Oct 6, 2020 8:00 AM

Neutrino Detectors Could Be Used to Spot Nuclear Rogues

In theory, the particles could reveal whether a reactor is building up plutonium for weapons. US energy experts are starting to take the idea seriously.

When Robert Svoboda was a young man in the Navy, his submarine carried 48 nuclear warheads, each packing many times the atomic power of the bomb the United States exploded over Hiroshima. Inhabiting the same ship as those destructive forces, he was forced to think about the reality of what they could do, and what they were there for.

Svoboda never learned to stop worrying and love the bombs. But dealing with them wasn’t his job. Instead, he was in charge of a different sort of nuclear device: the reactor that powered the sub through the dark ocean depths. Before he got that assignment, Navy officials had enrolled him in a year-and-a-half crash course on how reactors worked and how to run one. It was in this “nuclear power school” that Svoboda first met the particles that would intrigue him for the rest of his career: neutrinos.

One day, his reactor-theory professor was talking about how, yes, nuclear fission generates enormous power—but a certain percentage of the power essentially evaporates. It zips away, lost in the form of ghost particles with almost no mass that travel at nearly the speed of light. “That sounded sort of intriguing,” says Svoboda. “It just disappears into nothingness.”

Radioactive atoms emit these particles (technically “electron antineutrinos,” which nevertheless fall under the “neutrino” umbrella) in a two-step process when their nuclei rend themselves apart. This quality makes them a sign of nuclear fission—and, scientists now know, can reveal how far away a reactor is, what power level it’s operating at, and what fuel it’s burning. That information can reveal something its human operators might not: whether the reactor is being used to build up weapon-grade plutonium, or whether a country isn’t burning up its stored plutonium as promised.

When Svoboda—now a professor of physics at the University of California, Davis—left the Navy, he became a full-fledged neutrino scientist. These particles bridge the divide between his basic scientific research and his commitment—remembering always the bombs in the bay—to nuclear security.

These days, Svoboda and a growing number of colleagues are interested in using signals from neutrinos that burst from reactors for nuclear security: to perhaps detect undeclared devices and to ensure that known reactors are not being used to amass material for weapons. The neutrino signature that slams straight through a reactor’s walls changes with what’s going on inside, providing a window into how much plutonium is in there, and whether it matches what’s expected. If not, it’s a clue that some of it may have been diverted toward weapons programs. It’s a real-time measurement that could augment the inspections and measurements that officials from the International Atomic Energy Agency currently do.

Those “safeguards” measures take many forms: first, declarations from facilities about what they’re up to. And second, monitoring. That involves inspections both routine and ad hoc, satellite imagery, open-source analysis, camera surveillance, collection and analysis of sample nuclear material, and environmental analysis. But neutrinos never lie, can’t hide, and tell the truth as it unfolds.

The tech isn’t quite ready for prime time, but some scientists think neutrino monitoring might be practical, and they’re taking (small) steps toward making that happen: The Department of Energy recently commissioned a study on nuclear security applications for neutrino detectors, and this summer it spooled up a group that aims to determine where the technology might be useful to policy types. A project with the creepy name Watchman, which Svoboda works on, is also developing detectors and methods that officials could use to pick up neutrino signals from a reactor dozens of miles away, revealing its activities without being right next door.

Every second, 100 trillion neutrinos zip through your body. And “zip through” is right: They do not see skin, walls, buildings, nor even the Earth itself: They rocket through the planet unperturbed, barely interacting with the world, thanks to their near-nothingness.

At the same time, they represent a fundamental ingredient of the universe and could reveal why matter exists at all, how the insides of stars work, and what really happened right after the Big Bang. But because they’re hard to capture and analyze, scientists don’t know much about them.

Reactors, though, typically produce around 10²⁰ neutrinos per second, allowing detectors to reliably find them. Svoboda’s old boss, Fred Reines, actually picked up the very first definitive signs of neutrinos around a reactor—the Savannah River Facility in South Carolina—in the 1950s, a discovery that later won him the Nobel Prize. Svoboda did early work proving the utility of reactor neutrinos, following up on similar research by scientists in the former Soviet Union, in part to see if they could correctly monitor the evolution of fuel burning up at the Rovno nuclear power plant.

Svoboda was involved in an effort led by Lawrence Livermore National Laboratory and Sandia National Laboratories to set up a low-maintenance detector about 65 feet from the San Onofre nuclear generator in California. They ran it from 2003 to 2008, successfully detecting neutrinos and using them to tell whether the reactor was on or off, how powerful it was, and what fuel was burning. A collaboration called Nucifer showed similar positive results monitoring a research reactor in France, in a project led by the French Alternative Energies and Atomic Energy Commission, the Max Planck Institute for Nuclear Physics, and a research group called Subatech. They showed that a neutrino detector could operate inside a research reactor building. Researchers in China and Brazil, ditto, have seen positive results around their reactors. One project in Japan, called KAMLand, picked up neutrinos from reactors more than 100 miles away.

It was all welcome news. Officials could potentially use the ghost particle signal to tell, objectively, whether a reactor was acting as its officials claimed. “All reactors build up plutonium at some rate, so the amount that is there compared to operator declaration is the critical issue, rather than ‘Is there plutonium or no plutonium?’” says Svoboda. Back when these experiments were first going on, he says, “almost the only way you had to tell if people were making plutonium was to believe their documents.”

But the results suggested neutrinos could augment such safeguards. That was the start of the idea for Watchman, Svoboda continues. Watchman—a giant cylinder 50 feet tall and 50 feet across, filled with gadolinium-doped water—will, pending final approval, live at Boulby Underground Laboratory in England, more than a kilometer below Earth’s surface. Intended to be completed in 2023, it will watch for ghost signals from the Hartlepool nuclear plant, about 15 miles away. It’s a demonstration project, aiming to show officials what’s possible and develop the technology and analysis techniques. Ideally, the plant’s neutrinos will charge through and create a flash of light in the water, demonstrating that scientists can see a reactor they’re not right next to—which is useful if you want to monitor a reactor you can’t be right next to, for political or physical reasons. The team is finishing up its detailed engineering plan—it's no longer, as Svoboda says, “just cartoons on a blackboard.”

Not needing to be right next to a reactor could make monitoring easier. So, too, could a move aboveground. Over the past few years, scientists like Virginia Tech’s Patrick Huber have developed neutrino tech that doesn’t have to be underground. In the past, neutrino detectors have needed to live deep down to help screen out the background noise that might otherwise swamp small signals. But physicists have now created detectors that rely on lithium, rather than gadolinium. Lithium gives a sharper view of where a neutrino signature appeared. Rather than acting as a single, big detector, aboveground tech is essentially segmented into a series of smaller detectors, letting physicists better pinpoint the location of a signal. With those two innovations in place, they are able to more precisely map the sequence of a neutrino event—and to more easily tell what actually is a neutrino event, versus what's background.

Being able to place a detector aboveground also helps if you want to convince a reactor facility to get onboard. “Most operators of nuclear reactors don’t respond too kindly to building a 6-meter hole in the ground,” says Huber.

Proof that aboveground detectors could work came in 2018, when two projects—called Prospect and Chandler—on which Huber is a collaborator, did, in fact, catch the ghost particles at the surface. The combination of Watchman’s progress and this novel aboveground detection have helped kindle interest among officials who’d like to potentially put the technology to more than prototype, navel-gazing use. Recently, the Department of Energy commissioned a group, including Huber, to lay out where and how neutrino science could actually be useful for nuclear security. They looked at, for instance, whether the elusive particles could reveal nuclear tests, spent fuel, and reactor activity.

Although the official report isn’t ready yet, the team did, this spring, compile some of the findings into a publicly available paper titled “Neutrino Detectors as Tools for Nuclear Security.” The group found that, at least in the near term, neutrinos weren’t that useful for picking up explosions or spent fuel. But they could help, relatively soon, with reactor monitoring.

Bethany Goldblum, a nuclear engineer at the University of California, Berkeley, worked with Huber and others on the report. “We believe that using neutrinos for monitoring known reactors is the most immediate opportunity,” she says. Farther along, they could potentially hunt for hidden reactors. But the real opportunity, Goldblum thinks, is in checking up on the interiors of advanced reactors, like those that mix molten salt with the radioactive fuel, rather than using traditional solid fuel rods. “In existing reactors, we have adequate means,” she says, referring to the IAEA’s verification schemes. “States are comfortable, and we’re doing a good job with accounting. I don’t think neutrino monitoring really adds a whole lot there.”

That info in hand, the Department of Energy has also spun out a more practical study group, called NuTools, which aims to figure out where, in real life, their neutrino knowledge might be useful to nuclear-security practitioners who help enforce international safeguards. The discussions began this summer, with the webpage noting, pandemic-appropriately, “Where: All meetings virtual.”

“Research on this topic was driven by neutrino scientists interested in what, technologically, we have to do,” says Huber, who’s part of the group. “NuTools is saying, ‘Let’s talk to the people who are dealing with safeguards now to find out what would be useful to them.’ In a sense, it’s a market study.” The coalition’s officers hail from the Department of Energy’s national labs; the National Institute of Standards and Technology; and universities like MIT, Georgia Tech, and the Illinois Institute of Technology. Goldblum is also on that roster.

Goldblum, who trained as an applied nuclear physicist, became interested in security during a three-week public policy boot camp she took during graduate school. “I hadn’t really thought about the policy implications of my technical research,” she says. “After a few days at the boot camp, I was having nightmares of the nuclear holocaust.” She began to think about basic physics not as something neutral, but as something that has implications for the security of the whole world—something physicists have been struggling with at least since the Manhattan Project. Today, Goldblum shares her realizations with students and also as executive director of the Nuclear Science and Security Consortium, a research group sponsored by the National Nuclear Security Administration.

In fact, most people doing neutrino-nuclear work step over from “basic research”—or at least put a foot across the line—the way Goldblum did. It’s the kind of science-underlying-security work the consortium aims to enable.

Huber actually sees neutrinos as a national security recruitment tool. “This is a way to get your average physicist interested in nuclear security issues,” he says. Since the collapse of the Soviet Union, it’s been more difficult to bring young physicists aboard to do nuclear nonproliferation work. “When I grew up, nuclear weapons and the Cold War were at the forefront of people’s concerns,” says Huber. “Today, with Covid and climate change, we tend to become a little oblivious to the nuclear threat. Maybe because we also have lived with nuclear weapons for the past 75 years.”

That’s no guarantee we’ll live peacefully with them for the next 75. But these physicists are trying to help ensure safe passage, using phantom particles to chart the course. “We can’t put the genie back in the bottle,” Svoboda says. “But we can tame it a little bit.”