Scientists from the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg, Germany, have announced the first successful detection of antineutrinos from an inactive nuclear reactor.
The research team behind the historic achievement said that detecting antineutrinos, which they described as “the most elusive particle in the Universe,” could improve monitoring, maintenance, safety, and security of nuclear reactors.
Hunting for ‘Faint Flux’ of Antineutrinos Inside Shutdown Nuclear Reactors
In a statement announcing the detection, the researchers point out that even when nuclear fission reactors are turned off, “the story inside the reactor core still has a great deal to tell.”
For example, the long-lived fission products inside nuclear reactors continue to decay for months, or in some cases, years after a reactor is no longer in use. This ongoing radioactive decay can produce what the team termed a “faint flux” of the most elusive particle in the universe: antineutrinos.
Because antineutrinos interact only extremely weakly with matter, they pass almost unhindered through the reactor vessel and surrounding shielding. In theory, the right equipment could detect this activity, offering insights into the reactor’s radioactive inner core.
Fortunately, for MPIK study leader Anthony Onillon and co-leader Thierry Lassere from the independent research group OMINA, also located at MPIK, there is an underground detector around 400 meters from two reactor cores, which were part of the region’s Chooz reactor in northern France.
Dubbed ‘Double Chooz,’ the detector contains over 30 cubic meters of liquid “scintillator.” According to the study authors, when antineutrinos interact with this material, a detectable flash of light is emitted.
“Antineutrinos interact only extremely rarely with matter,” Lasserre explained. “However, when one interacts within the Double-Chooz detector, a characteristic double-light signal is produced that can be distinguished from background events.”
Observing the ‘Faint Glow’ of Radioactivity
To search for the weak antineutrino signal from the decaying Chooz nuclear reactor material, the research team evaluated recorded data from a 17.2-day period when both reactors were shut down. Because antineutrinos interact so rarely with matter, the researchers searched for the characteristic paired flashes produced when one underwent inverse beta decay inside the detector.
After scanning through the data, the team said they found evidence for roughly 100 “antineutrino candidate events” originating from within the reactors. This included signals from the cores and nearby spent-fuel cooling pools.
The researchers said the measured signal “agrees remarkably well” with simulations of the remaining fuel inventory and the decay of long-lived fission products.The team said this detection marks the first direct experimental validation of the antineutrino emissions predicted by the shutdown reactor models.
“With this new work, the experiment has achieved another first: observing the faint neutrino glow that remains after a reactor goes dark,” the research team explained.
New Perspectives for Reactor Monitoring, Nuclear Safety, and Safeguards
When discussing the significance of the achievement, Dr. Onillon noted that previous efforts have primarily focused on operational nuclear reactors, where the antineutrino flux is much larger. Conversely, their detection was made under challenging conditions using methods perfected by the team.
“Detecting the tiny residual signal after shutdown required exceptionally low backgrounds and careful analysis techniques developed by the Double Chooz collaboration over many years,” the researcher explained.
When discussing the experiment’s practical applications, the team noted that the work provides researchers and regulators with “the first published benchmark” of the antineutrino glow coming from shutdown reactors and spent fuel pools. In theory, such detection could provide information on a reactor during both operational and maintenance phases.
“The results demonstrate that antineutrino detectors can probe nuclear reactors even during shutdown periods, opening new perspectives for reactor monitoring, nuclear safety, and safeguards,” they conclude, adding that such detections “may become relevant for independent verification of reactor status and spent-fuel inventories.”
The study “First measurement of neutrino emissions from spent nuclear fuel by the Double Chooz experiment” was published in Physical Review Letters.
Christopher Plain is a Science Fiction and Fantasy novelist and has spent the last six years as Associate News Editor and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him at christopher@thedebrief.org