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 counter current flow may be very complex oscillatory behavior.
Three series of experiments were performed, using three types of tube bundle differing only in the number of spacer grids attached. Experimentally determined flooding points at various water film Reynolds number for each test sections are presented in graphical form and compared with entrainment criterion for co-current flow and instability criteria. In addition, empirical flooding correlations of the Kutateladze type are obtained for each type of test section using liquid penetration data.
이상역류유동과 플러딩현상은 원자로에서의 가상사고인 냉각재 상실사고 시 비상 노심냉각수 주입분석에 중요하다. 냉각재상실사고시 비상노심 냉각 계통의 역활은 원자로 하부구조물( Lower Plenum )과 노심에 물을 주입하여 냉각시키는 것이다. 그러나 플러딩현상으로 노심으로의 냉각수 주입에 상당한 제한이 따르게 되며 재관수시 원자로의 상부구조물 (Upper Structure)로 부터 흐르는 물에 의하여 상부노심 냉각 (Top Quenching)의 제한도 발생한다. 따라서 플러딩현상의 이해는 원자로 안전평가에 중요하다.
현재까지의 플러딩연구는 주로 수직단일 관내에서 공기와 물의 흐름에 대하여 이루어져있다. 원자노심의 실제적인 구조는 다중챈널 (Multi-Channel) 이므로 이에대한 연구는 부분적으로 이루어져있으나 이해할 수 없는 유동의 불안정 및 피로효과 (Hysteresis)가 관측되었으며, 수직단일관에서의 연구결과를 연장하여 물의 주입이 작은 경우는 다중챈널에서도 유사한 경향을 보일 것이라고 추정하고 있는 실정이다. 따라서 원자노심과 같은 다중챈널에서의 플러딩현상을 이해하기 위하여는 더 많은 연구가 필요하다.
