Plutonium-238 pellet
Plutonium-238 pellet. Credit: U.S. Department of Energy

After a 25-year hiatus, the U.S. Department of Energy (DOE) has produced its first plutonium-238, a nonweapons-grade radioactive isotope used as a power source for space probes.

“We’ve been working quite closely with DOE on a restart program,” NASA’s planetary science chief Jim Green said during a webcast Mars exploration planning group meeting Feb. 27.

“They’ve developed a series of processes that have encapsulated neptunium and put it into a reactor at Oak Ridge, radiated it for a month and now the analysis is clear that we did indeed generate plutonium. This is a major step forward,” Green said.

“We’re expecting reports back from them later this year on a complete schedule that would then put plutonium on track to be generated at about 1.5 kilograms a year, so it’s going quite well,” Green said.

In addition to replenishing the dwindling U.S. supply of plutonium-238, the newly made isotopes can regenerate older plutonium that has decayed to the point where it is no longer viable to power probes. The plutonium emits heat, which can be converted into electricity by a radioisotope thermoelectric generator, or RTG.

“For every 1 kilogram, we really revive 2 other kilograms of the older plutonium by mixing it. It’s a critical part of our process to be able to even utilize our existing supply at the energy density that we want it at,” Green said.

The United States has not produced plutonium-238 since the late 1980s when the DOE’s Savannah River Site in South Carolina was shut down. NASA turned to buying plutonium-238 from Russia, but that supply line ended about two years ago.

NASA and the DOE have been working on a more energy-efficient RTG called the Advanced Stirling Radioisotope Generator (ASRG). Green said it likely will not be available for use on the planned 2020 Mars rover.

Even if it were, duplicating the Mars Curiosity rover’s RTG, called the Multi-Mission RTG may be a better engineering choice.

“The [RTG] that’s used on Curiosity makes use of waste heat to warm the electronics in the interior of the rover. If you use an ASRG, part of the efficiency gain is that you don’t have as much heat for the interior of the rover, so you buy a second problem,” said Michael Meyer, NASA’s Mars Exploration program chief scientist.