包覆層厚度對核能電廠反應器壓力槽可靠度之影響

Abstract

核能電廠具有發電成本低與二氧化碳排放量少等優點，在面臨全球能源短缺與環保問題下，其為乾淨能源選項之一，然而核能電廠運轉首重結構與機械設備的安全，避免它們因老化而引起風險。本論文以反應器壓力槽為安全評估的對象，其為核能電廠蒸汽供應系統中極重要的壓力邊界組件，係由數片鋼板焊接而成，其中的焊道最有可能產生裂縫。輻射脆化為反應器壓力槽最主要的老化機制，當裂縫隨運轉時間成長至某一程度時，反應器壓力槽焊道即有可能產生脆裂。先前研究指出，包覆層厚度與裂縫尺寸為影響焊道脆裂的主要因素，因此本論文針對沸水式反應器壓力槽之垂直與水平焊道，利用破裂力學理論、並考慮輻射脆化現象，依隨機變數觀念進行模擬分析。分析結果顯示，當包覆層厚度超過0.35 in時，垂直焊道失效機率有上昇的趨勢；而當考慮檢測與修復時，使用適當的檢測能力相較於增加檢測週期或減少檢測範圍更能有效的降低失效機率；此外，在相同的環境負載下，水平焊道失效機率較垂直焊道來的低，就此，國外管制單位對於水平焊道檢測豁免的作法不失為可行之道。

Nowadays, we are facing problems of global energy shortage as well as the need of environmental protection. The advantage of low cost and small amount of CO2 discharge makes nuclear power an important choice for energy. However, the safety of structures, systems and mechanical components employed in a nuclear power plant has to be assured before a plant can be constructed. One of the most important pressure boundary components in the steam supply system of a nuclear power plant is the reactor pressure vessel (RPV). It is welded together by several steel plates. Cracks occur more frequently in welds rather than in base plates of a RPV. When a predominant crack grows along with operating time to a certain size, it may result in brittle fracture in the weld of a RPV. It has been pointed out that clad thickness and crack size affect the embrittlement and fracture of the weld. The present study employs a probabilistic fracture mechanics approach by taking into account radiation embrittlement to find fracture-failure probabilities of RPV welds. The result shows that, when the clad is thicker than 0.35 inch, the failure probability at axial weld should be paid more attention to. As for the effect of inspection and repair, it is found that adopting a more advanced inspection instrument reduces failure probability more than increasing inspection cycles or covering more inspection areas. It is also found the probability of failure at circumferential welds is smaller than that at axial welds. The finding reassures the proposition made by the United States Nuclear Regulatory Commission (USNRC) that inspection of circumferential welds of a RPV can be exempted.