Title: Status of the IVMR project: First steps towards a new methodology to assess In-Vessel Retention Strategy for high-power reactors
Publisher: ERMSAR
Publication Year: 2017
JRC N°: JRC106398
URI: http://ermsar2017.ncbj.gov.pl/conference-materials/
Type: Articles in periodicals and books
Abstract: The In-Vessel Retention (IVR) strategy for Light Water Reactors (LWR) intends to stabilize and isolate corium and fission products in the reactor pressure vessel and in the primary circuit. This type of Severe Accident Management (SAM) strategy has already been incorporated in the SAM guidance (SAMG) of several operating small size LWR (reactor below 500 MWe (like VVER440)) and is part of the SAMG strategies for some Gen III+ PWRs of higher power like the AP1000 or the APR1400. However, the demonstration of IVR feasibility for high power reactors requires using less conservative models as the safety margins are reduced. In Europe, the IVMR project aims at providing new experimental data and a harmonized methodology for IVR. During the first year of the project, the work was mostly dedicated to methodology and computer code analysis. A synthesis of the methodology applied to demonstrate the efficiency of IVR strategy for VVER-440 in Europe (Finland, Slovakia, Hungary and Czech Republic) was made. It showed very consistent results, following quite comparable methodologies. The main weakness of the demonstration was identified in the evaluation of the heat flux that could be reached in transient situations, e.g. under the “3-layers” configuration. Analyses have also started for various designs of reactors with a power between 900 and 1300 MWe. Large discrepancies of results were observed, which were due to the use of very different models for the description of the molten pool: homogeneous, stratified with fixed configuration, stratified with evolving configuration. The last type of model provides the highest heat fluxes (above 3 MW/m2) whereas the first type provides the lowest heat fluxes (around 500 MW/m2). Obviously, there is an urgent need to reach a consensus about best estimate practice to be used in the major codes for safety analysis, such as ASTEC, MELCOR, SOCRAT, MAAP, SCDAP/RELAP, etc. Despite the discovered model discrepancies, and leaving aside the unrealistic case of homogeneous pool, the average calculated heat fluxes in many cases are well above 1 MW/m2 which would threaten the integrity of the vessel considerably and require a detailed mechanical analysis. Therefore, it is clear that the safety demonstration of IVR for high power reactors requires a more careful evaluation of the situations which can lead to formation of either a very thin top metal layer provoking focusing effect or significantly overheated metal, e.g. after oxide and metal layer inversion. An international workshop was organized at the end of the first year of the IVMR project in order to gather the positions of several safety authorities and research organizations from all over the world. As a conclusion, it appears that the current approach followed by most experts for IVR is a compromise between a deterministic analysis using the significant knowledge gained during the last two decades and a probabilistic analysis to take into account large uncertainties due to the lack of data for some physical phenomena (as listed above) and due to excessive simplifications of models. A harmonization of the positions of safety authorities on the IVR strategy is necessary to allow decision making based on shared scientific knowledge. Currently, the acceptance criteria of a safety demonstration for IVR may be differently defined from one country to the other and the differences should be further discussed to reach harmonization on this important topic. This includes the accident scenarios to be considered in the demonstration and the modelling of the phenomena in the vessel. Such harmonization is one of the goals of IVMR project. The project will now focus on providing new experimental data for situations of interest like the inversion of stratification and the kinetics of growth of the top metal layer. The project will also provide new data about external vessel cooling from full-scale facilities: CERES for VVER-440 and a new facility built by UJV for VVER1000. It will also include an activity on innovations dedicated to increase the efficiency of the IVR strategy such as delaying corium arrival in the lower plenum, increasing the mass of molten steel or implementing measures for simultaneous in-vessel water injection.
JRC Directorate:Nuclear Safety and Security

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