Gas distribution in the containments of nuclear power plants in a case of severe accident currently is a high priority safety issue. One of the topics in this issue is a formation and break up of stratified conditions inside the containment. CEA (France) performed M5 test in their MISTRA test facility, to form a stratified atmosphere, which later was destroyed during MASP tests that followed. During M5 there was formed vertically stratified atmosphere using steam jet release. Application of lumped parameter codes for simulation of jet release is complicated due to limitations inherent in the lumped-parameter approach. However, measures exist which can be used to take these limitations into account when using lumped parameter approach.
This paper presents simulations and parametric study of M5 test, i.e. only formation of the stratified atmosphere. Presented simulations of the experiment were performed using lumped parameter code COCOSYS. The aim of the work is to investigate the capability of the code to simulate correctly jet release and formation of the stratified atmosphere in the M5 experiment and the impact of several parameters to the simulation results.