FE-simulation of the viscoplastic behaviour of different RPV steels in the frame of in-vessel melt retention scenarios

Abstract – Assuming the hypothetical scenario of a severe accident with subsequent core meltdown and formation of a melt pool in the reactor pressure vessel (RPV) lower plenum of a Light Water Reactor (LWR) leads to the question about the behav-ior of the RPV. One accident management strategy could be to stabilize the in-vessel debris configuration in the RPV as one major barrier against uncontrolled release of heat and radio nuclides.
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) have been performed at the Division of Nuclear Power Safety of the Royal Institute of Technology Stockholm. These experiments are simulating the behavior 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 com-pared to a common LWR.
Accompanying the experiments metallographical and numerical work is performed at the Forschungszentrum Rossendorf. An axisymmetric Finite Element model is devel-oped based on the multi-purpose code ANSYS®. To describe the viscoplastic defor-mation a numerical creep data base (CDB) is developed where the creep strain rate is evaluated in dependence on the current total strain, temperature and equivalent stress. In this way the use of a single creep law, which employs constants derived from the data for a limited stress and temperature range, is avoided. For an evalua-tion of the failure times a damage model according to an approach of Lemaitre is ap-plied. The microstructural investigations give insight to the material state of the vessel wall at different positions. This can be compared with the numerical damage value calculated in the Finite Element Model.
This paper deals with the experimental, numerical, and metallographical results of the creep failure experiment EC-FOREVER-4, where the American pressure vessel steel SA533B was applied for the lower head. For comparison the results of the experi-ment EC-FOREVER-3B, build of the French 16MND5 steel, are discussed, too. Em-phasis is put on the differences in the viscoplastic behaviour of different heats of the RPV steel. For this purpose, the creep tests in the frame of the LHF/OLHF experi-ments are reviewed, too. As a hypothesis it is stated that the sulphur content could be responsible for differences in the creep behaviour.