Most of the operating commercial reactors are loaded with ceramic uranium oxide fuel enriched to around 4% uranium-235 (U-235), with the remaining bulk being uranium-238 (U-238). The U-235 atoms begin to undergo nuclear fission when struck by neutrons as the reactor reaches criticality, with the fission of a U-235 atom into two smaller chemical elements releasing two to three neutrons and energy. The initial neutrons may come from an unstable chemical element loaded into the reactor core, and after the fission has begun, from those neutrons being produced by subsequent fissions in a nuclear chain reaction.

During nuclear fission, the U-235 is most commonly split into two substantially different fragments, with one heavier than the other within a size range of mass numbers that peak at the number of particles in the nucleus (or mass number) at about 96 and 135. Symmetrical fission which yields two identical products of 117 particles only occurs once in every 10,000 fissions in today’s reactors.

The fission products contain more neutrons than are necessary for them to remain stable. They will therefore transform into different chemical elements by emitting a series of electrons accompanied by energy over time until they eventually become stable. These fission products are therefore radioactive and their emissions are commonly known as nuclear radiation.

The radiation is highly intense and poses a hazard that requires proper shielding. The fission products continue to generate heat, meaning the reactor requires cooling even after shutdown. A few fission products that are strong neutron absorbers may remain temporarily in the reactor in sufficient quantities after shutdown and must be accounted for before restarting the reactor.

A small percentage of U-238 will absorb excess neutrons and convert into plutonium-239 (Pu-239). Pu-239 also undergoes nuclear fission so that the chain reaction will slowly shift to a ratio of two U-235 fission to one Pu- 239 fission after about 18 months of reactor operation. Some Pu-239 will further absorb neutrons and convert into other plutonium nuclides, so that when the nuclear fuel is finally discharged from the reactor, it contains about 1% plutonium, although only around two-thirds of this can readily undergo fission. The discharged nuclear fuel typically contains about 1% U-235.

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