Fuel-Composition Dependent Reactor Antineutrino Yield at RENO

We report a fuel-dependent reactor electron antineutrino (${\overline{\nu }}_e$) yield using six $2.8{\mathrm{GW}}_{\mathrm{th}}$ reactors in the Hanbit nuclear power plant complex, Yonggwang, Korea. The analysis uses 850 666 ${\overline{\nu }}_e$ 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 ${}^{235}\mathrm{U}$ dependent variation of measured IBD yields with a slope of $(1.51\pm 0.23)\times {10}^{-43}{\mathrm{cm}}^2/\mathrm{fission}$ and measure a total average IBD yield of $(5.84\pm 0.13)\times {10}^{-43}{\mathrm{cm}}^2/\mathrm{fission}$. The hypothesis of no fuel-dependent IBD yield is ruled out at $6.6\sigma$. The observed IBD yield variation over ${}^{235}\mathrm{U}$ 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 $(6.15\pm 0.19)\times {10}^{-43}$ and $(4.18\pm 0.26)\times {10}^{-43}{\mathrm{cm}}^2/\mathrm{fission}$ for two dominant fuel isotopes ${}^{235}\mathrm{U}$ and ${}^{239}\mathrm{Pu}$, respectively. The measured IBD yield per ${}^{235}\mathrm{U}$ fission shows the largest deficit relative to the HM prediction. Reevaluation of the ${}^{235}\mathrm{U}$ IBD yield per fission may mostly solve the reactor antineutrino anomaly (RAA) while ${}^{239}\mathrm{Pu}$ 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 ${}^{235}\mathrm{U}$.

Figure 1

Top: Effective ${}^{235}\mathrm{U}$ daily fission fraction ($F_{235}$) in the near detector according to Eq. (1). The daily $F_{235}$ is obtained from the reactor information provided by the Hanbit nuclear power plant. Bottom: Relative fission fractions for the primary fuel isotopes of ${}^{235}\mathrm{U}$, ${}^{239}\mathrm{Pu}$, ${}^{238}\mathrm{U}$, and ${}^{241}\mathrm{Pu}$. The numbers in the parentheses represent eight data groups with different fission fractions [24].

Figure 2

IBD yield per fission ${\overline{y}}_f$ as a function of the ${}^{235}\mathrm{U}$ effective fission fraction. The measured values (black dots) are compared to the scaled HM prediction (blue dotted line) and the best fit of the data (red solid line). The value of ${\overline{F}}_{235}$ for each data point is calculated as an average of fission fractions weighted by thermal power and a distance between reactor and detector. The error of ${\overline{F}}_{235}$ indicates the variation of ${}^{235}\mathrm{U}$ fission fraction. Errors of ${\overline{y}}_f$ are statistical uncertainties only [24].

Figure 3

Combined measurement of $y_{235}$ and $y_{239}$. The shaded contours are allowed regions and the dot is the best fit. The cross shows the prediction of the HM. The top and right side panels show one dimensional $\mathrm{\Delta }{\chi }^2$ profile distributions for $y_{235}$ and $y_{239}$ while the gray shaded bands represent the model predictions.

Figure 4

Allowed regions of the IBD yield per fission for the six pairs of fission isotopes. The dots indicate the best-fit IBD yields and the cross lines represent the model prediction. The three contours are allowed regions of 68.3, 95.5, and 99.7 % C.L.

Figure 5

Fraction of the 5 MeV excess as a function of ${\overline{F}}_{235}$. The red line is the best fit to the data and the dotted line represents no correlation of 5 MeV excess fraction with ${\overline{F}}_{235}$ [24].