Simulating structural collapse of a PWR containment

NNC Limited was sponsored by the Health & Safety Executive (UK) to conduct finite element analysis to simulate the over-pressurisation tests conducted on a 1/4 scale model of a pre-stressed concrete containment vessel (PCCV) by Sandia National Laboratories. The tests and the results indicated three distinct stages of structural response as the vessel approached its failure load. The first stage was essentially an elastic response. The second stage displayed nonlinear behaviour, which started after onset of substantial concrete cracking and involved local yielding or fracture of structural components. The third stage involved extensive concrete cracking and rapid increase in strains leading to global structural collapse.This paper discusses the engineering judgment used to develop the 3D finite element (FE) global model, which was successfully analysed to predict the failure mode. For the limit state test (LST), it was shown that the liner ruptures before structural collapse. For the structural failure mode test (SFMT), the asymmetric behaviour was successfully predicted, identifying the weak area in the vessel where the structural collapse was initiated. This was possible because the 3D model simulated the interaction of the essential structural features such as the tendons, buttresses, liner, penetrations and the basemat. Explicit modelling of the post-tensioned tendons allowed the model to accommodate frictional loss, simulating realistic reduction in the pre-stress levels in certain areas. Detailed modelling of the penetrations and buttresses simulated local stiffening of the cylindrical wall. The maximum deflection was expected at the mid-height of the wall in the area away from local stiffening and where the pre-stress levels had reduced due to friction. This weak area was identified by the FE model and was confirmed by the test.