After absorbing a neutron, a uranium-235 nucleus will split into two roughly equal parts.

One way to visualize this is to view the nucleus as a kind of liquid drop.

The mass distribution of the fragments shows that the two pieces are large, but usually unequal.

The energy release in a fission reaction is quite large. Also, since smaller nuclei are stable with fewer neutrons, several neutrons emerge from each fission as well.

These neutrons can be used to induce fission in other nuclei, causing a chain reaction.

In order to make a nuclear reactor, the chain reaction needs to be self-sustaining – it will continue indefinitely – but controlled.

A moderator is needed to slow the neutrons; otherwise their probability of interacting is too small. Common moderators are heavy water and graphite.

Unless the moderator is heavy water, the fraction of fissionable nuclei in natural uranium is too small to sustain a chain reaction, about 0.7%. It needs to be enriched to about 2-3%.

Neutrons that escape from the uranium do not contribute to fission.

There is a critical mass below which a chain reaction will not occur.

This is because, below this level, too many neutrons escape.

Control rods can be used.

They are usually cadmium or boron, that absorb neutrons and can be used for fine control of the reaction, to keep it critical but just barely.