A Detector in France Counted Ghost Particles Streaming Out of Nuclear Fuel That Had Already Been Switched Off

When a nuclear reactor is running, it pours antineutrinos out into the world at a rate no other human-built machine comes close to. When you switch the reactor off, the flood is supposed to stop. It does not entirely stop, and physicists have now measured what is left.

A team working with the Double Chooz detector in northern France has published the first measurement of antineutrinos coming from a nuclear plant while its cores were shut down and its spent fuel sat cooling in pools. The result appears in Physical Review Letters.

An antineutrino is the antimatter counterpart of the neutrino, one of the lightest and least sociable particles known. It carries no electric charge and passes through matter almost without noticing it. Trillions are streaming through you as you read this. Catching one requires a very large volume of very clean material and a great deal of patience. Double Chooz sits 400 meters from the Chooz nuclear power plant and holds more than 30 cubic meters of liquid scintillator — a fluid that emits a flash of light when a particle happens to interact inside it.

"Antineutrinos interact only extremely rarely with matter," said Thierry Lasserre of the Max Planck Institute for Nuclear Physics in Heidelberg, one of the researchers behind the analysis. "However, when one interacts within the Double Chooz detector, a characteristic double-light signal is produced that can be distinguished from background events." That double flash — one from the interaction itself, a second microseconds later — is what separates a genuine antineutrino from the constant drizzle of cosmic rays and radioactive decay in the surroundings.

The team, working with Anthony Onillon and the OMINA group at Heidelberg, examined 17.2 days of data taken while both reactors at Chooz were off. They found roughly 100 antineutrino candidate events. The signal matched simulations of what long-lived fission products should produce as they continue decaying inside shut-down cores and inside the fuel assemblies sitting in the cooling pools. The fuel is spent, in the sense that it can no longer sustain a chain reaction. It is not finished.

The practical interest is in safeguards. Verifying that a country is doing what it says with its nuclear material currently depends heavily on inspectors, seals, cameras and accounting. Antineutrinos cannot be shielded, blocked or faked — they come straight out of the fuel and straight through several meters of concrete, and their rate tracks what is actually inside. A detector that can read a reactor while it is running is useful. A detector that can also read the spent fuel next to it, during the months and years when the reactor is idle, closes a gap that is otherwise hard to monitor from outside the fence.

There is a long way from 100 events in 17 days at 400 meters to a portable instrument an inspection team could park outside a facility. But the measurement establishes that the shutdown signal is there, that it is large enough to see, and that it behaves the way the decay physics says it should.