MCCI analyses in support of the Probabilistic Safety Assessment level 2 for the French 900 MWe PWRs

Titre du congrès :MCCI Seminar 2007 Ville du congrès :Cadarache Date du congrès :10/10/2007

Molten Corium Concrete Interaction (MCCI) is one of the most important phenomena potentially occurring during a Severe Accident (SA) sequence in a nuclear power plant insofar as it can lead to the loss of integrity of the reactor confinement, radionuclide release into the environment and potential water table pollution. Thus, within the framework of the level 2 Probabilistic Safety Assessment (PSA) developed by the IRSN for the French 900 MWe PWRs, this hazard has been assessed. The loss of integrity of the reactor confinement resulting from MCCI may occur in two ways: the basemat failure due to its ablation by the corium or the over-pressurisation failure of the containment due to the release of gases produced during the ablation of the concrete. Thus, in a first time, a detailed support study has been carried out to assess occurrence and timing of the erosion of the basemat and of the walls of the reactor pit according to the accident scenario. In this study, it was supposed the absence of contact between corium and water. As the MEDICIS code developed at the IRSN for MCCI calculations was not fully operational and validated when this study started (beginning of year 2002), the CORCON code was used. The choice of CORCON has been done on the basis of considerations on the completeness, diffusion, maturity and validation of the existing codes. These criteria suggested that CORCON was at the level of the “state of the art” concerning MCCI. However, MCCI phenomenology is rather complex and, in spite of the considerable R&D work internationally carried out, the phenomenology is still not completely understood and research activities on MCCI are still going on. Consequently, although models and computer codes are presently available to describe and compute MCCI, their level of accuracy and validation reflects the lack of understanding of the phenomenology and is not fully satisfactory. On the other hand, a probabilistic safety assessment requires an estimation of uncertainties. Thus, in order to support the evaluation of the uncertainties affecting the MCCI calculations, a review of the elementary phenomena occurring during MCCI has been carried out. The phenomena considered as the most important has been then retained and the relative models implemented in CORCON has been deeply investigated in order to point out their limits and their representativity. This analysis carried out in the frame of a collaboration with CEA has consisted in a critical review of the models used in CORCON taking into account the available experimental results, their representativity of the reactor case, a comparison with other existing models and an analysis of their extrapolation to prototypic conditions. This work, if it has confirmed that CORCON well synthesizes the existing knowledge on MCCI, it has also pointed out the very high level of uncertainties existing on the used physical correlation and in consequence on prediction of concrete erosion kinetics. However this analysis, along with the results of preliminary CORCON calculations, has pointed out that uncertainties on the corium pool configurations have the largest impact on the basemat erosion time predicted by CORCON and encompass the whole uncertainties associated with these calculations. As a consequence the corium pool configuration has been retained as representative of the whole modeling uncertainties. Other important aspect of this type of studies is that MCCI initial conditions to be considered in a PSA may substantially change from one scenario to the other. In such situation, evaluation of MCCI and related uncertainties may not be disconnected with the identification of the main phenomena and parameters affecting it and with the evaluation of models accuracy and representativity. Several CORCON pre-calculations have then been performed in order to identify the main parameters affecting MCCI. A reduced number of the main parameter has then been retained and their values have been varied over a wide range in order to represent a large spectrum of MCCI initial conditions. About 200 calculations have been performed. In a second time, when the coupling of the codes MEDICIS and CPA has been available, some calculations have been performed to evaluate the containment pressurisation during MCCI. The results have shown that, in case of absence of spray systems, the containment pressurisation can be strong, particularly if the corium is in contact with water. This considerable effort allowed deriving interesting global information on timing and occurrence of basemat failure in the different accidental scenario and containment pressurisation during MCCI. However the observed very large discrepancy of results (due to the remaining uncertainties existing on MCCI mechanisms and modelling) challenged the confidence given to these results and showed the need for a further R&D effort on MCCI.