"highly enriched weapons grade Uranium 235 needs about 15 Kg. in a spherical configuration for a fissionable reaction to take place. "
I don't know the specifics, but a kilo of this stuff would, in my best judgement, make a dandy little "dirty bomb".
It would make an excellent dirty bomb, particularly if it were ground into fine dust before it was exploded and dispersed. Again, since it decays with production of an alpha particle, inhalation is the only way for significant risk other than shrapnel wounds with imbedded uranium.