ASTEC application for in-vessel melt retention modelling in VVER plants

The paper is devoted to extended ASTEC analyses of possible in-vessel retention (IVR) phenomena, based on simulation of main accident scenarios for VVER-1000 reactor design with the ASTEC 2.0 rev2 code. To reach the objective of the investigation, an assessment of two hypothetical low-pressure severe accidents scenarios has been carried out, which provided a basis for well-founded insight on key phenomena during IVR in a VVER-1000 plant. This examination allowed to obtain main characteristics of the corium that is relocated to lower plenum, along with necessary understanding of ex-vessel phenomena. Separate simulations of corium behaviour within the vessel lower head in conditions of external cooling show that the efficiency of IVR critically depends on residual nuclear power retained in the corium. Basing on calculation results it may be concluded that the IVR objective can be achieved for VVER-1000 at the level of residual power which corresponds to approximately 24 h of accident transient after the initial event. Potentially such conditions can be realized on new Generation III plants or in the course of modernization of the existing ones by increasing water supply from passive safety injection tanks. A code-to-code comparison between ASTEC and the SOCRAT Russian code confirms main results obtained. At the same time it revealed certain deficiencies in existing modelling approaches, which are the result of poorly examined thermodynamical properties of the corium at temperatures higher than 2200 K