LWR severe accident simulation: synthesis of the results and interpretation of the first Phebus FP experiment FPT0

The international Phebus Fission Product (FP) programme, initiated in 1988 and performed by the French Institut de Radioprotection et de Sûreté Nucléaire (IRSN), investigates key phenomena of severe water reactor accidents. Six in-pile experiments were planned. Four have been successfully performed in 1993, 1996, 1999 and 2000. The first experiment, called FPT0, used uranium dioxide fuel of 4.5% enrichment in-situ irradiated for 9 days to a burn-up of 230 MWd t −1. It was designed to reach significant fuel melting and to study low pressure fission products release and transport through the primary cooling system including a non-condensing steam generator and into the containment vessel. As the first test of the programme, FPT0 was intended to demonstrate the adequacy of the new, complex Phebus facility to simulate the anticipated phenomena and was the first attempt in using the new experimental results for verifying codes. The scientific results from FPT0 were sufficiently challenging that they deserve to be documented and interpreted. Since some of them did not correspond to the predicted and pre-calculated behaviour, the post-test analysis and interpretation period was rather long. Three years later, the second experiment FPT1, rather similar in its boundary conditions but using a fuel burned in a reactor (23 GWd t −1), confirmed certain FPT0 results, helping in their final interpretation and removing doubts about possible fundamental shortcomings of the Phebus facility. More detailed experimental results of the test are available in the final test report deliverable on CD-ROM support. It can be obtained upon request from IRSN. 1 Requests upon CD-ROM have to be addressed to georges.repetto@irsn.fr and to nathalie.girault@irsn.fr. 1 This report retraces the history of FPT0 and its general programme context, and briefly describes the layout of the facility, supporting separate effect tests and computational tools. It then presents the synthesis of the results and of the international understanding reached concerning their interpretation, with emphasis on fuel and fission product behaviour. Finally, conclusions are presented about the impact of FPT0 on severe accident modelling with implications on source term evaluation and on accident prevention and mitigation studies.