fission processes in the reactor core. These methods have rather strong dependence on theoretical models. Daya Bay has accumulated the world’s largest sample of reactor antineutrinos, at a rate of $\sim 1$ million per year. A direct measurement will provide the most precise and model independent reactor antineutrino spectrum.

In reactor cores, antineutrinos are emitted from four primary fuel isotopes: $^{235}$U, $^{238}$U, $^{239}$Pu, and $^{241}$Pu. Each fission releases about 200 MeV energy ((0.2-0.5)% uncertainty). Fission rates can be estimated with the thermal power measurement (0.5% uncertainty) and core simulation of the evolution of the fuel composition (0.6% uncertainty with the constraint of the total thermal power) [@cjflux]. The most uncertain part is the rate and spectrum of antineutrinos emitted from each fission of the four isotopes. By fitting the measured $\beta$ spectrum of the $^{235}$U, $^{239}$Pu, and $^{241}$Pu fuel [@illschr; @illvonf; @illhahn] with hypothesized virtual $\beta$ decays, plus theoretical calculation for $^{238}$U, two models, the ILL-Vogel and the Huber-Mueller models, have been developed [@vogel238; @mueller; @huber] with an uncertainty of (2-3)%.

The latest Daya Bay measurement on the absolute reactor antineutrino rate is $\sigma_f=(5.92\pm0.14)\times 10^{-43} {\rm cm}^2\, {\rm fission}^{-1}$, where the dominant uncertainty comes from the absolute efficiency (2.1%) [@DB_reactor]. 