Applying ANSYS/Multiphysics with Extended Creep Capabilities to an Integral Severe Nuclear Accident Experiment

Considering the hypothetical accident scenario of a core melt down for a Light Water Reactor the behaviour of the Reactor Pressure Vessel has to be investigated. The vessel behaviour is governed by multiaxial creep deformation of the three-dimensional vessel with highly non-uniform temperature and stress fields.
Therefore a Finite Element model is developed on the basis of ANSYS/Multiphysics®. Using FLOTRAN® the transient temperature field within the vessel wall is evaluated. The transient structural mechanical calculations are performed applying a creep model which is able to take into account great temperature, stress and strain variations within the model domain.
The user programmable features (UPF) of the finite element code ANSYS® are used to generate a customized ANSYS-executable including a more general creep behaviour of materials and a damage module. The numerical approach for the creep behaviour is not restricted to a single creep law (e.g. strain hardening model) with parameters evaluated from a limited stress and temperature range. Instead of this strain rate - strain relations can be read from external creep data files for different temperature and stress levels.
The damage module accumulates a damage measure based on the creep strain increment and plastic strain increment of the load step and the current fracture strains for creep and plasticity (depending on temperature and stress level). If the damage measure of an element exceeds a critical value this element is deactivated.
Post test calculations of a scaled core melt down experiment FOREVER-C2 were performed. If the temperature field in the vessel wall is fixed along creeping process, the calculated creep curves disagree with the measurement. Considering the time dependent variation of the temperature field, the calculated results show a good agreement with the measurements.
Currently the FE-Model is improved and validated in the frame of further scaled core melt down experiments.