Evaluation of an accident management strategy using an emergency water injection in a reference PWR SFP

Abstract

The Fukushima accident on March 11, 2011 has shown the relevance of examinations of severe accident inside a spent fuel pool (SFP) during beyond-design-basis external events, and the necessity for provisions to cope effectively with such events through a relevant severe accident management (SAM) strategy. Although the low decay heat of fuel assemblies and the considerable water inventory in an SFP can slow the progress of an accident compared to an accident in the reactor core, the numerous number of fuel assemblies stored inside it and the fact that the SFP building is not leak-tight present the potential for the formation of a direct path for fission products to rise from the SFP into the environment (i.e., a much greater severe accident risk). The purpose of this paper is to assess the effectiveness and success conditions of an emergency makeup water injection strategy, which is being as a representative SFP SAM measure after the Fukushima accident. Two typical accident scenarios (loss-of-cooling and loss-of-pool-inventory accidents) and two different reactor operating modes (normal and refueling modes) were considered in the analysis. For the foregoing SAM strategies, the analysis results and relevant insights are summarized in relation to two major aspects: (a) the key events of the progression of an accident (such as the exposure, heat-up and degradation of the fuel assemblies; the generation of combustible gases such as Hydrogen; and the over-pressurization of the SFP building) and (b) the release of radiological fission products (such as Cesium and Iodine) into the environment. A simulation tool for severe accidents, MELCOR1.8.6, was used in the present analysis.