The Japanese government plans to conduct decontamination tasks in radioactively contaminated areas. For such a situation, we developed a system that evaluates radiation dose rates in a wide radioactively contaminated area by utilizing our radiation dose evaluation technology. This system can not only generate present maps of radiation dose rate in the air based on the dose rate measured at the surface of the contaminated areas, but can also quickly calculate the reduction effect of dose rate due to decontamination tasks by entering decontamination factors. The system can then formulate decontamination plans and make it possible to plan measures to reduce radiation exposure for workers and local residents. Radioactive nuclides that contribute to gamma-ray dose rate are mainly Cs-134 and Cs-137 in soil, on trees, buildings, and elsewhere. Shapes of such radiation sources are assumed to be 10m square or 100m square. If it is unsuitable that the radiation sources assume to squares, the radiation sources can assume to point. The relation between distance from the surface or point source and the radiation dose rate is calculated using MCNP5 code (A General Monte Carlo N-Particle Transport Code - Version 5), and approximated using four-parameter empirical formula proposed by Harima et al. In addition, the system can consider shielding such as soil, concrete, and iron. When setting such shielding, the skyshine dose rate is taken into account in dose rate calculation.

Hitachi-GE Nuclear Energy, Ltd. has developed the Dose rate meter called "D-phod (Detector for photon dose)" using fiber optics as a radiation sensor to figure out the dose rate of Fukushima area which is contaminated by radioactive materials. D-phod is a tool that can rapidly measure linear or curved dose rate up to 20m along fiber optics. Element technology has already established, however, existing measurement system is unfit to measure environmental dose rate. Therefore, we improved it to be able to be used as a survey meter and completed D-phod. It enables to measure a wide range of environment reasonably. In this paper, we report the development of D-phod which solved the problems mentioned above and can be put into practical use.

The Japanese government plans to conduct decontamination tasks in radioactively contaminated areas. For such a situation, we developed a system that evaluates radiation dose rates in a wide radioactively contaminated area by utilizing our radiation dose evaluation technology. We also applied this system to actual decontamination model project and confirmed that this system was capable of calculating predicted dose rate after decontamination with sufficient accuracy.

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