Ver. 1は削除します。
This is version 2 draft of my message to people outside Japan.
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April 4, 2011
Several colleagues outside Japan asked us about the current situation of the accident at Fukushima Daiichi nuclear plants and its effects on the safety and health of residents of Japan and overseas visitors. I thought that it would be best to provide you with some facts in Japan so that you, as scientists, can make your own judgment.
A. Detectable fall out of radioactivity is now limited to the area near the reactors.
http://r.diim.jp/
This set of graphs shows regional difference in the current environmental radioactivity (white circles) compared to the past levels (green circles. Movements represent variation).
Note:
1. Radiation at ~0.15 µSv/h is considered to be the normal ambient level (Tokyo and surrounding areas are currently at this level).
2. Environmental radiation varies from place to place. For example, it generally increases with altitude. Denver Colorado (USA), where I spent 3 years as a postdoc, has a radiation level above 1µSv/h.
3. Graph based on data reported by Ministry of Education, Culture, Sports, Science and Technology (MEXT). http://www.mext.go.jp/english/radioactivity_level/detail/1304082.htm
B. The radiation level is constantly dropping (Graph 1).
Graph 1 "Radiation level in Kanto/Tokyo area] Till Apr-4, 9am. pm." http://plixi.com/p/89466733#
Note:
1. A major release of airborne radioactivity occurred in March 15-16. Another increase on March 21 is now considered to be due to rain.
2. 'Short bursts' could only be detected in the vicinity of the reactor (plots of Dai-ichi (1) monitors: http://dl.dropbox.com/u/16653989/NuclPlants/can.gif ), but not at distant monitoring sites.
3. A graph based on the data provided by MEXT Japan, prepared by nuclear theory group of Professor Ryugo Hayano (Univ. Tokyo). He regularly tweets update and useful information (@hayano).
4. Live monitor at KEK provides current level in Tsukuba (link below).
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The reactors are still in critical condition and leakage of radioactive substances into the seawater was detected (April 2). Water supply and foods in the market are controlled for radioactivity, and have been tested for safety (radiation level in water can be found here http://atmc.jp/water/).
Although details are still unavailable, it will likely take months to regain full control of the reactors, and years for the cleanup of the fuel materials. Importantly, however, the fission chain reaction was terminated immediately after the initial earthquake. The current problem is due to the heat generated from radioactive disintegration of spent fuels. Since radioactivity of materials with short half-lives decays rapidly, the heat level from the fuel will drop to less than 1% of the level during operation within one month. I think it is fair to assume that the risk of a major explosive event will continue to drop if the current efforts to cool the reactor are maintained.
The Tokyo and Tohoku areas are still experiencing frequent aftershocks. In addition, scheduled blackouts in the Tokyo area will cause some inconvenience for residents and industries. In contrast, most of the other areas in Japan, such as Western areas, including Nagoya, Kyoto, and Osaka, are completely fine. Narita and Haneda international airports are under normal operation. Even for the most cautious visitors, exposure in these areas (0.091µSv/h in Tokyo, 4/4/2011) is significantly lower than what one receives during a flight. Furthermore you can choose other international airports in Western Japan, such as Kansai, Nagoya, and others. .
In sum, the effect of radioactive leakage from Fukushima Daiichi nuclear plants is limited to relatively small area, and radioactivity levels in Tokyo area are in the normal range. Radiation levels, seismicity, transportation, power supplies, and the availability of daily living needs in other parts of Japan are essentially unaffected.
We appreciate your enthusiastic support and hope to see you in Japan, soon.
Useful Link
Information of radioactivity in Japan. Most comprehensive collection of information (Google translation available)
Live Monitor of radioactivity at KEK/ HIGH ENERGY ACCELERATOR RESEARCH ORGANIZATION at Tsukuba.
IAEA Fukushima Nuclear Accident Update Log (http://www.iaea.org/newscenter/news/tsunamiupdate01.html)
NARITA international airport (http://www.narita-airport.jp/en/index.html)
FAQ WHO (http://www.wpro.who.int/media_centre/jpn_earthquake/FAQs/): it provides useful information on food and radiation safety for travelers in Japan.
