Why does uranium explode?

Answer 1

Natural uranium, a heavy metal, is not explosive. It is, however, quite reactive chemically. As a fine powder, if it is was thrown into the air, it would spontaneously begin burning as it is pyphoric. (As an aside, it's a very toxic heavy metal.) No amount of uranium will just "blow up" like the detonation of a brick of plastique (plastic explosive). But there is a bit more to know.

If uranium is enriched to increase its U-235 concentration, it can be come explosive if critical mass is reached and "maintained" for a very short duration via explosives. This is the basis for the operation of a nuclear weapon. Note, however, that under normal circumstances, any assemblage of uranium to critical mass (for that concentration of isotopes and the physical conditions) will cause fission to begin. The fission will release enough energy to immediately force the critical mass apart and stop the reaction, though the "mess" resulting will be highly radioactive one. These so-called criticality accidents have occurred from time to time, and they are uniformly tragic (and almost always fatal).

Answer 2

Uranium has to be enriched to at least 20% Uranium-235 isotope, 93.5% is preferred, the Little Boy bomb used on Hiroshima was 80% enriched Uranium-235 isotope. Pure elemental Uranium metal is usually used to avoid moderation (slowing of fast neutrons) which could slow the fission rate and result in a dud/fizzle. Enrichment is a physical process, not a chemical process/reaction.

Some bombs use both enriched Uranium & Plutonium, this is called a composite bomb. The two metals could either be alloyed or cast as separate nested shells.

Some bombs use both fission (Uranium/Plutonium) & fusion (hydrogen isotopes) fuels. But these fuels are kept separate in the bomb, not combined.

Uranium metal by itself is pyrophoric and when used as armor penetrating bullets (usually depleted Uranium is used for this) it ignites and ricochets around inside the vehicle, burning. But this is just a chemical reaction and no explosion results, unless the burning Uranium chunks set off the vehicles weapons or ignite its fuel tanks.

BTW, small pieces of Uranium metal make very good cigarette lighter flints, they generate lots of sparks because it is pyrophoric and Uranium dust spontaneously ignites in air. However the radioactivity danger is too high for this use. This spontaneous ignition of the dust is a major hazard in plants that process Uranium metal!

Answer 3

Uranium explode when the critical mass of enriched uranium is reached and the uncontrolled nuclear fission reaction begin.