Fission products phase distribution and gas release characteristics of metallic and oxide matrices of nuclear fuel

This presentation covers the distinguishing features of the fission products' chemistry in oxide and metallic fuels. The fission products produced from the same fissile isotope of an actinide (235U, 233U, 239Pu) in these fuel matrices settle down in characteristically different chemical states. Whereas the metallic matrix incorporate the reactive fps in alloy forms, the oxide matrices incorporate them mostly as oxides. The gaseous fission products in the two cases are partly retained inside matrix; the released part accumulates mostly in plenum region. The gas retention is more in the oxide matrix. Comparatively lower retention of gas inside metallic matrix results from its significant swelling followed by restructuring and internal pressure relaxation through gas release and fission products redistribution. In contrast, the oxide matrix largely accommodates the gas in micro-dispersed state with a significant rise in internal pressure. Higher mechanical strength and relatively high void space inside the oxide lattice gives this distinction. The gas transport behaviors of the two types of fuel matrices are quite different. Xe transport studied inside the two matrices showed comparatively higher gas diffusion in the metallic case. With as cast matrix of U-10 wt%Zr, the Xe transport is shown to be facilitated (20-30 folds increase) by the oxygen ingress in trace concentration inside the metallic lattice. Phase distribution study of transition metals (Pd, Rh, Ru) and rare earths (Nd, Ce) in high burnup simulated composition of the base fuel (U-10 wt%Zr) showed the presence of several phases with distinctly different morphologies. Six phases noted at 700℃ in 30d annealed sample reduced to three (U-rich, Zr and Ru rich, Pd rich) on annealing at 1000℃. The total number of fission product phases being governed by the fuel temperatures, high burnup fuel is expected to have the low and high temperature phases distributed over its radial section. (author)