In the next few decades, many physicists are hopeful that nuclear fusion could become a realistic source of practically limitless energy. But before this can happen, it will be critical to ensure that reactors cannot be covertly misused to produce materials for nuclear weapons.

Through new analysis published in Physical Review Applied, a team led by Patrick Huber at Virginia Tech has shown that an existing type of particle detector could be used to flag any such misuse.

Dangerous tampering
Fusion reactors generate energy by forcing two hydrogen nuclei to fuse together, releasing enormous amounts of energy along with a stream of neutrons. Alongside their energy-related advantages, these reactors are also particularly desirable because, unlike conventional nuclear fission reactors, they generally don't require any weapons-grade materials.

However, the neutron stream produced in the reaction presents the possibility of a dangerous scenario: If uranium-238 were secretly introduced into the reactor, those neutrons would convert it into plutonium-239—a key ingredient in certain nuclear weapons. To ensure this can never happen, future fusion reactors will need reliable monitoring.

Detecting covert plutonium
In their study, Huber's team focused on whether this purpose could realistically be served with an antineutrino detector. Antineutrinos are chargeless, extremely low-mass particles that are abundantly produced during nuclear reactions. Crucially, it is impossible either to block them with a shield or to produce them with any non-nuclear process—making them ideal for covert monitoring.

When uranium-238 absorbs neutrons created in the fusion process, it undergoes fission, splitting into smaller nuclei and releasing a distinctive pattern of antineutrinos. The team ran simulations to see whether this signal could be distinguished from the antineutrinos produced during normal reactor operation, as well as those arriving from space.

Reassuringly, the team's results confirmed that even a relatively compact detector would be capable of confirming the production of just a few kilograms of plutonium over a 30-day period, enough to raise the alarm about any weapons program in its early stages.

Crucially, the detector would not need to be housed inside the reactor itself—it could operate at a distance, making the monitoring process minimally intrusive and practical to deploy at real facilities.