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[AGE28-10] Evaluation of dose rate reduction in the riverside park in Fukushima by using walk survey measurement
★Invited Papers
Keywords:air dose rate, natural attenuation, spatial analysis
The Fukushima Dai-ichi nuclear power plant accident in 2011 released a vast amount of radionuclides, especially radiocesium, into the environment. The air dose rate in the riverside area tends to be higher than that in the surrounding residential area due to fewer decontamination works and the accumulation of radiocesium with sediment from upstream forested areas. On the other hand, natural attenuation processes, such as the translocation of radiocesium with water flow, have a more substantial impact on the riversides. Therefore, estimating accumulation and attenuation effects should be considered for appropriate riverside management.
To evaluate the temporal evolution of the air dose rate in the riversides, we measured the air dose rate in riverside parks in Minamisoma City using a walk survey meter with a GPS receiver from 2015 to 2020. We constructed the air dose rate map using the ordinally kriging. Additionally, we sampled surface soil and sediments in 2015 and 2018 to obtain the depth profile of radiocesium concentration in the soil. Finally, we modeled three attenuation processes of the air dose rate: physical decay of radiocesium, vertical migration of cesium to the deeper soil layer, and accidental reduction due to extreme typhoons and remediation works.
In the initial measurement in 2015, the air dose rate in the riverside park was higher along the Niida River mainstream. The riverside area showed a four times larger radiocesium inventory than the lawn areas that were not flooded, clearly indicating radiocesium accumulation. After typhoons and remediation works, the air dose rate was greatly reduced in the entire area. Focusing on each accidental event, after two typhoons—Etau in 2015 and Hagibis in 2019—the riverside areas showed a more reduction in the dose rate, while after the remediation works, the dose rate at the center of the park strongly reduced.
From 2015 to 2020, the contribution of the reduction in the air dose rate by the three processes was estimated as follows: 35% by physical decay, 14% by vertical migration, and 51% by the typhoons and remediation works. Strong typhoons effectively reduced the air dose rate in the studied riverside park.
To evaluate the temporal evolution of the air dose rate in the riversides, we measured the air dose rate in riverside parks in Minamisoma City using a walk survey meter with a GPS receiver from 2015 to 2020. We constructed the air dose rate map using the ordinally kriging. Additionally, we sampled surface soil and sediments in 2015 and 2018 to obtain the depth profile of radiocesium concentration in the soil. Finally, we modeled three attenuation processes of the air dose rate: physical decay of radiocesium, vertical migration of cesium to the deeper soil layer, and accidental reduction due to extreme typhoons and remediation works.
In the initial measurement in 2015, the air dose rate in the riverside park was higher along the Niida River mainstream. The riverside area showed a four times larger radiocesium inventory than the lawn areas that were not flooded, clearly indicating radiocesium accumulation. After typhoons and remediation works, the air dose rate was greatly reduced in the entire area. Focusing on each accidental event, after two typhoons—Etau in 2015 and Hagibis in 2019—the riverside areas showed a more reduction in the dose rate, while after the remediation works, the dose rate at the center of the park strongly reduced.
From 2015 to 2020, the contribution of the reduction in the air dose rate by the three processes was estimated as follows: 35% by physical decay, 14% by vertical migration, and 51% by the typhoons and remediation works. Strong typhoons effectively reduced the air dose rate in the studied riverside park.