A programme on crack opening and propagation of a lower head during a hypothetical PWR severe accident

Titre du congrès :ICONE 15 15th International Conference on Nuclear Engineering Ville du congrès :Nagoya Date du congrès :22/04/2007

In PWR severe accident scenarios, involving a relocation of corium (core melt) into the lower head, the possible failure mode of the reactor pressure vessel (RPV), the failure time, the failure location and the final size of the breach are regarded as key elements, since they play an important part in the ex-vessel phase of the accident. In the framework of the LHF and OLHF programmes, the failure time and failure location predictions were obtained using numerical modelling and agreed reasonably well with the experimental values. However the final size of the failure is still an open issue. Analyses of both the LHF and OLHF experimental data (as well as of that from the Swedish FOREVER experiments) do not enable an assessment of the final size of the breach (in relation with the testing conditions and results). Indeed, the size of breach depends on the mode of crack propagation which is directly related to the variability in behaviour of the RPV material at high temperatures. To determine crack propagation and failure final size, 3D modelling would thus be needed with an adequate failure criterion which takes into account this material behaviour variability. This paper presents an outline of the methodology being used in a current research programme of IRSN, in partnership with CEA and INSA LYON. The aim is to model crack opening and crack propagation in French RPV lower head vessels under severe accidents conditions and to develop a new failure criterion for the 3D finite element models.