Influence of grid-to-rod fit on fuel rod fretting

Abstract

The fretting behavior of nuclear fuel assemblies is influenced by a number of parameters. One of these is the fit between the fuel rod and the spacer grid, which changes during the fuel residence time in the core. The influence of fit on fretting has been investigated by performing model tests on a typical fuel rod support configuration. The test rod was suspended in water during the tests and was excited electromagnetically to induce vibration. Fretting tests each lasting 100 hours were carried out at room temperature for each individual condition of fit. The results showed that the first small fretting marks, having a maximum depth of 13 µm, did not start to appear until the spacer spring preload has been reduced to approximately 0.5-3.5 N. Substantial amount of wear was only observed in case of gapped support. The depth and volume of the fretting marks increased significantly as the preload decreased and the grid-to-rod gap increased. It was found that the stiffness of the vibrating system decreases as the size of this gap increases, allowing the test rod to move more freely and more extensively at the same level of excitation. It was further shown that the probability of fretting damage is greater with a clearance fit than with an interference fit (spring preload), and increases as the gap becomes larger. The results of this work are to be applied in the development of future spacer grids and should provide a better understanding of fretting processes inside the reactor.