CFD analysis of hydrogen volumetric concentrations in a Hard Venting Containment System of a Mark II BWR

Abstract

After the Fukushima Daiichi Nuclear Power Plant (NPP) accident, the analysis of severe accidents (SA) has become an important field of study in order to define appropriate severe accident management and emergency planning. The Station Blackout (SBO) scenario, in which all the external power is lost, is a reference to the simulation of a severe accident initiating event. All nuclear power plants are designed to face partially a SBO. However, a long SBO scenario can result in a beyond design basis accident and evaluations in order to define severe accident management guidelines (SAMGS) must be performed. The hydrogen generation due to the high temperatures of the melting core in contact with steam is an issue in the assessments of SA. Usually, in boiling water reactors (BWR), the primary containment has an inert atmosphere (with Nitrogen) in order to prevent a detonable hydrogen–oxygen mixture. However in a SA, the primary containment is highly jeopardized and can fail with the subsequent release of the inert atmosphere to the secondary containment where the hydrogen can be in contact with the oxygen of the air and a detonation may occur (as in Fukushima Daiichi NPP). An option in order to avoid a detonation inside the reactor building in the case of a primary containment failure is by means of the installation of a Hard Venting Containment System (HVCS). A methodology in order to assess the transport of gases (especially hydrogen, steam and nitrogen) inside the hard venting pipe has been developed. Taking the results of the MELCOR code, mainly mass fluxes as well as temperature and pressure fields inside the primary containment, a source term for the computational fluid dynamics (CFD) code GASFLOW is calculated in order to predict the hydrogen behaviour inside the venting system. The calculations performed with GASFLOW showed the risk of the occurrence of a detonation inside the venting pipe due to the hydrogen volumetric concentrations entering in contact with the air inside the venting pipe (if it is initially filled with air).