Detailed Assessment of Combustion isk and PAR Efficiency in the Late Phase of a Severe Accident within the European AMHYCO Project

Abstract

The European AMHYCO project (Euratom 20192020, GA No 945057) aims at enhancing the understanding of H₂/CO combustion risk within the containment of a nuclear power plant in the late phase of a severe accident. The goal is to incorporate this knowledge into severe accident management guidelines and recommendations for long-term operation upgrades. Based on a critical review of existing methodologies and practices related to gas combustion risk, as well as the identification of accident sequences where the containment integrity may get challenged, experimental investigations were conducted to close knowledge gaps related to combustion characteristics and the operation of passive autocatalytic recombiner under late phase conditions.

To prepare the basis for the further assessment and refinement of existing SAMGs, systematic detailed analyses of the most challenging scenarios and possible mitigative measures were conducted for three generic European PWR containment designs, namely KWU, Westinghouse, and VVER. For each reactor type, one Loss of Coolant Accident and one Station Blackout scenario were selected for detailed analyses. Both scenarios cover a range of in-containment atmospheric conditions from potentially flammable at medium pressure to a steam-inertized atmosphere at high pressure, including the late phase with an active filtered containment venting system.

This paper outlines the employed methodology using a consecutive analysis chain consisting of three levels with increasing detail (system codes, 3D GOTHIC™ and CFD) to assess containment pressurization, efficiency and/or options of individual mitigation measures with respect to H₂/CO combustion risk and equipment and instrumentation survivability. As a common basis, the system code nodalization schemes and 3D models are developed from detailed CAD geometries. The paper summarizes the status of the work with a focus on the comparative assessment of the impact and effectiveness of mitigative measures (PARs, sprays, FCVS) on the combustion risk in the late phase. Concluding, challenges and lessons-learned are summarized.