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Testing the Perseverance rover's radioisotope power system.

Plutonium production for NASA carries avoidable risk, safety panel says

20 April 2023

An independent oversight group encourages the Department of Energy to minimize the risk of earthquake-driven contamination at Los Alamos by upgrading the glove boxes used to make fuel for spacecraft.

Dragonfly spacecraft illustration.
NASA’s eight-bladed rotorcraft Dragonfly is shown exploring Saturn’s moon Titan in this illustration. Slated to launch toward the outer solar system in 2027, the spacecraft will be powered by a plutonium-238 thermoelectric generator. Credit: NASA/Johns Hopkins APL/Steve Gribben

In the late 1960s, NASA began using plutonium to power some of its robotic spacecraft. Radioisotope power systems (RPS), which convert the heat from 238Pu’s radioactive decay into electricity, have fueled 24 missions, including the twin Voyager spacecraft, the New Horizons probe to Pluto, and the Perseverance and Curiosity rovers that continue to roam the surface of Mars. Soon RPS will power the Dragonfly mission to Saturn’s moon Titan.

Any mission that uses RPS involves not only NASA but also the Department of Energy, which, through its national laboratories, supplies the 238Pu. Having dealt with plutonium since humans synthesized it, DOE has the expertise and infrastructure to manipulate the actinide and turn it into space fuel.

But one of the labs that helps with that process, Los Alamos National Laboratory in New Mexico, does some of its work for NASA using old equipment. And an independent governmental oversight organization called the Defense Nuclear Facilities Safety Board (DNFSB) takes issue with that. In a public meeting held in Santa Fe in November, the board reiterated its more than decade-long position that the glove boxes that Los Alamos workers use in part to supply NASA’s RPS aren’t just old: They are unsafe due to their susceptibility to breaching in the event of an earthquake, and they need to be replaced.

The hazards of plutonium production

Despite 238Pu’s utility for space missions, the US has historically produced it as a secondary product in efforts to manufacture 239Pu for nuclear weapons. When the country stopped making weapons material in the late 1980s, relying instead on existing stores of 239Pu when needed, the infrastructure to make space fuel was shut down, according to a NASA statement to Physics Today. When the 2006 New Horizons mission and the 2011 Mars Curiosity rover required more of the substance than had been the case in the recent past, DOE began buying plutonium from Russia. “This was viewed as a short-term solution,” NASA says in its statement.

Acting on a 2009 recommendation from the National Research Council, Congress provided funds beginning in 2011 for DOE to restart domestic 238Pu production, with the goal of manufacturing 1.5 kilograms per year by 2026. NASA says the program is on track to meet that target.

NASA’s RPS technology uses a ceramic form of plutonium oxide, which is produced in a multistep process. Idaho National Laboratory ships some of its stores of neptunium to Oak Ridge National Laboratory in Tennessee. Oak Ridge turns the neptunium into pellets, which are then bombarded by neutrons at reactors in Oak Ridge and Idaho to produce plutonium. After the material has cooled, Oak Ridge separates out the plutonium and converts it to plutonium oxide. Los Alamos purifies the plutonium, mixes it with plutonium that’s in inventory, and transforms it into pellets, which are then encased in iridium cladding for protection. The pellets go to Idaho National Laboratory for final assembly and to sit in storage until launch.

Testing the Perseverance rover's radioisotope power system.
NASA technicians test the fit of the Mars Perseverance rover’s radioisotope power system at Kennedy Space Center in April 2020. Credit: NASA/JPL-Caltech/KSC/DOE

In a spacecraft, the decaying 238Pu releases alpha particles and—more important for this use case—heat, which is used to derive an electrical voltage. But preparing the isotope for flight is a dicey proposition, with exposure hazards that are greater than those for the 239Pu that goes into weapons. According to the Nuclear Regulatory Commission, 238Pu decays at a rate of 17 curies per gram of material, whereas 239Pu’s rate is 0.062 curies per gram. The isotopes’ half-lives are 88 years and 24 110 years, respectively. “It’s far more dangerous,” says Jay Coghlan, head of the watchdog group Nuclear Watch New Mexico, of 238Pu.

Phillip Broughton, a health physicist, says 239Pu poses a risk for criticality events that 238Pu does not. PF-4 has been the subject of criticality safety concerns, which the DNFSB has also reported on, and of a related pause on some plutonium work in 2013. But in terms of the risk to personnel working with the materials and to residents of the surrounding areas, “I would be much more concerned about a 238Pu exposure because of its specific activity, its alpha-decay mode, and much shorter half-life,” Broughton says. “While an internal uptake of 239Pu wouldn’t be great, it takes comparatively very little 238Pu to deliver a large internal dose.”

Safety and funding complications

At Los Alamos, workers handle 238Pu in a building called PF-4, inside glove boxes that look like the hoods under which chemists and biologists do research. The goal is to prevent what’s inside from contaminating the people and environment outside. Safety procedures include wearing radiation-shielding gloves when manipulating the actinide metal and the use of filtration and pressure systems to ensure particles don’t waft into the ambient air.

The DNFSB’s concern, laid out in a 2009 document about seismic safety at Los Alamos, is that the glove boxes might not hold up against the kind of earthquake that geologists now know the area could experience. The lab sits atop the Pajarito fault system, which has ruptured as many as three times in the last 10 000 years. The highest-magnitude quake the area could expect to encounter is 7.0. “Seismic issues are not on the same level as at Lawrence Livermore in California,” says Coghlan. “But nevertheless, seismic issues are definitely serious.”

If such tectonic activity took place near the lab, a glove box could topple, spill, or crash and release its plutonium contents. “At PF-4, the most challenging accident is a severe earthquake that also causes a fire,” Christopher Roscetti, the DNFSB’s technical director, said at the November hearing, speaking of the facility as a whole and including other radioactive work in addition to the work for NASA. In that scenario, workers and the offsite public likely would be exposed to unsafe amounts of radiation, he said.

At the hearing, Thomas Summers, vice chairman of the safety board, displayed two types of glove box for the audience, which included officials from Los Alamos and the National Nuclear Security Administration (NNSA). One version was, he said, “seismically qualified,” with bracings and footings to keep it securely locked down. The other was “seismically deficient.” It had feet like a laundry sink.

The lab has upgraded many of the glove boxes to the safer sort. But some of them—at least 180 at the time of the hearing, including about 40 that do heat-source plutonium work in support of NASA’s missions—“do not meet the required seismic capacity,” said Summers, a physicist and former NNSA official. Although the DNFSB provides recommendations and advice to the president and the secretary of energy, it does not have the authority to force DOE to address what it sees as safety problems.

Upgraded glove box.
An older glove box.
A glove box stand that is designed to withstand a significant earthquake is shown in the upper photo. Many of the glove boxes used for the production of fuel for NASA spacecraft, however, are older and less secure, like the one in the lower photo. Credit: Defense Nuclear Facilities Safety Board

“The most important glove boxes have already been upgraded in terms of the release hazard, associated particularly with molten plutonium operations,” Thom Mason, director of Los Alamos, told the safety board. “But much more,” he admitted, “remains to be done.” He added that the lab is installing new, safer glove boxes as fast as it ever has—so fast that the limiting factor is how quickly industry can produce and ship them.

Many of the release-conscious upgrades, however, are focused on the lab’s national security work, particularly making plutonium pits for nuclear weapons. The glove boxes shared with NASA are not on the current list for upgrades, according to a statement from DOE. “Currently the NNSA is upgrading those boxes required for plutonium pit manufacturing,” the statement continued. “The NNSA will continue to work with our partners to monitor risks and establish new efforts as resources become available.”

Coghlan thinks that’s backward. “It should be 238 operations that have the highest priority,” he says. “If something goes wrong with 238Pu operations, it could well impact the rest of the facility.”

A major complication is funding, NNSA administrator Jill Hruby told the board. Certain defense programs are financially supporting safety upgrades that the lab may not have historically had the resources for. But “in the facilities where we’re not doing mission-critical work right now, we haven’t yet obtained the funding,” she continued.

Summers said that includes the glove boxes used for NASA’s plutonium. Los Alamos charges outside customers like NASA only for day-to-day operations, not for supporting improvements to those operations. And according to hearing participants, it doesn’t currently have a mechanism for amortizing the cost of upgrades to those outside customers.

“You’re accepting the risk for NASA for not upgrading their heat-source plutonium glove boxes, even though that could actually hurt the mission for the deterrent?” asked safety board chair Joyce Connery, a former NNSA and DOE official, referring to nuclear weapons work. “That doesn’t make sense to me.”

Mason said the program is working on the problem, but because it entails navigating between NASA and the other national labs involved in batching its RPS fuel, change takes a while.

“Clearly,” said Connery, “since it’s been since 2009 that we’ve identified this problem.”

A DOE spokesperson tells Physics Today that there have been no new investments in glove-box infrastructure besides those used for plutonium pit manufacturing, but that DOE and Los Alamos “continue to engage their partners, including NASA, in establishing a priority list of glove boxes to perform seismic upgrades on.”

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