Discharged CANDU fuel is stored under water in irradiated fuel bays (IFBs) to remove their decay heat. If the fuel is exposed to air, a self-sustaining reaction could result when the Zircaloy-4 sheathing reaches temperatures sufficient for a breakaway oxidation. To predict when the transition occurs, a 2-D fuel bundle cross-section model in air was developed using the COMSOL Multiphysics® platform. Breakaway was predicted to occur at its earliest within 2.6 hours for a range of recently discharged bundle powers. It was concluded due to the time required for heat up and cracking of the oxide layer, sufficient margin exists for operators to intervene before a passively cooled, isolated bundle undergoes breakaway. To examine the effect of multiple bundles, a 3-D model based on a quarter of a stand-alone spent fuel rack was developed to calculate the steady-state temperature and mass fluxes of air. The model provided a lower bound for the ambient temperatures because the flow resistance of the bundle was not considered. The correct incorporation of flow resistance is a necessary step before conclusions could be made about the safety of IFBs. However, the analysis using a Computational Fluid Dynamics model for a 0.5 MW fuel rack, indicated that the maximum temperature of the air within the rack was 642 K and located at the centre of the outlet. This result is encouraging to support the safety of IFBs, as the temperature is well below the 873 K, which is approximately the minimum required for a breakaway reaction.
The event at the Fukushima Daiichi Spent Fuel Pools (SFPs) has renewed interest in quantifying the safety margins related to loss of coolant accidents in Irradiated Fuel Bays (IFBs). Thermal-hydraulic analyses of exposed spent CANDU fuel has been limited to a small number of bundles due to its complex bundle geometry and open rack design. This paper presents a process to predict the steady state temperature and velocity of air as it passes through a rack of spent fuel using analytical models and Computational Fluid Dynamics (CFDs) techniques. The scenario acts as lower bound estimate for the effectiveness of convection during a complete loss of coolant in a fuel bay by examining the heat-up of a stand-alone rack without flow resistance of the bundles. The correct incorporation of flow resistance is a necessary step before conclusions are made about the available safety margins of irradiated fuel bays.
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