Experimental Study for Effects of the Stud shape of the Core Catcher System

Abstract

In preparation of potential severe accidents, a nuclear power plant is equipped with diverse systems of engineering safety features or mitigation system dedicated to the severe accidents conditions. As a common strategy, a number of nuclear power plants adopt the in-vessel retention (IVR) and/or external reactor vessel cooling (ERVC) strategies. With the ERVC strategy, an additional system (core catcher system) to catch molten core penetrating the reactor pressure vessel (RPV) was proposed for advanced light water reactor. The core catcher system is for Ex-vessel in the European Advanced Power Reactor 1400 (EU-APR1400) to acquire a European license certificate. It is to confine molten materials in the reactor cavity while keeping coolable geometry in case that the RPV failure occurs. The system consists of a carbon steel body, sacrificial material, protection material and engineered cooling channel. As shown in Fig 1, the engineered cooling channel of the ex-vessel core catcher was adopted to remove sensible heat and decay heat of the molten corium using cooling water flooded from the In-Containment Refueling Water Storage Tank (IRWST) by gravity. A large number of studs are placed in the cooling channel to support the core catcher body. While installation of the studs is unavoidable, the studs tend to interfere in the smooth streamline of the core catcher channel. The distorted streamline could affect the temperature distribution and overall coolability of the system. Thus, it is of importance to investigate the effects of studs on the coolability of the core catcher system. In the current research, to evaluate the effect of a stud on the streamline and natural convective boiling performance, numerical and experimental approaches were taken. As a part of numerical approach, CFD simulation using ANSYS/FLUENT was carried out. The objective was to predict disturbance of the streamline and temperature distribution due to the interference of the studs. Through the CFD analysis, it was found that installation of studs affects the streamline by partially blocking the channel. In addition, it was observed that stagnant flow exists at the back of the studs. Such disturbances were expected to differ with the shape of the studs. Thus various stud shapes of rectangular, cylinder, and ellipse were investigated. With this preliminary CFD study, flow boiling experiments were designed and conducted to investigate the effects of the different stud shape on the critical heat flux (CHF). The occurrence of the CHF is anticipated at the back side of the stud due to the possible existence of the hot spot driven by the accumulation of the vapors. Final objective is, therefore, to confirm the effect of the core catcher stud on the performance of boiling heat transfer and to assure that the designed core catcher system may work as intended. In this study, CFD simulations for the different stud shapes of the core catcher system were carried out using ANSYS FLUENNT. With this preliminary CFD study, flow boiling experiments were designed. Final objective is to confirm the effect of the core catcher stud on the performance of boiling heat transfer and to assure that the designed core catcher system may work.