Editing Radioactive waste (section)

==== Fuel composition and long term radioactivity ====
[[File:Activityofuranium233.jpg|thumb|upright=1.4|Activity of [[Uranium-233|U-233]] for three fuel types. In the case of MOX, the U-233 increases for the first 650 thousand years as it is produced by the decay of [[Np-237]] which was created in the reactor by absorption of neutrons by U-235.]]
{{See also|Spent nuclear fuel|High-level waste}}
{{Main|Long-lived fission product}}
[[File:Activitytotal1.svg|thumb|upright=1.4|Total activity for three fuel types. In region 1, there is radiation from short-lived nuclides, in region 2, from [[Sr-90]] and [[Cs-137]], and on the far right, the decay of Np-237 and U-233.]]
The use of different fuels in nuclear reactors results in different [[spent nuclear fuel]] (SNF) composition, with varying activity curves. The most abundant material being U-238 with other uranium isotopes, other actinides, fission products and activation products.<ref name=":1">{{Cite web |title=Radioactivity : Spent fuel composition |url=https://www.radioactivity.eu.com/site/pages/Spent_Fuel_Composition.htm |url-status=dead |archive-url=https://web.archive.org/web/20200923005202/https://www.radioactivity.eu.com/site/pages/Spent_Fuel_Composition.htm |archive-date=2020-09-23 |access-date=2021-08-10 |website=www.radioactivity.eu.com}}</ref>

Long-lived radioactive waste from the back end of the fuel cycle is especially relevant when designing a complete waste management plan for SNF. When looking at long-term radioactive decay, the actinides in the SNF have a significant influence due to their characteristically long half-lives. Depending on what a [[nuclear reactor]] is fueled with, the actinide composition in the SNF will be different.

An example of this effect is the use of [[nuclear fuel]]s with [[thorium]]. Th-232 is a fertile material that can undergo a neutron capture reaction and two beta minus decays, resulting in the production of fissile [[uranium-233|U-233]]. The SNF of a cycle with thorium will contain U-233. Its radioactive decay will strongly influence the long-term [[radioactive decay|activity]] curve of the SNF for around a million years. A comparison of the activity associated to U-233 for three different SNF types can be seen in the figure on the top right. The burnt fuels are thorium with reactor-grade plutonium (RGPu), thorium with weapons-grade plutonium (WGPu), and [[Mixed oxide fuel]] (MOX, no thorium). For RGPu and WGPu, the initial amount of U-233 and its decay for around a million years can be seen. This has an effect on the total activity curve of the three fuel types. The initial absence of U-233 and its daughter products in the MOX fuel results in a lower activity in region 3 of the figure at the bottom right, whereas for RGPu and WGPu the curve is maintained higher due to the presence of U-233 that has not fully decayed. Nuclear reprocessing can remove the actinides from the spent fuel so they can be used or destroyed (see {{section link|Long-lived fission product|Actinides}}).