Effective Half-life of 134Cs and 137Cs in Fukushima Prefecture When Compared to Theoretical Decay Models

On 11 March 2011, a 9.0 magnitude earthquake struck the Tohoku region of Japan. The earthquake caused a 15 m tsunami that bombarded the east coast of the island nation. Among the losses was the damage to the Fukushima Daiichi nuclear reactor that lost onsite power and was unable to cool the reactor cores. The reactors melted down and released a plume of radionuclides into the environment. Radiocesiums (Cs and Cs) are the long-lived radionuclides of concern that were deposited along the plume and were distributed on the soil. Radiological decay models are typically used to determine the reduction in external radiation dose over time. However, these radiological decay models do not take into account physical removal by wind and water erosion, or sedimentation in soil outside expected depths. Thirty-five fixed dose-rate monitors were used to record dose rates at 1 mo intervals from the time of installation in Fukushima Prefecture in April 2012 until December 2018 and were used to estimate the effective half-life for radiocesium contamination based on external radiation dose rates. The effective half-life of cesium in the environment was calculated to be 3.2 ± 0.5 y, compared to a theoretical half-life of 7.8 y.