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Summary of “CHEMISTRY TODAY", No.489, pp.48-50 (2011) (Japanese), ISSN 0386-961X

Radioactive Contamination Map in Japan from Fukushima
----- Comparison of Contamination from Chernobyl -----


Masuchika Kohno
Yoshida, Sakyo, Kyoto 606-8501, Japan
Graduate school of Engineering, Kyoto University
Fumitaka Nishiyama
1-4-1, Kagamiyama, Higashi-Hiroshima 739-8527, Japan
Graduate school of Engineering, Hiroshima University


Introduction
Most people know the names Chernobyl and Fukushima. These are places where accidents have occurred at nuclear power plants and there has been extensive damage to people and the environment. The Chernobyl accident was a runaway reactor incident that occurred 25 years ago on April 26, 1986. It goes without saying that the radioactive material released into the environment contaminated the surrounding area, but it also resulted in contamination in Japan more than 8,000 km from Chernobyl. Radioactive cesium and radioactive iodine from the explosions at the nuclear reactor were detected in all prefectures of Japan [*1].
The Fukushima accident started with the Great East Japan Earthquake on March 11, 2011. Because of the loss of cooling functions due to the loss of their external power supply, the fuel in each of the No. 1 through No. 3 reactors of the Fukushima Dai-ichi nuclear power plant melted. It is presumed that some of the molten fuel collected underneath the reactor pressure vessels [*2], but there have also been reports that the situation in the reactors may have worsened even more (Asahi Shimbun, August 8, 2011). Large volumes of hydrogen were produced by a reaction between fuel cladding that reached high temperatures and steam (Zircaloy–steam reaction). A hydrogen explosion occurred in the containment vessel for the No. 2 reactor, and it is thought that part of the containment vessel was damaged [*2]. The widespread radioactive contamination of the environment was caused by damage to the No. 1, No. 3, and No. 4 reactor buildings: by a hydrogen explosion at the No. 1 reactor (March 12, 2011) and by an explosion at the No. 3 reactor (March 14, 2011) that also involved the No. 4 reactor.

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Sampling pine needles to determine the spread of radioactive contamination
The amount of radioactivity released into the environment by the accident at the Fukushima nuclear plant is presently thought to be 630,000–770.000 terabecquerels, or 12–15% of that from the accident at the Chernobyl nuclear plant [*3]. It is presumed that the radioactive materials released from the reactor buildings by the hydrogen explosions were sent up into the air by the force of the explosions and spread to various locations by the wind.
To gain an understanding of the relative states of contamination by radioactive fallout in the various parts of Japan, samples were collected from various locations, and Ge semiconductor detectors were used to measure radioactive substances originating in the Fukushima nuclear power plant accident. The measurements were carried out in Hiroshima and Kyoto. We used pine needles as samples for detecting the radioactive substances. Pines grow throughout Japan, and they are evergreens; therefore, the needles hold a record of the state of radioactive contamination from its onset to the time they are collected. In addition, it is possible to collect pine needle samples from all over Japan under substantially the same conditions (in this case, needles that emerged in the spring of 2010). In actual fact, we were able to collect pine needle samples from all prefectures and metropolitan areas in a short period of time with the cooperation of residents in the various areas. There was no fear that samples would include needles that had emerged after the accident because new pine needles had not emerged in Japan at the time of the accident.

Current State of Radioactive Contamination
To compare the radioactivity in pine needle samples collected in various areas and at different times after the Fukushima accident, we estimated the maximum value for radioactivity in the pine needles based on the results of the measurements after Chernobyl. These estimated values were found in consideration of the decay for each of the radioactive substances in pine needles (half-life, biological effects, and environmental effects) [*1]. When there were multiple sampling locations within a prefecture, the maximum value for each of the radioactive elements was used as the representative value for that prefecture. Naturally, the results of measurements for the one location chosen as the representative value for each prefecture does not show the situation in the prefecture as a whole. There are undoubtedly places with greater radioactive contamination and, naturally, places with less radioactive contamination. However, our thinking was that this approach would be sufficient for the purpose of getting an image of the situation of the radioactive contamination in Japan as a whole.
Figure 1 displays the levels of radioactivity (Bq/kg-fresh) for radioactive cesium (Cs-134 + Cs-137) in pine needles divided exponentially into a color gradient. The left side of the figure shows the effects on Japan as a whole from the Chernobyl nuclear power plant accident 25 years ago, and the right side shows the effects of the recent accident at the Fukushima Dai-ichi nuclear power plant. Speaking in terms of radioactive cesium, all Japanese prefectures were affected by radioactivity from both the accident at the Chernobyl nuclear reactor and the one at Fukushima Dai-ichi. Radioactivity from the Fukushima Dai-ichi nuclear power plant was detected in Okinawa Prefecture, 1,770 km away.
Figure 2 shows the levels of radioactivity in pine needles for radioactive iodine (I-131) in the same manner as shown for radioactive cesium. The figures on the left and right are after the Chernobyl nuclear power plant accident and after the Fukushima Dai-ichi nuclear power plant accident, respectively. In the measurements after the accident at the Chernobyl nuclear power plant, radioactive iodine was detected in all prefectures and metropolitan areas, but after the accident at the Fukushima Dai-ichi nuclear power plant, it was not possible to measure radioactive iodine in all prefectures and metropolitan areas because of the low contamination levels in western Japan at the time measurements began. In the areas in the figure where it was not possible to detect radioactive iodine, the amount of radioactive iodine was estimated based on amount of radioactive cesium.
From these figures, it can be seen that Japan as a whole was contaminated essentially uniformly by the radioactivity from the Chernobyl nuclear power plant accident. On the other hand, in the contamination of Japan due to the accident at the Fukushima Dai-ichi nuclear power plant, although there were prefectures with extremely high concentrations centered on Fukushima Prefecture, western Japan had a contamination level that was the same as or lower than the that from the Chernobyl nuclear power plant accident.
Multiplying the radioactive cesium contamination level (Bq/kg-fresh) in pine needles by 2.6 gives a rough estimate of the soil contamination (Bq/m2). This coefficient is a value found as a result of observations of radioactivity from radioactive cesium in pine needles in Japan and radioactive fallout after the accident at the Chernobyl nuclear power plant [*4]. After the accident at the Chernobyl nuclear power plant, the standard determined by the former Soviet Union for regions requiring radiation management was a soil contamination of 1 Ci/km2 (37,000 Bq/m2) of radioactive cesium. Judging from the levels of radioactivity in pine needles, there are three prefectures to which this standard is applicable. However, if we consider hotspot contamination such as that arising at the time of the Chernobyl nuclear power plant accident, it is predicted that more prefectures will include regions requiring management.

Concluding remarks
Even though the amount of radioactivity is said to have decreased to 1 / 2,000,000 compared with early time of the accident (Asahi Shimbun, July 20, 2011), radioactivity from the Fukushima Dai-ich nuclear power plant continues to be released into the atmosphere. This release of radioactivity will probably not immediately lead to a redrawing of our contamination map. However, we currently have not completely grasped the conditions inside the reactors at the Fukushima Dai-ichi nuclear power plant. We only hope there will not be a new large release of radioactive substances.

References
[*1] “Circumstances of Pollution by Radioactivity Released from Chernobyl in Japan and in Belorus”, M. KOHNO et al., Japanese Slavic and East European Studies, Vol.17 (1996) 53-66. 
[*2] “Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety - The Accident at TEPCO's Fukushima Nuclear Power Stations -” (2011)
[*3] “Fukushima Dai-ichi nuclear accident and its consequences”, National Diet Library ISSUE BRIEF NUMBER 718 (2011.6.28.) (Japanese)
[*4] “Radioactivity Survey Data in Japan”, No.76 (March 1986), ISSN 0441-2561


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