核能電廠反應器壓力槽焊道之可靠度評估

Reliability Assessment of Welds of Reactor Pressure Vessels in Nuclear Power Plants

林獻洲(Lin, Hsien-Chou)

指導教授 ： 吳文方

國立臺灣大學/工學院/機械工程學研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.03018

摘要

反應器壓力槽為核能電廠蒸汽供應系統中重要的壓力邊界組件，因壓力槽體積巨大，無法一體成型，所以由數片鋼板焊接而成，其中的焊道可依其走向分為水平與垂直焊道。因不同走向之焊道會有不同的焊接製程，所以水平與垂直焊道內的材料性質與可能裂縫方向也有所差異。事實上，焊道為反應器壓力槽中最可能產生裂縫之處，自有其安全疑慮，其中除了焊道可能預存裂縫外，輻射脆化亦會導致反應器壓力槽產生裂縫，或導致裂縫加速成長，進而影響安全。所幸目前國內電廠已依美國核能管制委員會所制定的規範，訂定一套反應器壓力槽輻射脆化問題的評估方法，以為安全把關。本論文針對沸水式反應器壓力槽之水平與垂直焊道，考慮破壞力學理論和輻射脆化現象，依隨機變數和隨機程序的基本觀念，自行建構一套模型並撰寫程式、進行分析。分析結果顯示，若每十年檢測並維修一次，本文所探討的反應器壓力槽於其四十年的設計壽命到達時，水平與垂直焊道失效機率分別為 、 ；若將分析模擬時間延長至六十年，則水平與垂直焊道失效機率分別變為 、 。由此可知，即使反應器壓力槽延役使用至第六十年，其失效機率的增加相當有限。此外，在同樣的環境負載下，垂直焊道之失效機率較水平焊道者高，且兩者相差甚鉅，所以在檢測修補時，應對垂直焊道施予較精密、詳細的檢測，而一些國外管制單位對水平焊道檢測豁免的作法亦不失為可行之道。

關鍵字

可靠度 ； 機率破壞力學 ； 反應器壓力槽 ； 應力腐蝕龜裂 ； 蒙地卡羅

並列摘要

The reactor pressure vessel (RPV) is the most important pressure boundary component in the steam supply system of a nuclear power plant. The RPV is too large to be manufactured as a single piece. Instead, it is welded together by several steel plates. Based on their orientations, the welds can be classified into circumferential ones and axial ones. Different orientations may require different weld processes. It results in different material properties and different directions of pre-existed cracks. In general, cracks occur more frequently in welds rather than in base plates of a RPV. Radiation embrittlement is considered the major cause of cracks. To evaluate the degree of radiation embrittlement and ensure the safety of a RPV, the domestic nuclear power plants adopt the standard set by the United States Nuclear Regulatory Commission (USNRC) and carry out aging assessment programs periodically. Aside from it, the present study employs a probabilistic fracture mechanics approach taking into account the radiation embrittlement to find crack failure probabilities of RPV welds. The result shows that, for the particular vessel studied in this thesis, if it is inspected and repaired every ten years, the probabilities of failure due to cracks at circumferential and axial welds are and respectively after forty years of usage. If its usage is extended twenty more years beyond its design life and a ten-year inspection period is considered again, the probabilities of failure of circumferential and axial welds become and respectively at the sixtieth year of usage. They are still considered low. However, the difference is large in comparison of failure probability of circumferential welds and that of axial welds. Based on this observation, axial welds should be inspected more frequently than circumferential welds. The latter can even be exempted from inspection as proposed by USNRC.

並列關鍵字

Reliability ； Probabilistic Fracture Mechanics ； Reactor Pressure Vessel ； Stress Corrosion Cracking ； Monte Carlo Simulation

參考文獻

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