The Last Of NASA's Fuel

Storms on Jupiter, Saturn’s spectacular rings, and even the outer reaches of the solar system have all been glimpsed thanks to the radioactive warmth of plutonium-238. That’s because the substance is at the heart of radioisotope power supplies that generate electricity for the spacecraft that journey to places where sunlight is faint—essentially anything past Mars.

But a shortage of ²³⁸Pu threatens to make historic missions such as Voyager, Cassini, and New Horizons the last of their kind. The U.S. hasn’t produced ²³⁸Pu since the late 1980s, and the remaining stockpile—both from U.S. production and from purchases from Russia—has already been allotted to a handful of missions.

Since 1994, scientists at NASA and the Department of Energy have tried to restart ²³⁸Pu production, says Stephen G. Johnson, director of Space Nuclear Systems & Technologies at Idaho National Laboratory (INL), in Idaho Falls. None of these efforts, however, has managed to get beyond the planning and funding stage.

“Looking at the NASA planning calendar, people are starting to realize that we really need to get started in order to get the material NASA is going to need eight years from now,” adds Bob Wham, technology integrator for fuels, isotopes, and nuclear materials in the Nuclear Science & Technology Division of Tennessee’s Oak Ridge National Laboratory (ORNL).

Making ²³⁸Pu is a big job though. The process begins by purifying the neptunium oxide starting material. Neptunium-237 is a long-lived isotope, but it does have a daughter product, protactinium-233, that produces a radiation field potentially harmful to workers, Johnson explains.

Once the ²³³Pa has been chemically removed, the neptunium oxide is mixed with aluminum powder, pressed into pellets, and loaded into an aluminum tube. The whole assembly is then placed into a reactor, where ²³⁷Np is left to absorb neutrons and make the jump one spot over on the periodic table.

Depending upon the level of neutron flux where the material is placed in the reactor, the irradiation process can take anywhere from a couple of months to several years, Johnson says. Also, because the ²³⁷Np acts as a neutron sponge, it must be positioned carefully so it doesn’t interfere with other experiments in the reactor.

Because ²³⁷Np to ²³⁸Pu conversion is only about 10–15%, a series of three chemical separations and a purification need to be done to recover and recycle unconverted neptunium and remove any fission products. The high levels of radiation involved mean these steps need to be carried out with remotely operated equipment in rooms with 4-foot-thick concrete walls.

In the past, ²³⁸Pu was made at the Savannah River National Laboratory, in Aiken, S.C. That reactor was shuttered years ago, and now only two reactor sites are suitable for producing plutonium: INL and ORNL.

“The good news and bad news about DOE research reactors is that a lot of people are using them,” Wham says. “To get the 5 kg of ²³⁸Pu that NASA needs each year, we’re going to have to use both reactors.”

Getting both sites ready to produce plutonium, however, requires major construction work as well as hiring and training personnel. “The timeline for doing that would be five to six years from the time that we got a green light,” Johnson says.

The project also carries a hefty price tag. Johnson estimates it would take $200 million to get started and an operating budget of $30 million annually. So far, that expense has proven to be a sticking point.

DOE hoped Congress would move forward with its request for funding the plutonium project following a National Research Council (NRC) report in May that recommended immediate action be taken to reestablish the ²³⁸Pu supply.

Congress’ response, however, was less than enthusiastic. Of the $30 million DOE requested for funding, the House Appropriations Committee approved $10 million. The Senate Appropriations Committee recommended no funding for the project. Both committees complained DOE hadn’t sufficiently explained how it would use the money.

Without ²³⁸Pu, NASA has little choice but to delay missions, NRC noted. “If restart of production continues to stall, our human and intellectual capital investments in this technology, made with great cost and effort over decades, will continue to fail as the only people knowledgeable in this technology age, retire, and die and facilities and infrastructure fail and are abandoned,” says Ralph L. McNutt Jr., a space scientist in the Space Department of the Johns Hopkins Applied Physics Laboratory and cochair of the NRC committee that put together the report.

“If the U.S. abandons its role of leadership in science and technology, there are others who will gladly step into that role,” McNutt says.