Monitoring air dose rates from a series of aircraft surveys across the two years after the Fukushima Daiichi NPS accident

Transcription

1 Monitoring air dose rates from a series of aircraft surveys across the two years after the accident June 5, 213 Radiation Monitoring Division The Secretariat of the Nuclear Regulation Authority, Japan

2 1 Contents Surveys were carried out in the 8km zone from on five/four occasions to create air dose rate maps for; (i) the 8km zone from (ii) evacuation-directed zones

3 2 Methods of measuring radiation dose Airborne Monitoring Aircraft such as helicopters equipped with radiation detectors are used for monitoring. The detailed method is described in the next page. Mobile Monitoring Cars equipped with radiation detectors are used for monitoring. Survey Meters Radiation doses are measured by survey meters. Monitoring Stations Radiation doses are measured at monitoring stations. Approximately 3 stations are located in Prefecture.

4 Airborne radiation dose monitoring by using aircraft Data collecting device Gamma rays from the ground are detected by NaI scintillator installed in an aircraft (e.g. helicopter) while flying approximately 15 to 3m above the ground with every 1 second in a row. Flight schedule (e.g. part of an evacuation zone) Detector Airborne monitoring devices (left) are installed in a helicopter (right) Flight height above the ground : approx.15-3m 1.85km Setting up the given points on the ground, gamma rays from these points were measured from the upper sky at different heights. The calibration curves (heights vs. those rates) were created and air dose rates were normalized using these curves at 1m from the ground surface to create the map. Approx. 3~6m Ground Nuclear Power Plant The data measured and interpolated were integrated to create the map. Survey data Analyzing the data with the processing software for airborne monitoring surveys 3

5 Air dose rates in the 8km zone from 1 month later ( ) 3 months later ( ) 7 months later ( ) Air dose rates at 1m from ground (µsv/h) Not measured 1 year and 3 months later ( ) 1 year and 8 months later ( ) The natural radionuclides are included. 4

6 Air dose rates surveyed on two occasions (7 months and, 8 months later) Air dose rates on 8 months later ( ) Air dose rate Y=.6x R 2 =.97 Air dose rates on 7 months later ( ) Modified; 5

7 6 The percentage of the area in terms of air dose rates in the 8km zone (Table) (%) Air dose rates (µsv/h) 7 months later ( ) 3 months later ( ) 8 months later ( ) 19. < (3.8 <) (.2-3.8) (.2)

8 Percentage of the area (%) The percentage of the area in terms of air dose rates in the 8km zone Air dose rates (µsv/h) 19. < months later 3 months later 8 months later

9 Conceptual diagram of evacuation-directed zones (as of May 28, 213 ) Date City Kawamata Town Iitate Village (212/7/17~) Katsurao Village (213/3/22~) Tamura City (212/4/1~) Kawauchi Village (212/4/1~) Iwaki City Minami-soma City (212/4/16~) Namie Town (213/4/1~) Difficult- to return zone Habitation-restricted zone Zone preparing to lift the evacuation directive Deliberate evacuation zone A town before rezoning Futaba Town (213/5/28~ ) Okuma Town (212/12/1~) Tomioka (213/3/25~ ) Naraha Town (212/8/1~ ) Hirono Town 2km Daini NPS Difficult-to return zone approx. 32km 2 #,$ Habitation-restricted zone; approx. 3km 2 $ Zone preparing to lift the evacuation directive; approx.46km 2 $ Deliberate evacuation zone; approx. 33km 2 $ #. An area within 3km from is excluded. $. Snow coverage areas are excluded. Revised Regarding rezoning of the evacuation-directed and restricted zones in Futaba Town issued by Nuclear Emergency Response Headquarters oadmap/pdf/1357_assistance.pdf 8

10 9 Air dose rates in the evacuation-directed zones 2 years later ( ) Air dose rates at 1m from ground (µsv/h) Not measured The natural radionuclides are included. * The white zones surrounded by the solid lines: Snow Coverage Areas

11 Air dose rates in the evacuation-directed zones Air dose rates at 1m from ground(µsv/h) 7 months later ( ) 3 months later ( ) Not measured The natural radionuclides are included. The white zones surrounded by the solid lines: Snow Coverage Areas 8 months later ( ) 2 years later* ( ) 1

12 The percentage of the area in evacuation-directed zones (Table) ref. P. 8 Difficult-to return zone Approx. 32km 2 Habitationrestricted zone Approx. 3km 2 Zone preparing to lift the evacuation directive Approx. 46km 2 Deliberate evacuation zone Approx. 33km 2 Air dose rates (µsv/h) 19. < < < < months later ( ) (Evacuation-directed zones: as May 28, 213) 3 months later( ) months later ( ) years later ( ) (%)

13 Percentage of the area (%) Percentage of the area (%) Percentage of the area (%) Percentage of the area (%) The percentage of the area in evacuation-directed zones Difficult-to-return zone 7 months months 8 months 2 years < Zone preparing to lift the evacuation directive 1 9 Air dose rates (µsv/h) Habitation-restricted zone months months months Deliberate evacuation zone 2 years months 3 months 8 months 2 years 7 months 3 months 8 months 2 years 12

14 Air dose rates (µsv/h) Air dose rates (average) in evacuation-directed zones Difficult-to return zone Habitation-restricted zone Deliberate evacuation zone Zone preparing to lift the evacuation directive ~ months after the accident #Standard deviations are omitted in the figure. 13

15 14 Summary Surveys were carried out on five/four occasions across the two years that followed the March 211 accident at for measuring air dose rates using an aircraft mounted with detectors. For that purpose, maps were created and compared each other. 1. Air dose rates in the 8km-zone reduced approx. 4% between 5 November 211 and 16 November 213, although physical decay was approx. 21%. The areas exceeding 3.8µSv/h decreased from 7% to 4%. 2. Air dose rates in the evacuation-directed zones between 5 November 211 and 11 March 213 are as follows: 2-1. Difficult-to-return zone: The areas exceeding 19.µSv/h decreased from 27% to 6%, and exceeding 3.8µSv/h, 88% to 75%, respectively Habitation-restricted zone: The area exceeding 3.8µSv/h decreased from 88% to 3% Zone preparing to lift the evacuation directive: The area was almost all below 3.8µSv/h, 2 years after the accident Deliberate evacuation zone: The area was almost all below 3.8µSv/h, 2 years after the accident.