Calculation of the thermal conductivity and of the contact characteristics of rough surfaces of fuel and jackets of fuel elements of a power reactor

The authors consider the contact of rough surfaces of the fuel and the jacket of a fuel element. Since the gap is small in comparison with the radii of curvature of the contacting surfaces, we consider the contact of nominally flat surfaces. We make the following assumptions: the unevenness of the surface is modeled by spherical segments with a constant radius R of curvature; two types of contacts of uneven structures exist: elastic contact is in accordance with the conclusions of the Hertz solution, with the average stress of a plastic contact equal to 3σ where σ (minimum yield strength of the materials of the pair) denotes the yield strength of the material of the jacket; the plastic contact of two uneven surfaces begins at a maximum contact stress 3σ; and the influence of surface films upon the heat conduction through the actual contact points is slight. Taking into account the mechanical properties of the fuel (uranium dioxide) and of the jacket (alloy of Zr + 1% Nb) at an average contact temperature of 6730K, we find that at a contact pressure > 0.04 MPa, the fraction of the plastic contacts of the uneven structures exceeds 0.87; it was concluded that the dominating plastic contact form is preserved even in repeated load applications