US Research on Direct Geologic Disposal of Spent Fuel Canisters

In the U.S., the need for SNF dry interim storage at the nuclear power plants started a little more than 10 years before the 1998 planned opening of Yucca Mountain, has continued now while Yucca Mountain remains suspended, and is expected to continue for several more decades until Yucca Mountain or an alternative repository is licensed by the U.S. Nuclear Regulatory Commission (NRC) and built and operated by the U.S. Department of Energy (DOE) or a new organization created by the U.S. Congress.  Thus, welded dual purpose canisters (DPCs) have been constructed to meet the needs of on-site dry interim storage and off-site transportation, but not necessarily geologic disposal.  Rather than cutting open the DPCs and then repackaging the SNF, which the DOE estimates could cost several billions of dollars and will result in increased SNF handling after an unknown duration in dry storage and increased doses to workers, the DOE with industry support is examining the possibility of disposal of the SNF in the existing DPCs, known as “Direct Disposal.”  DOE performed several studies related to the direct disposal of DPCs with the focus of these studies on addressing four technical elements: safety of workers and the public, engineering feasibility, thermal management, and post-closure criticality control.

Waste Package Multi-Assembly Glass Particulates Fill Test. ORNL 1997.

The general finding is that direct disposal of loaded DPCs without modifications, is technically feasible at least for some DPCs in some potential repository host media, with the remaining technical challenge associated with post-closure criticality in DPCs. The focus of this paper is to technically address post-closure criticality through one of four potential paths forward to cover the 3,200+ canisters currently loaded in the U.S.: (1) full burnup credit for the actual canister contents to demonstrate sub-criticality under credible disposal conditions; (2) ensuring the consequences of a criticality event are acceptable to the performance of the repository; (3) injection of fillers into the welded canisters as credit for moderator exclusion; and (4) inclusion of corrosion-resistant neutron absorber materials into the basket of a DPC prior to or after the loading of the canister with SNF. As Spain’s inventory of SNF in on-site interim dry storage grows and the potential use of welded canisters appears evident, this direct disposal path for welded dual purpose canisters could be of interest to future planning activities for SNF in Spain.

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