Choi, Y.; Ji, Y.-Y.; Joung, S., and Min, B.-I., 2023. Development of a portable underwater in-situ radiation detector (MARK-U4) for marine radioactivity monitoring. In: Lee, J.L.; Lee, H.; Min, B.I.; Chang, J.-I.; Cho, G.T.; Yoon, J.S., and Lee, J. (eds.) Multidisciplinary Approaches to Coastal and Marine Management. Journal of Coastal Research, Special Issue No. 116, pp. 176-180. Charlotte (North Carolina), ISSN 0749-0208.

The issue of the discharge of contaminated water became controversial after the Fukushima accident, and the necessity of monitoring radioactive contamination in the East Sea emerged. Existing methods for analyzing radioactive contamination in seawater have mainly focused involved laboratory analysis of collected samples. However, this is not suitable for screening a wide area and does not allow rapid detection because it takes a long time to confirm the analysis results, despite their high accuracy. In this study, a portable underwater radioactivity detector capable of in-situ measurement that can be used for radioactive contamination screening was developed and its performance and field applicability were verified. This detector, called the MARK-U4, is part of the Monitoring system series of Ambient Radiation of the Korea Atomic Energy Research Institute (MARK); it is a 2 L NaI detector in a cylindrical case, and is designed for use underwater via under-ship installation. Through a waterproof cable, measurement information can be transmitted to a PC in real time for confirmation; GPS location information can be collected at the same time, enabling on-site screening. To build an algorithm to analyze radioactivity and dose rate from the measurement results of this detector, an analysis of the detector characteristics was performed using Monte Carlo simulation. As a result of the simulation, the effective radius in water was calculated and found to be approximately 90 cm. Based on the effective radius, the radioactivity conversion coefficient for each radionuclide and the function of the absorbed dose rate conversion factor (G-factor) of the detector were calculated. The field applicability of the detector was verified through an actual measurement experiment using the MARK-U4 installed under a moving ship in both the East Sea of Busan and Lake Daecheong.