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Synthesis

The vast majority of the neptunium that currently exists on Earth was produced artificially in nuclear reactions. Neptunium-237 is the most commonly synthesized isotope due to it being the only one that both can be produced via neutron capture and also has a half-life long enough to allow weighable quantities to be easily isolated. It is by far the most common isotope to be utilized in chemical studies of the element.

Heavier isotopes of neptunium decay quickly, and lighter isotopes of neptunium cannot be produced by neutron capture, so chemical separation of neptunium from cooled spent nuclear fuel gives nearly pure Np. The short-lived heavier isotopes Np and Np, useful as radioactive tracers, are produced through neutron irradiation of Np and U respectively, while the longer-lived lighter isotopes Np and Np are produced through irradiation of U with protons and deuterons in a cyclotron.

Artificial Np metal is usually isolated through a reaction of NpF with liquid barium or lithium at around 1200 ° C and is most often extracted from spent nuclear fuel rods in kilogram amounts as a by-product in plutonium production.

By weight, neptunium-237 discharges are about 5% as great as plutonium discharges and about 0.05% of spent nuclear fuel discharges. However, even this fraction still amounts to more than fifty tons per year globally.