Descriptif du sujet
Severe accidents arising from the fusion of the nuclear reactor core must be anticipated to enhance the efficiency of its mitigation. Such accidents have occurred at TMI-2 in the USA- 1979, Chernobyl-1984 and in Fukushima, Japan-2011 where 3 reactors were destroyed.
Following a loss of coolant accident, the reactor core gets uncovered and starts to accumulate residual heat. As the accident evolves, core heating and oxidation of the fuel cladding by the coolant vapor provoke core degradation. In this case, injection of water into the core (reflooding) to remove the residual heat is vital for stopping the progressive degradation and saving the core from melting down.
Reflooding can cause a thermal shock and the embrittlement of the cladding, hence forming a porous debris bed in the core. The arrival of steam that is generated by cooling the lower zones may activate the oxidation of Zircaloy at high temperature zones, the reaction is very exothermic and leads to partial melting of materials. Those molten materials tend to move within the porous medium and thus reducing the porosity in the accumulation zones and increasing it in the zones from which they migrate. Due to this heterogeneity and the varying degrees of degradation, the coolant flow becomes multidimensional.
This is a multiphase system including 4 phases: gas, liquid (water), liquid (molten material) and solid particles constituting the porous debris bed. The first initiative was to establish the Mass, Energy and Species conservation equations in a form that accounts for porosity evolution and the chemical reaction (Zircaloy oxidation). Averaging techniques are applied to get the volume-averaged form of the governing equations, followed by considering several assumptions to simplify these equations.
This should integrate with future initiatives into improving the previously developed model in order to deal with local geometrical modifications consequent to partial melting of the porous medium, and include a refined structure of the medium as the two-phase flow is very sensitive to the evolution of this structure. This model would either be integrated into the ICARE/CATHARE code – a tool developed by IRSN for core degradation studies – or into a stand-alone code. The models developed in this thesis will be used by IRSN to improve the ASTEC code (IRSN’s reference code for simulating core meltdown accidents).