Modeling 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, 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 induce 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/>In such nuclear fuel rod applications where high cyclic loads are involved [1], surfaces with high yield strength and wear resistance are required. As surfaces with homogeneous or graded nanostructures have been shown experimentally to significantly increase the yield strength and enhance surface wear resistance [2-5], in this work, we systematically investigate the potential for such novel nanomaterials for nuclear fuel-rod applications. In particular, we examine the fretting behavior of graded nanomaterials and compare them with homogeneous structures through a comprehensive computational study coupled with dimensional analysis. Using dimensional analysis, a set of dimensionless functions that takes the loading condition, the mechanical properties of the constituent materials, and geometric parameters into account are constructed to characterize surface damage on the structure with the graded nanostructured surface upon frictional sliding. Analytical expressions are proposed to explain experimental and computational observations to provide more insights into surface damage mechanisms, including shakedown behavior for nanostructured surface layers upon fretting wear. For numerical modeling, fretting sliding simulations are performed by finite element method (FEM).<br/>A comprehensive theoretical/computational framework is established for identifying the influence of mechanical properties of the constituent materials and geometric parameters such as material hardening, friction coefficient, and gradient function vs. depth on the surface deformation and damage for structures with graded nanostructured surfaces upon fretting frictional sliding. Using this framework, the plastic shakedown behavior vs. strain hardening, friction and the plastic gradient is quantified. It is also demonstrated that graded nanomaterials exhibit enhanced resistance to frictional sliding wear with a smaller plastic deformation zone. Optimization of such nanomaterials and potential for their applications in nuclear fuel rods are discussed.<br/><b>References</b>:<br/>[1] Blau PJ. Wear 2014; 313(1):89-96.<br/>[2] Singh A, Dao M, Lu L, Suresh S. Acta Materialia 2011; 59(19):7311-7324.<br/>[3] Cao SC, Liu J, Zhu L, Li L, Dao M, Lu J, Ritchie RO. Scientific Reports 2018; 8(1):5088.<br/>[4] Bernoulli D, Cao SC, Lu J, Dao M. Surface and Coatings Technology 2018; 339:14-19.<br/>[5] Long J, Pan Q, Tao N, Dao M, Suresh S, Lu L. Acta Materialia 2019; 166:56-66.