Simulation of CANDU Fuel Thermal-Hydraulic Behaviour During Spent Fuel Bay Loss of Cooling Events

After the Fukushima accident, one of the highest priorities for CERNAVODA NPP was to investigate events that can lead to Spent Fuel Bay (SFB) loss of cooling and loss of coolant inventory. In CANDU plants, fuelling is performed on-power. Daily, fresh fuel bundles are loaded in core and spent fuel bundles are discharged from the core, transferred and stored in SFB. Due to the SFB limited storage capacity, bundles having 6 years or more of cooling time are transferred to the Dry Storage Facility. Thus, as per design, a maximum number of around 38 000 fuel bundles can be stored, at any time, in SFB. Following a loss of class III and class IV power sources (e.g. Station Blackout), the cooling and purification systems for SFB water become unavailable. Consequently, the bay water temperature increases up to the boiling conditions and, due to boiling and vaporization, the water inventory and level will decrease in time. The decrease of coolant level can leave uncovered a number of fuel bundles, degrading their cooling. The present paper reviews the analysis methodology and results for a typical event of Spent Fuel Bay loss of cooling. Methodologies used in the analysis and results presented are focused upon the CANDU fuel thermal-hydraulic behaviour during the event and upon its potential radiological hazard. (author)