Engineering Approach for the Reactor Pressure Vessel Integrity in PTS Conditions: Thermal Hydraulic and Fracture Mechanics Studies of a Safety Injection in a Three Loop Power Plant — French Benchmark With EDF and AREVA-NP Tools

Abstract

For the Reactor Pressure Vessel (RPV) assessment and Lifetime evaluation of the nuclear plants, French Utility applies a series of calculations including thermal-hydraulic, thermo mechanical and fracture mechanics studies in order to study the Pressurized Thermal Shock (PTS) in the down comer caused by the safety injection. Within the frame of the plant life time project, integrity assessments of the French 900 MWe (3-loops) series reactor pressure vessel (RPV) have been performed. We found that the modeling of thermal-hydraulics loads is a source of gain. Considering the length of local 3D calculation and the large number of cases, E.D.F and AREVA-NP decided to share the effort. However the two chains of software differ: EDF uses Code_Saturne (coupled with the thermal solid code Syrthes) and Cuve 1D and AREVA-NP uses Star_CD and CALORI codes respectively for thermal hydraulic and thermo mechanical computations. According to this approach, comparison between the two chains of tools have been performed. Moreover this action contributes to the verification and the validation of each code in accordance with the OECD Best Practise Guidelines (BPG). The study has been achieved by two independent teams from EDF and AREVA-NP. It should be emphasized that this benchmark helped to strengthen the accuracy of CFD and the adapted methodology (working progress). The investigated configuration corresponds to the injection of cold water in the vessel during a penalizing representation of a primary break transient and its impact on the solid part formed by cladding and base metal. Numerical results are given in terms of fluid temperature and velocity fields in the cold legs and in the downcomer. The obtained numerical description of the transient (internal pressure and temperature field within the vessel) is used as boundary conditions for a full mechanical computation of the stresses. This thermal mechanical transient is obtained on a 3D mesh of the vessel, covering the two core shells and their circumferential welds, as well as the internal cladding. The results show that such a complete thermal–hydraulic and mechanic 3–dimensional analysis improves the evaluation of the consequences of the loading on the stress fields and eventually the margins to fast fracture of the RPV. The good agreement observed between EDF and AREVA-NP results and their accordance with the validation computations, confirm the robustness of the approach.