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

Abstract

SUMMARYAn ex-reactor study of the thermal gap conductance between Uranium Dioxide (U0 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 Dmp = 5.9 ~m (light contact); and, Dmp = 23.1 ~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. Although the determinant 3a uncertainty in the measured gap conductance, Hg, is the same as reported in a previous topical report (PNL-2696, NUREG/CR-0330;, the 3a uncertainty associated with reproducibility of measurement was determined to be greater than 12% especially at high gas pressures and temperatures. As a result, the 3a uncertainty in the reproducibility of measurement was selected as being most representative of the overall experimental error and is calculated for each datum in this study.The Hg results are found to depend on gas pressure, temperature, average mean-plane separation distance, and gas composition. The following general features were observed:Hg decreases with increasing xenon or argon gas concentration at any given temperature and gas pressure.The gap conductance initially increases very rapidly with increasing gas pressure (i.e. gas density) over the range between 0.103 and 1 MPa. The rate of increase in Hg with gas pressure over this range decreases with additions of xenon or argon gas or increasing gap separation for a helium atmosphere. For all atmospheres and gap SP.parations studied, the rate of increase in Hg with gas pressure over the same range increases with increasing temperature.Hg exhibits a pronounced hump at gas pressures between 0.1 and 2 MPa for the helium and helium-xenon atmospheres. The hump is most pronounced for iii • helium at Dmp = 5.9 ~m at higher temperatures and progressively diminishes with decreasing temperature and/or argon addition. No hump is observed at elevated temperature (473K) or with increased argon content (both 48.21% or 100%). The unusual behavior of H with pressure is similar to reported g behavior of the thermal conductivity of helium with gas pressure. Discussion of a mechanism to account for the observed behavior based on the nature of velocity, time, and space of the gas molecules is given. At higher gas pressures between 1 and 7 MPa, Hg decreases with increasing gas pressure for both the helium and He:Xe(89.6:10.4) atmospheres at all temperatures. With the addition of Argon gas, however, H exhibits no g dependence on gas pressure for the He:Ar(51.79:48.21) gas composition. However, a linear increase with increasing gas pressure for the argon (100%) atmosphere is observed and is in accordance with reported increases in the thermal conductivity of Argon between 1 and 7 MPa.In this report the predicted gap conduc...

Analysis of fuel relocation for the NRC/PNL Halden assemblies IFA-431, IFA-432, and IFA-513

“…Substituting equations (17) and (IS) into equation 19, the axial fuel strain of equation {16) can be determined from the cladding elongation data (czcs): ( 20) The thermal strains for fuel and cladding may be determined from rod power and fuel temperature measurements.…”