The most common nuclear fuels are 235U (the isotope of uranium with an atomic mass of 235 and of use in nuclear reactors) and 239Pu (the isotope of plutonium with an atomic mass of 239). These fuels break apart into a bimodal range of chemical elements with atomic masses centering near 95 and 135 u (fission products). Most nuclear fuels undergo spontaneous fission only very slowly, decaying instead mainly via an alpha/beta decay chain over periods of millennia to eons. In a nuclear reactor or nuclear weapon, the overwhelming majority of fission events are induced by bombardment with another particle, a neutron, which is itself produced by prior fission events.

The most important isotope of plutonium is Pu-239. It's virtually nonexistent in nature. It is produced by bombarding uranium-238 with slow neutrons. This forms neptunium-239, which in turn emits a beta particle and forms plutonium-239. Plutonium-239's principal mode of decay is alpha decay. Various sources give slightly different figures for the half-life. The values found include 24,360, 24,400, 24,110, and 24,000 years. None of theses measurements agree. It is because there are many factors that can affect the accuracy of this measurement. Plutonium-239 is produced artificially, and every time it is produced, it is mixed with varying amounts of other isotopes, notably plutonium-240, plutonium-241 and plutonium-242. Since all the isotopes have nearly the same chemical characteristics, it is very difficult to separate isotopes from each other by chemical techniques. This means, it is virtually impossible to study the properties of pure Plutonium-239. Therefore, the results might be the average half-life of it being mixed with a small amount of the other isotopes (''Half-Life of Plutonium-239'' http://hypertextbook.com/facts/JaniceChing.shtml).