Relationship between Individual External Doses, Ambient Dose Rates and Individuals' Activity-Patterns in Affected Areas in Fukushima following the Fukushima Daiichi Nuclear Power Plant Accident.

Wataru Naito,Chie Yamada,Motoki Uesaka,Tetsuo Yasutaka,Hideki Ishii,Tadahiro Kurosawa

doi:10.1371/journal.pone.0158879

Abstract

The accident at Fukushima Daiichi Nuclear Power Plant on March 11, 2011, released radioactive material into the atmosphere and contaminated the land in Fukushima and several neighboring prefectures. Five years after the nuclear disaster, the radiation levels have greatly decreased due to physical decay, weathering, and decontamination operations in Fukushima. The populations of 12 communities were forced to evacuate after the accident; as of March 2016, the evacuation order has been lifted in only a limited area, and permanent habitation is still prohibited in most of the areas. In order for the government to lift the evacuation order and for individuals to return to their original residential areas, it is important to assess current and future realistic individual external doses. Here, we used personal dosimeters along with the Global Positioning System and Geographic Information System to understand realistic individual external doses and to relate individual external doses, ambient doses, and activity-patterns of individuals in the affected areas in Fukushima. The results showed that the additional individual external doses were well correlated to the additional ambient doses based on the airborne monitoring survey. The results of linear regression analysis suggested that the additional individual external doses were on average about one-fifth that of the additional ambient doses. The reduction factors, which are defined as the ratios of the additional individual external doses to the additional ambient doses, were calculated to be on average 0.14 and 0.32 for time spent at home and outdoors, respectively. Analysis of the contribution of various activity patterns to the total individual external dose demonstrated good agreement with the average fraction of time spent daily in each activity, but the contribution due to being outdoors varied widely. These results are a valuable contribution to understanding realistic individual external doses and the corresponding airborne monitoring-based ambient doses and time-activity patterns of individuals. Moreover, the results provide important information for predicting future cumulative doses after the return of residents to evacuation order areas in Fukushima.

Highlights

The accident at the Tokyo Electric Power Company (TEPCO) Fukushima Dai-ichi Nuclear Power Plant (F1NPP) in March 2011 discharged radionuclides into the environment and contaminated large areas of land in Japan
Of the 12 municipalities in which the entire or part of the population was forced to evacuate after the F1NPP accident [1], as of March 2016 the evacuation order has been lifted on only three municipalities (i.e., Tamura City, Kawauchi Village, and Naraha Town) and residents are not permitted to live permanently in most of the areas affected by the accident
Time spent outdoors contributed on average approximately 20% of the total individual external dose, but for some farmers it contributed more than 70% of the total exposure due to the farmer spending long periods outdoors in fields with relatively high ambient doses. These results suggest that quantification of individual external doses obtained while at home is essential for examining dose reduction measures for individuals, and that detailed monitoring data are required to understand the contribution of outdoor activities to the total individual external dose

Summary

Introduction

The accident at the Tokyo Electric Power Company (TEPCO) Fukushima Dai-ichi Nuclear Power Plant (F1NPP) in March 2011 discharged radionuclides into the environment and contaminated large areas of land in Japan. The additional individual external dose is calculated by assuming that the background radiation, i.e., the national average exposure due to natural sources of radiation, is 0.04μSv h-1, that individuals spend 16 hours indoors and 8 hours outdoors each day, and that the shielding effect of a wooden house is 0.4 times [3]. To express it the additional individual external dose is calculated by the cumulative ambient dose multiplied by a reduction factor of 0.6. In July 2014, the Ministry of the Environment proposed a new policy that will determine decontamination needs by using radiation exposure data collected using personal dosimeters [8]

Methods

Results

Conclusion