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.
