Uranium silicides have been considered for use as reactor fuels in both high power and low enrichment applications. However, U3 Si was found to become amorphous under irradiation  and to become mechanically unstable to rapid growth by plastic flow [2,3]. U3Si2 appears to be stable against amorphization at low displacement rates, but the extent of this stability is uncertain. Although the mechanisms responsible for plastic flow in U3Si and other amorphous systems are unknown, as is the importance of crystal structure for amorphization, it may not be surprising that these materials amorphize, in light of the fact that many radioactive nuclide - containing minerals are known to metaminctize (lose crystallinity) under irradiation . The present experiment follows the detailed changes in the crystal structures of U3Si and U3Si2 introduced by neutron bombardment and subsequent uranium fission at room temperature. U-Si seems the ideal system for a neutron diffraction investigation- since the crystallographic and amorphous forms can be studied simultaneously by combining conventional Rietveld refinement of the crystallographic phases with Fourier-filtering analysis  of the non-crystalline scattering component.