Aging assessment and risk-informed management of feedwater heaters in nuclear power plants
Date Issued
2005
Date
2005
Author(s)
Wu, Chih-Hsien
DOI
zh-TW
Abstract
本研究係針對飼水加熱器殼側的老化與維修管理問題，進行「老化評估模式」與「風險告知管理技術」兩方面問題的探討，期能根據合理的老化評估模型評估飼水加熱器運轉之風險，並在風險的考量下協助電廠決策者擬定適當之維護管理方案。在老化評估模式方面，我們根據電廠的檢測技術報告推斷飼水加熱器殼側的薄化係雙相流的流動加速腐蝕(Flow Accelerated Corrosion, FAC)所造成的問題，並選定Siemens/KWU所發展出的雙相流FAC薄化率經驗式作為預估模型，同時在考量電廠運轉環境的隨機性以及壓力容器的相關法規後，擬定適當的失效準則，評估飼水加熱器殼壁的失效機率變化趨勢。在風險告知管理技術方面，本研究以機率風險評估(Probabilistic Risk Assessment, PRA)技術為主要的工具，根據電廠機組的運轉方式，完成飼水系統的故障樹分析，再利用分析結果配合法塞爾-維斯利(Fussell-Vesely, FV)指標、風險增加值(Risk Achievement Worth, RAW)指標以及差分(Difference Importance Measure, DIM)指標，針對飼水系統各元件，將其風險值量化並為相對之風險排序。透過上述的分析，我們發現KWU雙相流模式應用在飼水加熱器的薄化率預估方面有不錯的準確性，同時也發現針對所探討的飼水系統案例，PRA分析所提供的風險量化值及風險排序結果，確可協助廠方決策人員鑑定出飼水系統當中的LP6B、LP6A、LP5B等飼水加熱器為需要優先施予預防性檢測維護的元件。本研究的分析方法及所得的結果，相信有利於電廠決策者擬定更合乎經濟效益的檢測維護方案。
The risk-informed management of feedwater system in a nuclear power plant starts from the identification of failure mechanism of the heaters. It is found from all sources, including examination of previous inspection reports, that flow accelerated corrosion (FAC) occurred on shell sides near the inlet nozzle is the major failure mechanism. Therefore, a semi-empirical formula proposed by Siemens/KWU is used in the present study to predict the corrosion amount of the heater-wall caused by FAC. The formula is, in fact, a modified one that considers two-phase flow rather than the single-phase-flow formula developed previously by the same organization. It is found the model predicts the amount of corrosion very accurately. The sensitivity analysis indicates that, aside from other factors, fluid velocity, steam quality and oxygen content have great influence on the corrosion rate. A Monte Carlo method that considers randomness of possible parameters is then employed to simulate amounts of wall-corrosion of individual feedwater heaters and evaluate their relative risks. Under the assumption that risks for other components of the feedwater system are also known, probabilistic risk assessment (PRA) is applied to the entire feedwater system. Several importance measures including Fussell-Vesely (FV), risk achievement worth (RAW), and difference importance measure (DIM) are adopted for risk ranking of individual heaters and other components. It is found that although RAW is simpler to use, it is sometimes more difficult to tell the importance differences among individual components, it is also found that the LP6B, LP6A, LP5B would be the most critical feedwater heaters in entire feedwater system. Since the risk-significance of each component obtained or assumed evolves with time, a proper and economical inspection or management plan that emphasizes certain components at certain times can be made, and it is shown in the present study.
Subjects
飼水加熱器
老化評估
風險管理
feedwater heater
aging assessment
risk-informed management
Type
thesis
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