[blackdog]

To set off a nuclear bomb, the first explosion compresses the fissile material which causes the chain reaction. It's like holding an egg in your hands and trying to squeeze it with microspecific pressure on all sides at precisely the same instant. Any variation of pressure from the explosive charge meant to trigger the nuclear reaction(mostly a timing function) results in nuclear dud, but irradiates an area instead.

[Don Joe]

To set off a nuclear bomb, the first explosion compresses the fissile material which causes the chain reaction. It's like holding an egg in your hands and trying to squeeze it with microspecific pressure on all sides at precisely the same instant. Any variation of pressure from the explosive charge meant to trigger the nuclear reaction(mostly a timing function) results in nuclear dud, but irradiates an area instead.

You're thinking of an implosion bomb (uranium or plutonium). A simple "gun type" bomb (uranium) has none of those requirements, and is crudeness personified -- but the Japanese can attest to the fact that even with its greatly reduced yield (compared to an implosion device) it can definitely ruin your day.