Validation of in situ underwater radiation monitoring detector

Following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, 137 Cs released into the environment accumulated in the underwater sediment in Fukushima Prefecture due to the high affinity for soil minerals. This study aimed to validate the performance of the MARK-U1 (Monitoring of Ambient Radiation of KAERI - Underwater), an in situ underwater radiation detector, for estimating the 137 Cs activity concentration in river and reservoir sediments at predicted contamination sites. Additionally, underwater core samples were collected to measure radioactivity using a high-purity germanium (HPGe) detector. For rapid estimation of radioactivity during on-site survey, a conversion factor was derived by comparing the measured gamma-ray energy spectrum with the 137 Cs activity in the samples. A Monte Carlo N-Particle (MCNP) simulation was performed to determine the effective source geometry for in situ measurements. The simulation results correlated well with the on-site MARK-U1 monitoring results, with a deviation of 13.1%. These findings validate the performance of the in situ detector, demonstrating that the MARK-U1 can effectively estimate 137 Cs activity concentration in the underwater sediment through on-site monitoring, without the need for sample collection.