Phenomenological Analysis of Melt Progression in LWRS: Proposal of an In-Vessel Corium Progression Scenario in the Unit 1 of Fukushima Dai-ichi Nuclear Power Plant

In the field of severe accident, the description of corium progression events is mainly carried out using integral calculation codes. However, these tools are usually based on bounding assumptions because of the high complexity of phenomena. The limitations associated with bounding situations [1] (e.g., steady-state situations and instantaneous whole core relocation in the lower head) led CEA to develop an alternative approach to improve the phenomenological description of the 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 (MCCI). This phenomenological approach is based on the available data (including learnings from TMI-2) on physical models and knowledge about the corium behavior. It provides emerging trends and best-estimate 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 (3D), 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 the available results derived from codes such as the MELCOR calculation code [2]. The core degradation could then follow different ways: (1) Axial progression of the debris and the molten fuel through the lower support plate, or (2) lateral progression of the molten fuel through the shroud. On the basis of the Bali program results [3] and the TMI-2 accident observations [4], 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 some number of issues. Random calculations and statistical analysis of the results could be performed with calculation codes such as LEONAR–PROCOR codes [5]. This work was presented in the frame of the OECD/NEA/CSNI Benchmark Study of the Accident at the Fukushima Dai-ichi Nuclear Power Station (BSAF) project [6]. During the years of 2012 and 2014, the purpose of this project was both to study, by means of severe accident codes, the Fukushima accident in the three crippled units, until 6 days from the reactor shutdown, and to give information about, in particular, the location and composition of core debris.