Assessment of the French Reactor Pressure Vessel Integrity in PTS Conditions. Thermalhydraulic and Thermomechanical Studies of the Small Break LOCA in a Three Loop PWR Plant

This paper deals with the Research and Development program started by E.D.F. about the study and risk characterization of a PWR vessel (RPV) submitted to a Pressurized Thermal Shock (PTS). The PTS is described by simulation with the thermalhydraulic Finite Element (FE) code N3S 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 three loop PWR plant which is the more common running plant in the world. In this study, the simulated finite element mesh takes into account as much as possible the actual 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 transients 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 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 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 elastic stress intensity factor K_β 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 K_Ic/K_β, where K_Ic 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 the classical uni–dimensional method. The results show that such a complete thermal–hydraulic and mechanic 3–dimensional analysis allows to reduce considerably the severity of the flaws, thus improving the margins regarding brittle fracture.