Overview

Neptunium was the first transuranium element to be made, synthesised in 1940 at Berkeley by Edwin McMillan and Philip Abelson, who produced it by bombarding uranium with neutrons. It was named after the planet Neptune. Neptunium is a dense, silvery, radioactive metal, the first element heavier than uranium and a bridge into the wholly synthetic reaches of the periodic table; small amounts arise in nuclear reactors. Neptunium is a by-product of the reactions that make plutonium, and although it can in principle sustain a chain reaction, its chief practical interest is as a precursor, since bombarding it with neutrons yields plutonium-238, the isotope that powers the generators of deep-space probes.

McMillan began the experiment but was pulled away to work on wartime radar research before it was finished, leaving the chemist Philip Abelson to complete the delicate work of separating the new element from uranium, a division of labour that meant the physicist who conceived the idea did not personally finish confirming it. Neptunium-237, with a half-life of over two million years, accumulates in spent nuclear fuel and is one of the longer-lived hazards nuclear waste management must plan for, since it persists across timescales that dwarf any human institution.