Papers by Author: Young Ho Lee

Abstract: Recently, a dual-cooled fuel (i.e. annular fuel) which is compatible with current operating PWR plants has been proposed in order to increase both power densities and safety margins. Due to the design concept that is compatible with current PWR plants, however, when compared with a current solid nuclear fuel it shows a narrow gap between fuel rods and needs to modify spacer grid shapes and their positions. Because a flow-induced vibration by fast primary coolant is inevitable phenomenon, it is necessary to examine the fretting wear behavior between an annular fuel and designed spacer grids. In this study, fretting wear has been performed to evaluate the wear resistance of the annular fuel by using specially designed spring and dimple of spacer grids that have a cantilever type and a hemispherical shape, respectively. At the spring specimen with relatively small stiffness value, fretting wear was initiated at both end regions and then proceeded gradually to center region. Based on the test results, the fretting wear behavior of annular fuel was compared with the current solid nuclear fuel and a comparative factor of its reliability was proposed.

Abstract: A dual-cooled fuel (i.e. annular fuel) has been proposed to substantially increase in power density and safety margins compared to a solid fuel in operating PWR plants. As this fuel rod has larger outer diameter than the conventional solid rod to accommodate sufficient internal flow, new supporting structure geometries should be designed and their reliabilities (i.e. vibration characteristics, fretting wear resistance, etc.) are also examined with both analytical and experimental methods. In this study, the supporting structure characteristics and fretting wear behaviors are analyzed and examined by using two kinds of simulated supporting structures that have embossing and cylindrical shapes. Their supporting structure characteristics were examined by using a specially designed test rig and their results were compared with that of analytical method. Also, fretting wear behaviors of simulated supporting structures were experimentally examined with considering the effect of contact shapes and their stiffness values. Based on the test results, the relationship between the supporting structure characteristics and their fretting wear behaviors was discussed in detail.

Abstract: In order to evaluate the effects of a variation of a supporting springs' shape on the wear behavior of a nuclear fuel rod, sliding wear tests have been performed in room temperature air and water. The objective of the tests is to quantitatively evaluate the relationship between a worn area and a wear volume, and the formation behavior of a worn area with a variation of the slip amplitudes, applied normal loads and supporting spring shapes. The results indicated that the variation behavior of the volume and the wear scar size was influenced by the contact shape between the springs and the fuel rods. Also, it was found to be possible to evaluate a critical ratio (Tc) for each spring shape and test condition when the T was defined as the ratio of an applied normal load (Ln) to a wear scar size (At). Below this Tc, the wear volume was rapidly increased and the Tc was determined by a variation of the At under the same applied normal load condition. This result enables us to evaluate a wear resistant spring shape by using an analysis of a wear scar after wear tests have been completed. Based on the above results, the relationship between At and a worn area (Aw), a wear mechanism and an evaluation method for a wear resistance were discussed.

Abstract: An effort has been made in the present investigation to evaluate the wear resistance of nuclear fuel rods with a variation of the supporting spring shapes and their stiffness by conducting fretting wear tests in room temperature air and water. With increasing slip amplitude, the wear volume and maximum wear depth are increased with increasing slip amplitude. However, these are not linearly increased with increasing spring stiffness. After the wear test, the worn surfaces were observed to investigate the debris behavior and wear mechanism by using an optical microscope (OM). The results indicated that almost all of the wear debris remained between the contacting surfaces and the wear debris layers were well developed in room temperature air. Besides, some of the debris also remained on the worn surface in room temperature water. This result shows that the remaining debris effect on the worn surface was more dominant than the spring stiffness one. So, in order to improve the fretting wear resistance of a nuclear fuel rod, it is necessary to consider the debris behavior between contacting surfaces even though the supporting spring shape was optimized by considering the contact mechanics, material compatibility, etc. From the experimental results, the fretting wear mechanisms and the effect of spring properties were discussed.

Abstract: A new device for measuring an acceleration in a fuel rod has been developed. The primary purpose is to apply it to the experiments for a nuclear fuel fretting, which is caused by a fuel rod and grid interaction due to a flow-induced vibration of the rods. A bi-axial accelerationdetecting device of a cylindrical shape for an insertion into a cladding tube is designed. Two unimorph piezoelectric accelerometers of small size and for special use in a high temperature condition were attached to the housing’s inner wall of the mounting device, which were oriented perpendicularly with each other to accommodate the acceleration signal during a fretting. Verification is carried out by a displacement and a frequency response analysis. The verification results of the bi-axial acceleration-detecting device showed a good response for the amplitude and bi-directional trace. This paper mainly presents the detailed design of the bi-axial accelerationdetecting device which is developed by using the design optimization of a cylindrical type and applying it to the lower and upper plugs to sustain the device in the tube wall. The verification is carried out by a displacement analysis and a frequency response analysis. An elaboration of the development procedure, calibration method and results are also given in this paper.

Abstract: New system (load cell) for measuring a contacting force in a high temperature and pressure water condition (i.e. 320
, 15 MPa) has been developed. The primary purpose is to apply it to the experiments of a nuclear fuel fretting wear, which occurs on the contacts between the fuel rods and the spacer grid spring/dimples due to a flow-induced vibration of the rods. A bi-axial load cell of a cylindrical shape is specially designed. Strain gages for a special use in a high temperature condition were attached on to the sensitive region of it to accommodate the bi-axial loading condition in fretting. The full scale of this load cell is ±50 N in axial force and ±50 N in bending force, respectively. In order to increase the sensitivity and to compensate for an ambient temperature effect, the load cell consists of two Wheatstone full bridge arrangements. The calibration results of the load cell show that the coupling effects of each force were almost negligible. This paper mainly presents the experimental techniques used during the development of the new load cell system. The techniques are characterized by a design optimization of a jig of a cylindrical type, the application of a metallic sealant for a waterproofing, a free welding fabrication process and a temperature compensation circuit. Details on the development procedure, calibration method and application results are also given in this paper.

Abstract: In this study, the variation of spring characteristics with increasing temperature was examined and the effect of their variations on the wear behavior of a nuclear fuel rod in both room and high temperature (300°C) water conditions was evaluated. From the results of the load-displacement tests, the spring stiffness was remarkably varied with increasing temperature. The results of the wear tests indicated that the wear damages are decreased at high temperature water when compared with the room temperature result. These results indicated that the removal mechanisms of wear debris at high temperature water are dependent on not only the formation of the wear particle layer but also on the changed contact conditions such as the contact length or area due to the stiffness drops.

Abstract: Slipping characteristic on the contacts between the tubes and the supports is investigated to study the fretting wear of a vibratory tube. Tests were carried out to simulate the vibration of the fuel rods supported by the springs and dimples. A tube was forced to vibrate with 30Hz. The supporting condition was varied artificially: positive contact force or gap existence. During the tube vibration, amplitudes were measured continuously in the vicinity of the supports. Simple equations were derived to evaluate the slip displacement on the contacts. As a result, it was found that the supporting condition affected the vibration characteristic near the contacts. In the positive contact condition, the phase difference of the vibration signals at both sides of the contact was 180 degrees. It is altered in the gap condition and the higher frequency components than 30Hz appeared. The severer wear in the case of the gap existence is discussed with the evaluated slip distance per cycle.