During a severe nuclear accident, a part of the molten corium resulting from the core degradation may relocate down to the lower plenum of the reactor vessel.
The interaction with residual water in the lower plenum leads to a fragmentation of the corium and formation of particles (order of magnitude : 1-5 mm ).
In order to predict the safety margin of the reactor under such conditions, the coolability of this porous heat-generating medium and the possibility to reflood the particle bed are studied in this paper and compared with other theoretical or experimental results.
The detailed description of two-phase flow in a debris bed is addressed in the French Institut de Radioprotection et de Sûreté Nucléaire (IRSN) by a special module of the ICARE/CATHARE code. This thermalhydraulic module is three-dimensional and is able to deal with a non-homogeneous debris bed.
Calculations of one-dimensional reflooding are compared with two-dimensional calculations. As expected, water supply is greater considering multi dimensional flow in the bed and the dryout heat flux is larger than predicted by 1-D modelling. Conditions for reflooding are also more favourable if large-scale non homogeneities exist in the debris bed. This leads to a flow pattern where steam can exit the debris bed in preferential channels and there is no limitation by counter-current flow. The results also show the importance of using a non-equilibrium model for temperatures. The impact of metallic debris oxidation during reflooding is also presented.
(1) : Uv. Toulouse