Splitting an atom, and usually it is a uranium-235 atom, will yield neutrons that will encounter more of such atoms and lead to more splitting to keep the process going. But it will need enough uranium-235 atoms and neutrons to sustain the process or the "chain reaction" and keep the uranium mass or the reactor core "critical".

Obviously the reactor core needs either to be large enough, or its concentration or "enrichment" of uranium-235 high enough, to produce enough splitable or "fissile" atoms. It is also necessary to have enough neutrons available even though a typical uranium-235 fission will yield 2 or 3 neutrons. Since other materials forming the core will absorb some of the neutrons and the reactor core is necessarily limited in size, some neutrons will escape at its edge. Attention is therefore taken not to introduce too much neutron absorbing material into the reactor core, either for slowing down the neutrons to enable the chain reaction or for providing the structural strength. A layer of material is also installed at the edge of the core to reflect back some of the escaping neutrons.

For the Pressurised Water Reactor which represents some 60% of the commercial reactors in the world, nuclear fuel with enriched uranium-235 is loaded into the reactor core which then operates practically continuously for 12 to 18 months. To keep the reactor at the same level of criticality throughout the process, the chain reaction for a newly fuelled reactor is suppressed by loading short-lived neutron absorbing material with the fuel into the reactor and by adding boric acid as another neutron absorbing material into the reactor cooling water. As the uranium-235 in the nuclear fuel is gradually depleted during operation, the concentration of boric acid is gradually reduced for balance until the replacement of the fuel.

The reactor can be readily restarted after shutdown if the nuclear fuel is fresh. However, if the fuel is depleted, it may be necessary to wait for certain short lived fission products that are strong neutron absorbers and are produced during operation to disappear enough before a restart is possible.

This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.