Ex-Reactor Determination of Thermal Gap Conductance Between Uranium Dioxide and Zircaloy-4

An ex-reactor study of the thermal gap conductance between Uranium Dioxide (UO{sub 2}) and Zircaloy-4 (Zr4) was performed under varying conditions of gas pressure (0.1 to 7 MPa); temperature (283 to 673 K); gas composition (He (100); Ar (100); He:Ar (51.79:48.21); and He:Xe (89.4:10.6)); and, average mean-plane separation distance D{sub mp} = 5.9 {micro}m (light contact); and, D{sub mp} = 23.1 {micro}m). In this report a description of the high pressure autoclave, specimen holder and associated apparatus is given together with experimental results. In conjunction with the experimental apparatus an assessment of determinant and indeterminant errors is made. In this report the predicted gap conductance based on the "ideal" expression for the gap conductance, H{sub g} = K{sub gas}/(d + g{sub 1} + g{sub 2}) and assuming D{sub m}p = d and g{sub 1} = g{sub 2}, is compared to the H{sub g} results and found not to be in agreement. Use of the ideal gap conductance expression under these conditions is seen to under-estimate the value of H{sub g} results. A discussion of possible reasons for the differences between the observed and predicted behavior is made. In addition to experimental errors resulting from gaseous convection, a detailed examination of the ideal gap conductance expression reveals that the basic assumptions (upon which the ideal gap conductance expression is derived) are not valid for real surfaces in close proximity and/or contact