Fission products are the remains of a heavy uranium or plutonium nucleus which splits apart after capturing a passing neutron. As the 'ashes' of a nuclear reaction, they provide the bulk of the radioactivity found inside the spent fuel in nuclear reactors.
An uranium or plutonium nucleus generally splits into two unequal pieces - a veritable Laurel and Hardy double act with one light nucleus containing between 80 and 110 nucleons and another heavier one made up of between 130 and 155 nucleons.
A nucleus of uranium 235 (made up of 143 neutrons and 92 protons) contains 61% neutrons, whereas the stability level for heavy or light nuclei is under 57%. As a result, the two new fragments are highly radioactive at the moment of their creation, as the excess number of neutrons they inherit from the parent nucleus renders them unstable. These two new nuclei gradually regain stability by undergoing a series of beta decays, which transform some of the excess neutrons into protons.
This 'cascade' of nuclear transformations starts off very quickly, but the time taken to reach stability varies with the decaying fragment. A nucleus with 140 nucleons will stabilise in a few days, whereas one with 137 nucleons will take 30 years and a nucleus with 99 nucleons will remain radioactive for 210 millennia.
Around two thirds of all fission products stabilise in the time they are stockpiled inside the core of a nuclear reactor. When the spent fuel is moved after a couple of years, the radioactive fuel therefore still contains a substantial proportion of unstable nuclei. The most radioactive of these, with the correspondingly shortest half-lives, will disappear fairly quickly. It is paradoxically the least radioactive nuclei that are the most problematic, as they will continue to be a relatively low key nuisance for many generations from now.
In the two hundred years immediately following a fission reaction, the fission products are much more radioactive than the actinides (the heavy elements which are also found in spent fuel and later in nuclear waste). After these 200 years, however, most of the fission products will have stabilised and it is the actinides that become the main hazard.
The term 'fission products' applies to a very wide range of chemicals. Many intereact with oxygen and can be found as solid oxides in the spent fuel. The most mobile of the fission products are gases, such as iodine-131 decays rapidly . Very mobile fisson products such as krypton-85 are noble gases, but not forming molecules, their radioactivity is the less toxic.
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