Significance of Phébus-FP results for plant safety in Switzerland

•Prototypicality of Phebus is unique for severe accident integral code assessment.•MELCOR and SCDAP show good simulation capability.•Phebus revealed knowledge gaps concerning organic iodine source.•Complementary studies seek to reduce uncertainties.•New engineering technology is developed to resolve organic iodine issue.

Switzerland, in common with other countries, uses source term estimates to formulate emergency plans in the unlikely event of an accident with release of activity to the environment. In the past estimates have typically been based on conservative treatments of the accident sequence, necessitated by the limited validation status of models used for the phenomena that control the reactor accident evolution. Although analyses using best-estimate tools frequently indicated substantially smaller releases – knowledge and data were insufficient to place reliance on the methods or the calculated results.The Phébus programme is unique in providing a source of integral transient data on fission product release, transport and chemical behaviour under prototypic conditions, capturing the entire portfolio of processes – transport, material and chemical – and their causal interaction. To this day, no other source of such data is available. These data provide a means to assess, improve and validate methods for source term evaluation and establish the needs for establishing new processes to mitigate the source term. There are strong synergies and complementarities between the Phébus project, the International Source Term Project, the iodine-related studies at PSI and the aerosol retention projects at PSI, and current moves toward improved management/mitigation of severe accidents.The Federal Nuclear Safety Inspectorate (ENSI), Paul Scherrer Institute (PSI) and the Swiss utilities running five nuclear power plants share in the findings. The need to strengthen the foundation on which we perform best-estimate evaluations of reactor accidents, identified in the early start of the Phébus programme and which motivated Switzerland’s participation, still remains at this point at the closure of Phébus. Likewise, there is a clear need to enable retention of volatile iodine in the venting filters so as to make venting a feasible accident management measure. Indeed the events at Fukushima Daiichi in 2011 make this need particularly pressing in the context of present challenges to continuation and further growth in nuclear electricity generation.