For the project, the team analyzed 17.2 days of data collected while both reactor units were completely shut down<\/a>. Over that period, the detector recorded about 100 antineutrino candidate events originating from radioactive decay inside the reactor cores and nearby spent-fuel cooling pools.<\/p>\n The signal closely matched detailed simulations of the remaining nuclear fuel<\/a> and the decay of long-lived fission products. It provided the first direct experimental confirmation of theoretical predictions for antineutrino emissions from shutdown reactors and spent nuclear fuel.<\/p>\n Nuclear reactor monitoring<\/p>\n The findings suggest antineutrino detectors could become useful not only while reactors are generating power, but also during refueling outages, maintenance work, and long-term storage of spent nuclear fuel.<\/p>\n \u201cUntil now, reactor antineutrino experiments have mainly focused on operating reactors, where the antineutrino flux is much larger,\u201d Onillon concluded in a press release<\/a>. \u201cDetecting the tiny residual signal after shutdown required exceptionally low backgrounds and careful analysis techniques developed by the Double Chooz collaboration over many years.\u201d<\/p>\n According to the researchers, these measurements could provide an independent way to verify whether a reactor is operating and help confirm inventories of spent fuel without relying solely on conventional inspection methods.<\/p>\n Meanwhile, interest in this approach is already growing. Results from the JUNO-TAO experiment, presented at the Neutrino 2026 conference, have also explored the detection of antineutrinos produced by spent nuclear fuel during reactor shutdowns. The Double Chooz measurement<\/a> now provides the first published benchmark for those efforts.<\/p>\n Double Chooz was created to study neutrino oscillations. It played a key role in measuring the neutrino mixing angle \u03b813, one of the fundamental parameters governing how neutrinos change between different types as they travel through space. It has now shown a new capability outside its original mission.<\/p>\n![\"\"](\"https:\/\/www.europesays.com\/ie\/wp-content\/uploads\/2026\/08\/Z3_5442d2.jpg\" "\\"Antineutrinos")
The Double-Chooz detector. Credit: Double-Chooz collaboration<\/a><\/p>\n![\"\"](\"https:\/\/www.europesays.com\/ie\/wp-content\/uploads\/2026\/08\/Z2_1aa812.jpg\" "\\"Antineutrinos")
Researchers Thierry Lassere and Anthony Onillon discussing the data.\u00a0
Credit: R. Lackner \/ MPIK<\/a><\/p>\n