Protactinium

Protactinium is a radioactive chemical element with atomic number 91. This dense, silvery actinide occurs in trace amounts in uranium ores.

Actinide: An actinide is an element in the series from actinium to lawrencium, characterized by filling 5f electron orbitals. Protactinium belongs to this series and shares its characteristic radioactive chemistry.
Uranium-235 decay chain: The uranium-235 decay chain is a sequence of radioactive transformations ending in stable lead-207. Protactinium-231 appears in this natural sequence as uranium-235 decays.
Kasimir Fajans: Kasimir Fajans was a Polish-American physical chemist who helped establish radioactive displacement laws and identified brevium. He and Oswald Göhring detected the short-lived isotope later recognized as protactinium-234.
Thorium fuel cycle: The thorium fuel cycle converts fertile thorium-232 into fissile uranium-233 through neutron capture and radioactive decay. Protactinium-233 is an intermediate whose behavior affects uranium-233 production.
Uranium: Uranium is a radioactive chemical element with atomic number 92, naturally occurring in minerals and used as nuclear fuel. Uranium is more abundant and better known, while its decay produces trace protactinium.
Atomic number: Atomic number is the number of protons in an atom's nucleus and defines the chemical element. Protactinium's 91 protons distinguish it from every other element.
Thorium-231: Thorium-231 is a radioactive isotope that decays into protactinium-231 with a half-life of about 25.5 hours. Its decay is the immediate natural source of protactinium-231 in the uranium-235 series.
Oswald Göhring: Oswald Göhring was a German physicist who collaborated with Kasimir Fajans on radioactive substances. Their 1913 work identified a short-lived uranium decay product first named brevium.
Molten salt reactor: A molten salt reactor uses a molten salt mixture as fuel, coolant, or both. Some proposed thorium designs remove protactinium-233 from circulating fuel to limit neutron capture.
Neptunium: Neptunium is a radioactive chemical element with atomic number 93 and the first transuranium element. Neptunium lies just beyond protactinium and is generally human-produced rather than naturally abundant.

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