No abstract

Within the European Integrated Project NURESIM, the simulation of PTS is investigated. Some accident scenarios for Pressurized Water Reactors may cause Emergency Core Coolant injection into the cold leg leading to PTS situations. They imply the formation of temperature gradients in the thick vessel walls with consequent localized stresses and the potential for propagation of possible flaws present in the material. This paper focuses on two-phase conditions that are potentially at the origin of PTS. It summarizes recent advances in the understanding of the two-phase phenomena occurring within the geometric region of the nuclear reactor,that is, the cold leg and the downcomer, where the “PTS fluid-dynamics" is relevant. Available experimental data for validation of two-phase CFD simulation tools are reviewed and the capabilities of such tools to capture each basic phenomenon are discussed. Key conclusions show that several two-phase flow subphenomena are involved and can individually be simulated at least at a qualitative level, but the capability to simulate their interaction and the overall system performance is still limited. In the near term, one may envisage a simplified treatment of two-phase PTS transients by neglecting some effects which are not yet well controlled, leading to slightly conservative predictions.

This paper explains the numerical program concerning the new thermalhydraulic Code_Saturne qualification for Safety Injection studies. Within the frame of the plant life time project, an analysis has shown that the most severe loading conditions are generated by a pressurised injection of cold water in the downcomer of a Reactor Pressure Vessel. For this kind of transients, a thermal hydraulics study has to be carried out in order to better adjust the accurate distribution of the fluid temperature in the downcomer. For that, the numerical tools have to be able to simulate the physical phenomena present during the Pressurised Thermal Shock. (PTS). For this qualification task, we have investigated one configuration related to an injection of cold water particularly in cold leg but also in a downcomer. One experiment test case has been studied and this paper gives a comparison between experiment and numerical results in terms of temperature field.

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 loading PTS conditions due to the emergency cooling during SBLOCA transients. This paper presents the Research and Development program started at E.D.F on the CFD determination of the cooling phenomena of a PWR vessel during a Pressurised Thermal Shock. The numerical results are obtained with the thermalhydraulic tools N3S and Code_Saturne, in combination with the thermal-solid code SYRTHES to take into account the coupled effect of heat transfer between the fluid flow and the vessel. We first explain the recent improvement of the thermalhydraulic analysis with the global definition of the SBLOCA transient and the local analysis in the downcomer. Then, the qualification task of the EDF numerical tools is described. In order to reach this purpose, we have investigated several configurations related to an injection of cold water and focused our analysis particularly in the cold leg but also in a the downcomer. Two experiment test cases have been studied. A comparison between experiment and numerical results in terms of temperature field is presented. On the whole, the main purpose of the numerical thermalhydraulic studies is to accurately estimate the distribution of fluid temperature in the downcomer and the heat transfer coefficients on the inner RPV surface for a fracture mechanics computation which will subsequently assess the associated RPV safety margins.

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.