Neutron Irradiated Uranium Silicides Studied by Neutron Diffraction and Rietveld Analysis
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NEUTRON IRRADIATED URANIUM SILICIDES STUDIED BY NEUTRON DIFFRACTION AND RIETVELD ANALYSIS R. C. BIRTCHER, M. H. MUELLER* and J. W. RICHARDSON, Jr. *Argonne National Laboratory, Argonne, IL60439 INTRODUCTION The irradiation behavior of high-density uranium silicides has been a matter of interest to the nuclear industry for use in high power or low enrichment applications. Transmission electron microscopy studies have found that heavy ion bombardment renders U3Si and U3Si2 amorphous at temperatures below about 250 C [1], and that U3Si becomes mechanically unstable suffering rapid growth by plastic flow [2,3]. In this present work, crystallographic changes preceding amorphization by fission fragment damage have been studied by high-resolution neutron diffraction as a function of damage produced by uranium fission at room temperature. Initially, both silicides had tetragonal crystal structures. Crystallographic and amorphous phases were studied simultaneously by combining conventional Rietveld refinement of the crystallographic phases with Fourierfiltering analysis of the non-crystalline scattering component [4]. For U3Si, damage introduced by irradiation results in a monotonic expansion of the a-axis and contraction of the c-axis with increasing irradiation dose. The net effect is an expansion of the tetragonal unit cell. The expansion, coupled with a concomitant shift of midplane/midcell uranium atoms to symmetric positions, would result in a new cubic unit cell of the Cu3Au type. The phase transformation appears to be homogeneous and continuous without indication of coexistence of the initial and final phases. For U3Si2, both the a- and c-axes contract with increasing irradiation dose. Positional shifts of both uranium and silicon atoms continuously transform the U3Si2 toward a yet undetermined crystal structure. For both silicides, increases in background scattering reflect damage accumulation and/or amorphization. Damage in U3Si saturates after a dose of 0.02 displacements per atom (dpa) and amorphization begins after an expansion of 1 at.% at 0.05 dpa. EXPERIMENTAL Powdered U3Si and U3Si 2 specimens were fabricated from high purity Si and highly depleted Uranium, 0.022 at % 235U. The high dose irradiation behaviors of these powders, roll-bounded between aluminum plates, have been previously studied (2]. For this study, each powder, 50 to 150 gIm in diameter, was doubly encapsulated in thin wall Vanadium cans that had been evacuated and filled with He gas. The U3Si specimen also contained precipitates of U3Si 2 (
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