Cladding Behaviour Under Severe Accident Conditions in SFP

While in a reactor accident the reactor pressure vessel and the containment are barriers for the fission product release, the cladding of the fuel in a spent fuel pool is the only barrier between the fission products and the environment. Therefore cladding integrity is of high importance for the wet and dry storage of nuclear spent fuel. Since more than 10 years Paul-Scherrer-Institute (PSI) is involved in research activities to investigate and model phenomena, which are of great importance for the cladding integrity under spent fuel pool conditions. Part of the phenomena like air oxidation and break away of oxide layer were not modelled in the severe accident codes and so a new model, which was primarily thought for air ingress during a reactor accident or a mid-loop accident during maintenance, was developed using separate-effect test data from IRSN, France and KIT, Germany. This new model was implemented in the severe accident codes SCDAP/RELAP5 and MELCOR. Then the focus of the severe accident research group (SACRE) concentrated more on the "Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Station (BSAF) Project" and on the spent fuel pool and possible accident scenarios with the participation in the OECD/NEA SFP project. After the Fukushima accident, the SACRE group was involved in an IAEA project to collect, analyze and assemble data from Fukushima Daiichi spent fuel storage facilities. The knowledge gained during that time was applied in several international projects. A NUGENIA project with a severe accident code benchmark exercise followed these activities and finally a phenomena identification and ranking table was produced during an OECD/NEA project. As a result of the ranking table, a project for the investigation of the nitrogen influence on the cladding degradation was started with an experimental program at KIT, Germany, and a nitriding model development at PSI, Switzerland.