Fretting wear behavior of a nuclear fuel rod under a simulated primary coolant condition

Grid-to-rod fretting wear is one of the main causes of failure in a nuclear fuel rod. A key factor affecting the fretting wear resistance is the spacer grid geometry under the primary coolant conditions of a nuclear reactor. This paper describes the experimental results of grid-to-rod fretting carried out to examine the effects of flow rate and rod support conditions. Fretting wear tests were performed in a specially designed test section with a simulated primary coolant and then verified with a rod vibration simulator of a hydraulic actuating system at room temperature. The objective is to examine the effects of the fluid flow on the rod vibration behavior and grid-to-rod fretting with different contact conditions. As a result, the fuel rod vibration under the simulated primary coolant rapidly increased with increasing grid-to-rod gap and flow rate. However, the fretting wear results showed that the amount of wear damage under the primary coolant flow conditions is dramatically increased compared with the simulated rod vibration results in stagnant water. This means that, for a coolant flow, it is almost pointless to explain the spring shape effects based on wear debris behavior according to the grid geometry in grid-to-rod fretting tests. Thus, an evaluation of grid-to-rod fretting should be performed by considering the variation in the flow field with different spacer grid geometries. Based on the test results, the grid-to-rod fretting behavior was evaluated by comparing the rod vibration behavior with different simulated primary coolant conditions and the variations in the contact conditions. Finally, the grid-to-rod gap effects on the fretting wear of a nuclear fuel rod under primary coolant conditions were discussed.