The Importance of Modelling for Severe Accidents in PHWR SFPS

A spent fuel pool and its storage racks serve the safety functions of cooling irradiated fuel bundles removed from the reactor, while maintaining them in a sub-critical configuration and providing a safe means of their reception, storage, and removal to a longer term storage solution. The potential for fuel heatup and subsequent Zircaloy fires in overheating fuel assemblies upon a sustained, accidental loss of cooling (and/or cooling water) from spent fuel storage pools is a safety concern and has required special investigations for all reactor designs since the Fukushima accident. CANDU fuel bundles are rather densely stored in stacked, 'fish-basket' like, relatively tightly packed, congested configurations that may severely limit the potential heat removal by the natural circulation of air after a loss of liquid water envelope. Potential exists for significant off site doses, should an accidental and sustained uncovery in air of spent fuel bundles cause energetic reactions leading to sheath failures and releases of radioactive fission products.