The fuel, known as plutonium-238, is a radioactive isotope of plutonium that’s been used in several types of NASA missions to date, including the New Horizons mission, which reached Pluto earlier in 2015. While spacecraft can typically use solar energy to power themselves if they stick relatively close to Earth, missions that travel farther out in the solar system — where the sun’s radiation becomes more faint — require fuel to keep themselves moving.
Plutonium-238 satisfies this need by producing heat as it decays, which can then be converted into electricity by NASA’s radioisotope power system, a kind of nuclear battery called the Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG. Excess heat from the MMRTG can also be used to keep some spacecraft systems from freezing in cold environments — a service it’s been providing for the Curiosity rover on Mars, for instance.
While other isotopes could theoretically also get the job done, plutonium-238 is ideal because of its “unique combination of properties,” said Rebecca Onuschak, a program director in the Department of Energy’s Office of Space and Defense Power Systems. Most notably, it’s safer to work with than many other types of radioactive materials.
“It’s easier to work with and get close to than other kinds of radiation,” Onuschak said. “You can make it into a ceramic material that’s very safe. It has a long enough half-life that we can do long missions, but it also produces a lot of heat per unit mass.”
But despite its importance for space exploration, the fuel has been out of production for close to 30 years now. It was originally produced at the Savannah River plant in South Carolina as a byproduct in the production of defense nuclear isotopes during the Cold War, Onuschak said. But production was discontinued in the late 1980s when the defense missions ended and the reactors were shut down.
Fortunately, the government had some extra fuel stocked, and in the intervening years also was able to purchase a little more from Russia to add to its stores, which NASA has been drawing on for missions ever since. But “we knew we were going to eventually need more,” Onuschak said. So in fiscal year 2011, Congress allocated funding for a new project.
Over the past several years, scientists at Oak Ridge National Laboratory have worked on designing new ways of creating plutonium-238. They’ve had to work with smaller reactors than the ones formerly used at Savannah River, said Bob Wham, a plutonium-238 technology integration manager at Oak Ridge National Laboratory.
In order to create plutonium-238, scientists have to apply certain nuclear reactions to a substance called neptunium-237. “We had to design new ways to introduce the neptunium into the reactor, and then obviously pull that material out of the reactor and go through the chemical processing steps to recover and purify that plutonium,” Wham said.
Eventually, their labors paid off. On Dec. 22, the Department of Energy announced that researchers at Oak Ridge had managed to produce 50 grams of plutonium-238 — a feat that hasn’t been performed since production was halted at Savannah River.
It’s a big step forward for future space missions. Currently, there are only about 35 kilograms (or around 77 pounds) of stored plutonium-238 left, and only half of that is immediately usable. As the fuel ages, it cools off and becomes less useful — but Onuschak said that old fuel can be mixed with new fuel as it’s produced to extend the substance’s life.
What’s available now will still be enough to get NASA through its next planned Mars mission — the Mars 2020 rover — but if NASA wants to continue sending missions like New Horizons into deep space, it will need new stores of fuel in the future.
Researchers at Oak Ridge plan to collaborate with facilities at Idaho National Laboratory and Los Alamos National Laboratory to begin scaling up production, Wham said. By the end of the decade, they’re hoping to be producing several hundred grams of fuel per year — and by the early 2020s, they hope to be up to a kilogram and a half.
In the meantime, NASA’s Radioisotope Power Systems program will also be funding research into the development of more efficient MMRTGs — that nuclear battery used to supply electricity to spacecraft. According to the Department of Energy, researchers in the program are hoping to produce a new MMRTG that would be able to provide about 25 percent more power at the beginning of a mission and up to 50 percent more at the end.
The revived interest in fuel production and efficiency signals the start of a new era for space exploration — one which many enthusiastic scientists have had a hand in, Wham said.
“We’ve got a lot of great people in Oak Ridge that are working on [the project], as well as people in Idaho and Los Alamos national labs,” he said. “It’s great to know that there are that many people that are excited and enthusiastic about it.”
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