SIGNIFICANCE OF LEVEL OF STRESS IN PIPE ON RISK-INFORMEDINSERVICE INSPECTION

摘要

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