When a heavy atom (such as uranium or plutonium) undergoes fission, it splits into two lighter fission products. This splitting process also yields two or three neutrons, which can cause other heavy atoms to fission, as well as a huge amount of energy, which nuclear engineers convert into electric power.
The two fission product atoms are not the same two atoms every time. Nuclear scientists can predict the distribution of fission products through physical models, but generally this is measured experimentally to ensure accuracy.
When fission products are first produced, they are highly unstable and rapidly decay (usually β- decay) multiple times until they become relatively stable nuclides with long half-lives. All this decaying generates quite a bit of energy, which we call decay heat. So even after the fission reaction completely stops (which it did immediately following the earthquake), fission products continue to produce energy for a long period of time. This energy is large enough to melt the fuel if the fuel is not cooled, and cooling the fuel has been what the reactor operators at Fukushima have been struggling with for several days. The fission reaction was never out of control – only the decay heat cooling systems were out of control.
Although most fission products are considered waste, some are very important to the operation of a nuclear reactor and have specific uses. Two of the fission products, xenon-135 and samarium-149, are prolific neutron absorbers (called “neutron poisons”) and can substantially affect control of the fission reaction during normal operation. Others, especially molybdenum-99 which eventually decays to technetium-99m, are used to produce “medical isotopes” that are essential for diagnostic testing for numerous life-threatening illnesses. Each year, 40 million people worldwide undergo necessary testing with technetium-99m. If you’ve ever had a nuclear medicine procedure, the chances are high that what they put into your body came straight out of a nuclear reactor – and if they hadn’t put it into your body, it would have been considered nuclear waste!
Fission products remain inside the fuel under normal circumstances. When fuel resides in the core, it contains an amount of fission products proportional to the total energy it generated. When the fuel is depleted, it is moved to spent fuel pools and ultimately to dry cask storage, long-term repositories, or reprocessing facilities. At the Fukushima nuclear power plants, fuel inside the core (and possibly the spent fuel pools) is suspected to have likely been damaged. Because of this, some fission products, especially the gaseous products, have likely been released. We do not currently have enough information to know exactly which (or in what amount) fission products have been released.
Not all fission products are harmful. Although a few are gaseous, which enables them to travel long distances through the atmosphere, most are not highly mobile and will thus remain localized near the reactor site. Although nearly all fission products emit radiation, only some are potentially harmful to humans.
The chart below lists various important fission products along with their yields – the frequency at which they are produced from fission. For example, 6.3% of fission events (on average) will produce xenon-135 (after the highly unstable fission products rapidly decay). The half-life is a general time scale for how long the listed radioactive fission product will exist before decaying to a more stable fission product. Note that cesium and iodine, which were detected near the Fukushima site, are by far the most frequently occurring radioactive fission product elements.Yield Fission Product Half-life 6.8% cesium-133/134* 2 years 6.3% iodine-135 / xenon-135 7 hours 6.3% zirconium-93 1.5 million years 6.1% cesium-137 30 years 6.1% molybdenum-99 / technetium-99** 200,000 years 5.8% strontium-90 30 years 2.8% iodine-131 8 days 2.3% promethium-147 3 years 1.1% samarium-149 not radioactive 0.7% iodine-129 15 million years 0.4% samarium-151 90 years 0.4% ruthenium-106 1 year 0.3% krypton-85 11 years 0.2% palladium-107 7 million years
*Cs-133 is stable but has a high fission yield, but it will then produce Cs-134 from absorbing neutrons in the reactor and Cs-134 is radioactive with a ~2 year half-life.
**Half-life reported in the table is for Tc-99. Mo-99 has a half-life of ~66 hours, which then decays to Tc-99m (metastable form of Tc-99) with a half-life of ~6 hours. The Tc-99m then decays to the Tc-99 with the 200,000 year half-life reported in the table.
Note that longer half-lives do not necessarily mean more danger. Some fission products have extremely long half-lives but emit very little radiation at any given time, so they are not dangerous. Other fission products emit huge amounts of radiation but exist for such a short period of time that they are not dangerous. How harmful a given fission product is to humans is a complicated function of half-life, radiation intensity, and various human biology factors.