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On the basis of environmental radiation monitoring results at the Nuclear Fuel Cycle Engineering Laboratories, Japan Atomic Energy Agency, following the Fukushima Dai-ichi Nuclear Power Plant accident, the dry deposition velocities of radionuclides on the ground were estimated. As a result, the estimated dry deposition velocities for Iodine-131, Cesium-134, Cesium-136 and Cesium-137 were in the order of 1 mm/s, and were the same order of those estimated in other studies. The estimated dry deposition velocities varied according to the variations of the ratios of the particulate and gaseous forms of their radionuclides and meteorological conditions such as wind speed.

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The Nuclear Fuel Cycle Engineering Laboratories (NCL), JAEA operates the Tokai Reprocessing Plant (TRP), plutonium fuel fabrication facilities and the supplemental facilities. In the terrestrial environment, environmental gamma-ray dose rate was measured continuously using an energy-thermo-compensation-type NaI(Tl) scintillation detector. The data of environmental dose rate measured in 5 monitoring stations and 8 monitoring posts were collected and analyzed to monitor on real-time. This paper summarizes the monitoring data of environmental dose rate around the NCL during the past 10 year (fiscal 1998-2007). The averages of the environmental dose rate for all monitoring stations and posts were in the range of 32-45 nGy h -1 . The long-term variation in the environmental dose rate was not found. The cases of the short-term increases occurred by the operations of the facilities in the NCL were concluded to the discharges of 85 Kr from the TRP, transportation of MOX fuels or radioactive solid wastes, X-ray generator to calibrate the radiation monitoring instruments. The other cases to affect the change of the environmental dose rate were the scavenging of the airborne natural radionuclides, a patient cured by the radiation therapy and the shield effect by cars.