Severe accident analysis of plant-specific spent fuel pool to support a SFP risk and accident management

Abstract

The Fukushima accident that happened on March 11, 2011 has raised the possibility of severe accidents in an SFP under beyond design basis external events (BDBEEs) that might lead to a loss of all safety functions and need a way to cope effectively with such an event through a SFP risk and accident management. From a SFP safety point of view, the low decay heat of the fuel assemblies and large water inventory in an SFP may make the accident processes slow compared to an accident in the reactor core, but a huge number of fuel assemblies stored inside it and no containment in a SFP building (especially for PWR types) might be exposed to much greater risk. A quantitative analysis for SFP accidents can give more insights into which aspects play dominant roles in an accident progression. To date, however, few studies have been made for severe plant-level SFP accidents. This paper provides the plant-specific MELCOR analysis results for potential severe accidents that can be expected in a typical PWR. The SFP of the reference plant is located adjacent to the primary containment (outside primary containment), and its bottom is placed above the plant grade. Three representative accident scenarios (loss of cooling accident, loss of coolant inventory, and complete loss of coolant accident) and two different reactor operating modes (normal operation and refuelling mode) have been considered for the analysis. From the risk and accident management point of view, phenomenological analysis was mainly focused on (a) the level of the coolant, (b) the cladding temperature, (c) the mass of hydrogen generated during accident progressions, and (d) MCCI (Molten Corium Concrete Interaction).