Results from a new scientific study may shed light on a mismatch between predictions and recent measurements of ghostly particles streaming from nuclear reactors—the so-called “reactor antineutrino anomaly,” which has puzzled physicists since 2011.
The anomaly refers to the fact that scientists tracking the production of antineutrinos—emitted as a byproduct of the nuclear reactions that generate electric power—have routinely detected fewer antineutrinos than they expected. One theory is that some neutrinos are morphing into an undetectable form known as “sterile” neutrinos.
But the latest results from the Daya Bay reactor neutrino experiment, located at a nuclear power complex in China, suggest a simpler explanation—a miscalculation in the predicted rate of antineutrino production for one particular component of nuclear reactor fuel.
Antineutrinos carry away about 5 percent of the energy released as the uranium and plutonium atoms that fuel the reactor split, or “fission.” The composition of the fuel changes as the reactor operates, with the decays of different forms of uranium and plutonium (called “isotopes”) producing different numbers of antineutrinos with different energy ranges over time, even as the reactor steadily produces electrical power.
The new results from Daya Bay—where scientists have measured more than 2 million antineutrinos produced by six reactors during almost four years of operation—have led scientists to reconsider how the composition of the fuel changes over time and how many neutrinos come from each of the decay chains.
The scientists found that antineutrinos produced by nuclear reactions that result from the fission of uranium-235, a fissile isotope of uranium common in nuclear fuel, were inconsistent with predictions.
“The model predicts almost 8 percent more antineutrinos coming from decays of uranium-235 than what we have measured,” said Kam-Biu Luk, a Daya Bay Collaboration co-spokesperson who is a faculty senior scientist at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and a physics professor at UC Berkeley.