Finite Element Pre- and Posttest Calculations of the Forever-Experiments

To get an improved understanding and knowledge of the melt pool convection and the vessel creep and possible failure processes and modes occurring during the late phase of a core melt down accident the FOREVER-experiments (Failure Of REactor VEssel Retention) are currently underway at the Division of Nuclear Power Safety of the Royal Institute of Technology Stockholm. These experiments are simulating 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 depressurization scenario. The geometrical scale of the experiments is 1:10 compared to a common Light Water Reactor (LWR). During the first series of experiments the creep behaviour (FOREVER-C) of the vessel under the thermal attack of the melt pool and varying internal pressure loads is investigated. It is intended to enforce the creep process until vessel failure. Due to the multi axial creep deformation of the vessel with a non-uniform temperature field these experiments are on the one hand an excellent source of data to validate numerical creep models which are developed on the basis of uniaxial creep tests. On the other hand the results of pre-test calculations can be used to optimize the experimental procedure with considerations of the uncertainties in the applied models and assumed boundary conditions. Therefore a axis-symmetric Finite Element (FE) model is developed based on the multi-purpose code ANSYS/ Multiphysics®. Using the Computational Fluid Dynamics (CFD) module the temperature field within the melt pool and within the vessel wall is evaluated. The transient structural mechanical calculations are then performed applying a creep model which takes into account large temperature, stress and strain variations.