The insoluble residues produced during the reprocessing of certain nuclear fuel elements containing aluminum, silicon, and uranium were investigated with respect to particle size, shape and distribution, composition, and surface- active tendencies. The fuel material samples studied contained from 0.4 to 7.0% silicon by weight (the high analysis represents a cast base AlSi alloy). The fuel materials were dissolved in mercury-catalyzed nitric acid. Two types of solid residue were produced by actual fuel dissolution. One was a finely divided material, brown in color and the other was black, crystalline material relatively large in size. Only the black crystalline material was obtained when cast AlSi was dissolved in nitric acid. Spectrographic analysis of the residues showed that silicon and alumimum were the major constituents. X-ray diffraction analysis of each type indicated the presence of elemental silicon only. The x- ray pattern obtained with the brown material showed lower intensities indicating the presence of amorphous material. Under a magnification of 970 diametems, the crystals observed appeared to be non-uniform, shallow platelets of irregular shape. The bulk of the residue is remmed by a 10-micron filter: however, the resulting filtration rates are very low. (2.0 gallons per hour per square foot). Sedimentation data …