Pu-238: Fuel for Deep Space JourneysRobert Springer | February 02, 2017
The U.S. space agency NASA is particular when it comes to the fuel used to power its longest space missions – only plutonium-238 will do.
Yet the Cold War-era fuel that powered the New Horizons journey to Pluto and will drive the Mars 2020 Rover mission is in short supply, as the United States hasn’t produced any since the late 1980s.
The agency has been working with the Department of Energy to produce more of the vital substance, and scientists at the Oak Ridge National Laboratory (ORNL) have produced two 50 gram (1.8 ounce) batches, with another planned for later in 2016.
ORNL plans to scale up production to 1.5 kilograms (3.3 pounds) per year in the early 2020s. The Pu-238 generates heat that is converted to electricity by a multi-mission radioisotope thermoelectric generator (MMRTG). The units measure about 64 by 66 centimeters (25 by 26 inches), weigh approximately 45 kg (94 lb) and contains 4.8 kg (10.6 lb) of Pu-238.
The Mars Rover 2020 mission is the only mission currently slated to use the nuclear fuel, but with the multi-year lead time needed to produce sufficient fuel, NASA decided it should start producing Pu-238 sooner rather than later.
Pu-238: The Ideal Space Fuel?
The MMRTG’s that Pu-238 powers provide NASA with reliable, relatively safe, and maintenance-free power for space missions. Although it’s expensive to produce and dangerous to use, engineers are familiar with Pu-238. Indeed, they have been using it to fuel space missions for more decades. (The unit in Voyager 1 will mark its 40th anniversary in early September.)
“It’s very nice because it has a very high energy density,” says Casey Dreier, director of space policy at the Planetary Society. “It’s also relatively safe for a radioactive isotope. It’s stable, so they can make it in a ceramic form that is not easily destroyed and not easily vaporized, and it’s very, very hot, so it gives off a ton of energy.”
It’s fortunate that Pu-238 does emit a lot of energy, as the process of turning it into electricity is not very efficient “at the few percent to an upper limit of about 5%,” says Ralph McNutt, a space physicist at the Johns Hopkins Applied Physics Laboratory.
Another Pu-238 advantage is that the Cold War-era infrastructure to manage, process, and move it is already in place “so you don’t have to create a whole new everything for it,” Dreier says.
Handling Pu-238 is relatively safe, compared to other radioactive isotopes. Dreier says it’s primarily an alpha particle emitter, and “it’s little helium atoms without nuclei can be stopped with a piece of paper. You can hold it in your hand and you wouldn’t get a dangerous dose. You would burn your hand, but you wouldn’t get a dangerous dose of radiation.”
Pu-238 throws off a lot of heat, which is good news for spacecraft that need to keep operating at low temperatures.
“I can make use of that heat in order to keep the spacecraft itself from freezing, to keep the electronics at a usable temperature, and also from keeping fuel lines and oxidizer lines from freezing up,” McNutt says.
Solar is the other main option for powering a spacecraft. It’s an excellent solution for the International Space Station, for example, which is powered by an acre of solar panels. The current outer limit for more distant solar-powered missions appears to be Jupiter; beyond that and solar radiation is too weak to produce sufficient power. Pu-238 also keeps producing energy in a Mars dust storm; dust-covered solar panels do not.
It Starts with Neptunium-237
The process of producing Pu-238 is elaborate and involves three of the United States’ national laboratories. It starts with obtaining neptunium oxide feedstock from the Idaho National Laboratory (INL), says Bob Wham, a chemical engineer who leads the Plutonium-238 for Space Missions Project for the Nuclear Security and Isotope Technology Division at ORNL.
Wham’s team at ORNL then conditions the neptunium oxide to remove some of the radioactive decay daughters (protactinium-233) so they can get as high an assay as possible. Next, the neptunium is blended with aluminum powder and pressed into pellets. The pellets are loaded in an aluminum tube backfilled with helium.
Aluminum is an ideal medium for the powder and tube as “it’s transparent to neutrons and it gives very high thermal conductivity,” Wham says.
Once the tubes pass inspection, they are placed into ORNL’s high flux isotope reactor and irradiated until 85% of the plutonium produced from the process is Pu-238 (the rest is plutonium-239). The process fissions a small percentage of the neptunium. That means the irradiated material needs to age (cool down) for up to a year before it’s moved from the reactor pool to the hot cells.
In the hot cells, engineers dissolve the aluminum away from the neptunium and plutonium oxides, and then separate the neptunium from the plutonium and fission products. Only about 14-15% of the neptunium is converted to plutonium, according to Wham. The rest can be reused.
Once they’ve finished processing it (which can take up to three years), ORNL ships the Pu-238 to Los Alamos National Laboratory (LANL) in New Mexico. LANL takes the Pu-238 powder and performs additional conditioning steps, presses it into pellets and puts the pellets into an iridium clad. After passing inspection, the clads are shipped to INL, which assembles and tests the MMRTGs for NASA. The complete cycle, from neptunium-237 to ready-to-use MMRTGs, takes several years.
Pu-238 as a Last-Resort Fuel
As suitable a fuel as Pu-238 is for certain space missions, it’s not without downsides. The decision to use nuclear fuel adds significant cost to a mission and can delay it for years while it goes through the qualification process.
The United States National Space Policy states that nuclear power systems can be used if they “significantly enhance space exploration or operational capabilities,” so it’s incumbent upon mission planners to look for another power source. Advances in using solar technology in space allowed Barry Goldstein, project director of NASA’s planned Europa mission, to consider it as an alternative to a Pu-238 powered mission.
The Europa team performed a “significant number of tests, on various solar cells, to see if they would survive the combined impacts of the radiation dose, as well as the temperature extremes” that the mission would require, Goldstein says. Their calculations, and data from NASA’s solar-powered Juno mission, showed that “solar power would work very well for our mission,” he says.
Despite its cost and dangerous nature, there’s no real alternative to Pu-238 for some space missions. It’s not for lack of trying, says McNutt. “People have really looked hard at this trying to figure out if there are alternatives, and a lot of money has been spent, and a lot of effort.”
Many of the thousands of known radioisotopes have been investigated, but all have been found wanting. The European Space Agency has been investigating americium-241 as a possible Pu-238 substitute. McNutt says that Am-241 is more readily available than Pu-238 but lacks its energy density.
Engineers need sure things to power billion-dollar spacecraft, Dreier says, and are understandably conservative about what energy sources power them. As a result, NASA has invested in the development of more efficient radioisotope generators.
Although it produced a small amount of the material, Dreier says the initial demonstration of a new process for creating plutonium-238 is “a very good step in progress to getting full production back online.”