Minor actinides
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The minor actinides are the actinide elements in used nuclear fuel other than uranium and plutonium, which are termed major actinides.
The minor actinides will be responsible for the bulk of the radiotoxicity of used nuclear fuel in the medium term (300 to 20,000 years in the future).
The plutonium from a power reactor tends to have a greater amount of Pu-241 than the plutonium generated by the lower burn up operations designed to create bomb grade plutonium. Because the reactor grade plutonium contains so much Pu-241 the presence of americium-241 makes the plutonium less suitable for making an atom bomb. The ingrowth of americium in plutonium is one of the methods for identifying the origin of an unknown sample of plutonium and the time since it was last separated chemically from the americium.
Americium is commonly used in industry as both an alpha particle and low photon energy gamma radiation source. For instance it is used in many smoke detectors. Americium can be formed by neutron capture of Pu-239 and Pu-240 forming Pu-241 which then decays by beta decay to Am-241. In general, as the energy of the neutrons increases, the ratio of the fission cross section to the neutron capture cross section changes in favour of fission. Hence if MOX is used in a thermal neutron reactor such as a boiling water reactor (BWR) or pressurized water reactor (PWR) then more americium can be expected in the used fuel than that from a fast neutron reactor.
Minor actinides include neptunium, americium, curium, berkelium, californium, einsteinium, and fermium.
Some of them have been found in fallout from bomb tests. See Actinides in the environment for details of the actinides in the environment.
