Use of a CFD-tool for assessment of the reactor pressure vessel integrity in pressure thermal shock conditions

Integrity evaluation methods for nuclear Reactor Pressure Vessels (RPVs) under Pressurised Thermal Shock (PTS) loading are applied by French Utility. They are based on the analysis of the behaviour of relatively shallow cracks under PTS loading conditions due to the emergency cooling during SBLOCA transients. This paper explains the Research and Development program started at E.D.F about the cooling phenomena of a PWR vessel after a Pressurised Thermal Shock. The numerical results are obtained with the EDF Thermal Hydraulic code (CodeSaturne) coupled with the thermal-solid code SYRTHES to take into account the conjugate heat transfer on the cooling of the vessel. We first explain the numerical program concerning the qualification task of this CFD-Tool for Safety Injection studies. We have investigated several configurations. Two experiment test cases have been studied and we present a comparison between experimental and numerical results in terms of temperature field in cold leg but also in a down comer. Then, for the Thermal-Hydraulic reactor study, the geometries used represent a three and a four loop PWR. In these calculations, the simulated mesh takes into account as much as possible of the exact geometry of the lower plenum. Numerical results are given in terms of temperature field in the cold legs and in the down comer, as well as in the solid part formed by cladding and base metal. On the whole, the main purpose of the numerical thermalhydraulic studies is to accurately estimate the distribution of fluid temperature in the down comer and the heat transfer coefficients on the inner RPV surface for a fracture mechanics computation which will subsequently assess the associated RPV safety margins. Lastly, a parametric study has been carried out in order to assess the relative importance or influence of some physical data. Two cases are suited. The first part of this paper reports the effects of the thermal coupling between the RPV and the fluid flow. To this end, two computations have been realised the first one taking account of the thermal coupling, the other one neglecting the thermal feedback of the wall to the fluid during the transient. In the second study, the safety injection temperature is increased from the usual value of 9 degree C in the previous calculations up to 35 degree C. (authors)