Experimental investigation of flooding in air-water counter-current flow with a vertical adiabatic multi-rod bundle

Abstract

The phenomena of counter current flow and flooding are important in emergency core cooling of light water reactors following a postulated loss of coolant accident. In case of such accidents, the primary task of emergency core cooling system is to refill the reactor lower plenum and subsequently reflood the reactor core. However the flooding phenomenon may considerably limit the penetration of the emergency coolant into the core region and the downward penetration of water into the core from the upper plenum during reflood process determines the top quenching behavior. Therefore, an understanding of the counter current flow and the flooding phenomenon is of major importance for the assessment of the reactor safety system performance. Many studies have been performed with single pipes which consists of falling annular liquid film and upward gas flow, although the actual physical situations often consist of large pin array or parallel channels. Although the existing literature on pin bundle and multichannel behavior is sparse the previous study has uncovered instability and hysteresis phenomena for which no explanation was offered. Only by extrapolating the present knowledge of single-channel counter-current flow, it was concluded that the multiple-path flow would exhibit similar characteristics as that of single channel flow for low liquid-holdup cases. Thus a detailed investigation on Counter Current Flow Limitation(CCFL) of rod bundles will be required to improve the understanding of interactions between parallel channel in the reactor core. The purpose of the present study is to discuss the effects of spacer grids and multi-channel flow interactions in the fuel bundles. Although the effect of upper structure on the fuel bundles is somewhat important, it``s study was out of considerations in this experiment. Because the upper structure of the core is not only limited to the top nozzle but also includes complex support structures, and its interaction with the co...