Antineutrino detector could monitor spent nuclear fuel for clandestine activity

Scientists have recently measured antineutrino emissions from spent nuclear fuel for the first time, showcasing that monitoring can continue even after reactors are shut down. This is crucial as traditional detection methods only function when reactors are active. The Double Chooz collaboration has demonstrated the ability to detect residual antineutrino emissions during complete reactor shutdowns, including from spent fuel in cooling pools.

Antineutrinos are subatomic particles that, like neutrinos, are incredibly elusive and can be detected even at a distance from the power plant. Their detection features make them valuable for non-intrusive monitoring of nuclear reactors. The concept of using antineutrinos to monitor the fissile content of a reactor core and detect illicit material for nuclear weapon production has been around since the 1970s.

During reactor shutdowns, long-lived fission products continue to decay, generating a residual neutrino flux, though it is around 1% as strong as during operation and is affected by background activities. The Double Chooz team, led by Thierry Lasserre and Anthony Onillon, managed to measure both the residual neutrino flux and its energy spectrum despite these challenges.

They utilized the Double Chooz neutrino detection experiment, situated near the Chooz B nuclear power plant in France. This experiment features two detectors, "near" and "far," located approximately 400 meters and 1.05 kilometers from the reactors, respectively. The reactors at Chooz B contain 205 fuel assemblies, primarily composed of enriched uranium dioxide.

In typical operation, these reactors generate additional fissile isotopes, 239Pu and 241Pu, after neutron capture and subsequent decay processes involving 238U. Following full power operation for over a year, reactors are shut down for refueling every six to eight weeks, during which some spent fuel assemblies are transferred to storage pools.

The Double Chooz detector consists of over 30 cubic meters of liquid scintillator, designed to emit light flashes when antineutrinos interact with it. The detector is shielded from background radiation by layers of steel, water, and mineral oil. In 2017, during a 24.4-day shutdown for refueling and maintenance, the researchers collected data to measure the residual antineutrino signal.

They observed 106 ± 18 events, which aligns with the predicted 88 ± 7 events based on detailed simulations of the remaining nuclear fuel inventory and decay of long-lived fission products. This result, obtained during 17.2 days of measurements for the near detector and 22.2 days for the far detector, is a significant achievement in demonstrating the feasibility of antineutrino monitoring for spent nuclear fuel.