Thermal-Hydraulic Effects of Stud Shape and Size on the Safety Margin of Core Catcher System

Abstract

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 newly engineered corium cooling system, that is, an ex-vessel core catcher system has been designed and adapted in some nuclear power plants such as VVER-1000, EPR, ESBWR, EU-APR1400 to mention a few. For example, Russia adopted a crucible-type core catcher for VVER-1000. On the other hand, a way to catch melt spreading is adopted by several countries, such as EPR in France, ESBWR in USA, ABWR in japan, and EU-APR1400 in Korea In Korea, the core catcher system has been designed and implemented for 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 maintaining a coolable geometry in case that RPV failure occurs. The core catcher system consists of a carbon steel body, sacrificial material, protection material and engineered cooling channel. 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 overall thermal-hydraulic performance including two-phase heat transfer coefficient and critical heat flux (CHF) of the system. Thus, it is of importance to investigate the thermal-hydraulic effects of studs on the coolability, especially the CHF of the core catcher system. With aforementioned importance, pool boiling experiments were carried out with stud shape of, rectangular, cylinder, and elliptic and for stud sizes of 10, 15, 20, and 25 mm under the condition of atmospheric saturated water. A particular attention was focused on observing local vapor behavior around the studs and finding any hot spots, where the vapors are accumulated. The occurrence of the CHF is anticipated at the back side of the studs. The visual observation and CHF measurements indicate that the effect of studs on the performance of boiling heat transfer is significant. In this work, an experimental study has been conducted to investigate the effects of stud shape and size on the pool boiling CHF of saturated water under the atmospheric pressure. The major conclusions from this study are summarized as follows. (1). The experimental result confirmed that the installation of the stud influenced the CHF value due thermal-hydraulic restriction. With the presence of studs, the CHF value was measured lower than the bare specimen up and the reduction ranged from 18% to 55% depending on the stud shape and size. (2). It was observed that CHF data generally decreases as the stud size becomes larger. Especially, when the size of the studs was 25 mm, the CHF value drastically decreased in all studs' shape