High Quality Content by WIKIPEDIA articles! Protactinium-231 is the longest-lived isotope of protactinium, with a half life of 32760 years. In nature, it is found in trace amounts as part of the actinium series which starts with the primordial isotope uranium-235; the equilibrium concentration in uranium ore is 46.55 Pa-231 per million U-235. In nuclear reactors, it is one of the few long-lived radioactive actinides produced as a byproduct of the projected thorium fuel cycle, as a result of (n,2n) reactions where a fast neutron removes a neutron from thorium-232 or uranium-232, and can also be destroyed by neutron capture though the cross section for this reaction is also low.

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