Ga Hyun Choi

To ensure the integrity and safety of steam generator tubes in nuclear power plants, eddy-current testing is periodically employed. Because steam generators are equipped with thousands of thin-walled tubes, the eddy current is tested using a bobbin probe that can be used at high speed. Steam generator heat pipes in nuclear power plants have different sizes and shapes depending on their row number. In particular, heat pipes in the first row are located inside the steam generator and are of the U-bend type because the radius of the curved pipe is the smallest. A steam generator heat pipe has a thickness of about 1 mm, so a geometrical cross-sectional area change may occur due to residual stress when manufacturing the curved pipe. It is difficult to determine an exact shape because the change in cross-sectional area generated during the manufacturing process varies depending on the position of the pipe and the distortion rate. During eddy-current testing (ECT), to ensure the integrity and safety of the steam generator tubes, the shape change of the bend may cause a noise signal, making it difficult to evaluate defects in the pipe. However, the noise signals generated in this situation are difficult to analyze in a real measurement environment, and difficult to verify by producing a mock-up, which complicates distinguishing a noise signal from a defective signal. To solve this problem, various noise signals were obtained using the electromagnetic analysis method of COMSOL Multiphysics, a commercial program based on numerical analysis of the finite element method, to simulate the environment, including the change in cross-sectional area of the heat pipe. When compared to the actual measurement signal, the accuracy of the simulations improved, and various types of noise signals were detected, which may be helpful for accurate evaluations of defects during actual inspections.

원자력발전소 증기발생기 전열관은 고온 · 고압 환경에 노출되어 있어 건전성 및 안전성의 확보를 위하여 와전류 검사를 주기적으로 수행하고 있다. 국내 원전에 도입된 U-자형 전열관의 곡관부는 bending 공정에 따른 기하학적 변형에 의해 와전류 검사 시 큰 노이즈 신호를 유발하여 결함의 검출능을 현저히 저하시킨다고 보고되고 있다. 따라서 전자기적 해석을 통하여 곡관부에 대한 다양한 노이즈 신호를 시간적, 경제적으로 확보한 후 결함 검출능을 향상시키기 위한 데이터베이스로 활용하고자 한다. 본 연구에서는 bending 공정으로 인한 관의 변형 정도를 관찰하기 위하여 U-bend 시편에 대하여 computed tomography 분석을 수행하여 내부 단면 형상을 분석하였다. 곡관부의 노이즈 신호 유발 요인인 코일 프로브의 위치 변화와 관의 형상 변화가 와전류 신호에 미치는 영향을 파악하기 위하여, 유한요소 수치해석 기법을 기반으로 하는 상용 프로그램인 COMSOL Multiphysics로 노이즈 신호 유발 요인을 모사하여 전자기적 해석을 수행하였다. 이를 통해 mock-up 제작을 통해서 직접적으로 얻기 어려운 신호들을 취득하고 평가하였으며, 향후 노이즈 신호 제거에 활용 가능할 것으로 판단된다.

Thin ITO/Au/ITO lamellar films were prepared by a low-temperature process, and its microstructure development was analyzed to find an optimum thickness of the metallic layer to improve the optoelectrical properties. Crystalline ITO on the Au layer with a preferred orientation occurred with increasing thickness of the Au layer. The crystalline ITO had lower average surface roughness than the amorphous ITO layer. The sheet resistance of the ITO/Au/ITO lamellar film was lower than that of the amorphous ITO film. The ITO deposit on the gold layer with 10-nm thickness had a relatively homogeneous and uniform surface of ITO thin films. As the thickness of the gold layer of the ITO/Au/ITO increased, the carrier density tremendously increased, whereas the mobility and the transmittance were slightly decreased. Thin amorphous Au layer was effective to improve the transmittance.