Aging assessment and risk-informed management of feedwater heaters in nuclear power plants

• Main Authors: Chih-Hsien Wu, 吳志賢
• Other Authors: 吳文方
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/20303267436870569113

碩士 === 國立臺灣大學 === 機械工程學研究所 === 93 === 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.