Influence of Non-Destructive Examination Performance on Reliability of Pipes Having Stress Corrosion Cracks

Abstract

The fitness for service of the facilities of operational nuclear power plants is secured by proper inspections and evaluations. Defects took place in components will be detected by periodical inservice inspection, and fitness for service of cracked components during an evaluation period is evaluated. Although variation exists in the conditions used for inspection or evaluation, it is difficult to grasp about those influences on the reliability of cracked components by the deterministic evaluation like Fitness-for-Service Codes. In this study, influences of inspection parameters (defect sizing error, percentage of defect oversight, detectable defect size, successive tests) and an evaluation parameter (crack growth rate) on failure probability were evaluated for primary loop recirculation system piping of boiling water reactors in which many stress corrosion cracks took place. From these evaluations, the dominant parameters for the reliability of piping having stress corrosion cracks were clarified, and the requirements for the inspection and evaluation for revising Fitness-for-Service Codes were proposed. 1. 緒 言 2000 年の日本機械学会原子力設備規格維持規格 (1) (以下， 維持規格) の発行と前後して， 沸騰水型原子炉 (Boiling Water Reactor．以下，BWR)の原子炉再循環系(Primary Loop Recirculation system．以下，PLR)において応力腐 食割れ(Stress Corrosion Cracking．以下，SCC)が顕在化し，既に 100 以上の SCC き裂が検出されている (2) ．供

Stress Measurement of Stainless Steel Piping Welds by Complementary Use of High-Energy Synchrotron X-rays and Neutrons

“…High axial stress around the HAZ near the inner surface and relatively high radial stress around the same HAZ were observed. These observations align with FEM analysis results reported in the literature [11]. Additionally, the residual stress maps suggest that SCC initiated at the HAZ tends to propagate towards the weld metal, a phenomenon corroborated by observations of the PLR piping of BWRs [1].…”

“…A consistent axial stress pattern was observed at all measurement locations, characterized by a decrease in stress from tension on the inner surface side towards the central area, followed by an increase towards the outer surface. In the weld heat-affected zone (HAZ), the axial stress on the inner surface side tended to diminish with increasing distance from the weld line, aligning with the reported FEM analysis results [11]. Figure 12 shows triaxial stress maps, which were developed from the data presented in Figure 11 and superimposed onto the DEM specimen.…”

“…Furthermore, large-diameter piping featuring relatively thick walls may still present challenges for neutron penetration depth. Consequently, residual stress distributions in welds are typically estimated using the finite element method (FEM), factoring in welding conditions rather than through direct measurements [11][12][13].…”