In 2015, Oak Ridge National Laboratory produced the first plutonium fuel in the US in nearly 30 years. Now it’s headed to another planet. //

At the heart of Perseverance is a small “nuclear battery” the size of a beer keg called a radioisotope thermoelectric generator, or RTG. Unlike the nuclear reactors that create electricity on Earth, RTGs don’t have to initiate or sustain a fission reaction to generate power. They don’t even have any moving parts. Instead, they passively harvest the natural heat produced by the decay of plutonium-238 and convert it into electricity. They can reliably provide energy and heat to a spacecraft for decades—the two plutonium-powered Voyager probes launched in the late 1970s are still transmitting from interstellar space—and have been NASA’s go-to power source for more than two dozen deep-space missions.

“Plutonium-238 is a unique isotope of plutonium that principally decays by alpha radiation, and because of that, it generates a lot of heat,” says Robert Wham, the plutonium supply program manager at Oak Ridge National Laboratory, which is now responsible for making the stuff for NASA. “For a small spacecraft like Perseverance, you don’t want fission power. You just want thermal decay.” //

When the US got out of the plutonium business, it left NASA with a cache of a few dozen kilograms of plutonium-238 to ration for all future missions. It wasn’t much; the Perseverance rover alone uses nearly 5 kilograms of plutonium. At some point, this stockpile was bound to run out; a 2009 report by the National Academy of Sciences predicted that the US had only enough plutonium for a few more deep-space missions. That left the US with a few unpalatable options: Abandon exploration of the outer solar system, purchase plutonium from abroad, or start making it again domestically. //

With concerns about a plutonium shortage mounting—Russia was also running low—NASA policymakers decided the agency would foot the bill on its own. And since 2011, NASA has borne almost the entire cost of producing plutonium at the Department of Energy’s Oak Ridge National Laboratory in Tennessee. The investment soon paid off. By 2015, chemists at Oak Ridge produced the first sample of plutonium-238 in the US in nearly 30 years. At the same time, the lab invested heavily in automated production systems that would allow it to produce enough plutonium to meet NASA’s future needs. //

The process starts when researchers at Idaho National Lab send neptunium-237, itself a radioactive metallic oxide, to Tennessee, where automated machines press it into pellets the size of pencil erasers. Next, 52 of these pellets are stacked into metal rods called targets and placed in a nuclear reactor at either Oak Ridge or Idaho National Lab, where they are bombarded with neutrons to produce plutonium. After it’s left to cool for a few months, the plutonium is shipped to Los Alamos National Laboratory in New Mexico, where another machine presses the small plutonium pellets to form larger ones the size of marshmallows. Then they’re ensconced in a casing made out of iridium, a virtually indestructible metal that would prevent radioactive contamination in case of an accident when the rover is launched. Finally, the armored plutonium is shipped to Idaho National Lab, where 32 pellets are loaded into the rover’s nuclear battery before it’s installed on the vehicle.

Today, Oak Ridge is only producing about half of its target of 3.5 pounds of plutonium a year, a milestone Wham and his colleagues plan to hit by the mid-2020s.