Results of Airborne Monitoring Survey by MEXT in Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama Prefectures, and Revision of thepast airborne monitoring Results by refrecting the influences of natural radionuclides（Nov.11,2011)

Size: px

Start display at page:

Download "Results of Airborne Monitoring Survey by MEXT in Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama Prefectures, and Revision of thepast airborne monitoring Results by refrecting the influences of natural radionuclides（Nov.11,2011)"

あいりもてぎ
5 years ago
Views:

1 November 11, 2011 Results of Airborne Monitoring Survey by MEXT in Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama Prefectures, and Revision of thepast airborne monitoring Results by refrecting the influences of natural radionuclides The results of the airborne monitoring survey by MEXT in Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama prefectures (Iwate announced on September 13, 2011; Shizuoka announced on September 22, 2011; Nagano announced on September 22, 2011; Yamanashi announced on September 29, 2011; Gifu announced on October 4, 2011; and Toyama announced on October 6, 2011) were concluded today, so they are provided here. Furthermore, past airborne monitoring results were revised to assess influences of natural radionuclides in more detail by using methods which were applied for those of the prefectures mentioned above. Revised results are also provided. 1. Objective of monitoring in Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama prefectures MEXT has conducted airborne monitoring* within the 100km zone from the Fukushima Dai-ichi NPP (up to around 120km in the southern part of the Fukushima Dai-ichi NPP) and in neighboring prefectures so as to ascertain the wide-area distribution of radioactive substances and evaluate future doses and deposition of radioactive substances in evacuation zones, etc. In addition to this, airborne monitoring was also conducted in Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama prefectures for the first time in order to ascertain influences of radioactive substances in wider areas. This monitoring was conducted under the following systems in respective prefecturess. The airborne monitoring survey in Iwate was conducted by the staff of the OYO Corporation and the Japan Atomic Energy Agency using a dedicated private helicopter that can be equipped with an airborne monitoring system of the OYO Corporation. The airborne monitoring survey in Shizuoka and Gifu was conducted by the staff of the FUGRO Airborne Surveys (Australian company) and the Japan Atomic Energy Agency using a private helicopter equipped with an airborne monitoring system owned by the FUGRO

2 Airborne Surveys. The airborne monitoring survey in Nagano was conducted by the staff of the Nuclear Safety Technology Center using a private helicopter equipped with an airborne monitoring system of MEXT. The airborne monitoring survey in Yamanashi and Toyama was conducted by the staff of the Japan Atomic Energy Agency using a private helicopter equipped with an airborne monitoring system borrowed from the U.S. Department of Energy. These measurement results were analyzed by the Japan Atomic Energy Agency. *Airborne monitoring is a method to measure gamma rays from radioactive substances accumulated in the ground quickly over a large area by using aircraft equipped with highly sensitive, large radiation detectors in order to confirm the influences from the surface deposition. 2. Details of monitoring Monitoring period: (i) Iwate: September 14 to October 13 (70 flights in total) (ii) Shizuoka: September 23 to September 30 (13 flights in total) (iii) Nagano: September 24 to October 7 (23 flights in total) (iv) Yamanashi: September 30 to October 4 (15 flights in total) (v) Gifu: October 5 to October 12 (15 flights in total) (vi) Toyama: October 7 to October 9 (7 flights in total) Aircraft: (i) Iwate: a private helicopter (AS350B3) (ii) Shizuoka: a private helicopter (AS350B1) (iii) Nagano: a private helicopter (BELL412) (iv) Yamanashi: a private helicopter (BELL412SP) (v) Gifu: a private helicopter (AS350B1) (vi) Toyama: a private helicopter (BELL412SP) Monitoring Items: Air dose rate at 1 m from the ground surface and deposition of radioactive cesium on the ground surface in Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama prefectures 3. Results of monitoring and revision of the past monitoring results Maps showing the distribution of air dose rates at 1m from the ground surface and maps showing the deposition of radioactive substances on the soil surface in Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama prefectures (Attachments 1 to 24) are based on the results of this monitoring. Furthermore, to verify the spread of radioactive substances, we also developed maps integrating the results of the past airborne monitoring surveys that MEXT had conducted so far. The results are

3 as shown in References 1 to 4. Maps were devloped under the following conditions. The flight altitudes were between 150m and 300m from the ground and the values of this monitoring results are the averages of the measured values in circles with a diameter of around 300 to 600m (varies by flight altitude) below the aircraft. The width of the track is around 3km in this monitoring survey. As flights are difficult in mountainous areas over 2000m in height, there were no data available for such areas. When developing maps of air dose rates and maps of deposition amounts of radioactive cesium (Attachments 1 to 24), the attenuation-compensated values as of the final day of the monitoring in respective prefectures were used. Maps as shown in References 1 to 4 were comprehensively based on the following results. When developing these maps, the attenuation-compensated values as of the final day of the monitoring in Iwate (October 13) were used. Within 80km from the Fukushima Dai-ichi NPP: results of the third airborne monitoring Within 80 to 100km from the Fukushima Dai-ichi NPP (regarding the southern part of Fukushima Dai-ichi NPP, up to around 120km): results of the second airborne monitoring Prefectures other than Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama: results of airborne monitoring conducted in respective prefectures so far Iwate, Shizuoka, Nagano, Yamanashi, Gifu, and Toyama: results of this monitoring As attenuation-compensation methods, air dose rates arecalculated by subtracting the average air dose rates due to natural radionuclides throughout all of East Japan from the measured values and further considering physical attenuation of Cs-134 and Cs-137 from the measurement timeto a specified time. On the other hand, deposition amounts of Cs-134 and Cs-137 are evaluated by taking into account physical attenuation from the measurement time to a specified time. As preparation works for developing the maps of the deposition of radioactive cesium, this monitoring newly assess characteristics of energy spectra of gamma-rays measured in the air with respect to each type of helicopter and measuring instruments used, and sort out areas according to whether energy spectra of radioactive cesium (Cs-134 and Cs-137) were detected significantly or not. Finally, the maps were developed by using the following methods. (see Attachment 25 for details). (i) With regard to areas where energy spectra of radioactive cesium were detected significantly, the deposition amounts of Cs-134 and Cs-137 on the ground surface were calculated in the same manner used so far, by subtracting the average air dose rates due to natural radionuclides throughout all of East Japan from the values measured at respective airborne monitoring points, and then reevaluated based on the correlation between air dose rates and the results of the in-situ measurement using germanium semiconductor detectors, which was conducted by the Japan Chemical Analysis Center in the course of the project,

4 the 2011 Strategic Funds for the Promotion of Science and Technology, entitled Establishment of the Base for Taking Measures for Environmental Impact of Radioactive Substances Study on Distribution of Radioactive Substances. (ii) Areas where energy spectra of radioactive cesium were not detected significantly are indicated on maps as areas showing the minimum range of radioactive cesium ( 10kBq/m 2 ) for the sake of simplicity. Contacts: Horita, Oku Emergency Operation Center, Tel.: Ext.: 4604, 4605

5 別紙 1 ( 岩手県内の地表面から 1m 高さの空間線量率 ) 本マップには天然核種による空間線量率が含まれています

6 別紙 2 ( 岩手県内の地表面へのセシウムの沈着量の合計 )

7 別紙 3 ( 岩手県内の地表面へのセシウム134の沈着量 )

8 別紙 4 ( 岩手県内の地表面へのセシウム137の沈着量 )

9 ( 静岡県内の地表面から 1m 高さの空間線量率 ) 別紙 5 本マップには天然核種による空間線量率が含まれています

10 別紙 6 ( 静岡県内の地表面へのセシウムの沈着量の合計 )

11 別紙 7 ( 静岡県内の地表面へのセシウム 134 の沈着量 )

12 別紙 8 ( 静岡県内の地表面へのセシウム 137 の沈着量 )

13 別紙 9 ( 長野県内の地表面から 1m 高さの空間線量率 ) 本マップには天然核種による空間線量率が含まれています

14 別紙 10 ( 長野県内の地表面へのセシウムの沈着量の合計 )

15 別紙 11 ( 長野県内の地表面へのセシウム 134 の沈着量 )

16 別紙 12 ( 長野県内の地表面へのセシウム 137 の沈着量 )

17 別紙 13 ( 山梨県内の地表面から 1m 高さの空間線量率 ) 本マップには天然核種による空間線量率が含まれています

18 別紙 14 ( 山梨県内の地表面へのセシウムの沈着量の合計 )

19 別紙 15 ( 山梨県内の地表面へのセシウム 134 の沈着量 )

20 別紙 16 ( 山梨県内の地表面へのセシウム 137 の沈着量 )

21 別紙 17 ( 岐阜県内の地表面から 1m 高さの空間線量率 ) 本マップには天然核種による空間線量率が含まれています

22 別紙 18 ( 岐阜県内の地表面へのセシウムの沈着量の合計 )

23 別紙 19 ( 岐阜県内の地表面へのセシウム 134 の沈着量 )

24 別紙 20 ( 岐阜県内の地表面へのセシウム 137 の沈着量 )

25 別紙 21 ( 富山県内の地表面から 1m 高さの空間線量率 ) 本マップには天然核種による空間線量率が含まれています

26 別紙 22 ( 富山県内の地表面へのセシウムの沈着量の合計 )

27 別紙 23 ( 富山県内の地表面へのセシウム 134 の沈着量 )

28 別紙 24 ( 富山県内の地表面へのセシウム 137 の沈着量 )

29 ( 参考 1) 文部科学省がこれまでに測定してきた範囲及び岩手県静岡県長野県山梨県岐阜県及び富山県内における地表面から 1m 高さの空間線量率本マップには天然核種による空間線量率が含まれています

30 ( 参考 2) 文部科学省がこれまでに測定してきた範囲 ( 改訂版 ) 及び岩手県静岡県長野県山梨県岐阜県及び富山県内の地表面へのセシウムの沈着量の合計

31 ( 参考 3) 文部科学省がこれまでに測定してきた範囲 ( 改訂版 ) 及び岩手県静岡県長野県山梨県岐阜県及び富山県内の地表面へのセシウム 134 の沈着量

32 ( 参考 4) 文部科学省がこれまでに測定してきた範囲 ( 改訂版 ) 及び岩手県静岡県長野県山梨県岐阜県及び富山県内の地表面へのセシウム 137 の沈着量

33 Attachment 25 Methods to Assess the Deposition of Radioactive Cesium in Areas where Energy Spectra of Radioactive Cesium are not Detected Significantly 1. Background/Objective In airborne monitoring, measurements of count rates (cps) at 150 to 350m in the air (at 150 to 350m from the ground) and air dose rates (μsv/h) at 1m from the ground surface by NaI survey meters were firstly carried out to set the conversion factor (cps/μsv/h) between them at a selected test line in each prefecture. Based on this conversion factor, air dose rates at 1m from the ground surface are calculated from the count rates measured in the air at respective monitoring points. Deposition amounts of radioactive cesium on the ground surface are calculated from air dose rates which were subtracted the average air dose rates due to natural radionuclides throughout all of East Japan from the results of airborne monitoring by using the conversion factor between air dose rates (μsv/h) and deposition of radioactive cesium (Bq/m 2 ).. The conversion factor between air dose rates and deposition of radioactive cesium is based on the correlation between air dose rates and the results of the in-situ measurement using germanium semiconductor detectors, which was conducted by the Japan Chemical Analysis Center in the course of the project, the 2011 Strategic Funds for the Promotion of Science and Technology, entitled Establishment of the Base for Taking Measures for Environmental Impact of Radioactive Substances Study on Distribution of Radioactive Substances. Although measuring time is too short in airborne monitoring surveys to directly measure deposition amounts of radioactive cesium accurately based on energy spectra of gamma rays emitted from the ground surface, data of energy spectra of gamma rays are collected in the air. As a result of examining such data, it was found that there are areas, such as Niigata prefecture, where energy spectra of radioactive cesium (Cs-134 and Cs-137) were not detected significantly but which were indicated on a map as if showing a significant amount of radioactive cesium, due to influences of natural radionuclides, such as potassium, uranium, and thorium. Therefore, in the airborne monitoring survey in Niigata prefecture, energy spectra measured every second were checked mainly in areas showing higher air dose rates than surrounding areas. With regard to areas where energy spectra of radioactive cesium were not detected significantly, the measurement results in these areas were not used but interpolated values based on deposition of radioactive cesium measured in their surrounding areas were used for mapping. However, while continuing monitoring, areas where energy spectra of radioactive cesium were not detected significantly were found widely in Nagano, Shizuoka, Gifu, and Toyama prefectures as well. In this reason, it became difficult to sort out such areas in the same manner as that used in developing maps for Niigata prefecture.

34 Therefore, we conducted a detailed survey on radiation energy spectra (data by second) for each helicopter and measuring instrument used and examined means to identify areas where energy spectra of radioactive cesium were not detected significantly. 2. Results of the examination In the areas where energy spectra of radioactive cesium were detected significantly or not, *1 we compared the count rates of energy spectra in the following energy ranges between both areas and found its ratio is depend on existence of radioactive cesium. (i) the energy range including energy spectra of radioactive cesium (the range in which the gamma ray energy is 450keV or more) and in (ii) the energy range without radioactive cesium but only including energy spectra of natural radionuclides (the range in which the gamma ray energy is 900keV or more). Therefore, we examined whether it is possible to utilize such ratios for identifying areas where energy spectra of radioactive cesium were not detected significantly. *1: Differences in in where of radioactive cesium were detected or were not detected significantly (i) Example of spectra in areas where energy spectra of radioactive cesium were not detected significantly Counts (arbitrary unit; logarithmic display) 0 No significant peaks of cesium Gamma ray energy

(ii) Example of spectra in areas where energy spectra of radioactive cesium were detected significantly Counts (arbitrary unit; logarithmic display) *2: Count Rate Ratio Count rate ratio Significant

35 (ii) Example of spectra in areas where energy spectra of radioactive cesium were detected significantly Counts (arbitrary unit; logarithmic display) *2: Count Rate Ratio Count rate ratio Significant peaks of cesium can be confirmed. Gamma ray energy Count rate of 450keV or more (incl. radioactive cesium) (i) Count rate of 900keV or more (not incl. radioactive cesium) (ii) (i) Count rates in the energy range in which the gamma ray energy is 450keV or more Peaks of radioactive cesium Counts (arbitrary unit; logarithmic display) Radioactive potassium (peaks of natural radionuclides) Uranium series (peaks of natural radionuclides) Count rate of 450keV or more Thorium series (peaks of natural radionuclides) Gamma ray energy

(ii) Count rates in the energy range in which the gamma ray energy is 900keV or more Counts (arbitrary unit; logarithmic display) Peaks of radioactive cesium Radioactive potassium (peaks of natural

36 (ii) Count rates in the energy range in which the gamma ray energy is 900keV or more Counts (arbitrary unit; logarithmic display) Peaks of radioactive cesium Radioactive potassium (peaks of natural radionuclides) Count rate of 900keV or more Uranium series (peaks of natural radionuclides) Thorium series (peaks of natural radionuclides) Gamma ray energy As a result of the examination, regarding areas where energy spectra of radioactive cesium were not detected significantly (but energy spectra of natural radionuclides were detected), *3 the average count rate ratios and the variation in count rate ratios varied by type of helicopters and measuring instruments used, but in any case, they were confirmed to be normally-distributed, centering around the average count rate ratios.

*3: Count Rate Ratios in Areas where Radioactive Cesium were not Detected Significantly Count rate ratio (Count rate of 450keV or more/ Count rate of 900keV of more) Count rates being

37 *3: Count Rate Ratios in Areas where Radioactive Cesium were not Detected Significantly Count rate ratio (Count rate of 450keV or more/ Count rate of 900keV of more) Count rates being normallydistributed Count rate ratio (Count rate of 450keV or more/ Count rate of 900keV of more) Count rate ratios (Count rate of 450keV or more / Count rate of 900keV or more) Average +3σ* Average Count rate ratios (Count rate of 450keV or more / Count rate of 900keV or more) Frequency Count rate *σ: Standard deviation, which means the degree of variation in count rates When count rates are normally-distributed, 99.7% of them are within a range not exceeding the value adding the average and the three times of the standard deviation (σ). (Reference) Count Rate Ratios in Areas where Energy Spectra of Radioactive Cesium were Detected Significantly Count rate ratio (Count rate of 450keV or more/ Count rate of 900keV of more) Count rate ratios (Count rate of 450keV or more / Count rate of 900keV or more) Standard values Range in which energy spectra of radioactive cesium were detected significantly Range in which energy spectra of radioactive cesium were not detected significantly Count rate

38 In order to identify areas where energy spectra of radioactive cesium were not detected significantly (but energy spectra of natural radionuclides were detected), we set the standard values of count rate ratios (the average count rate ratios + the variation 3 *4 ) by type of helicopters and measuring instruments used, and decided that only the airborne monitoring results over thestandard values should be judged to indicate the existence of a significant amount of radioactive cesium (see Attachment 25 (Reference)). *4: A range that contains 99.7% of count rates in areas where energy spectra of radioactive cesium were not detected significantly Through the abovementioned means, we could identify 99.7% of the areas where energy spectra of radioactive cesium were not detected significantly, but the remaining 0.3% were left unidentified. As a result, for some of these areas, although energy spectra of radioactive cesium were not detected significantly, measured air dose rates were rather high due to influences of natural radionuclides, and deposition amounts of radioactive cesium were wrongly indicated to be over 10kBq/m 2. Although we could identify most of the areas where energy spectra of radioactive cesium were not detected significantly, we cannot determine deposition amounts of radioactive cesium, as no significant energy spectra were detected in these areas, and we do not have accurate air dose rates due to natural radionuclides in these areas. Therefore, we decided to evaluate air dose rates in these areas, by firstly subtracting the average air dose rates due to natural radionuclides throughout all of East Japan from the measured values, as before, and then considering physical attenuation of Cs-134 and Cs-137 from the measurement time to a specified time.

39 Attachment 25 (Reference) Means Adopted to Judge Airborne Monitoring Results Measurement of the count rate data Calculation of count rate ratios Larger than the standard value Comparing with the standard count rate ratios (the average count rate ratios + the variation 3) in areas where energy spectra of radioactive cesium were not detected significantly Smaller than the standard value It should be considered that radioactive cesium was detected. It should be considered that radioactive cesium was not detected.* A significant amount of radioactive cesium was detected in these areas, but there might be influences of natural radionuclides. Therefore, deposition amounts of radioactive cesium were calculated by first subtracting the average air dose rates due to natural radionuclides throughout all of East Japan, and then based on the conversion factor between air dose rates and deposition of radioactive cesium. * In order to prepare maps indicating the deposition of radioactive cesium in areas, including those where energy spectra of radioactive cesium were detected significantly, areas where energy spectra of radioactive cesium were not detected significantly are indicated on maps as areas showing the minimum range of radioactive cesium ( 10kBq/m 2 ) for the sake of simplicity.

スライド 1

スライド 1 September 12, 2011 Results of Airborne Monitoring Survey by MEXT in the Western Part of Fukushima Prefecture The results of the airborne monitoring survey by MEXT in Fukushima prefecture (announced on