Comparative study on rod fretting behavior of different spacer spring geometries

The resistance to fretting wear is one of the most important parameters of fuel assembly behavior in the reactor. To choose and optimize the designs with regards to this aspect, it is necessary to carry out lots of model tests. This paper describes the results of some of them carried out to determine the fuel rod fretting behavior for different support conditions. The tests were carried out in a special autoclave in water and at room temperature, with the test excitation being applied by an electromagnetic force excitation system. The test objects consisted of a test rod made of zirconium alloy, supported in three spacer grid cells with convex spring profiles of different geometries (i.e. different radii of curvature). The grid-to-rod diametral gap measured 0.1 mm at all locations. Due to these conditions of support, both impact and sliding motions can simultaneously occur at the interface between the contact surfaces. The dimensions of the fretting marks found after testing for 100 h were measured using mechanical instruments and an interferometer. The results showed that fretting increases the more, the smaller the radius of curvature of the convex support elements. This is due, on the one hand, to the smaller contact surface and, on the other, to certain features of the spring design. Given that the shapes of the fretting marks can vary widely, evaluation of fretting on the basis of fretting depth alone is insufficient. Instead, it is important to determine the entire geometry or shape of the fretting mark and thus the wear volume. The conclusions of this study are limited to the explored parameters and must not be extrapolated to any operational product without further investigations.