Editing Protactinium (section)

===In nuclear reactors===
Two major protactinium isotopes, <sup>231</sup>Pa and <sup>233</sup>Pa, are produced from thorium in [[nuclear reactor]]s; both are undesirable and are usually removed, thereby adding complexity to the reactor design and operation. In particular, <sup>232</sup>Th, via [[Nuclear reaction#Reactions with neutrons | (''n'', 2''n'')]] reactions, produces <sup>231</sup>Th, which quickly decays to <sup>231</sup>Pa (half-life 25.5 hours). The last isotope, while not a transuranic waste, has a long half-life of 32,760 years, and is a major contributor to the long-term [[radiotoxic]]ity of spent nuclear fuel.<ref name="b1" />

Protactinium-233 is formed upon neutron capture by <sup>232</sup>Th. It either further decays to <sup>233</sup>U, or captures another neutron and converts into the non-fissile <sup>234</sup>U.<ref>{{cite book|author=Hébert, Alain|title=Applied Reactor Physics|url=https://books.google.com/books?id=sibA5ECQ8LoC&pg=PA265|date=July 2009|publisher=Presses inter Polytechnique|isbn=978-2-553-01436-9|page=265}}</ref> <sup>233</sup>Pa has a relatively long half-life of 27 days and high [[cross section (physics)|cross section]] for neutron capture (the so-called "[[neutron poison]]"). Thus, instead of rapidly decaying to the useful <sup>233</sup>U, a significant fraction of <sup>233</sup>Pa converts to non-fissile isotopes and consumes neutrons, degrading [[neutron economy|reactor efficiency]]. To limit the loss of neutrons, <sup>233</sup>Pa is extracted from the active zone of thorium [[molten salt reactor]]s during their operation, so that it can only decay into <sup>233</sup>U. Extraction of <sup>233</sup>Pa is achieved using columns of molten [[bismuth]] with lithium dissolved in it. In short, lithium selectively reduces protactinium salts to protactinium metal, which is then extracted from the molten-salt cycle, while the molten bismuth is merely a carrier, selected due to its low [[melting point]] of 271&nbsp;°C, low vapor pressure, good solubility for lithium and actinides, and [[Miscibility | immiscibility]] with molten [[halide]]s.<ref name="b1">Groult, Henri (2005) [https://books.google.com/books?id=dR2DA50PUV4C&pg=PA562 Fluorinated materials for energy conversion], Elsevier, pp. 562–565, {{ISBN|0-08-044472-5}}.</ref>