Coupled thermal structural analysis of LWR vessel creep failure experiments

Considering the hypothetical core melt down scenario for a light water reac tor (LWR) the failure mode of the reactor pressure vessel (RPV) has to be i nvestigated to determine the loadings on the containment. The failure of re actor vessel retention (FOREVER)-experiments, currently underway, are simul ating the thermal and pressure loadings on the lower head for a melt pool w ith internal heat sources Due to the multi-axial creep deformation of the v essel with a non-uniform temperature field these experiments are an excelle nt source of data for validation of numerical creep models. Therefore, a fi nite element (FE) model has been developed based on a commercial multi-purp ose code. Using the computational fluid dynamics (CFD) module the temperatu re field within the vessel wall is evaluated. The transient structural mech anical calculations are performed using a new numerical approach, which avo ids the use of a single creep law employing constants derived from the data for a limited stress and temperature range. Instead of this a three-dimens ional array is developed where the creep strain rate is evaluated according to the values of the actual total strain, temperature and equivalent stres s. Care has to be exercised performing post-test calculations particularly in the comparisons of the measured data and the numerical results. Consider ing the experiment FOREVER-C2, for example, the recorded creep process appe ars to be tertiary, if a constant temperature field is assumed. But, small temperature increase during the creep deformation stage could also explain the observed creep behavior. Such considerations provide insight and better predictive capability for the vessel creep behavior during prototypic seve re accident scenarios. (C) 2001 Elsevier Science B.V. All rights reserved.