- 學位論文
核能電廠管路之預覆焊影響評估
Effects Assessment of Preemptive Weld Overlay of Nuclear Pipes
摘要
現今覆焊(Weld Overlay, WOL)技術已經相當成熟，且在美國核能電廠中廣泛使用，其係針對電廠組件在其較常發生洩漏或產生裂縫之異種金屬焊接(Dissimilar Metal Welds, DMWs)處，實施預覆焊(Preemptive Weld Overlay, PWOL)，以期降低失效的可能性。國外對此議題已有相當多的研究報告與評估，惟國內業界尚少涉獵。本論文即著眼於此，考量國內電廠實際運轉條件與環境，進行核電廠管路預覆焊之效果研究及其影響評估，以求迎合國際技術發展的趨勢，並期能提供國內核能工業實務上的參考。本研究主要針對壓水式反應爐(Pressurized Water Reactor, PWR)噴嘴管路的焊道，預先評估其實施預覆焊的效果及其對管路安全的影響，文中分別以有限元素法(Finite Element Method, FEM)與機率破壞力學(Probabilistic Fracture Mechanics)分析焊道施工後的殘留應力與失效機率。研究結果發現，在施予預覆焊後，管路內表壁焊道處之殘留應力會由原來的張應力轉變為壓應力，表示預覆焊對內管壁裂縫有抑制其成長之保護作用；而在焊道因疲勞裂縫延伸導致失效方面，噴嘴管路在正常運轉40年後，其焊道失效機率將由未預覆焊的10-4大大降低至有實施預覆焊之10-6，表示預覆焊確能大幅降低管路因疲勞導致失效的機率，也更加說明預覆焊的可行性。
並列摘要
Weld overlay (WOL) repair technique has been used in U.S. nuclear industry for years. In particular, preemptive weld overlay (PWOL) is frequently applied to areas of dissimilar metal welds (DMW) to avoid fracture of nuclear power plant components. Although a lot of researches and assessments related to PWOL have been carried out in recent years, they are primarily done in foreign countries, and the environmental situation may not be the same as the local one. In the present study, domestic environmental and operational conditions are taken into consideration to assess the effect of preemptive weld overlay of nuclear power plant components. Finite element method (FEM) is employed to find the residual stresses and probabilistic fracture mechanics analysis is adopted to evaluate the failure probabilities due to fatigue crack growth. Welds at the pipe of spray nozzle of a pressurized water reactor (PWR) is illustrative as an example. It is found from finite element analysis that residual stresses at the weld on inner surface of the pipe is changed from tensile to compressive after carrying out the weld overlay. It indicates that PWOL reduces the possibility of crack growth. The result of the probabilistic fracture mechanics analysis also indicates that, after 40 years of operation, the failure probability at welds of the pipe changes from order of 10-4 to 10-6 after employing PWOL. It further ensures the applicability of PWOL to local nuclear power plant components or, at least, to the pipe of the studied spray nozzle.