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Ground deposition maps of Iodine 131 discharged from Fukushima Daiichi were produced using a newly developed method for analyzing airborne monitoring data
- Joint study with JAEA and DOE/NNSA -

Jun. 27, 2013

[Key point of announcement]

The of Ministry of Education, Culture, Sports, Science and Technology (MEXT) requested that President Shojiro Matsuura of the Japan Atomic Energy Agency (JAEA) further analyze the airborne radiological survey data. The JAEA obtained the airborne monitoring data collected during the early stage ĀEMarch 17 to April 5, 2011ĀEby the United States Department of Energy, National Nuclear Security Administration (DOE/NNSA), and analyzed the spectral information in a joint research project with DOE/NNSA. Within this data, the energy peak corresponding to iodine 131 was observed. JAEA and DOE/NNSA have jointly developed a new method to analyze the iodine 131 to determine the ground deposition concentration and create distribution maps.

Since this was the first trial of the analytical methods, JAEA and DOE/NNSA also analyzed cesium 134 using the same method. Cesium 134 has a longer half life (2 years) which makes it available for detection in subsequent aerial monitoring. The results from different surveys should be consistent. As the result of analysis, high concentrations of iodine 131, cesium 134, and cesium 137 were shown to be present to the northwest of the Fukushima Daiichi NNP. South of the Fukushima Daiichi NNP, high concentrations were also detected. However, the difference between the areas northwest of the NPP and those to the south suggest a difference in the ratio of cesium and iodine deposited. Comparisons were performed between the iodine 131 and cesium 134 results from the analysis of the airborne measurements and soil sampling results (June 14, 2011) of iodine 131 and cesium 134 by EOC (Emergency Operation Center) of MEXT conducted June 14, 2011. Comparison of the cesium 134 distribution with results based on subsequent airborne monitoring (July 2, 2011) was also performed. When results from different time periods and collection or analysis methods are corrected for radioactive decay, the results are almost identical. From this analysis, the iodine 131 deposition distributions at the early stage of the accident and its maps were created for the first time.

High concentrations of iodine 131 depositions to the northwest of Fukushima Daiichi NPP were identified in the same way as radioactive distributions of cesium 134 and 137. Iodine 131 distributions were also found to spread to the south of the NPP.

A result of this analysis will be released in the August issue of Health Physics and its On LINE version will appear on June 26, 2013 (U.S. time)

[Background of the study]

The radioactive substances were spread in a wide area in the aftermath of the Fukushima Daiichi NPP accident. After the accident DOE/NNSA conducted airborne monitoring from March 17 to April 5, 2011. The Ministry of Education, Culture, Sports, Science and Technology and DOE/NNSA jointly conducted the primary airborne monitoring (areas within 80km) from April 6, and the secondary airborne monitoring(areas within 80 to 100km, and parts extending to 120km). Beginning with the third monitoring the airborne survey activity is conducted only by MEXT (JAEA participated in the third monitoring).

The general analysis of radioactive substance deposition concentration was conducted by measuring air dose rate 1m above the ground by a gross count (GC) method which counts signal from large Sodium Iodine ( NaI ) scintillation detectors loaded on an aircraft. The cesium deposition concentration was calculated by multiplying the dose rate by a conversion factor. For these reasons, without measured results on the ground, each nuclide could not be identified. If several nuclide including as iodine 131 and cesium 134 and 137 were present and proportion of its substances differs from area to area, it was difficult to evaluate deposition concentrations according to each nuclide.

One the other hand, JAEA, which received the request from the MEXT, EOC about the possibility of analyzing iodine 131 deposition concentrations based on spectral data obtained from airborne monitoring by DOE/NNSA at the early stage after the accident. JAEA has conducted R&D of a new analytical method with DOE/NNSA for use on the early data from DOE/NNSA. However, assessing ground surface deposition concentration was difficult because the results at the early stage cannot exclude the effects of scatter component of γ-ray released by other nuclides (cesium 134 and 137) and effects of radioactive substance added to an aircraft.

[Research and results]

Our main focus was on 3 airborne monitoring flights on April 2 and 3, 2011 and the energy peak (365keV) of iodine 131. Iodine 131 deposition concentrations were analyzed by developing methods to calculate the response to ground deposited activity at aircraft heights, or detectorís characteristic. This was done by using Monte Carlo simulations which extracts a trace of iodine peak in data and simulates the behavior of γ-ray.

Fig. 2 shows the results for the deposited iodine 131 concentration corrected for radioactive decay to April 3 when the monitoring was finished. Analysis of cesium 134 was conducted using the same method as for the iodine 131.

In order to verify the validity of the method, a comparison was performed with the MEXT/EOCís soil sampling data on June 14, 2011. The results from the current analysis were corrected for radioactive decay for this comparison. The deposition concentrations of iodine 131 and cesium 134 measured on the ground were well matched with these airborne monitoring results. (Fig. 3).

The cesium 134 having long half life was compared with the third monitoring results (conducted on July 2, 2011). The third monitoring was performed with using GC method which is not the same analysis method used in the above mentioned way. But they were well matched by comparing with results amended with half life as of July 2 when the third airborne monitoring finished.

Ground deposition maps of iodine 131 discharged from Fukushima Daiichi were produced using a newly developed method for analyzing airborne monitoring data

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