Nuclear fission

An atom’s nucleus contains protons and neutrons. Some large nuclei are unstable and can split into two smaller nuclei. This splitting is called nuclear fission; the smaller products are called daughter nuclei. Energy is released in the process.

For fission to occur, the unstable nucleus usually first absorbs a neutron. For example, a uranium-235 nucleus can absorb a neutron and become a highly unstable uranium-236 nucleus. It can then split. The products include two smaller nuclei and, typically, two or three free neutrons. Gamma radiation may also emerge.

Where does the released energy go?

The daughter nuclei and emitted neutrons move away from the split with kinetic energy. As they collide with surrounding particles, they transfer energy to them, causing heating. Some energy can also leave as gamma radiation. Fission therefore does more than emit radiation: it transfers energy to other particles.

How can one split lead to another?

A neutron released in one fission may be absorbed by another nucleus that can undergo fission. That nucleus may split and release further neutrons, which may trigger still more fissions. This continuing sequence is a chain reaction. It depends on enough released neutrons reaching and being absorbed by suitable nuclei; a neutron may instead escape or be absorbed without causing another fission.