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Ryu K.H.,Seoul National University | Lee T.H.,Seoul National University | Kim J.H.,Seoul National University | Hwang I.S.,Seoul National University | And 4 more authors.
Nuclear Engineering and Design | Year: 2010

The flow accelerated corrosion (FAC) phenomenon persistently impacts plant reliability and personnel safety. We have shown that Equipotential Switching Direct Current Potential Drop (ES-DCPD) can be employed to detect piping wall loss induced by FAC. It has been demonstrated to have sufficient sensitivity to cover both long and short lengths of piping. Based on this, new FAC screening and inspection approaches have been developed. For example, resolution of ES-DCPD can be adjusted according to its monitoring purpose. The developed method shows good integrity during long test periods. It also shows good reproducibility. The Seoul National University FAC Accelerated Simulation Loop (SFASL) has been constructed for ES-DCPD demonstration purposes. During one demonstration, the piping wall was thinned by 23.7% through FAC for a 13,000 min test period. In addition to the ES-DCPD method, ultrasonic technique (UT) has been applied to SFASL for verification while water chemistry was continually monitored and controlled using electrochemical sensors. Developed electrochemical sensors showed accurate and stable water conditions in the SFASL during the test period. The ES-DCPD results were also theoretically predicted by the Sanchez-Caldera's model. The UT, however, failed to detect thinning because of its localized characteristics. Online UT that covers only local areas cannot assure the detection of wall loss. © 2009 Elsevier B.V. All rights reserved. Source

Baek C.-H.,Yonsei University | Kim H.-I.,Yonsei University | Hwang J.Y.,Yonsei University | Jung An S.,Yonsei University | And 3 more authors.
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2011

The gamma camera system was designed for monitoring the medical fields such as a radiopharmaceutical preparation lab or a patient waiting room (after source injection) in the division of nuclear medicine. However, gamma cameras equipped with a large-angle pinhole collimator and a thick monolithic crystal suffer from the degradation of the spatial resolution at the periphery region due to parallax error by obliquely incident photons. To improve the uniformity of the spatial resolution across the field of view (FOV), we proposed a three-layer crystal detector with a maximum-likelihood position-estimation (MLPE) method, which can measure depth-of-interaction (DOI) information. The aim of this study was to develop and evaluate the performance of new detector experimentally. The proposed detector employed three layers of monolithic CsI(Tl) crystals, each of which is 50.0×50.0×2.0 mm3, and a large-angle pinhole collimator with an acceptance angle of 120°. The bottom surface of the third layer was directly coupled to an 8×8 channel position-sensitive photomultiplier tube (PSPMT, Hamamatsu H8500C). The PSPMT was read out using a resistive charge divider, which multiplexes 64 anodes into 8(X)8(Y) channels. Gaussian-based MLPE method has been implemented using experimentally measured detector response functions (DRFs). Tc-99 m point source was imaged at different positions with and without DOI measurements. Experimental results showed that the spatial resolution was degraded gradually as the source moved from the center to the periphery of the FOV without DOI information but the DOI detector showed the marked improvement in the spatial resolution, especially at off-center by correcting the parallax error. In this paper, our new detector with DOI capability proved to characterize reliably the gamma event position with the high and uniform spatial resolution, so that the large-angle pinhole gamma camera could be a useful tool in contamination monitoring. © 2010 Elsevier B.V. All rights reserved. Source

Kim H.-I.,Yonsei University | Baek C.-H.,Yonsei University | Jung An S.,Yonsei University | Kwak S.-W.,Korea Institute of Nuclear Nonproliferation and Control | Hyun Chung Y.,Yonsei University
Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment | Year: 2013

The purpose of this study was to develop and evaluate a gamma camera with a newly designed diverging collimator for monitoring radiation fields in nuclear medicine. Simulations using the Geant4 Application for Tomographic Emission (GATE) were performed to model the gamma camera system designed to monitor Tc-99m radioactive isotopes usually used in nuclear medicine. A gamma camera consists of a diverging collimator, a CsI(Na) scintillation crystal with dimensions of 50.0 mm×50.0 mm×6.0 mm and Hamamatsu H8500 PSPMT. The diverging collimator is composed of two layers of diverging slats stacked directly above each other, and the front layer is rotated by 90° with respect to the back layer. The point source at different positions was simulated, and the optimal slat thickness and slat height were determined by evaluating the spatial resolution and sensitivity. The slat thickness is 1.0 mm, the slat height is 40.0 mm and the angle of slats ranges from 0 to 22.5 °. The front and back layers are composed of 40 and 18 slats, respectively, to achieve equal spatial resolution in the x and y directions. The diverging collimator improves the uniformity of the spatial resolution and sensitivity across the field of view and the count rate better than the pinhole collimator. Experimental measurements were performed, and the results agreed well with simulations in terms of spatial resolution and sensitivity. The results demonstrated that the two-layer diverging-slat collimator is suitable for large area monitoring of the radiation fields. © 2012 Elsevier B.V. Source

Baek C.-H.,Dongseo University | An S.J.,Yonsei University | Kim H.-I.,Yonsei University | Kwak S.-W.,Korea Institute of Nuclear Nonproliferation and Control | Chung Y.H.,Yonsei University
Radiation Measurements | Year: 2013

The collimator design for a nuclear monitoring system should be considered differently from the collimator design for medical environments because it has to be used in high-energy radiation environments. The purpose of this study was to determine the optimum pinhole design and to evaluate its performance for acquiring good-quality image in a high-energy radiation field. Simulations using the Geant4 Application for Tomographic Emission (GATE) were performed to model the pinhole gamma camera system. The gamma camera consists of a pyramid-shaped lead collimator with a tungsten pinhole insert, and a CsI(Tl) scintillation crystal with thickness of 6.0 mm and area of 50.0 mm × 50.0 mm. The acceptance angle of the pinhole collimator and the distance from pinhole to scintillator crystal were set to 45 and 60 mm, respectively. The intrinsic spatial resolution and sensitivity were simulated by changing the pinhole diameter and channel height. The point source was located 60 mm above the center of the pinhole, and the transmitted image was estimated for pinhole diameter values from 2.0 mm to 4.0 mm, while the channel heights were fixed between 2.0 mm and 6.0 mm. The optimal ranges of channel height and pinhole diameter were determined by evaluating the intrinsic resolution and sensitivity tradeoff curves. The pinhole parameters were selected based on these analyses, and we verified the simulation results through experimental tests of three types of collimators (general purpose, high sensitivity, and high resolution). The simulated and experimental results agreed, with discrepancies of 4.5% and 6.4% in the sensitivity and spatial resolution, respectively. The results demonstrate that the pinhole collimator designed in this study could be utilized to perform radiation monitoring. © 2013 Elsevier Ltd. All rights reserved. Source

Ryu H.-W.,Korea University | Han J.-J.,Korea Institute of Nuclear Nonproliferation and Control | Kim Y.-J.,Korea University
American Society of Mechanical Engineers, Pressure Vessels and Piping Division (Publication) PVP | Year: 2015

This study provides the application of damage model to complex cracked pipes which can be found especially in weld overlay region. From the perspective of structural integrity, enough basic and large-scale tests are required to accurately evaluate the components containing a crack-like defect. In this case, damage model using finite element (FE) method can be effectively used for the assessment of full-scale cracked pipes with minimum basic experiments data. The proposed method in this research is based on the stress-modified fracture strain damage model with stress reduction technique. In this paper, Battelle full-scale complex cracked pipe tests are simulated by the proposed damage model with reasonable procedure. FE simulation is conducted for basic experiments to determine failure criteria with calibrations. Then, crack initiation and maximum loads are predicted to characterize the fracture behavior of full-scale complex cracked pipes. Damage model is applied to both of carbon and stainless steel materials and verification with comparing to test data is conducted. Copyright © 2015 by ASME. Source

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