As nuclear fuel ages, an antineutrino anomaly changes

For several years now, detectors at three experiments—Daya Bay in China, RENO in South Korea, and Double Chooz in France—have been measuring the flux of electron antineutrinos emanating from nearby nuclear reactors. The experiments’ main purpose has been to help quantify the oscillations of neutrinos from one flavor to another, but along the way their precise measurements have sharpened a mystery that has troubled particle physicists and reactor physicists alike: The overall antineutrino flux is about 6% less than theory predicts. One explanation involves a “sterile” neutrino with a mass of about 1 eV that, unlike the three standard neutrinos, is not subject to the weak force that governs nuclear decay.

Now the Daya Bay group has analyzed 1230 days of data and identified a new factor that influences the antineutrino flux: the evolving isotopic composition of the nuclear fuel in the cores of the reactors at the Daya Bay and Ling Ao nuclear power plants. (The image shows the inside of one of the group’s detectors.) The new results contradict the notion that sterile neutrinos are the source of the antineutrino flux anomaly.

Uranium-235 and plutonium-239 are the key components of the fuel at the Daya Bay and Ling Ao power plants. But over the course of the reactors’ typically 18-month-long fuel cycles, the relative amount of ²³⁹Pu increases. By monitoring how the antineutrino flux evolves during the fuel cycles, the Daya Bay researchers determined the rate of change of flux with changing ²³⁹Pu fraction. They also teased out the individual contributions of ²³⁹Pu and ²³⁵U to the antineutrino flux.

In the low-mass sterile-neutrino picture, the shortfall in antineutrino flux is independent of the isotopic mix that generates the antineutrinos. Therefore, the statistically significant evolution of flux measured over the course of a reactor cycle is inconsistent with such a neutrino. Moreover, when the researchers compared their reactor-physics model with the measured antineutrino fluxes from the two types of nuclear fuel, they found a good fit for the ²³⁹Pu and a poor fit for the ²³⁵U. They concluded that an improper understanding of the ²³⁵U contribution to the antineutrino flux is the likely cause of the flux anomaly. (F. P. An et al., Phys. Rev. Lett., in press, doi:10.1103/PhysRevLett.118.251801 .)