"The presence of a stratification in a NPP containment is a source of concern, as pockets of hydrogen in high concentration could lead to a deflagration or detonation risk. The objectives of the project are two folds: one is to establish whether in a test sequence representative of a severe accident in a LWR a hydrogen (helium) stratification can be established during part of the transient starting from the initiation of the loss of coolant accident (LOCA) blowdown until the end of bulk hydrogen release from the reactor vessel into the containment, and the second is how this stratification can be broken down by the operation of Severe Accident Management systems (SAMs); sprays, coolers and Passive Auto-catalytic Recombiners. Experiments will be performed in the “smaller scale” TOSQAN (IRSN, Saclay), ""medium scale"" in the MISTRA (CEA, Saclay) and PANDA (PSI, Villigen) facilities. The specifications of the counterpart tests to be designed for the desired tests in the KMS facility (NITI, St Petersburg) will provide basis for code benchmarking at ""nearly prototypical scale”. State of art tools, lump parameter to CFD codes, are planned for pre- and post-test calculations and scenario analyses. A synthesis report to be produced will integrate the outcome of test results from the facilities as well as the code analysis in view of highlighting the anticipated behaviour in a LWR and point out to strengths and deficiencies in the predictive capabilities of the tools. To be coupled at the technical and administrative levels, the research will be conducted by two parallel running projects. One project consortium is composed of PSI (Switzerland), IRSN and CEA (France), KIT (Germany), NRG (The Netherlands) and AECL (Canada) and the second project to be run by the Russian organizations: IBRAE, SPbAEP, IPPE, and NITI (the Russian Federation). These two strongly interlinked and coordinated research projects will foster cooperation and provide means to reach a common safety goal."