Allowable Maximum Storage Temperatures for CANDU Fuel in Air in Interim Storage Systems and Used Fuel Containers for a DGR

Abstract

The use of ambient air in the storage of used fuel instead of an inerting atmosphere such as helium is a desirable engineering feature. The potential presence of undetected defective fuel with through the wall fuel element cladding defects requires the determination of the allowable maximum temperature fuel can be exposed to during its storage. This is because oxidation of the UO₂ fuel pellets in the defective fuel elements by air can lead to the unzipping of the fuel element cladding containing the fuel.

Both defective and non-defective CANDU fuel stored in air at constant temperature of 150°C from 1980 to 1997 at the former AECL-Whiteshell Laboratories as part of the Controlled Experiment Program (CEX-1) maintained its mechanical integrity with insignificant change. Defective CANDU fuel showed gross oxidation of the UO₂ pellets to U₃O₇/U₄O_9+x without breaching the fuel cladding. These experiments failed to provide a kinetic relation for the oxidation of defected fuel. However, the USA DOE developed an empirical relation for the oxidation of LWR fuel. Because of the similar oxidation mechanisms in both fuels, the DOE relation was expanded to CANDU fuel and applied to typical storage systems for CANDU fuel including permanent storage in the proposed NWMO Used Fuel Containers (UFCs) in a DGR.

The temperature of the used fuel in storage is a function of time, the storage system design characteristics, the age of the fuel and the fuel heat of decay. It was determined that fuel older than 7 years can be stored for up to 100 years at an initial maximum fuel temperature of 160°C without the fuel cladding being breached, and indefinitely in UFCs in a DGR if the initial fuel peak temperature is below 150°C.