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International audienceIn the field of severe accident, the description of corium progression events is mainly carried out by using integral calculation codes. However, these tools are usually based on bounding assumptions because of high complexity of phenomena. The limitations associated with bounding situations ([J.M. Seiler, B. Tourniaire, A phenomenological analysis of melt progression in the lower head of a pressurized water reactor, Nucl. Eng. Des. 268, 87 (2014)] e.g. steady state situations and instantaneous whole core relocation in the lower head) led CEA to develop an alternative approach in order to improve the phenomenological description of melt progression. The methodology used to describe the corium progression was designed to cover the accidental situations from the core meltdown to the molten core concrete interaction. This phenomenological approach is based on available data (including learnings from TMI2), on physical models and knowledge about the corium behavior. It provides emerging trends and best estimated intermediate situations. As different phenomena are unknown, but strongly coupled, uncertainties at large scale for the reactor application must be taken into account. Furthermore, the analysis is complicated by the fact that these configurations are most probably three dimensional, all the more so because 3D effects are expected to have significant consequences for the corium progression and the resulting vessel failure. Such an analysis of the in-vessel melt progression was carried out for the Unit 1 of the Fukushima Dai-ichi nuclear power plant. The core uncovering kinetics governs the core degradation and impacts the appearance of the first molten corium inside the core. The initial conditions used to carry out this analysis are based on available results derived from codes like MELCOR calculation code [R. Ganntt, D. Kalinich, J. Cardoni, J. Phillips, A. Goldmann, S. Pickering, M. Francis, K. Robb, L. Ott, D. Wang, C. Smith, S. St. Germain, D. Schwieder, S. Phelan, Fukushima Daiichi Accident Study (Status as of April 2012), Sandia Report Sand 2012-6173, Unlimited Release Printed August, 2012]. The core degradation could then follow different ways: axial progression of the debris and the molten fuel through the lower support plate; lateral progression of the molten fuel through the shroud. On the basis of the Bali program results [J.M. Bonnet, An integral model for the calculation of heat flux distribution in a pool with internal heat generation, in Nureth7 530 Conference Saratoga Springs, NY, USA, September 10–15, 1995 (1995)] and the TMI-2 accident observations [D.W. Ackers, J.R. Wolf, Relocation of Fuel Debris to the Lower Head of the TMI2 Reactor Vessel-A possible scenario, TMI 2 pressure vessel investigation project, in Proceedings of the Open forum OECD/NEA and USNRCm, Boston, USA, 20–22 October 1993 (1993)], this work is focused on the consequences of a lateral melt progression (not excluding an axial progression through the support plate). Analysis of the events and the associated time sequence will be detailed. Besides, this analysis identifies a number of issues. Random calculations and statistical analysis of the results could be performed with calculation codes such as LEONAR–PROCOR codes [R. Le Tellier, L. Saas, F. Payot, Phenomenological analyses of corium propagation in LWRs: the PROCOR software platform, in ERMSAR 2015, Marseille, France, 24–26 March, 2015 (2015)]

Possible in-vessel corium progression way in the Unit 1 of Fukushima Dai-ichi nuclear power plant using a phenomenological analysis