Even when the lights go out and nuclear reactors are turned off, the story inside the reactor core still has a great deal to tell. Radioactive, long-lived fission products continue to decay for months or even years, producing a faint flux of a specific type of particle known as antineutrinos. (Anti)neutrinos are the lightest and most elusive known particles in the Universe, allowing them to escape unhindered from both the reactor and the surrounding shielding.
Researchers of the Double Chooz collaboration have now measured this residual antineutrino emission for the first time. The study, which was recently published in Physical Review Letters, was led by Anthony Onillon and Thierry Lassere from the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg, Germany. The results demonstrate that antineutrino detectors can probe nuclear reactors even during shutdown periods, opening new perspectives for reactor monitoring, nuclear safety, and safeguards.
The measurement was performed at the Chooz nuclear power plant in northern France. The Double Chooz detector is located underground at a distance of about 400 metres from the two reactor cores. The detector contains more than 30 cubic metres of liquid scintillator, a material that emits tiny flashes of light when an antineutrino interacts inside the detector.
"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", explains Thierry Lasserre from the independent research group OMINA, also located at MPIK. This allows researchers to identify the reactor antineutrinos.
The collaboration analysed 17.2 days of data recorded while both reactor units were completely shut down. During this period, the detector observed around 100 antineutrino candidate events originating from residual radioactivity in the reactor cores and in nearby spent-fuel cooling pools.
The measured signal agrees remarkably well with detailed simulations of the remaining nuclear fuel inventory and the decay of long-lived fission products. This constitutes the first direct experimental validation of predictions for antineutrino emission from shutdown reactors and spent fuel.
"Until now, reactor antineutrino experiments have mainly focused on operating reactors, where the antineutrino flux is much larger. Detecting the tiny residual signal after shutdown required exceptionally low backgrounds and careful analysis techniques developed by the Double Chooz collaboration over many years", adds Dr Onillon.
Recent first results from JUNO-TAO , presented at Neutrino 2026 , show that this new direction is already being explored by other experiments. By analysing reactor-off data, TAO aims to isolate the faint antineutrino signal from spent nuclear fuel. The Double Chooz result now provides the first published benchmark for understanding this residual glow from shutdown reactors and spent-fuel pools.
The result demonstrates that antineutrino detectors could, in the future, provide information not only during reactor operation, but also during maintenance periods and after shutdown. Such measurements may become relevant for independent verification of reactor status and spent-fuel inventories.
Double Chooz was originally designed to study neutrino oscillations and played a key role in measuring the neutrino mixing angle θ13 , a fundamental parameter that describes how neutrinos transform between different types as they travel. This measurement opened the way to future studies of matter-antimatter asymmetries in the neutrino sector. With this new work, the experiment has achieved another first: observing the faint neutrino glow that remains after a reactor goes dark.
