Feature

# Neutrino Detectors for National Security

Physics 13, 36
Detecting neutrinos offers a new way to monitor the potential bomb materials inside a nuclear reactor, but the technology’s practicality remains uncertain.

Nuclear reactors pose a dilemma. They provide carbon-free power and currently generate 14% of the world’s electricity, but many also produce plutonium, an essential component of some nuclear bombs. Physicists have recently demonstrated neutrino detectors that can continuously watch for the removal of plutonium, and a paper published today in Reviews of Modern Physics (RMP) outlines the state of play for the technology, including its limitations [1].

“Neutrinos are never the easiest solution to a problem,” says Patrick Huber, a physicist at Virginia Tech in Blacksburg, who has spent his career studying the particles and who coauthored the paper in RMP [1]. “You would only use them in a context where other solutions don’t work.” But there are countries, like Iran and North Korea, where the options for monitoring are limited.

Nuclear reactors produce a slew of neutrinos (actually antineutrinos, but the distinction only matters when discussing the details of nuclear reactions). While working on fundamental physics experiments over the last two decades, physicists have also been developing the theory and technology needed to make neutrino detectors that can monitor reactors. Now that these systems exist, there are practical challenges to their use. For example, the current generation of detectors, nearly all of which weigh upwards of a ton, have to be placed within tens of meters of a reactor’s core—inside a facility’s fence.

##### Beyond a Nuclear Security Tool

Despite these limitations, researchers are confident that the technology will find use as a nuclear safeguard. Making that step will require both finding the right scenario and updating current nuclear safeguarding protocols, which Goldblum sees as the biggest hurdle. “We have the capability right now to build a full-scale mobile system,” she says. For such a detector to be adopted “really comes down to the IAEA seeing value in the system and then the host country accepting this kind of monitoring.”

Those hurdles, Huber thinks, could be overcome for countries such as North Korea, where there is a lack of trust, and the stakes are high. “In this context, the cost of a detector wouldn’t be prohibitive,” he says. Glaser agrees. If concerns arise that the IAEA might be denied access to a reactor, then “a neutrino detector could suddenly become this magic piece of equipment that does something important,” he says. But not everyone is convinced that those conditions would warrant a detector’s hefty price tag.

“The cost of [building] many detectors is just so high that it’s not practical within a real-world context,” Carr says. Carr thinks implementation of these detectors will only happen if they find a role beyond that of a monitoring tool. “If [physicists] can come up with some value added, like the ability to reemploy weapons scientists, that maybe makes neutrino detectors a bit more practical,” she says. Giving scientists at North Korean nuclear facilities a tool to probe neutrino oscillations could shift their focus from weapons development to scientific discovery and also provide a connection to the scientific community. “It’s a stretch,” she says. But such a scenario could be more likely if the North Korean political situation improves.

Jonathon Coleman, a nuclear physicist at Liverpool University, UK, who helped build VIDARR, is more positive about the tool’s uptake, envisioning a future where neutrino detectors are stationed at every reactor around the world. And Littlejohn, who was involved in PROSPECT, thinks that the devices could find use at new reactors that are cooled with opaque liquids such as molten sodium, rather than water. These liquids block a security camera’s view of the fuel rods. Traditional temperature sensors and flow meters—used to measure reactor power—have the problem that their metal components quickly corrode. “You either have to create brand new monitors that work in liquid sodium, or you could use an antineutrino detector,” Littlejohn says. “It doesn’t need to be a tool exclusively for nuclear security.”

–Katherine Wright

Katherine Wright is a Senior Editor for Physics.

Correction (16 March 2020): The article was corrected to clarify the details of the process by which antineutrinos are detected. The figure illustrating the detection was also updated.

## References

1. A. Bernstein et al., “Colloquium: Neutrino detectors as tools for nuclear security,” Rev. Mod. Phys. 92, 011003 (2020).

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