Evaluation of the effect of radioactivity to the environment in wooden houses within the evacuation area at Fukushima.

The accident at Fukushima Daiichi Nuclear Power Plant caused radioactive materials to spread outside the plant. To limit exposure in the area, air dose rates have been measured. When the dose rate exceeded the acceptable upper bound, the area was decontaminated by stripping the soil and washing roads. Immediately after the accident, it was not clear how much of an effect outdoor contamination had on dose rates inside houses. This paper discusses the effect of outdoor contamination on indoor air dose rates and factors influencing those dose rates based upon actual measurements taken of a wooden house standing on flat land in the area around the power plant. In addition, the effects of indoor horizontal distribution, height distribution, and decontamination are discussed using simple model simulations. As a result, the following was found, although within the range of air dose rates and location conditions measured in this study. Horizontal variation in indoor air dose rates decreases monotonically toward the centre of the house. Once decontamination has been performed, the overall dose rate decreases and variation becomes smaller. This trend is thought to be mainly due to expansion of a low-dose area. Vertical changes in indoor air dose rates are not straightforward. It is estimated that the dose rate decreases near the ground surface, then tends to increase going higher and finally decreases even higher up. This change in dose rates is thought to be due to the effect of radioactive materials permeating the soil. Recognition of the effects of these factors is important when estimating indoor air dose rates because indoor air dose rates are affected by the degree of decontamination, house size, and degree of penetration of radionuclides into the soil.

Measuring individual external doses of Tokyo Electric Power Company Holdings employees living in Fukushima prefecture.

Since the accident at Fukushima Daiichi Nuclear Power Plant, individual external doses of residents have been investigated. To accurately analyse survey data, a variety of information, including the activity patterns of many residents, needs to be integrated. However, such large-scale surveys have not yet been conducted and actual individual external doses in Fukushima are unclear. In this study, the individual external doses of approximately 300 Tokyo Electric Power Company Holdings employees, who live and work in Fukushima Prefecture outside the Fukushima Daiichi Nuclear Power Plant, were measured. The employees carried GPS loggers and personal dosimeters capable of measuring dose in counts per minute. The employees' individual external doses were compared along with their activity patterns. It was found that the annual additional individual external dose estimated based upon actual measurements was 1 mSv or less, and the influence on the individual external dose was also revealed.