This study presents a comprehensive simulation-based investigation into the detection and directional localization of reactor antineutrinos (ν_e) in deep-sea environments using a water Cherenkov detector. The work addresses the significant challenges posed by the low energy of reactor antineutrinos (2-10 MeV), their tiny weak-interaction cross-sections, and the presence of intense natural radioactivity in seawater. A cylindrical detector (10 m diameter × 10 m height) filled with pure or gadoliniumdoped water and instrumented with 8inch photomultiplier tubes (PMTs) spaced 40 cm apart is modeled within the WCSim/Geant4 framework. The simulation fully reconstructs both elastic scattering (ν_ee^-) and inversebetadecay (IBD, ν_ep → e⁺n) events, accounting for Cherenkovphoton production, propagation, and PMT response.
Vertex reconstruction is performed by maximizing a likelihood based on time residuals, while the direction of the recoil electron is extracted via a Houghtransform technique that identifies the Cherenkov cone axis. To suppress the dominant IBD background, which lacks directional correlation with the incident neutrino, a coincidencebased identification strategy is developed: a primary signal satisfying a PMTmultiplicity threshold (M_pmt > 20) is followed by a secondary neutroncapture signal within a 0.5 ms (pure water) or 0.2 ms (Gddoped water) window. The neutroncapture signal is discriminated from the ubiquitous ⁴⁰K background by applying an optimal PMTmultiplicity cut (M_pmt ≈ 5). With 0.06% natural Gd doping, the IBD identification efficiency rises to ~72%, nearly doubling that of pure water. Furthermore, a novel correlationbased purification method is introduced to enhance the signaltobackground ratio during source reconstruction. By pairing events and assigning positive weights to smallangle pairs (cosine > 0.75) and negative weights to largeangle pairs, the background is effectively suppressed without sacrificing signal statistics, reducing the required number of detector modules by about onesixth for the same localization confidence.
Simulation results show that the reconstruction efficiency for reactorspectrum antineutrinos (2-10 MeV) is 2.65%, with vertex and angular resolutions suitable for directional analysis. Under realistic seawaterradioactivity conditions, the expected elasticscattering rate from a 1 GW reactor at 1 km distance is 0.35 counts/day per module, while the total radioactive background (dominated by ²⁰⁸Tl and ²¹⁴Bi) amounts to ~5 counts/day per module. With an array of 100 identical modules, the cumulative signal reaches 35 counts/day, yielding a backgroundtosignal ratio of 13. Applying the correlationbased purification and Houghtransform source reconstruction, the array can localize the reactor direction with > 90% confidence (angular cosine > 0.8) within one day. The study demonstrates the feasibility of deepsea water Cherenkov detectors for kilometerscale monitoring of reactor antineutrinos and outlines a path toward extending the monitoring range to 10 km through further optimization of detector parameters and array size.