스페클 간섭법을 이용한 압력배관의 내부 결함검사 기술개발

Abstract

As nuclear power plants (NPPs) are aged, the degradation of component integrity induced by increasing susceptibility of defect has become to be an important issue on the aspect of ensuring safety and operability and life extension of NPPs. In particular, it is known that the current non-destructive techniques have limitations on the inspection of cracks in wall-thinning defects in pipe bends and elbows. For these types of defects, the integrity assessment methodologies and acceptance criteria are incomplete up to now. Therefore, it is needed to develop new techniques for inspection and to improve the basis technologies related to this area. The reliable methods and acceptance criteria to assess the integrity of components containing these types of defects should be also established. Since the integrity assessment procedure and methodology for wall-thinning defect in pipe bends have not been developed, especially, it is necessary to develop the integrity evaluation methodology for wall-thinned pipe bends, and the related numerical and experimental studies are required to support the development of the methodology. Therefore, the objectives of this project are to develop non-contact laser application inspection technique for applying to the inspection of wall thinning defects in pipe bends, and to establish the defect estimation and integrity evaluation methodologies for these defects. Most of the wall-thinning defects in pipe bends and elbows are caused by flow-accelerated corrosion (FAC), so that it is mainly observed in carbon steel piping systems. The thinning defect has occurred at various locations, for example, extrados, intrados, and crown, which depend on flow patterns in the pipe system. Also, the wall-thinned pipe bends have various bend angles and are subjected to in-plane and out-of-plane bending loads as well as internal pressure under various restraint conditions. Finite element analysis for not-thinned sound pipe bend showed that the deformation behavior of pipe bends is affected by the bend angle, type of bending load, and constraint conditions. This study developed integrity evaluation methods based on artificial intelligence techniques. In this method, PCA was used for a preprocessor (denoising and data selection) of input signals, and fuzzy neural-networks (FNNs), support vector regression (SVR), and fuzzy support vector regression (FSVR) methods were employed for integrity evaluation of components containing defects. The developed methods were applied to estimate the collapse moment of wall-thinned pipe bends using finite element analysis data. The results showed that the proposed methods give an accurate estimation of collapse moment of wall-thinned pipe bends whose RMS error is less than 1.0%. Also, the uncertainty analysis of the artificial intelligence techniques was performed, which provided the reliability of the estimation methods. This study designed and established specimens and testing system for a failure test of wall-thinned elbow, and performed the burst tests using the elbow specimens under various thinning geometries, thinning locations, and loading conditions. Failure pressures for 25 test cases were obtained and failure behavior were investigated from the results of experiments. Local failure criteria for wall-thinning defect, which are considered the stress states at defected area, were derived based on tensile tests using notched bar specimens and associated finite element simulations. By applying the criteria to the assessment of failure pressure of local wall-thinned straight pipes and elbows, the reliability and applicability of the failure criteria were verified. Also, their reliability at operating temperature of nuclear power plants (NPPs) were confirmed using the tensile data obtained from the notched bar and grooved plate specimens tests at the operating temperature of NPPs. Parametric finite element analyses on the wall-thinned elbows were conducted under various thinning geometries, thinning locations, and elbow geometries. Combining the results of the analyses and local failure criteria suggested by this study, failure pressure was calculated for each condition and failure behavior was investigated. A failure pressure evaluation model was suggested based on the calculated failure pressure for wall-thinned elbows. Finally, this study suggested a criteria for failure mode transition from axial cracking to circumferential cracking at wall-thinned area, and provided a map that presents the defect geometries to occur circumferential cracking at wall-thinned area using the failure mode transition criteria and parametric finite element analyses results.