Neptunium (Np)

Element 93 has the odd distinction of having been discovered wrongly, celebrated wrongly, and then found for real six years later by people who were looking for something else. It was the first element made past the natural end of the periodic table, and the mistake that preceded it is more instructive than the discovery.

Ausonium, hesperium, and a Nobel Prize for the wrong thing

In 1934 Enrico Fermi's group in Rome was bombarding every element they could get with slow neutrons. Uranium gave them a confusing spray of new activities that behaved like nothing in the known chemistry of the neighbourhood, and the natural reading — the one everybody made — was that neutron capture had pushed uranium up into elements 93 and 94. Fermi's collaborators floated the names ausonium and hesperium. The 1938 Nobel Prize in Physics cited, among other things, his demonstration of new radioactive elements produced by neutron irradiation.

The activities were fission fragments. Uranium had been splitting roughly in half, and the "transuranic" chemistry was the chemistry of barium, lanthanum and their neighbours, sitting in the middle of the table rather than past its end.

One person said so at the time. Ida Noddack published a paper in 1934 pointing out that nobody had excluded the possibility that the nucleus had broken into several large fragments, and that the chemical tests being used could not have detected it if it had. She was ignored for four years, until Hahn and Strassmann's barium result forced the same conclusion on everyone else.

The 2.3-day activity

The real element 93 turned up at the Berkeley 60-inch cyclotron in 1940. Edwin McMillan and Philip Abelson were studying the fragments thrown out when uranium fissions, and noticed an activity with a half-life of about 2.3 days that stubbornly refused to recoil out of the target the way fission products did. It was not a fragment. It was sitting where it had been made.

They showed it was chemically distinct from uranium and from every element above rhenium, and that it was the daughter of the short-lived uranium-239 formed by simple neutron capture. Neutron in, two beta decays out, and the table had a new square. Following Klaproth's planetary logic, the element past uranium took the name of the planet past Uranus.

McMillan left for radar work almost immediately, which is how the follow-on work on the next element passed to other hands. He and Glenn Seaborg shared the 1951 Nobel Prize in Chemistry for the transuranium elements — the prize Fermi's 1938 citation had, in a sense, awarded too early.

Not quite synthetic

Neptunium is usually described as an artificial element, and it very nearly is, but trace quantities occur naturally. Uranium ores contain a small flux of neutrons from spontaneous fission and cosmic-ray interactions, and a fraction of those are captured by uranium-238 to make neptunium-239, which decays onward. Neptunium-237 itself was detected in pitchblende in the early 1950s, at concentrations of a few parts in 10^12 relative to uranium. It is present, in the same sense that a whisper is present in a hurricane.

There is a much larger reservoir of it in domestic hardware. Americium-241, the isotope in ionisation smoke detectors, alpha-decays to neptunium-237. Every one of those detectors is very slowly turning into element 93, and after a few thousand years the neptunium in one would outweigh the americium that made it.

The isotope that owns the far future

Practically all neptunium chemistry is the chemistry of one isotope. Neptunium-237 has a half-life of 2.14 million years, which is enormous by the standards of anything beyond uranium and short enough that the isotope is intensely worth tracking. It builds up in reactor fuel, and it is separated from spent fuel in reprocessing plants.

Its half-life puts it in an unusual position in long-term waste modelling. Over the first few centuries, the dose from buried spent fuel is dominated by fission products; over tens of thousands of years, by plutonium. Push the model out past a hundred thousand years and neptunium starts to matter, for a reason that is chemical rather than nuclear. Under oxidising conditions neptunium sits happily in the +5 state as the neptunyl ion, which is soluble and does not bind strongly to most minerals. Where plutonium is largely immobilised by the rock around it, neptunium can travel with the water. Whether that matters in a real repository depends entirely on whether the deep groundwater there is oxidising, which is one reason site selection arguments turn on redox chemistry rather than on geology alone.

Neptunium-237 is also reportable material under international safeguards, one of only a handful of substances outside the uranium and plutonium families to be treated that way.

The feedstock nobody sees

Neptunium's single genuine industrial role is as a starting material. Plutonium-238, the isotope that has powered deep-space missions for six decades, is not economically extracted from spent fuel — the isotope mixture there is wrong. It is made deliberately, by irradiating targets of neptunium-237 and separating the product.

That gives element 93 a curious profile: an element with no application of its own, whose entire production chain exists to supply the element above it. When the United States restarted plutonium-238 production at Oak Ridge in the 2010s after a gap of decades, the practical constraint was the neptunium inventory and the throughput of the target fabrication line, not any shortage of reactor time. The spacecraft power supply begins with a stockpile of element 93 in storage.

Beyond that, neptunium has been used to make a small number of neutron-detection instruments and essentially nothing else. It is a metal with three crystalline forms, a place in every reactor's inventory, and almost no life outside the fuel cycle.