19.8 For the graphite-uranium mixture of Example 19.3, determine the minimum physical size and the minimum fuel mass for criticality for (a) a cube, (b) a sphere, and (c) a cylinder. Assume that the extrapolation distance is negligible. Material buckling of this device is calculated as Bm2 =0.0028/cm2. Its density is 0.0324g/cm3. (solve for (a) only) 19.8 For the graphite-uranium mixture of Example 19.3, determine the minimum physical size and the minimum fuel mass for criticality for (a) a cube, (b) a sphere, and (c) a cylinder. Assume that the extrapolation distance is negligible. Material buckling of this device is calculated as Bm2 =0.0028/cm2. Its density is 0.0324g/cm3. (solve for (a) only)