Fuel-composition dependent reactor antineutrino yield at RENO

Abstract

We report a fuel-dependent reactor electron antineutrino (${\overline{\nu }}_e$) yield using six 2.8 GW${}_{\text{th}}$ reactors in the Hanbit nuclear power plant complex, Yonggwang, Korea. The analysis uses $850666$ ${\overline{\nu }}_e$ candidate events with a background fraction of 2.0 % acquired through inverse beta decay (IBD) interactions in the near detector for 1807.9 live days from August 2011 to February 2018. Based on multiple fuel cycles, we observe a fuel ${}^{235}$U dependent variation of measured IBD yields with a slope of $(1.51\pm 0.23)\times 10^{-43}$cm${}^2$/fission and measure a total average IBD yield of $(5.84\pm 0.13)\times 10^{-43}$cm${}^2$/fission. The hypothesis of no fuel-dependent IBD yield is ruled out at 6.6 $\sigma$. The observed IBD yield variation over ${}^{235}$U isotope fraction does not show significant deviation from the Huber-Mueller (HM) prediction at 1.3 $\sigma$. The measured fuel-dependent variation determines IBD yields of $(6.15\pm 0.19)\times 10^{-43}$cm${}^2$/fission and $(4.18\pm 0.26)\times 10^{-43}$cm${}^2$/fission for two dominant fuel isotopes ${}^{235}$U and ${}^{239}$Pu, respectively. The measured IBD yield per ${}^{235}$U fission shows the largest deficit relative to the HM prediction. Reevaluation of the ${}^{235}$U IBD yield per fission may mostly solve the Reactor Antineutrino Anomaly (RAA) while ${}^{239}$Pu is not completely ruled out as a possible contributor of the anomaly. We also report a 2.9 $\sigma$ correlation between the fractional change of the 5 MeV excess and the reactor fuel isotope fraction of ${}^{235}$U.

6 pages and 5 figures, v4 note: (1) data files for figure 1, figure 2, figure 5 are included for arXiv users (2) minor modifications in abstract and main text for clarification

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