Multidimensional modelling of debris bed coolability and quenching

Consideration on severe accidents with core melting in LWRs pose the question whether and by what means even such events can be managed and safety cooled states can be reached. In the past, mainly ex-vessel core-catcher concepts have been considered for this, especially with respect to the EPR, as well as the concept of ex-vessel flooding for cooling of a melt pool in the RPV with respect to special lower power reactor types. In the latter case, not considering the coolability of melt states before reaching the final pool situation means to neglect the potential of coolability and reaching safe states before running into the ultimate condition. This becomes especially problematic if coolability for this condition cannot be guaranteed. The concept as single measure appears especially not to be feasible for most existing reactors, due to higher power than allowable and specific design problems. The EPR concept yields no solution for existing reactors. In order to check the coolability under realistic multidimensional situations, calculations with the WABE-2D are used in the present contribution. The WABE-2D model describes the transient boil-off and quenching behaviour of debris beds and aims at problems of coolability of degraded core material during a severe accident in a LWR. It is developed in the frame of the KESS code system, which is considered to describe the processes of core heatup, melting, degradation and relocation to the lower plenum as well as the subsequent behaviour. The models developed in this frame are also being integrated in the German system code ATHLET-CD. Main emphasis of the present contribution lies on multidimensional aspects of cooling behaviour, from which a significantly improved coolability is expected than still concluded based on 1D analyses. Such analyses - mainly oriented only at top cooling conditions - additionally miss the expected importance of interfacial drag in co-current flow situations due to bottom flooding. The latter situation plays a role in the multidimensional behaviour expected under realistic conditions. Thus, a further emphasis in the present contribution lies on the constitutive drag laws including such configurations. (authors)