파열시험을 통한 다중 부분관통균열이 존재하는 증기발생기 전열관의 손상거동 평가

Abstract

Steam generator tubes in nuclear power plants sustain the pressure difference between the primary reactor coolant system and the secondary system of nuclear power plants. However, steam generator tubes have been suffered from various types of degradation induced by mechanical and environment stressors while in-service. Therefore, a number of experimental and numerical studies have been conducted to evaluate the rupture of steam generator tubes. Most studies were performed on a single axial through-wall flaw. However, in practice, steam generator tubes with stress corrosion cracks were found by multiple part-through-wall flaws. Up to now, studies of steam generator tubes with multiple parts were insufficient. This study carried out burst tests on steam generator tube specimens containing multiple part-through-wall flaws. The specimens were machined from steam generator tube of alloy 690TT. All flaws were made by electro-discharge machining method on the outer surface of steam generator tubes, and depth of 50% of steam generator tubes. In the experiment, 9 types of multiple part-through-wall flaws, including collinear axial flaws, non-aligned axial flaws, parallel axial flaws, combined axial and circ. flaws, collinear circ. flaws, non-aligned circ. flaws and single flaw were considered. The tests were performed at room temperature with a pressurization rate of 1MPa/sec. The interaction effect of axial collinear part-through-wall flaws diminished with increasing the separating axial ligament length. However, the failure pressure by interaction effect of axial collinear part-through-wall flaws deceased when the ligament between flaws was less than 5mm. In case of non-aligned axial part-through-wall flaws, the failure pressure diminished with increasing the separating circumferential ligament length. For parallel axial part-through-wall flaws, the failure pressure decreases the minimum at 90o. However, the failure pressure increases when the ligament between flaws was more than 15mm. For circumferential part-through-wall flaws, the failure pressure was similar to the failure pressure of sound steam generator tubes. For combined axial and circ. part-through-wall flaws, the failure pressure of length of 25.4mm was not the interaction effect. Multiple part-through-wall flaws with length of 6.3 or 12.7mm failed by coalescence and unstable tearing. The flaws with length of 25.4mm or 50.8mm failed by rupture at one of multiple part-through-wall flaws.