It can just go Bang. Uranium stored in too large Amounts (without any shielding inbetween) can cause a nuclear chain reaction and go Boom. Guess how Nukes work. They don't work like Explosion = Nuclear Chain Reaction, they work like explosion compresses two clumbs of it to critical mass, which then goes Boom.
Ever heard of technitium? Formed when a small fission reaction happens in a certain uranium ore, which is why its so rare to find it naturaly. Huh, wonder why it hasnt gone boom, and why The uranium/plutonium needs to be Compressed, maybe becuase the energy Dissipates way to fast for an explosion on its own? 
