However, in the case of an atomic bomb this cannot be done by relying on spontaneous decay. Instead stimulated fission is used, where a uranium-235 nucleus is struck by a neutron from an external source and this triggers fission. The fission releases three neutrons and these strike other uranium nuclei and trigger their fission, and so on causing a rapid chain reaction and nuclear explosion.
Here is some info on this from https://en.wikibooks.org/wiki/Wikijunior:How_Things_Work/Nuclear_Bomb
There are two types of nuclear bombs, fission bombs and fusion bombs. Fission means to break apart and fusion to merge.
The fission bomb works on the principle that it takes energy to put together a nucleus with many protons and neutrons. Sort of like rolling a heavy cart up a hill. Splitting the nucleus up again then releases some of that energy. Some atoms have unstable nuclei which means that they tend to break apart with little or no nudging.
You may have heard of uranium and plutonium and that they are radioactive elements. These two have such unstable nuclei which causes their radioactivity. When a nucleus breaks into two smaller nuclei, a couple of neutrons shoot out. This is the radiation. Naturally occurring uranium and plutonium have atoms constantly undergoing radioactive decay. These are spaced sufficiently far apart so that the neutrons rarely bump into other unstable nuclei.
When a neutron, however, does hit an unstable nucleus, just like someone bumping into a cart at the top of a hill, it causes that nucleus to break apart and send out another couple of neutrons. By increasing the concentration of these unstable atoms, the probability that a neutron from one decay causes another one increases. The concentration where the reaction sustains itself is called critical mass and the reaction then called a chain reaction.
With each step of the reaction, energy is released and another step or two is started, and so an avalanche of reactions and energy release continues until the fissile (unstable) material is spent.
Basically atom bombs (which start with an atom exploding) are incredibly powerful and deadly weapons that are hugely destructive and very dangerous. However the theoretical science of atomic bonds and atomic fission is very interesting. So thank you for your interesting question!