Reactor Flux at Daya Bay Liang Zhan, Institute of High Energy Physics 1st Workshop on Reactor Neutrino Experiments, Seoul, Oct. 16-18, 2016

Outline Focus on oscillation-related flux prediction instead of absolute measurement. Reactor flux prediction Power Fission fraction (core simulation) Spent fuel/non-equilibrium Energy per fission and IBD reaction per fission (correlated) Error propagation and contribution in total uncertainty Predicting expected flux/spectrum for each detector and comparison with data. Correlated: completely canceled Uncorrelated uncertainty ~0.9%, reduced by a factor of 20 in the near and far relative measurement.

Neutrino Flux Calculation J.Cao, Neutrino2010 E : Neutrino energy fi : Fission rate of isotope i Si(E) : Neutrino energy spectra/f Neutrino Flux (fi /F): Fission fraction Wth : Reactor thermal power ei : Energy release per fission Heat balance test Online calibration Thermal Power Wth Core configuration Thermal power Operations Temperature pressure … … Energy release/fission Core Simulation fi/F Flux Spent fuel Non-equilibrium Spectra of Isotopes Si(E) Measurements Calculations

Daya Bay Thermal Power J.Cao, Neutrino2010 KME, thermal power, Secondary Heat Balance Method. The most accurate measurement. Offline measurement, weekly or monthly Generally cited with (0.6-0.7)% uncertainties in literature. KIT/KDO, thermal power. Good for analysis. Primary Heat Balance Online Weekly calibrated to KME power. RPN, nuclear power Ex-core neutron flux monitoring Safety and reactor operation control Daily calibrated to KIT/KDO power

Power Uncertainties Chooz 0.6%, Palo Verde 0.7%. J.Cao, Neutrino2010 Chooz 0.6%, Palo Verde 0.7%. Uncertainties of secondary heat balance is dominated by the flow rate. Venturi flow meter. Most US reactors. Uncertainty is often 1.4%. It can be as low as 0.7% if properly calibrated and maintained, but suffering from fouling effects, which could grow as high as 3% in a few years. (RENO reactor?) Orifice plate. French EDF reactors. Typically 0.72%. No fouling effects. Could be improved to 0.4% with lab tests. Note: Above flow meter uncertainties are at 95% C.L. as defined in ISO 5167. Unless specified, the thermal power uncertainty given by the power plant is also at 95% C.L. Ultrasonic. Start to use in some US and Japan reactors. Type I TT 0.45%, Type II TT 0.2% (Djurcic et al.)

An example J.Cao, Neutrino2010 EPRI document prepared by EDF, Improving Pressurized Water Reactor Performance Through Instrumentation:…… (2006) For EPR reactor (Chooz type) with 4 steam generators: Empirical formula and uncertainty specified in ISO 5167-1-2003. Correlated or Uncorrelated for the 4 flow meters? Orifice Plate

Daya Bay Power Use lived time weighted power; however, the maximum daily difference from the simple average < 0.1% All 6 reactor cores are the same type (M310) in the nuclear island. Each core has 3 steam generators  3 orifice plate flow meters To be conservative (valid for near-far relative analysis), we assume that the 3 flow meters in the same core are correlated, while flow meters in different cores are uncorrelated  yield the largest uncertainty in all possible assumptions. We take 0.5% uncertainty for a single core (which is by chance similar to the power plant number 0.48%, where they take 95% CL but assume that the 3 flow meters are uncorrelated)

Core Simulation Fission fractions, as a function of burn-up, is a by-product of the refueling calculation, provided by the power plant. Daya Bay: SCIENCE/APOLLO code Meanwhile, Daya Bay did standalone simulations with DRAGON Two Daya Bay reactors replaces 1/3 fuel elements every 18 months Four Ling Ao reactors replaces 1/4 fuel elements every 12 months. One analysis of Apollo 2.5 Fission fraction uncertainty ~ 5% Fission fraction vs. reactor burn-up in one cycle

Correlation among fuels Standalone DRAGON simulation developed in Daya Bay yields similar fission fraction uncertainty as Apollo2 validation (arXiv:1405.6807) The 5% fission fraction uncertainty yield 0.6% uncorrelated uncertainty with the constraint of the total thermal power and the correlation among fuels. The correlation is evaluated with DRAGON by adding perturbation to the normal core simulation. Fission fraction uncertainty is conservatively assumed to be uncorrelated between reactors in the oscillation analysis via relative measurement.

Energy Release per Fission Isotopes Energy (MeV) U-235 201.7±0.6 U-238 205.0±0.9 Pu-239 210.0±0.9 Pu-241 212.4±1.0 Kopeikin et al, Physics of Atomic Nuclei, Vol. 67, No. 10, 1892 (2004) M.F. James, J. Nucl. Energy 23, 517 (1969) 1. using updated nuclear databases 2. considering the production yields of fission fragments from both thermal and fast incident neutrons 3.updated calculation of the average energy taken away by antineutrinos. DYB number: X. B. Ma et al., Phys.Rev. C88, 014605 (2013).

Non-equilibrium Isotopes and spent nuclear fuel ILL spectra are derived after 1.5 days exposure time. Long-lived fission fragments have not reached equilibrium. Contribute only to low energy region. These long-lived fission fragments will be accumulated in the reactors and produce additional antineutrinos. Six chains have been identified, with half lives from 10h to 28y. (Kopeikin et al.) Contributions to IBD events SNF ~ 0.3% Non-equilibrium ~ 0.6% 90Sr 90Y Fission 89Sr neutron capture 89Y neutron capture neutron capture 28.78y 0.546MeV 64.1h 2.284MeV

Error Cancellation with N/F detector To cancel the reactor uncorrelated uncertainty, two strategies can be used. Strategy 1: the near and far detector detect the identical fraction of flux from each reactor by ideal geometry placement (iso-flux) Double Chooz uses this strategry Strategy 2: use N+1 detectors for N reactors. For any geometry placement, a combination of N near detectors can yield identical fraction of flux from N reactors (PhysRevD.73.053008 ) Daya Bay uses this strategy with two near halls and one far hall for two groups of reactors 2 reactors + 3 detectors scheme No requirement on the symmetry of geometry placement by one additional detector Iso-flux: Lf12/Lf22 = Ln12/Ln22 R1 R2 ND FD Lf1 Lf2 Ln1 Ln2 R1 R2 ND1 FD ND2

Error Cancellation for Daya Bay Double Chooz near detector is not at the ideal location, and a residual of 10% uncorrelated uncertainty remains. Daya Bay is also not a ideal 2-reactor + 3-detector experiments. A residual of 5% uncorrelated uncertainty remains. A combination of EH1 and EH2 to predict the flux at far hall Fraction of flux from DYB and Ling Ao Weighting determined by Considering 8 detectors + 6 reactors, we get the residual uncertainty vs. the weighting of EH2 to EH1 0.05

Predicting for each detector Two methods Detector Full Monte Carlo simulation (with power, spectrum, neutrino direction, etc.) Analytical calculation of core flux, and convert to detector spectrum with detector response matrix The energy model is covered in energy response talk. Response matrix includes effects of energy leakage, non-linearity, and energy resolution

Comparison of predicted and measured reactor rate Global fit of data/prediction Measured IBD yield at 8 ADs

Comparison of antineutrino spectrum Obvious bump in 4-6 MeV Statistical evaluation of the significance of the whole spectrum yields 2.9 σ deviation Significance in the energy window of 4-6 MeV is 4.4 σ

Investigation of the sources of bump The rate of the excess events in the bump region is ~1.5% Reject the detector response issue by studies of the beta decay spectra of natural radioactivities. A beta decay branch of a mono-energetic peak can not reproduce the bump. The events in the bump region have the same characteristic as the other region 4.5-5.5 MeV event rate is proportional to other region. The neutron capture time The delayed energy spectrum The vertex distribution

Summary Reactor flux prediction and the uncertainties are reviewed Rate and spectrum discrepancies between data and predicted was observed in Daya Bay