Critical mass 

Fissionable material is a mass of heavy radioactive isotope (or mixture of isotopes) large enough to sustain an uncontrolled chain reaction when suddenly assembled from smaller parts by a dynamite implosion. 50 kg's of ²³⁵U is fissionable. Natural uranium ,which is mostly ²³⁸U, is not. Plutonium is made in a breeder reactor. Now that is a worry.