This should be in the Mole Day forum, but anyway, no you do not need to increase the density (the density of uranium is already really high at 18.95g/cm3), you're thinking about plutonium-239, which does need to be in a sphere with explosives to implode it to get to the required density, but uranium-235 is EXTREMELY fissionable, and will completely explode by simply having enough mass.
[edit] Oh, so in the end, is Brian O. ok now?
