Finite Element Analysis for Local Creep of a Tube Coolant Piping System in Light Water Reactor Due to Local Heating Under Severe Accident Condition

During severe accident of a light water reactor (LWR), reactor coolant piping would be damaged when the piping is subjected to high internal pressure and very high temperature due to heat transfer from high-temperature gas and decay heat from wall-deposited fission product (FP), both from degraded core. In such a case, high-temperature fast creep deformation could be the main cause for the pipe failure. For the evaluation of piping integrity during severe accidents, a method to predict such high-temperature fast creep deformation should be developed, using a creep constitutive equation considering tertiary creep behavior which has not been considered well in the pipe failure analyses. In this paper, a creep constitutive equation was developed, which is based on the Kachanov-Ravotnov isotropic damage rule considered the tertiary creep behavior. Japan Atomic Energy Research Institute (JAERI) creep tensile test data for nuclear-grade cold-drawn SUS316 material was used to determine coefficients of the developed constitutive equation. Using the developed constitutive equation, finite element analyses were performed for local creep deformation of coolant piping under two temperature conditions; uniform temperature and temperature gradient. The analyses results indicated the damage variable being integrated following the evolution of creep damage can indicate pipe wall internal damage condition quantitatively. The damage variable was confirmed further to be able to reproduce the observation in JAERI piping failure tests; pipe failure from the wall outside.