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In-core-instrumentation methods for 3-Dimensional distribution information of reactor core temperatures and Melt-down   SHIN Yeong Cheol 1, CHAE Myoung Eun2, and KIM Sung Jin 3   1. Central Research Institute KHNP, 1312 Gil, 70 Yuseongdaero, Daejeon, 305-353, Korea (ycshin@khnp.co.kr) 2. Woojin Inc., 110,Dongbu-daero 970beon-gil, Dongtan, Hwaseong, Gyeonggi, 445-813, Korea (chaeme9@woojininc.com) 3. Woojin Inc., 110,Dongbu-daero 970beon-gil, Dongtan, Hwaseong, Gyeonggi, 445-813, Korea (sjkim@woojininc.com)   Abstract: The tsunami-induced nuclear accident at Japanese Fukushima power plants in March 2011 has revealed some weaknesses in the severe accident monitoring system. The plant instrumentation did not provide utility, safety experts, and government officials with adequate and reliable information. The information on the reactor core damage and coolability is critical for making decisions correctly as well as in a timely manner during the course of the mitigation of severe accidents. Current Pressurized Water Reactor (PWR)s have an In-Core-Instrumentation (ICI) system that measures the temperature distribution of the top surface (i.e. Core Exit Temperatures) of the reactor core mainly to indicate when to begin Severe Accident Mitigation Guidelines (SAMG). This design concept giving only the core exit temperature has limitations in terms of sufficiency as well as availability of the information necessary for diagnosis on the status of the degraded core and the effectiveness of the measures taken as mitigation strategies. The reactor core exit temperatures are not sufficient to support the assessment of the degree of the core damage and the location of the molten core debris and recognition whether the core damage progresses on or it is mitigated. The ICI location being at the top of the reactor core also makes the ICI thermocouples vulnerable to melt-down because the upper part of the reactor core uncovers first, thereby melt down at the early stage of the accident. This means that direct indication of reactor core temperature will be lost and unavailable during the later stages of severe accident. To address the aforementioned weaknesses of the current ICIs, it is necessary to develop a new ICI system that provides information that is more expanded and more reliable for accident mitigation. With the enhanced ICI information available, the SAMG can be prepared in more refined and effective way based on the direct and suitable indication of status of damages and the 3-dimensional temperature profile of the core rather than guesses and assumptions. Furthermore, this goal needs to be achieved economically and with minimal changes to current design of reactor and its instrumentation that has been proven and well established through many years of operation. In this paper, methods for a new ICI system to provide three-dimensional view of the reactor core temperatures and melt-down are introduced. Keyword: in-core-instrumentation;core exit temperature;severe accident, core cooling
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