encountered during operation of the repository is likely to differ from the ELWS used in this study. However, the ELWS is a plausible basis for identifying limiting thermal loading conditions that define a “thermal envelope” for analysis.
The results of this study (Section7) show that the postclosure thermal reference case used for total system performance assessment (TSPA) (DTN: MO0702PASTREAM.001 [DIRS 179925]) is an upper bound on the ELWS, when both are expressed as average thermal line loads. This proves in principle that the ELWS can be controlled so as to meet the postclosure temperature limits for the mid-pillar, drift wall, and waste package surface. Previous work already demonstrated that these limits are met by the postclosure thermal reference case for TSPA (SNL 2007 [DIRS 181383], Tables 6.3-49[a] and 6.3-51[a]). This result also shows that far-field system responses are adequately represented by existing analyses and models, if the waste packages are emplaced to maintain the overall average thermal load over distances corresponding to the drift spacing. Far-field geomechanical, hydrogeologic, and geochemical behaviors respond to the average thermal load and are not sensitive to drift-scale variability of waste package heat output.