Editing Protactinium (section)

==Occurrence==
Protactinium is one of the rarest and most expensive naturally occurring elements. It is found in the form of two isotopes – <sup>231</sup>Pa and <sup>234</sup>Pa, with the isotope <sup>234</sup>Pa occurring in two different energy states. Nearly all natural protactinium is <sup>231</sup>Pa. It is an [[Alpha particle|alpha emitter]] and is formed by the decay of uranium-235, whereas the [[beta particle|beta-radiating]] <sup>234</sup>Pa is produced as a result of [[:File:Decay chain(4n+2, Uranium series).PNG|uranium-238 decay]]. Nearly all uranium-238 (99.8%) decays first to the shorter-lived <sup>234m</sup>Pa isomer.<ref name="ANL">[http://www.ead.anl.gov/pub/doc/protactinium.pdf Protactinium] {{webarchive|url=https://web.archive.org/web/20080307023843/http://www.ead.anl.gov/pub/doc/protactinium.pdf |date=7 March 2008 }}, Argonne National Laboratory, Human Health Fact Sheet, August 2005</ref>

Protactinium occurs in [[uraninite]] (pitchblende) at concentrations of about 0.3-3 [[parts-per notation|parts]] <sup>231</sup>Pa per million parts (ppm) of ore.<ref name="Emsley" /> Whereas the usual content is closer to 0.3&nbsp;ppm<ref name="brit" /> (e.g. in [[Jáchymov]], [[Czech Republic]]<ref>{{cite journal|last1=Grosse|first1=A. V.|last2=Agruss|first2=M. S.|journal=[[Journal of the American Chemical Society]]|volume=56|pages=2200|date=1934|doi=10.1021/ja01325a507|issue=10|title=The Isolation of 0.1 Gram of the Oxide of Element 91 (Protactinium)|bibcode=1934JAChS..56Q2200G }}</ref>), some ores from the [[Democratic Republic of the Congo]] have about 3&nbsp;ppm.<ref name="CRC" /> Protactinium is homogeneously dispersed in most natural materials and in water, but at much lower concentrations on the order of one part per trillion, corresponding to a radioactivity of 0.1 picocuries (pCi)/g. There is about 500 times more protactinium in sandy soil particles than in water, even when compared to water present in the same sample of soil. Much higher ratios of 2,000 and above are measured in [[loam]] soils and clays, such as [[bentonite]].<ref name="ANL" /><ref>Cornelis, Rita (2005) [https://books.google.com/books?id=1PmjurlE6KkC&pg=PA520 Handbook of elemental speciation II: species in the environment, food, medicine & occupational health, Vol. 2], John Wiley and Sons, pp. 520–521, {{ISBN|0-470-85598-3}}.</ref>

===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>