Methodology of safety evaluation of In-Vessel Retention

Molten corium stabilization following a severe accident is of crucial importance in order to ensure containment integrity on a long-term basis and minimizing radioactive elements releases outside the plant. Among the possible options, In-Vessel Retention (IVR) through external cooling appears as an attractive solution that would limit the dispersion of corium in the plant and minimize the risks of containment failure. Nevertheless its feasibility has to be proved.The IVR strategy is already adopted in Europe for some VVER 440 type 213 reactors thanks to thorough research work started in the '90s for the Finnish Loviisa power plant, and subsequently extended to Bohunice and Mochovce (Slovakia), Dukovany (Czech Republic) and Paks (Hungary) power plants. The strategy is also included in the design of some high power new Gen.III reactors such as AP1000, APR 1400 and Chinese HPR1000 and CAP1400. It has also been studied in the past for other reactor concepts like KERENA (1250 MWe - BWR), AP600 or VVER-640.Current approaches for reactors with relatively small power, such as VVER 440 or AP600, use conservative assumptions for the safety demonstration. However, for higher power reactors (around 1000 MWe), the safety margin is reduced and it is necessary to evaluate the IVR strategy with best-estimate methods in order to reduce the uncertainties associated with the involved phenomena. Additional R&D as well as a revision of the methodology are needed to ensure and demonstrate adequate safety margins, including, in particular, best-estimate evaluations of thermal load applied on the vessel and mechanical resistance of the ablated vessel.The IVMR project (In-Vessel Melt Retention) was built with the goal of providing new knowledge (experimental, theoretical and technical) and a new methodology able to provide a best-estimate evaluation of IVR strategy for large power reactors. The main objective of Task 2.1 within WP2 was to define a common methodology to analyse IVR Severe Accident Management (SAM) strategy for the different types of EU NPPs. It started by reviewing the status of existing methodology and aimed at elaborating a more general, updated and less conservative one applicable to several types of reactors.This paper describes the proposed new methodology. It starts with the identification of the deficiencies of the standard methodology when it is applied to a high power reactor. It introduces the minimum vessel thickness as a parameter representing the cumulated imbalance between internal heat load and external cooling. Then it explains how to use that parameter in the evaluation of the safety margin. Although some examples are given as illustrations, it must be kept in mind that this paper proposes a generic methodology but there cannot be any generic conclusion: any reactor design must be evaluated independently.