of performance among the ADs at the same site can actually “measure" the relative uncertainty between them. By comparing the components of the detector responses, the relative uncertainty of two ADs was found to be 0.2% [@ad12]. The measured ratio of the antineutrino rates in two ADs was $0.981\pm0.004~(1.019\pm0.004)$ while the expected ratio was 0.982 (1.012) for the Daya Bay (Ling Ao) near site [@DB_2015], validating the uncertainty estimation. The deviation from unity is due to slightly different baselines of the two ADs to the reactor cores. Furthermore, the uncertainties of the ADs are found to be largely uncorrelated. Therefore, the total relative detector uncertainty is statistically reduced by $1/\sqrt{N}$, where $N$ is the number of ADs at a given site.

The water pool is divided into two water Cherenkov detectors, as shown in Fig. \[fig:det\]. The outer one is 1 m thick and the inner one is 1.5 m. The top of the water pool is covered by RPC tiles. The outer Cherenkov counter and the RPC play important roles in determining the fast neutron background originating from muon spallation in the surrounding rock. The efficiency of the water Cherenkov detectors for detecting muons was designed to be 95%, 