The plutonium-238 that powers NASA’s rovers on Mars crisscrosses the United States first on a tour of national laboratories.
Department of Energy officials outlined the path and process of manufacturing the Pu-238 for the Perseverance Rover that launched in July and is already about two-thirds of the way to the Red Planet.
“Perseverance’s nuclear heart completed its own journey of seven years and nearly 5,000 miles before finally meeting up with the rover at Kennedy Space Center in Florida,” said Matt Dozier, the host of DOE’s Direct Current podcast.
NASA uses a solid-state nuclear battery, called a Multi-Mission Radioisotope Thermoelectric Generator, instead of, say, solar arrays, so the rover can keep operating during dust storms and the Martian night. With a halflife of 90 years, Pu-238 can keep a craft powered for decades.
“What’s the secret to their longevity?” Dozier asks. “It’s not turmeric, or acai berries, or wheat germ—the Mars rovers, and dozens of other NASA missions, run on a diet of pure plutonium-238.”
The $75 million MMRTG produces about 110 watts of electricity from the heat of decaying PU-238.
Dozier interviewed DOE officials tasked with processing and securing the dangerous isotope during each step of its manufacture:
1 Idaho National Laboratory: The fuel begins its journey as neptunium-237, a by-product of nuclear reactors that’s stored at Idaho National Laboratories. “The Oak Ridge National Lab essentially calls up the Idaho National Lab and phones in a shipment, let’s say, for neptunium,” said Robert Wham, program manager for the Pu-238 Supply Program at Oak Ridge. “We get neptunium on a just-in-time basis; it’s shipped to us, and then we do the chemical processing here.”
2 Oak Ridge National Laboratory: Oak Ridge mixes the Np-237 with aluminum and bombards it with radiation in a reactor for 50 to 60 days. Some of the Np-237 turns into Pu-238, which Oak Ridge technicians separate, collecting plutonium as a powder. Oak Ridge also builds an iridium cladding that should keep the plutonium contained.
“One of the things that we worry about is that if there’s an accident either during launch or shortly after launch, that when these generators come back to Earth, and they’ll crash into Earth at very high velocities, very high speeds,” said Easo George, an alloys expert who serves as the governor’s chair at the Department of Energy’s Oak Ridge National Laboratory and the University of Tennessee. “And we want to make sure that if—such a very low-probability event—but if something like that were to happen, that the iridium would contain the plutonium fuel and prevent it contaminating areas around where it strikes.”
3 Los Alamos National Laboratory: The Pu-238 then heads 1,400 miles west to Los Alamos