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.

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