Integral Vessel Failure Experiment Analysis and Investigation of a Possible Vessel Support

Integral scaled coupled melt pool convection and vessel creep failure experiments are being performed in the FOREVER program at the Royal Institute of Technology, Stockholm. These experiments are simulating the lower head of a pressurized reactor vessel under the thermal load of a melt pool with internal heat sources. Due to the multi axial creep deformation of the three-dimensional vessel with a highly non-uniform temperature field these experiments offer an excellent opportunity to validate numerical creep models. A Finite Element model is developed and using the Computational Fluid Dynamic module, the melt pool convection is simulated and the temperature field within the vessel wall is evaluated. The transient structural mechanical calculations are then performed applying a new creep modeling procedure. Additionally, the material damage is evaluated considering the creep deformation as well as the prompt plasticity.
Pre- and post-test calculations of at least 4 FOREVER experiments have been performed successfully. Taking into account both - experimental and numerical results - gives a good opportunity to improve the simulation and understanding of real accident scenarios.
After analyzing the calculations, it seems to be advantageous to introduce a vessel support which can unburden the vessel from a part of the mechanical load and, therefore, avoid the vessel failure or at least prolong the time to failure. This can be a possible accident mitigation strategy. Additionally, it is possible to install an absolutely passive automatic control device to initiate the flooding of the reactor pit to ensure external vessel cooling in the event of a core melt down. This idea has been patented.