After a 27-year gap, the U.S. Department of Energy has resumed producing plutonium-238, the radioactive fuel that powers NASA’s Curiosity rover on Mars and the New Horizons mission to Pluto and beyond.
The material will be used in radioisotope thermoelectric generators, or RTGs, for future space missions. The RTGs generate electricity as well as heat – as shown in “The Martian,” a movie in which an RTG is repurposed to keep a marooned astronaut warm. They’re particularly suited for missions to the outer planets, where solar-powered probes face greater challenges. Future plutonium-powered missions may include flights to study the mysterious ice-covered moons of Jupiter (such as Europa) or Saturn (such as Enceladus).
“This significant achievement by our teammates at DOE signals a new renaissance in the exploration of our solar system,” John Grunsfeld, NASA’s associate administrator for science, said today in a news release. “Radioisotope power systems are a key tool to power the next generation of planetary orbiters, landers and rovers in our quest to unravel the mysteries of the universe.”
Twenty-seven U.S. space missions have used RTGs, but as the Cold War wound down, the infrastructure for plutonium production wound down as well. When the Savannah River Plant in South Carolina shut down its K Reactor in 1988, that cut off the supply of U.S.-made plutonium-238. The United States then had to buy plutonium from post-Soviet Russia.
Other radioactive isotopes of plutonium are used in nuclear weapons, and they’re produced using a different process.
The Energy Department said a 50-gram, golf-ball-sized lump of plutonium-238 was made in a reactor at the Oak Ridge National Laboratory in Tennessee to demonstrate a new procedure for fuel production. Neptunium-237 from the Idaho National Laboratory is mixed with aluminum and then irradiated in a reactor to produce neptunium-238, which quickly decays to become plutonium-238. The plutonium is then converted to a powdery oxide and shipped to Los Alamos National Laboratory for further processing and storage.
The first sample will be analyzed for chemical purity and plutonium-238 content. When production starts in earnest, NASA and the Energy Department plan to produce about 12 ounces (300 to 400 grams) of plutonium oxide per year. Eventually, the operation will be ramped up to a rate of 3.3 pounds (1.5 kilograms) per year.
The effort to revive plutonium-238 production has cost about $15 million a year, the Energy Department says.
The current U.S. stockpile of plutonium-238 amounts to about 77 pounds (35 kilograms). Only about half of that supply provides enough heat to meet specifications for future spacecraft. The other half is too old and cold, but it can be blended with newly produced plutonium to add to the inventory. Each RTG requires somewhere around 10 to 18 pounds of plutonium-238.
The next mission to use an RTG is NASA’s 2020 Mars rover, due for launch in four years. The plutonium for that mission is being produced from the existing stockpile.