Nuclear fission requires the mass of the fissile material above the critical mass. So that the explosion takes place at least in the case of a nuclear bomb. But once a single nucleus got involved in the reaction, the reaction can't be stopped easily if I'm right. A single nucleus how do know about the critical mass of the entire fissile material?
There's spontaneous fission, a rare decay mode for some superheavy nuclei like uranium and plutonium. There's also induced fission, where some interaction with the environment (typically neutron capture) causes a nucleus to split. Both fissions typically produce a couple of free neutrons, which may be captured on other fissionable nuclei, or may be captured on some nearby neutron absorber (a "control rod" or a "neutron poison"), or may escape from the reaction volume to infinity.
The "critical mass" is the amount of fissionable material needed so that each fission is likely to trigger, on average, one other fission. It's not a property of a single nucleus, but of the fuel assembly, its geometry, and its environment.