KTH experiments on severe accident phenomena relevant to Swedish BWRs

A significant fraction of national electricity production in Sweden is supplied by nuclear power plants with BWR reactors. Severe accident management concept of Swedish BWRs, which has been developed in 70-80s, envisages ex-vessel fragmentation and quenching of corium melt in a deep water-filled reactor pit and long term coolability of corium debris bed with water natural convection through the bed open porosity. The paper deals with experimental studies of several severe accident phenomena critical for the chosen SAM strategy: Corium melt jet fragmentation in water; debris bed formation and its properties; debris bed evolution and particle spreading; debris bed coolability; steam explosion during FCI in stratified configuration. Some observations, results and main conclusions from the listed experiments with high temperature corium simulants are presented. The experimental data were used for development and validation of different models and tools, such as MEWA and DECOSIM simulating melt arrest and coolability at the late phase of severe accident under quench and boil-off conditions. The studies were carried out in the Division of Nuclear Power Safety at the Royal Institute of Technology (KTH), Stockholm in the frames of different national and international projects and programs supported by industry, regulators, research organizations and EU. The EU part of the research was coordinated by the SARNET network. The SARNET collaboration will be continued in NUGENIA format. (author)