Assessment of the Reactor Pressure Vessel Integrity in PTS Conditions: ThermalHydraulic and ThermoMechanical Studies of a Safety Injection in a Four Loop PWR Plant

Abstract

This paper explains the Research and Development program started at E.D.F about the cooling phenomena of a PWR vessel after a Pressurized Thermal Shock. The numerical results are obtained with the thermalhydraulic code Code_Saturne coupled with the thermal-solid code SYRTHES to take into account the conjugate heat transfer on the cooling of the vessel. The geometry used represents a four loop PWR plant. In this calculation, the simulated geometry takes into account as much as possible the exact geometry of the lower plenum such as its columns and plates instrumentation. The configuration investigated is related to the injection of cold water in the vessel during a penalizing operating transient and its impact on the solid part formed by cladding and base metal. Numerical results are given in terms of temperature field in the cold legs and in the down comer. 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 by F.E. simulation using the F.E. code Code_Aster on a 3D mesh of the vessel, covering the two core–shells and their circumferential welds, as well as the internal cladding. Based on an analytical method specially established for underclad flaws, the corrected stress intensity factor Kβ during the transient is evaluated for an hypothetical flaw, by extracting the stresses along a radial segment. The severity of the flaw with respect to the transient is quantified by the minimum of the ratio KIc/Kβ, where KIc refers to the base metal fracture toughness for brittle initiation. The evolution of the severity with the position of the hypothetical flaw is studied and compared with the results given by a classical uni–dimensional method. The results show that such a complete thermal–hydraulic and mechanical 3–dimensional analysis allows to reduce considerably the severity of the flaws, thus improving the integrity of the RPV.