It is generally recognised that when a uranium 235 nucleus is hit by a neutron, it will split at the right conditions into two chemical elements and release energy as well as two or three neutrons in a process known as nuclear fission.  Some of these neutrons will hit more uranium 235 nuclei to maintain the nuclear fission and, in the case of an operating nuclear power reactor, a steady stream of energy is released.  Uranium 235, a uranium nuclide that contains 235 particles in its atomic nucleus, is the key ingredient of the nuclear fuel for most of the nuclear power reactors operating today.

Most of the neutrons produced in nuclear fission emerge practically instantaneously, within a hundred trillionth of a second if needed, to be more exact.  They are known as “Prompt Neutrons” and they sustain the continuity of the nuclear fission.

The two chemical elements produced by splitting the uranium nucleus are not equal.  They are known as fission products and generally fall within a range of 40% to 60% ratio.  They are unstable as well, in that they will release some of their contents in the form of subatomic particles or energy that are collectively known as nuclear radiation, and that they will release their contents in steps known as radioactive decay and become stable chemical elements in a short period to over a very long time.

Neutrons are among the contents released by a small number of fission products at the early stages of their radioactive decay, and because these fission products decay at different rates, these small percentage of neutrons are also released at different rates though within a short time range after the fission.  They are known as “Delayed Neutrons” that, for uranium 235, make up about 0.6% of the total and are released from within 1/4 second to nearly a minute afterwards. When there is a change in the reaction rate of a nuclear reactor , for example, due to a power change, delayed neutrons will provide a small delay in the emission of neutrons in a reactor.  It is through this delay in reactor response that the power level of a nuclear reactor can be effectively controlled.

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