Fretting behavior in three different model support configurations

Abstract

Fretting damage to fuel rods reduces the service lives of the fuel assemblies and therefore increases reactor operating costs. The future development of fretting-resistant spacer grids necessitates the investigation of fretting behavior by means of model tests. This paper presents the results of such tests carried out to investigate fretting in three different fuel rod support configurations. The tests were performed in water at room temperature in a fretting test stand (referred to below simply as “autoclave”) using test rods with zirconium-alloy cladding tubes. The test rod was excited by means of electromagnet to induce vibration. The depth of the fretting marks and their volume increased as testing progressed. The resulting increase in the grid-to-rod gap caused changes in rod dynamic behavior and in the intensity of rod motion. Fretting rate is affected by rod motion, and the presence of edges at the point of contact between rod and support accelerates fretting wear. Spring design affects not only the degree of fretting but also the time history of the fretting process. Steady-state fretting was identified in the case of rods supported by convex springs without edges at the point of rod-to-support contact. The results of the model tests should provide a better understanding of fretting processes inside the reactor. They should not, however, be used to describe real conditions inside a reactor.