Computational validation of the EPR™ combustible gas control system

During a postulated severe accident in a pressurized water reactor which involves the melting of the reactor core, hydrogen is released into the containment. In order to limit the pressure and temperature loads in case of a possible combustion of hydrogen and to prevent a challenge to the containment integrity, the EPR™ containment is equipped with a combustible gas control system. The multi-step approach for analytically validating the performance of this system is presented. The starting point for the analysis is a study of a large number of core melt accidents using a lumped-parameter code, which yields the release characteristics from the primary circuit into the containment. Based thereupon, a set of representative scenarios and bounding scenarios is selected for numerical calculations with a lumped-parameter code, and a smaller subset is then chosen for an additional in-depth analysis with computational fluid dynamics codes. Together, these calculations yield the detailed distribution of hydrogen in the containment, the depletion of hydrogen by recombiners, and the risk of the occurrence of potentially hazardous combustion modes. For cases where flame acceleration after hydrogen ignition cannot be ruled out by simple criteria, additional calculations with a dedicated computational fluid dynamics combustion code demonstrate that a transition from deflagration to detonation in the containment can be excluded. It is also shown that the temperature and pressure loads remain within the acceptable limits. Furthermore an in-depth analysis of the individual recombiner performance and methods for optimizing hydrogen depletion by the arrangement of the recombiners in the containment is presented. The potential impact of an early activation of the spray system on the hydrogen risk is also discussed.