[en] Analyses on external cooling of a melt pool in the lower head of the reactor pressure vessel (RPV) or of gap cooling inside the RPV means to consider limiting configurations for coolability. However, before running into such configurations, the formation of particle agglomerations (debris) can be expected in the most relevant scenarios. Analysis of such configurations shall help to identify conditions to avoid such limiting cases, to assess the available time spans to reestablish coolability by water injection and to evaluate constructive or procedure means to support it. The potential of long term coolability of debris beds can in principle be investigated by the modules WABE-2D and MESOCO-2D developed at IKE in the frame of the KESS code system for implementation in ATHLET-CD. WABE-2D describes separated two phase flow of water and steam in a debris bed submerged in a water pool under various boundary conditions of coolant inflow (gravity induced inflow from an overlying water pool yielding counter-current flow of water and steam in the bed or imposed inflow from the lateral boundaries respectively the bed bottom result in co-current flow situations). Coolant boil-off, dry zone formation and extension in the bed are considered by WABE-2D. If a sufficient coolant supply can not be established the dry zone will expand up to total bed dryout and temperatures will rise up to particle melting. The thermal non-equilibrium between particles, liquid and steam is taken into account by WABE-2D. This extends the range of coolability investigations and enables to analyse quenching of hot debris beds. For the simulation of melting, melt relocation and refreezing including melt pool formation on crusts in a porous solid matrix the module MESOCO-2D has been developed. Gravity as well as capillary forces are taken into account for the melt flow. Forced and natural convection gas flows are also described in 2D. Thermal non-equilibrium between solid matrix, melt and gas is taken into account. Up to now WABE-2D and MESOCO-2D have been developed as separate modules. Both modules will be combined to an integrated model for safety related analyses to evaluate conditions for a long term coolability as well as the safety margin to vessel failure. The effectiveness of cooling mechanisms and accident management measures to prevent RPV failure can be checked. (authors)