Greg Miller

The strangest product catalog on earth belongs to the Isotope Business Office, which manages the sale of atomic isotopes produced at Department of Energy labs around the country. It's got your calcium, platinum, and titanium. Your ytterbium, your strontium-90, and of course uranium-235 and plutonium-239 (responsible for Hiroshima and Nagasaki).

In this catalog, plutonium-238 is the only-on-one-model-year carburetor from the old car you've somehow kept running: useful, unique—and discontinued. And the old car is a spacecraft: NASA missions bound beyond the influence of the sun, where solar power isn't an option, use radioisotope power systems (RPS) to create electricity from the heat of atomic decay. RPS that use 4.8 kilograms of fuel have powered experiments on the moon and Mars and launched golden records etched with the essence of human civilization to the edge of the solar system. They run only on plutonium-238, which is (relatively) cheap, has a suitable half-life, and gets plenty hot. But it was a by-product of Cold War–era nuclear weapon factories—now decommissioned—and by the 2010s there were fewer than ten RPS' worth remaining for NASA to use. So scientists lobbied the DOE to start making it again. Tennessee's Oak Ridge National Laboratory produced a 50-gram sample in late 2015—the first since 1988. This year, having refined the process, the lab expects 300 grams. The goal? 1.5 kilograms per year.

The water that cools and helps stabilize the HFIR glows blue because of Cherenkov radiation—the result of excited electrons moving through water faster than light. Greg Miller

1 RECEIVE NEPTUNIUM-237

The precursor to plutonium- 238 is neptunium-237, a radioactive by-product of nuclear power plants. Oak Ridge gets its neptunium trucked in from Idaho National Laboratory in a powdery form called oxide. When it arrives, it's deposited via a dumbwaiter-like system in a shielded room called a hot cell. Some of the neptunium oxide will have already decayed into a more dangerous radioactive material called protactinium, so small quantities are moved into a separate hot cell plumbed for radioactive liquids, where scientists can do the chemistry needed to remove it.

2 ELIMINATE PROTACTINIUM

Watching through a four-and-a-half-foot-thick lead window, scientists use manipulators—grabber arms controlled from the outside—to pour the powdered neptunium into a beaker, add nitric acid, and cook it until it has dissolved and the solution takes on a dark green color. Then the liquid is poured through a column of silica glass beads, whose surface attracts protactinium. The remaining liquid is moved to a glove box.

3 CREATE TARGETS

In the glove box, the neptunium is processed with a technique invented at Oak Ridge called modified direct denitration. The liquid solution is rotated in a heated kiln until it sifts out, again in a powdered oxide form. This powder is mixed with powdered aluminum and pressed into pellets the size of a 5/8-inch socket, which are loaded into aluminum rods—targets for Oak Ridge's experimental high flux isotope reactor (HFIR).

Oak Ridge staff in front of the lead window and manipulators that allow them to work with the radioactive materials inside a shielded hot cell. Greg Miller

4 IRRADIATE NEPTUNIUM

The HFIR offers much higher flux—the rate at which targets are bombarded with neutrons—than the reactor of a nuclear power plant. Once target rods are loaded into the reactor, they're bombarded for a period of three to twelve months. As neutrons collide with the targets, some of them are absorbed by neptunium atoms. That creates a new neptunium isotope, neptunium-238, which radioactively decays into plutonium.

5 EXTRACT TARGETS

When irradiation is complete, the targets go back into a hot cell. The rods are dissolved with a caustic solution and the radioactive material inside, now 12 to 14 percent plutonium-238, is again dissolved in nitric acid.

6 SEPARATE PLUTONIUM

A process called solvent extraction isolates the plutonium and neptunium: Solvents are added to the solution that dissolve only those elements. Then scientists induce the solution to separate—like oil and water—so that they can remove the solvent that's bound to them. At this point, neptunium is separated and can be passed through the cycle again. The plutonium is purified through a process called ion exchange, which Oak Ridge is still refining—a key step to reaching the 1.5-kilogram per year delivery goal.

7 SEND TO LOS ALAMOS

Fully refined, the plutonium powder is packed into stainless-steel canisters designed for transporting radioactive materials. It's shipped to Los Alamos National Laboratory, where it's turned into fuel capsules for RPS, then to Idaho National Laboratory—right back where it started as neptunium—where the RPS are built.

Less radioactive steps in the manufacturing process happen in a glove box, which is slightly less intimidating than a hot cell. Here, pouring recycled neptunium. Greg Miller

This story appears in the September 2017 issue.

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