Analysis of changes over time in surface dose rates on contaminated concrete located along the coast

Abstract

  The accident at the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station resulted in the release of radioactive cesium (r-Cs) into the environment, contaminating concrete structures. Surface dose rates were measured on concrete columns at the former Fukushima Prefectural Aquafarming Research Institute, located along the Pacific coast, and were found to have decreased by half between December 2015 and January 2024. Concrete cores were collected, and immersed in pure water and artificial seawater for four months, and the surface radioactivities were evaluated using an imaging plate. There were no changes before and after immersion in pure water and stored in a plastic bag, but a 21% decrease was observed in artificial seawater immersion on average. The r-Cs reached the concrete in March 2011 is believed to have been through an ion exchange between rainwater and concrete and adsorbed onto the concrete. Although it is unlikely that r-Cs would leach from the concrete by exposure to rain with low salt concentrations, the concrete structures under evaluation are located facing the Pacific Ocean and may have been affected by sea salt, leading to leaching.