SIGNIFICANCE OF LEVEL OF STRESS IN PIPE ON RISK-INFORMED INSERVICE INSPECTION

摘要

Recently, some US nuclear power plants are switching over to risk-informed inservice inspection (RI-ISI) program for piping. Piping in many US light water reactor plants is still being subjected to inservice inspection (ISI) based on certain deterministic criteria set by Section XI of the ASME Boiler and Pressure Vessel Code and US federal regulations as mandated by US Nuclear Regulatory Commission. Many recent works have established that the switch to RI-ISI provides both safety and economic benefits. The RI-ISI program for piping has been developed by Westinghouse Owners Group (WOG) and EPRI based on earlier ASME research methods. A typical development of a RI-ISI program (WOG method) involves a scope definition with identification of piping segments in the plant, a risk evaluation utilizing failure probability of piping components along with a consequence evaluation. The risk importance of the segments is based on the core damage frequency and large early release frequency. Subsequently, the results are reviewed by an expert panel familiar with the plant operation, maintenance and testing. Based on the recommendations of the expert panel, inspection locations are selected. Like any other periodic inspection program the RI-ISI program contains an element of feedback to enhance the program going forward. A significant part of the scope for setting-up a RI-ISI program involves extracting and using stress analysis results from design analysis into the structural failure probability, which can be expensive depending upon the level of availability of pipe stress documentation. A sensitivity study was performed for piping in a PWR plant by varying the level of stress in piping segments to study the changes in the probability of leakage and the probability of break which form an element in the risk evaluation process. The core damage frequencies were evaluated using the stresses in the piping segment right up to the applicable code allowables. The results of the evaluation show some minor changes in the structural failure probability. However, utilization of these structural failure probabilities in the risk evaluation causes only insignificant change to the core damage frequency and virtually no change to the risk ranking of the piping segment for inspection. Therefore, it is concluded that the structural failure probability of the piping segments can be adequately determined using the Code allowable stress when developing a RI-ISI program.