Insights from the FOREVER-Programme and the Accompanying Finite Element Calculations

The hypothetical scenario of a severe accident with core meltdown and formation of a melt pool in the lower plenum of a light water Reactor Pressure Vessel (RPV) can result in the failure of the RPV and the discharging of the melt to the containment. To obtain an improved understanding and knowledge of the melt pool convection thermal loads, the vessel creep, and vessel failure modes occurring during the late phase of a core melt down accident the FOREVER-experiments have been performed at the Royal Institute of Technology, Stockholm. These experiments simulated the behaviour of the lower head of the RPV under the thermal loads of a convecting melt pool with decay heating, and under the pressure loads that the vessel experiences in a depressurised vessel scenario.
Due to the multi-axial creep deformation of the vessel with prototypic non-uniform temperature field these experiments are on the one hand an excellent source of data to validate numerical creep models that are developed on the basis of uniaxial creep tests and on the other hand they provide reasonably accurate insights into the course of a prototypic accident in which vessel failure may occur.
A Finite Element model was developed simulating melt pool convection and calculating the temperature field within the melt pool and within the vessel wall. The transient structural mechanical calculations are then performed applying a creep model, which takes into account the large temperature, stress and strain variations.
After performing successful pre- and post-test calculations, a discussion about the lessons learned from the experiments and the analyses led to the idea of providing a vessel support and an external water-flooding device.