Spent Nuclear Fuel and Graphite Management for Salt-Cooled Reactors: Storage, Safeguards, and Repository Disposal

Abstract

The fluoride salt–cooled high-temperature reactor (FHR) is a new reactor type that combines the graphite-matrix coated-particle fuel and graphite moderator from high-temperature gas-cooled reactors (HTGRs) with a clean liquid fluoride salt coolant. No FHR has yet been built. The proposed fuel cycle is a once-through fuel cycle—essentially identical to that of HTGRs. There is the option of adopting closed fuel cycles. Relative to light water reactor (LWR) spent nuclear fuel (SNF), all graphite-matrix coated-particle SNFs share the common characteristics of superior proliferation resistance and long-term performance as a waste form in a geological repository. The allowable HTGR and FHR SNF storage temperatures are much higher than allowable LWR SNF storage temperatures. These SNF characteristics are (a) a consequence of the high-temperature fuel form with a graphite matrix and SiC coating of the fuel microspheres and (b) to a first-order approximation independent of the reactor type in which the fuel is used.There are differences. The FHR reactor core power density is four to ten times higher than in an HTGR, so the short-term decay heat of the SNF per unit volume upon discharge is four to ten times higher. The volume of FHR SNF is one-half to one-third that of an HTGR per unit energy output because (a) the salt provides some neutron moderation thus reducing the carbon-to-uranium ratio of the fuel and (b) the economic optimization with higher power densities increases the fuel loading. The FHR SNF volume is about four times that of a LWR per unit of electricity. The coolant generates significant tritium that is partly absorbed by the graphite and can be partly desorbed at higher temperatures. Last, any residual solid salt coolant with the SNF at low temperatures can undergo radiolysis with the potential generation of fluorine gas. The presence of the salt coolant on the SNF and graphite moderator will require treatment, removal of residual coolant salt, or demonstration that the small quantities of radiolysis products of frozen salt do not impact long-term performance of storage or disposal facilities.