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Scientists have detected, for the first time, a faint stream of antineutrinos emitted by a nuclear reactor after it has been shut down, opening up new possibilities for reactor monitoring, nuclear safety and safeguards.
The measurement was made by the Double Chooz collaboration at the Chooz nuclear power plant in northern France. The findings, published in Physical Review Letters, show that nuclear reactors can continue producing detectable antineutrino signals long after their cores are switched off.
Even after a reactor stops operating, long-lived radioactive fission products inside its core continue to decay for months or even years. This decay produces low levels of antineutrinos, extremely elusive particles that can pass through the reactor and surrounding shielding with little interference.
The Double Chooz detector is located underground about 400 metres from the plant’s two reactor cores. It contains more than 30 cubic metres of liquid scintillator, which produces tiny flashes of light when an antineutrino interacts with the material.
“Antineutrinos interact only extremely rarely with matter. However, when one interacts within the Double Chooz detector, a characteristic double-light signal is produced that can be distinguished from background events,” said Thierry Lasserre of the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg, Germany.
Researchers analysed 17.2 days of data collected while both reactor units were completely shut down. During that period, the detector recorded about 100 antineutrino candidate events associated with residual radioactivity in the reactor cores and nearby spent-fuel cooling pools.
The measured signal closely matched detailed simulations based on the remaining nuclear fuel inventory and the radioactive decay of long-lived fission products.
The result provides the first direct experimental confirmation of theoretical predictions for antineutrino emissions from shutdown reactors and spent nuclear fuel.
“Until now, reactor antineutrino experiments have mainly focused on operating reactors, where the antineutrino flux is much larger,” said Anthony Onillon, who led the study with Lasserre. He said detecting the much weaker post-shutdown signal required exceptionally low background levels and sophisticated analysis techniques developed by the Double Chooz collaboration.
The findings could eventually help scientists monitor nuclear reactors even when they are not operating, offering a potential new tool for nuclear safety and international safeguards.
Other experiments are also beginning to investigate this area. Initial results from JUNO-TAO, presented at Neutrino 2026, indicate that researchers are using reactor-off data to study the faint antineutrino emissions from spent nuclear fuel.
While TAO is working to isolate the weak signal from spent fuel, the Double Chooz measurement provides the first published benchmark for studying residual antineutrino emissions from shutdown reactors and spent-fuel pools.