核三廠機組斷然處置之洩壓注水執行策略分析

日本福島事件後，台電提出「機組斷然處置程序指引 (Ultimate Response Guideline, URG)」，使核電廠面臨如福島事件之複合式災害時，能透過所謂「DIVing」，即降壓 (Depressurization)、注水 (Injection)、排氣 (Venting) 等一系列操作，防止燃料護套溫度上升超過 1500 °F，避免放射性物質外釋。本論文使用熱水流模擬程式 RELAP-3D 對核三廠進行事故暫態分析，了解核三廠在電廠全黑 (Station Blackout, SBO) 的情況下，各項重要參數對燃料護套受損時間及可用救援時間之影響，各項重要參數包括：蒸汽驅動輔助飼水運轉時間；蒸汽產生器窄幅水位；軸封洩漏流量；蓄壓槽 (Accumulator, ACC) 是否補水；生水注水量。重要結論如下：蒸汽驅動輔助飼水若能在 SBO 發生後運轉至少1小時，則燃料護套受損時間將延長 1.5 小時以上，輔助飼水運轉時間延長後，衰變熱功率雖持續降低，但窄幅水位維持在特定高度，蒸汽產生器內總水量變化不大，因此未能有效延長燃料護套受損時間。若能將蒸汽產生器窄幅水位維持在高水位，蒸汽產生器內總水量增加，燃料護套受損時間也將大幅延長。軸封洩漏流量增加將減少燃料護套受損時間，且整體暫態時間越長，一次側因軸封洩漏減少之水量越多，燃料護套受損時間將大幅降低。蓄壓槽正常補水將增加燃料護套受損時間，在軸封洩漏流量較大及整體暫態時間較長的案例中，因補水量較大且補水時間較長，燃料護套受損時間將增加較多。較大的生水注水量可將可用救援時間延長，由 200 gpm (約 12.6 kg/s, 27.81 lbm/s) 提升至 1500 gpm (約 94.64 kg/s, 208.65 lbm/s)，平均能增加可用救援時間 500 秒。

關鍵字

斷然處置； RELAP-3D ；核三廠；電廠全黑

並列摘要

On March 11th of 2011, Fukushima Daiichi Nuclear power Station of Tokyo Electric Power Company was hit by earthquake and tsunami. The attack caused loss of all onsite and offsite AC power. The core of unit 1 ~ 3 were severely damaged and large amount of radionuclide was released to the environment. Ultimate Response Guidelines (URGs) was proposed by Taiwan Power Company (TPC) to mitigate the so called “Fukushima type accident” of nuclear power plants (NPPs). The guidelines direct the emergency response staff of the plant set up portable electric supplies and line up the injection paths of unconventional cooling water, such as sea water, immediately after the initiation of station blackout (SBO). As specified in URGs, if it is necessary, operators will depressurize the reactor coolant system to bring in the low pressure water. Decay heat is then removed via containment venting. These collective actions are called DIVing (depressurization, injection, and venting) in URGs. The injection damages the economic value of the plant. However, the injection prevents the cladding temperature from rising above 1500 F, which is the temperature that the cladding breaches and the release of volatile fission products from gap is initiated. The actions are designed to avoid large scale evacuation of public around the damaged plant during the accident. In the present study, the effectiveness of DIVing to prevent the large release of radionuclide to the environment is assessed using RELAP5. The plant analyzed is Maanshan Nuclear Power Satation of Taiwan Power Company. The station employs Westinghouse designed three-loop Pressurized Water Reactor with a rated thermal power of 2822 MWt. The purpose of the study is to identify the time margin for operators to carry out DIVing actions after the termination of water injection of turbine driving auxiliary feedwater system (TDAFWS). Sensitivity studies were performed to assess the impact of operating time of TDAFWS, steam generator secondary side water level, leakage rate of reactor pump seal, whether accumulator injection is isolated on the time margin available. The results provide guidance to operators in the decision of execution time of DIVing.