The latest results from source term research: overview and outlook

Source term research has continued internationally for more than 30 years, with the overall aim of increasing confidence in the methods used in calculating the potential radioactive release to the environment after a severe reactor accident. Important data have been obtained from small- and large-scale experiments, mainly under international frameworks such as OECD/NEA, European Framework Programs of EURATOM and specific consortia. In particular, Phébus FP and associated studies provide outstanding insights into fission product release and transport and, particularly, containment iodine chemistry, which have been and are being encapsulated in recent versions of severe accident analysis codes like ASTEC 2.1, MELCOR 2.1 and MAAP-EDF, while data from newer projects such as VERDON, BIP and THAI are being interpreted with a view to further improvements in code capability. This paper briefly synthesizes the recent main outcomes from source term research concerning the above topics, and also source term mitigation. It highlights the knowledge gaps remaining and discusses ways to proceed, addressing those items considered high priority, taking as a basis the most recent source term workshop held in April 2015 under the international non-profit association for Gen. II and Gen. III reactor research, NUGENIA, sub-task 2.4 (Source Term) of technical area 2 (Severe Accidents). Two major issues generally affect experimental data from source term research: analytical exploitation and scale-up. Concerning fission product release, oxidizing environments potentially leading to increased release of harmful nuclides, like Ru-103/106, are considered of utmost interest. For transport, potential revolatilisation of fission products embedded in deposits needs further study. Containment iodine chemistry is and has been extensively investigated in recently ended, ongoing and upcoming OECD projects, like BIP3, STEM2 and THAI3, and the net results need evaluation. For mitigation, long term filter behavior regarding fission products, existing capabilities to remove Ru and the scrubbing capacity of pools undergoing saturation need further research. Aside from further knowledge-driven research, there is consensus on the need to assess the source term predictive ability of current system codes.