IRSN used the ASTEC code to perform numerical simulations of the Fukushima-Daiichi accidents in the frame of the OECD BSAF project. Presently, simulations are available for the three units and for six days from the earthquake. A clear lesson from the project phase 1 was that the uncertainties on the safety systems functioning and accident progression are still large and many ways are liable to explain the measured thermohydraulics behavior. Rather than focusing on the thermo-hydraulics key-parameters for which comparisons with measurements are available, the presentation will address melt composition computation results which may provide insights relevant for the decommissioning process.
When the molten corium relocates from the core down to the vessel lower head, the melt jets interact with water and may be totally or partially fragmented depending on the level of water. A U–Zr–O–Fe molten pool may form in the lower head and due to chemical reactions, separation between non-miscible metallic and oxide phases may occur. The models implemented in ASTEC enable to simulate these phenomena. Up to five different axisymmetric corium layers in the vessel bottom head can be formed, which are, from bottom to top: a debris layer, a heavy metallic layer, an oxide layer, a light metallic layer and another debris layer. An important process is the UO2 fuel reduction to metallic uranium by non-oxidized zirconium which results in uranium transport to the dense metallic layer as demonstrated in the MASCA Program.
Complex melt compositions before vessel failure will be presented for the current “bestestimate” cases with a special focus on Unit 1 due to the large consensus on the accident progression for this Unit.
It should be underlined that in case of vessel bottom failure a part of this complex melt will be relocated to the pedestal and a molten core concrete interaction will take place enhancing other complex physical phenomena with possible large consequences on the melt chemical composition and behavior.