Radiological Impact of the Fukushima Daiichi Accident
Immediately after the earthquake the Japanese Government established various systems such as the Nuclear Emergency Response Headquarter to respond to the emergency. The ministry of Education, Culture, Sports, Science and Technology (MEXT) was, and currently the Nuclear Regulation Authority (NRA) is, responsible for environmental monitoring.
The accident at the Fukushima site occurred soon after the occurrence of the natural disasters of the earthquake and tsunami, which caused great difficulties in emergency response including environmental monitoring of the nuclear disaster. For instance, 23 of 24 monitoring posts in Fukushima Prefecture became unavailable, and communication and transportation of necessary resources became very difficult. In addition, after the loss of external power supply on March 11, TEPCO became unable to perform measurements at monitoring posts and air stack monitors, although these difficulties were resolved gradually.
In spite of the difficulties, nationwide environmental monitoring was started on March 12 by MEXT with assistance from universities and various organizations. Results of daily measurement of air dose rate, radioactivity in fallout at the fixed time, and radioactivity in tap water were reported on news media as well as Internet websites (NRA ). Air monitoring was done by MEXT with the help of USDOE, and a map based on air dose rate was prepared. Also, radioactivity in soil, radioactivity in seawater and ocean sediment, and radioactivity of air dust were analyzed. Measurements of foodstuffs were done by the Ministry of Health, Labor and Welfare and measurements of farmland soil and fertilizer by the Ministry of Agriculture, Forestry and Fisheries. The results of these measurements and monitorings were used for decision making of various protective measures such as evacuation and inhouse sheltering, the control of consumption and shipping of foodstuffs, as well as for dose assessment. Almost all the monitoring was done periodically, which can provide very useful information for dose assessments.
After the Fukushima accident, the public received doses through four major exposure pathways: external dose from radionuclides in the radioactive plume, external dose from radionuclides deposited on the ground, internal dose from inhalation of radionuclides in the radioactive plume, and internal dose from ingestion of radionuclides in food and water. Currently most basic information available for the dose assessment is the gamma dose rate in the air (μSv/h), the density of radioactivity deposited per unit area (Bq/m2) in areas, and the concentration of radionuclides in soil samples collected (Bq/kg).
Dose assessment for individuals and groups of population is carried out using all available information.
For external dose from the ground, a dose estimation system was developed by NIRS using a time-series set of ambient dose rate maps and individual behavior with help of shielding factors of the house based on personal interview or questionnaire. External doses from a radioactive plume as well as internal dose through inhalation of the radioactive plume are calculated with help of atmospheric dispersion simulation at the time of major release of radioactivity in the affected area.
For internal dose, environmental transfer models of radioactivity from soil to human body were used. The upper soil layer, crops and vegetables, tea, milk and meat, fish and sea products, as well as tap water were important samples. Radioactivity in the total diet rather than an individual food can usually provide better information for dose assessments, although radiological survey of the total diet was not done on a wide scale by the Government after the present accident. Further, radioactivity in human body can provide the best information for dose assessment.
Each step contains some uncertainty, so total uncertainty increases with number of steps. It means that the measured data close to the human body is very important for dose assessment with smaller uncertainty. It is very important, therefore, to emphasize that individual dosimeters for external exposure and in vivo measurements of whole body or thyroid for internal exposure can provide the best information for the dose assessments. In reality, especially in the early stages after an accident, it is not easy to make these measurements, but I think it should be emphasized.