A nitrogen bomb
I thought that BWR primary containments were inerted with nitrogen. I am confused as to why we would need to do so, if it is already inerted.
Nitrogen is used in high explosives, but I think only when bonded in a solid molecule. You could check with a chemist, but I think it's the release of the atom into the gaseous form that causes the high explosion and heat. If it's starting out as a gas, then I don't think it can explode. I don't know if loose nitrogen molecules would bond with anything in the reactor environment.
Inert gas. A noble gas.