Quantitative Assessment of 131I in Soil Originating from the Fukushima Daiichi Nuclear Power Plant Disaster Using a Germanium Semiconductor Detector

In the accident at the Fukushima Daiichi Nuclear Power Plant (NPP) that accompanied the Great East Japan Earthquake on the 11th of March, 2011, large amounts of radionuclides were released into the atmosphere. Among these released radionuclides, there was an especially large amount of 131I and 137Cs. Two years have passed since the accident, and the decontamination of the deposited radionuclides has become a significant task. In the present study, a quantitative evaluation of the depth distribution of 131I and 137Cs in the soil using a high-purity germanium semiconductor detector was performed with soil samples collected at Adatara, Fukushima on April 30, 2011. Along with 131I and 137Cs, the concentration of radionuclides showed a maximum at a depth of 1 cm under the soil surface, and this attenuated to approximately 1/6 the concentration at the surface layer at a depth of 3 cm. In other words, the deposition of radionuclides was in a comparatively shallow portion of the soil. Additionally, soil surface samples were obtained from various observation sites, and a quantitative evaluation of the concentrations of radionuclides such as 131I was carried out. The results confirm a high concentration, more than 7000 Bq/kg, of 131I in samples collected at Nihonmatsu City on April 17, 2011 and samples collected on May 2, 2011 at Iwaki City. The concentration became higher as the distance from the NPP became smaller. However, the measurement values were variable also depending on the diameter of soil particle.