The reason you need explosives is that you want that release of energy to be as rapid as possible to make the contraption pass `bomb` object class duck typing.

The reason you need explosives is that you want that release of energy to be as rapid as possible to make the contraption pass `bomb` object class duck typing. If you kept criticality to a stable level and let energy of fuel release over e.g. 20 years, it's called a nuclear power plant. If you let it run away and let the material melt itself, it's called a meltdown situation. If you instead take highly purified fissile material and compressed it instantly into size of a peanut or however small you could, the material compressed experience nuclear chain reaction everywhere inside that peanut, and spontaneous release of that insane amount of energy resemble behaviors observed with conventional chemical explosive material exploding, and such a contraption that do this is somewhat metaphorically called an atomic "bomb".