By George Dowell
1. The act or process of splitting into parts.
2.A nuclear reaction in which an atomic nucleus, especially a heavy nucleus such as an isotope of uranium, splits into fragments, usually two fragments of comparable mass, releasing from 100 million to several hundred million electron volts of energy.
A nuclear power reactor relies on fission to produce the heat that makes the steam that drives the electric generators.
Consider the most common nuclear fuel, Uranium 235 (U-235). When a stray neutron is absorbed into a U-235 nucleus, U-236 is formed, which is a highly unstable atom. U-236 will usually split or fission into at least two smaller fragments called Fission Products, which themselves can be radioactive and decay by releasing nuclear fragments (alpha particles and electrons)
Of the many possible fission products possible, the ones of main concern in analyzing fallout are Cesium 137 (Cs-137), and Strontium 90 (Sr-90) due to their long persistence (Half-Life). Cs-137 and Sr-90 also occur in fallout from nuclear weapons detonations.
Another version of radioactive cesium is peculiar only to fallout from a nuclear reactor, not a bomb: Cesium 134. The reason for this is that Cs-134 is formed over a period of time in a reactor by neutron activation, not fission.
The Fukushima situation was unique is another respect, we believe now that the hydrogen chemical explosion vented large quantities of cesium in its gaseous form, but not strontium. Cesium is very much more volatile than strontium, therefore gasifies much easier, and transports on the wind to greater distances. Our tests are only on small environmental samples i.e. soil and vegetation, taken at a distance from the reactor site.
Sr-90 was tested using a magnetic beta spectrograph and found to be missing. Those tests are the subject
of another article.
About Cesium Radioisotopes:
Cs-137 has a half-life of over 30 years. Its presence can still be detected in Trinitite from the first nuclear explosion at Trinity site, and is the main contaminant left in Chernobyl fallout.
Cs-134 has a half-life of about 2 years. By rule-of-thumb it will only 1/64 th as active after 12 years (6 halflives) and be virtually undetectable after 20 years (10 half-lives).