Research on spatial distribution principle of underwater Čerenkov light field outside the floating power plant’s reactor compartment shell in two types of marine condition based on Geant4 modeling

Based on Geant4 modeling, we simulated the underwater Čerenkov light field in open seawater and coastal seawater, which was excited by different kinds of γ-rays. The γ-rays were originally released by fission products and activation products in the reactor coolant. The physical characteristics of the underwater Čerenkov light field were mathematically analyzed, including the spatial distribution principle, the correlation between the energy density and elevation angle, the energy density functions of Čerenkov photons, and the Čerenkov photon spectrum. Compared with other published research results, it was proved that the Čerenkov photons are in the range of 200–810 nm in both seawater types, but the peak wavelength in these seawater types is 420 nm (open seawater) and 570 nm (coastal seawater), and the strong absorption and scattering of suspended particles caused large attenuation in the ultraviolet range and infrared range. Čerenkov photons were symmetrically distributed along the emitting direction of γ-rays, and the maximum energy density of Čerenkov photons appeared at the center of the emission direction of γ-rays. The Čerenkov halo might only be observed in clear open seawater. The research results also show us that in the following field experiment, to collect the Čerenkov photons in water, deploying underwater photo-electric detection devices outside the reactor compartment of the floating nuclear power plants would be an effective way to locate the suspected coolant leakage position and leakage rate.