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In nuclear reactors

Two major protactinium isotopes, Pa and Pa, are produced from thorium in nuclear reactors; both are undesirable and are usually removed, thereby adding complexity to the reactor design and operation. In particular, Th, via (n, 2 n) reactions, produces Th, which quickly decays to 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 radiotoxicity of spent nuclear fuel.

Protactinium-233 is formed upon neutron capture by Th. It either further decays to uranium-233, or captures another neutron and converts into the non-fissile uranium-234. Pa has a relatively long half-life of 27 days and high cross section for neutron capture (the so-called " neutron poison "). Thus, instead of rapidly decaying to the useful U, a significant fraction of Pa converts to non-fissile isotopes and consumes neutrons, degrading reactor efficiency. To limit the loss of neutrons, Pa is extracted from the active zone of thorium molten salt reactors during their operation, so that it can only decay into U. Extraction of 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 °C, low vapor pressure, good solubility for lithium and actinides, and immiscibility with molten halides.