核三廠MUR後預期暫態未急停RCS壓力安全分析

本研究分析了核三廠在小幅度功率提升後反應爐冷卻水系統壓力受限制的預期暫態未急停事故，主要是確定反應爐冷卻水系統的壓力峰值是否仍能維持在3200psig內(根據ASME Code Level C service limit criterion)。在此研究中，使用廣泛應用於PWR事故分析之RETRAN程式及核三廠雷傳分析模型，來進行在小幅度功率提升後的預期暫態未急停分析及相關靈敏度分析。文件SECY-83-293的報告中，將所有的初始事故分為兩類，分別為汽機跳脫與非汽機跳脫事故，並根據此兩類事故評估其發生預期暫態未急停時的風險；本研究於兩類事故中各選擇喪失主飼水與喪失主冷凝器真空為極限事故來進行評估。為了評估反應爐冷卻水系統壓力，在分析模式中加入緩和劑密度係數模式來觀察緩和劑溫度與系統壓力變化對反應度的影響，進而分析反應爐冷卻水系統壓力尖峰值達到限值的可能性。除此之外，本研究同時也做了以下的靈敏度分析：(1)緩和劑溫度係數設定，(2)假設反應爐冷卻水泵跳脫，(3)蒸氣產生器出口蒸氣壓力初始條件，(4)蒸氣產生器的模型驗證。分析結果顯示，在小幅度功率提升後即使使用的較保守的緩和劑溫度係數設定(MTC=-4 pcm/℉)兩個預期暫態未急停事故的反應爐冷卻水系統壓力尖峰值仍能維持在3200psig內。

關鍵字

核三廠；小幅度功率提升；預期暫態未急停

並列摘要

The reactor coolant system (RCS) pressure transients under ATWS for Maanshan PWR with MUR conditions were analyzed in this study. The main concern was to observe whether the peak RCS pressure still maintained within 3200 psig limit (defined by ASME Code Level C service limit criterion under 100% rated power). The Maanshan RETRAN model, which had been developed for PWR transients, was used for ATWS safety analysis and related sensitivity analysis under MUR conditions. In document SECY-83-293, all initializing events were classified as either turbine trip or non-turbine trip events and their ATWS risks were also evaluated according to these two events. Loss of normal feedwater (LONF) and Loss of condenser vacuum (LOCV) ATWS were identified as limiting transients of non-turbine trip and turbine trip events in this study. In order to analyze the RCS pressure variations, a moderator density coefficient (MDC) model was included to observe the influences of moderator temperature and system pressure to the reactor reactivity, and the possibility of the peak RCS pressure reaching its limit. In addition, this study also covers the following sensitivity studies, (1)MTC settings, (2)RCP trip assumption, (3)initial conditions of steam generator outlet pressure, and(4)the model of steam generator. The peak RCS pressure was found to stay within the criterion of 3200 psig even at a conservative MTC setting of -4 pcm/℉.