Graded Nanomaterial Architectures for Enhanced Fretting Wear Resistance of Surfaces Exposed to Extreme Conditions in Nuclear Reactors

The materials used in several parts of a nuclear reactor experience a combination of extreme conditions, i.e., mechanical stresses, radiative conditions and corrosive environments. For example, the pressurized water nuclear reactors typically contain a large number of fuel rods (about 50,000). These fuel rods are cylindrical in shape, contain radioactive material in their cores and are surrounded by a cladding layer. These fuel rods are kept in place with spacer grids such that there is space between the fuel rods for water to flow through and absorb the heat from the fuel rods, which can then be used to produce power. At the regions where the spacers are in contact with the exterior of the fuel rods, (i.e., the surface of the cladding that surrounds the radioactive fuel rod core), vibrational loads caused by turbulence in the water induces fretting damage which can lead to cracking and deterioration of the cladding layer, resulting in serious radioactive leak issues. It has been reported that, of all the causes of such leaks, grid-to-rod contact fatigue damage is the most prominent, accounting for more than 70% of the problems.<br/><br/>In such nuclear fuel rod applications where high cyclic loads are involved, surfaces with high yield strength and wear resistance are required. As surfaces with nanograins have been shown experimentally to significantly increase yield strength and enhance surface wear resistance, in this work, the potential for graded nanomaterial architectures for nuclear fuel-rod applications is systematically investigated. A three-dimensional finite element model is developed to analyze the characteristics of fretting sliding and shakedown behavior, considering different levels of contact friction and gradient layer thicknesses. The results obtained using 304 stainless steel as a representative model material demonstrate that metallic materials with graded nanostructured surfaces exhibit a significant reduction of over 80% in plastically deformed surface areas and volumes. This reduction significantly enhances the material's resistance to fretting damage when compared to homogeneous coarse-grained metals. It is noteworthy that the graded nanostructured material can exhibit either elastic or plastic shakedown behavior, depending on the contact friction coefficient. By reducing the friction coefficient (e.g., from 0.6 to 0.4 in 304 stainless steel), the graded nanostructured material achieves optimal fretting resistance, resulting in elastic shakedown behavior. This behavior is characterized by the absence of any increment in the accumulated plastic strain in the plastically deformed volume and area during subsequent sliding. These findings, derived from the investigation of graded nanostructured materials using 304 stainless steel as a model system, can be further refined to engineer optimal fretting damage resistance in nuclear fuel-rod applications.