Scientists have measured antineutrino emissions from spent nuclear fuel for the first time, demonstrating that nuclear safeguarding efforts need not stop when reactors do. The finding, reported by members of the Double Chooz collaboration in Physical Review Letters, shows it is possible to detect residual antineutrino emissions during a complete reactor shutdown, including from fuel assemblies stored in cooling pools.
Antineutrinos are tiny, nearly massless fundamental particles that come in three flavours: electron, muon and tau. The largest human-made sources of electron antineutrinos are the reactor cores of nuclear power plants, which produce them via beta decay of neutron-rich fragments generated during the fission of heavy elements such as uranium and plutonium. Because antineutrinos interact so rarely with other particles, their emissions cannot be shielded and their signatures cannot be altered, and they can be detected some distance from the plant itself. These features make them valuable for nonintrusive nuclear reactor monitoring, an idea that dates back to the 1970s for detecting illicit production of material that could be used in nuclear weapons manufacturing.
When a reactor is shut down, long-lived fission products in burnt fuel assemblies continue to decay, producing a residual neutrino flux. However, this residual signal is only around 1 percent as strong as the signal from an operating reactor, and it lies in an energy region strongly affected by background activities. The team, led by Thierry Lasserre and Anthony Onillon from the Max-Planck-Institut für Kernphysik in Heidelberg, Germany, succeeded in measuring the residual flux and its energy spectrum despite these difficulties.
The researchers collected data in 2017 when both of Chooz’s reactors were simultaneously shut down for 24.4 days for refuelling and maintenance. This unusual double shutdown was long enough for the team to obtain the statistics and low background signals needed to extract, for the first time, a quantitative residual reactor antineutrino spectrum. After accounting for muon veto-induced dead time, the researchers clocked up 17.2 days of measurements for the near detector and 22.2 days for the far detector. They mainly focused on the near detector because it is closer to the reactor cores and spent fuel pools and therefore more sensitive to the residual antineutrino flux.
The Double Chooz experiment is located near the two 4.25 GWth cores of the Chooz B nuclear power plant in the French Ardennes. It uses a pair of detectors, labelled near and far and located around 400 m and 1.05 km from the cores, to study the θ₁₃ neutrino mixing angle that describes how neutrinos transform between different types as they travel. The Chooz plant uses a pair of pressurized water reactors, each containing 205 fuel assemblies consisting of roughly 600 kg of enriched uranium dioxide. During operation, these reactors also produce additional fissile isotopes, plutonium-239 and plutonium-241, through neutron capture and subsequent decay processes involving uranium-238.
The detector consists of more than 30 m³ of liquid scintillator, a material that emits short, double flashes of light when an antineutrino hits it. The outer part is shielded from background radiation by 15 cm of demagnetized steel for the far detector and 1 m of water for the near detector, while the inner part is shielded by a thick layer of mineral oil. An outer muon veto consisting of segmented scintillator modules eliminates contributions from cosmic muons that can mask the antineutrino signal.
In the 1–3 MeV range, where the residual neutrino signal is strongest, the researchers observed 106 ± 18 events, a 5.9 sigma excess over the background. This value is in very good agreement with the 88 ± 7 events predicted by detailed simulations of the remaining nuclear fuel inventory and the decay of long-lived fission products. The researchers note that in 2003, IAEA safeguards met with the neutrino community and raised the question of whether antineutrinos could also provide information on spent nuclear fuel. The main difficulty has been detecting the faint residual signal, which required data during periods when both reactors were off, very low and well-controlled detector backgrounds, and a detailed simulation of the irradiation and cooling history of relevant fuel assemblies.