Nuclear reactor containment flows – modelling of stratified layer erosion by a wall-deflected jet

A number of international computational benchmarks were devoted to assessing the capability of commercial and in-house Computational Fluid Dynamics (CFD) codes to predict transient evolution of the concentration and velocity fields of the nuclear power plant containment atmosphere in the course of severe accidents. In these benchmarks the velocity and concentration fields were measured during the transient. Usually, a vertically directed jet eroded an air-helium mixture stratified layer, set in the top part of the experimental vessel, which mimics the hydrogen-air mixture in a real life nuclear power plant in the course of a severe accident. As the jet direction can differ from vertical, the mixing process is anticipated to differ from the one observed in the preceding benchmarks. The present study focuses on modelling a recent benchmark exercise that was organized and performed in the MISTRA facility, CEA, Saclay. In this benchmark, a hot air jet impinges horizontally onto a cylindrical compartment wall. After the impingement, the air flows towards and erodes a stably stratified helium-air mixture set in the top part of the MISTRA vessel. A correct prediction of the integral mixing process relies on modelling of a number of separate flow situations, including free and buoyant horizontal jets, round jet impingement onto a cylindrical surface, jet to cylinder heat transfer, turbulent erosion in the presence of buoyancy, and more. The present study includes validation of the separate flow features versus experimental studies from the literature. After finalizing the modeling approach based on the separate-effect numerical studies, the MISTRA benchmark exercise is modelled and the computational and measured results are compared.