Fuel-Composition Dependent Reactor Antineutrino Yield at RENO

Abstract

We report a fuel-dependent reactor electron antineutrino ((nu) over bar (e)) yield using six 2.8 GW(th) reactors in the Hanbit nuclear power plant complex, Yonggwang, Korea. The analysis uses 850 666 (nu) over bar (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 U-235 dependent variation of measured IBD yields with a slope of (1.51 +/- 0.23) x 10(-43) cm(2)/fission and measure a total average 1BD yield of (5.84 +/- 0.13) x 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 U-235 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 +/- 0.19) x 10(-43) and (4.18 +/- 0.26) x 10(-43) cm(2)/fission for two dominant fuel isotopes U-235 and Pu-239, respectively. The measured IBD yield per U-235 fission shows the largest deficit relative to the HM prediction. Reevaluation of the U-235 IBD yield per fission may mostly solve the reactor antineutrino anomaly (RAA) while (PU)-P-239 is not completely ruled out as a possible contributor to 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 U-235.