CEA programme to model the failure of the lower head in severe accidents

The failure process of the reactor vessel lower head in severe accident conditions is likely to be complex, combining local melting, metallurgical transformations, plastic and viscoplastic flow, material damaging, plastic instability and rupture. A comprehensive research programme has been initiated in 1994 at Commissariat a lʹEnergie Atomique to work out the data and models necessary for the analysis of the mechanical phenomena involved. It relies on four activities: modelling based on continuous damage mechanics coupled with plastic or viscoplastic flow under large displacements and large strains conditions: material characterisation, consisting of tensile and creep tests at temperatures ranging from 600 to 1300°C; model validation based on analytical or semi-analytical experiments performed on tubes and hemispheres submitted to an internal pressure at very high temperatures, metallurgical investigations to analyse the role of metallurgical phenomena in the mechanical behaviour of the vessel and quantify the influence of the metallurgical history of the material. The first three activities are now well advanced and have produced significant results regarding the correlation between experiments and computations. They are now oriented towards the influence of geometrical discontinuities, complex loading histories, and strong thermal gradients. The metallurgical investigations have underlined the variability of the mechanical behaviour one can observe according to the various possible microstructures of the material. © 1999 Elsevier Science S.A. All rights reserved.