Nuclear fission does not happen by itself very often. It is more likely to occur if:
When a uranium nucleus is hit by a neutron, it splits to form two smaller nuclei. Up to three more neutrons are released in the process. These can go on to hit other uranium nuclei, causing them to split too. This process is called a chain reaction.
The radiation released by nuclear fission may be hazardous. Energy in radiation is transferred to the surroundings during nuclear fission, and is transferred by heating as well.
In a nuclear power station, this is used to turn water into steam, which then turns turbines connected to electrical generators. Much more energy is transferred by a nuclear fuel during nuclear fission than is transferred by the same mass of chemical fuel during combustion.
It would be dangerous to allow a chain reaction to run out of control. This is how an atomic bomb works. In a nuclear reactor, the chain reaction is controlled by ‘control rods’. These slow down or absorb some of the neutrons.