>>507239922
Nuclear reactions require very specific conditions to happen. You need radioactive material (unstable atoms that eject particles in order to become more stable) to essentially be so close together and compressed that the atoms flinging particles out of themselves are certain that those particles will hit other atoms, destabilizing them to split and emitting more particles, each ejection/splitting causing a release of energy with it as the atomic bonds are broken. On it's own, it's a tiny, tiny tiny bit of energy, but it scales exponentially with mass. So big chunk of radioactive material, critical state, chain reaction of atoms splitting, huge explosion.