The Possibility of Multiple Temperature Maxima in Geologic Repositories for Spent Fuel From Nuclear Reactors

Heat transfer studies for CANDU spent fuel repositories showed that two temperature maxima at the source could occur: one at ∼60 and another one at ∼13,000 yrs. Spent fuel from pressurized water reactors (PWRs) did not exhibit this phenomenon. Because CANDU spent fuels display a monotonically decreasing heat generation rate, it is not immediately obvious why such behavior occurs. This paper demonstrates that the “double-peak” phenomenon is due to the presence of the right mixture of short and long-lived nuclides in the fuel and that the second peak does not occur for repository sizes less than ∼1 km². For larger repositories, radial and surface heat transfer reduces the magnitude of the second peak.