Changes in environmental radiation levels in Katsushika Ward, Tokyo after the Fukushima Daiichi Nuclear Power Plant accident

Abstract

Katsushika Ward, located in northeastern Tokyo, had the highest deposition in Tokyo of artificial radionuclides after the Fukushima Daiichi Nuclear Power Plant accident. A car-borne survey for measuring absorbed dose rate in air was carried out in the ward in each of the years 2015-2020. The average dose rates measured in 2015-2018 decreased every year but percentage reductions were smaller after 2018 due to the decrease in 134 Cs amount; this radionuclide decays with a half-life of 2.065 years. Its ecological half-life was estimated to be 1.6 y and that value was shorter than the physical decay life (3.2 y).