Optimization of a water-intake system for monitoring radionuclides in aquatic environments

We designed and characterized a water-intake radiation monitoring system using a 3" × 3″ NaI(Tl) detector for real-time monitoring of, for example, nuclear power plant radioactive effluents. The MCNP6 code was used to determine the optimized container size and detector position for homogeneous 131 I and 137 Cs volumetric sources. Based on the simulation results, the dimensions of the monitoring system were determined. The simulation model was initially verified by comparison with the measurement results of a paper filter certified reference materials in water. Subsequently, it was further validated by comparison with the detection efficiency for 40 K in a KCl volumetric source measurement experiment, which confirmed that the detection efficiency well matched experimental data for volumetric sources. Using the validated simulation model, the detection efficiency of 131 I and 137 Cs volumetric sources in a water-filled container was assessed. To consider background radioactivity, three types of water samples—freshwater, brackish water, and seawater—were experimentally measured. Based on these measurements, the minimum detectable activity for 131 I and 137 Cs volumetric sources was determined for each of the three water sample types.