Rethinking High-Level Waste Disposal: Separate Disposal of High-Heat Radionuclides (⁹⁰Sr and ¹³⁷Cs)

An alternative approach for disposal of high-level waste (HLW) is proposed. HLW would be separated into two fractions: (a) the high-heat radionuclides (HHRs), e.g., ⁹⁰Sr and ¹³⁷Cs, and (b) the low-heat radionuclides (LHRs), which are all the remaining radionuclides. These two categories of waste would be disposed of separately in different sections of the repository or different facilities.

The LHRs in the HLW contain the long-lived radionuclides that control the repository performance requirements that in turn necessitate (a) expensive waste packages (WPs) and (b) limiting the repository temperatures to avoid repository performance degradation. To limit repository temperature, the amount of HLW per WP is limited and the WPs are spread over a large area. If the decay-heat-generating HHRs are removed from HLW, the repository design is not controlled by decay heat. The resultant LHR repository size (area, number of WPs, total tunnel length) may be reduced to <20% of the size of a conventional repository. With a waste partitioning and transmutation process that includes removal of the minor actinides (americium and curium) from the LHR wastes, significant further reductions in repository size are possible. The minor actinides are the next largest heat generators in LHR wastes.

Separate management of HHRs does require (a) separation of the HHRs from the HLW and (b) a separate HHR disposal facility. The HHRs are disposed of in a separate lower-cost facility made possible by the limited lifetimes (T_1/2 ~ 30 yr) of the HHRs. There are potentially significant gains in economics and repository performance for separate management of HHRs and LHRs in some types of fuel cycles.