Even when shut down, a nuclear reactor does not become completely inert. Its internal fuel continues to emit very faint particles, called antineutrinos, and for the first time the Double Chooz experiment directly measured this residual signal. This advance could one day help monitor spent fuel without permanent on-site inspection.
Antineutrinos are produced by the radioactive decay of fission fragments. They pass through matter very easily and are therefore currently impossible to block. Their emission reflects the real nuclear activity of the fuel, including inside the reactor and after shutdown.
Chooz nuclear power plant.
Image Wikimedia
This property is of interest for safety and nuclear material control. Today, spent fuel stocks are tracked with inventories, inspections and surveillance devices. An antineutrino detector could provide independent control, carried out remotely and without entering storage pools.
The international team used data from Double Chooz, installed near the Chooz B power plant, in the Ardennes in France. The closest detector was about 400 m from the two cores and from the pools containing the removed fuel. The measurements covered just over 17 days during which both reactors were shut down.
The measured signal represents less than 1% of that produced during normal reactor operation. It comes from fission products with relatively long half-lives.
The researchers estimate that 56% of the signal came from the cores and 44% from the pools during the studied periods. About 98.7% of detectable residual antineutrinos had an energy below 3 MeV. Their intensity gradually decreases as the fuel cools down.
Ultimately, this method could verify that a fuel stock remains consistent with the declared inventory, and it could also monitor residual activity after a shutdown or an incident.