Fuel-Composition Dependent Reactor Antineutrino Yield at RENO

Abstract

We report a fuel-dependent reactor electron antineutrino (νe) yield using six 2.8 GW th reactors in the Hanbit nuclear power plant complex, Yonggwang, Korea. The analysis uses 850 666 νe 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 ± 0.23) × 10 −43 cm 2 /fission and measure a total average IBD yield of (5.84 ± 0.13) × 10 −43 cm 2 /fission. The hypothesis of no fuel-dependent IBD yield is ruled out at 6.6 σ. The observed IBD yield variation over 235 U isotope fraction does not show significant deviation from the Huber-Mueller (HM) prediction at 1.3 σ. The measured fuel-dependent variation determines IBD yields of (6.15 ± 0.19) × 10 −43 cm 2 /fission and (4.18 ± 0.26) × 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 σ correlation between the fractional change of the 5 MeV excess and the reactor fuel isotope fraction of 235 U.A definitive measurement of the smallest neutrino mixing angle θ 13 is a tremendous success in neutrino physics during the last decade [1,2]. The measurement has been achieved by comparing the observed ν e fluxes with detectors placed at two different distances from the reactors. As reactor ν e experiments suffer from large reactor related uncertainties of the expected ν e flux and energy spectrum [3][4][5][6][7], identical detector configuration is essential to cancel out the systematic uncertainties. The RAA, ∼6 % deficit of measured ν e flux compared to the HM prediction, is an intriguing mystery in current neutrino physics research and needs to be understood [4][5][6][8][9][10][11]. There have been numerous attempts to explain this anomaly by incorrect inputs to the fission β spectrum conversion, deficiencies in nuclear databases, underestimated uncertainties of reactor ν e model, and the existence of sterile neutrinos [3,[12][13][14][15][16][17][18][19]. Moreover, all of ongoing reactor ν e experiments have observed a 5 MeV excess in the IBD prompt spectrum with respect to the expected one [8,9,20,21]. This suggests that reactor ν e model is not complete at all.In commercial nuclear reactor power plants, almost all (> 99 %)ν e 's are produced through thousands of β-decay branches of fission fragments from 235 U, 239 Pu, 238 U, and 241 Pu. The ν e flux calculation is based on the inversion of spectra of the β-decay electrons of the thermal fissions which were measured in 1980s at ILL [10,11]. The reactor ν e models using these measurements as inputs have large uncertainties [5][6][7]. Therefore, ree...

Extraction of the U235 and Pu239Adey1, An2, Balantekin3 et al. 2019Phys. Rev. Lett.6861100View full textAdd to dashboardCite

“…The spectral measurements indicated a new anomaly ("5-MeV bump") when compared with theoretical calculations, an observation further confirmed by the NEOS Collaboration [18], and by reexamination of earlier reactor antineutrino data [19]. Observation of the evolution of the reactorν e spectrum from commercial reactors [20][21][22][23] and measurement of the 235 Uν e spectrum from highly enriched uranium research reactors [24,25] have also been performed, providing first glimpses at the dependence of spectral features on reactor fuel content. Interpretations of the reactorν e flux and spectrum anomalies reveal the complexes in the fission beta spectrum conversion and nuclear databases [26][27][28][29][30][31][32].…”

Reevaluating reactor antineutrino anomalies with updated flux predictions

“…The Rate Anomaly: We begin with combined analyses of the inverse beta decay (IBD) event rates measured at the short-baseline experiments at Bugey [35,36], Gösgen [37], ILL [38,39], Krasnoyarsk [40][41][42], Nucifer [43], Savannah River [44] and Rovno [45,46]. Additionally, we analyze Chooz [47], Double Chooz [34] and Palo Verde [48,49] at medium baselines, as well as fuel evolution results from Daya Bay [50] and RENO [51,52]. We highlight the salient features of our analysis here; see Ref.…”

Light sterile neutrinos: a critical overview

“…What is more, in Ref. [25], the RENO collaboration showed that the relative amplitude of the 5 MeV bump is correlated with the amount of 235 U in their nuclear reactor. These results again shed doubt on the validity of the theoretical flux calculations for this isotope.…”

“…Despite the evidences presented in the previous section, there are also experimental results that support flux miscalculations as the origin of the reactor antineutrino anomaly. In more detail, both the Daya Bay [24] and RENO [25] experiments were able to monitor the abundance of the two most common β-decaying isotopes in their nuclear reactors: 239 Pu and 235 U. Therefore, they could disentangle whether the observed flux deficit affects both isotopes in the same way -the expected result if it were due to sterile neutrino oscillations -or not.…”