Out-of-pile study of lanthanide migration in U-Zr alloy fuel - 15311

In metallic fast reactor fuel, fission product lanthanides (La, Ce, Pr, Nd, etc) accumulate in the fuel and migrate to cladding. The lanthanides are most reactive with Fe-base cladding than the fuel constituents, and the resulting reaction products lower the cladding melting point, reduce effective cladding wall thickness and therefore increase stresses in cladding. Out-of-pile tests of U-Zr alloys containing cerium, simulating irradiation of high burnup metallic fuel, was performed to examine the migration mechanism. The sample had a blind central hole to simulate the porous nature of irradiated fuel. The sample is an alloys disk of U-10wt%Zr-2wt%Ce with dimensions of 6 mm D * 5 mm L. The sample was tested with the hotter side maintained at 700 C. degrees and the cooler side at 400 C. degrees, which was to obtain a thermal gradient of 80 K/mm. With this thermal gradient, the sample was heated for about 90 days. After the test, the sample was cut in half along the central hole, and SEM analysis was performed on the cross section to measure the concentrations of Zr and Ce along the length direction, and Ce concentration along the surface of the axial center hole. Using the measured data, the mechanism of Ce migration was assessed. The results show that the migration of Zr, U and Ce by diffusion through the fuel medium is negligible