An Overview of the Pressurized Thermal Shock Issue in the Context of the NURESIM Project

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.

From the Direct Numerical Simulation to System Codes - Perspective for the Multi-Scale Analysis of LWR Thermalhydraulics

“…In two-phase flow conditions, RANS and URANS have been applied within the NURESIM project for boiling flow [17 to 26] and for two-phase PTS scenarios [33].…”

Extension of CFD Codes Application to Two-Phase Flow Safety Problems

“…The measurements for steam-water were done in round ducts, annuli, and rod clusters. The data were reported in [10].…”

“…The identification of all basic flow processes was made in the NURESIM [26] project (see Figure 2) and many phenomena were identified. In the ECCS jet area:…”

“…This illustrates the benefit of having a multi-scale approach to two-phase CFD. Several experimental data sources were identified [26] and used for the development, and a partial validation, of physical models. Free surface flows without mass transfer were investigated in three experiments ranging from smooth interface up to wavy interface and KelvinHelmholtz instability [32].…”

Multiscale Thermal Hydraulic Study under the Inadvertent Safety Injection System Operation Scenario of Typical Pressurized Water Reactor

“…In recent years, one of the main applications of CFD method for NRS is the PTS study, which is regarded as high priority in the Phenomena Identification and Ranking Table (PIRT) [2]. Therefore, most scholars focused on the detailed coolant mixing mechanism study for either single-phase or two-phase flow.…”