Not the news that weary radiation-vigilant residents of Japan (including Tokyo Brown Tabby) wanted to hear.
But according to a new research paper by the international team of scientists to be published on the electronic version of the Proceedings of the National Academy of Sciences of the United States of America (PNAS), cesium-137 (half life 30 years) may have deposited in the soil in much wider areas including Hokkaido, Chubu, Chugoku, Shikoku in one month starting March 20. The cesium deposition in these regions is considered to be in much lower concentration than in Tohoku or Kanto, but the researchers say there may be radiation "hot spots" in these regions.
It's no surprise to anyone who used to look at the dispersion simulations by Austria's ZAMG, Germany's DWD and Norway's NILU in the early days of the nuclear crisis in March, April and May. I remember seeing large plumes engulfing Hokkaido a number of times, and occasional small plumes swiping Shikoku.
From Jiji Tsushin News (11/15/2011; links added):
東京電力福島第1原発事故で放出された放射性物質のうち、半減期が約30年と長いセシウム137が、3月20日からの1カ月間に中部や中国、四国地方の山 岳地帯や北海道の土壌に沈着した可能性があることが分かった。米大学宇宙研究協会（USRA）や名古屋大、東京大などの国際チームが14日までに行ったシ ミュレーションの結果で、米科学アカデミー紀要電子版に発表される。
Of radioactive materials released from the Fukushima I Nuclear Power Plant accident, cesium-137, whose half life is 30 years, may have deposited in the soil in the mountains in Chubu and Shikoku regions, and Hokkaido, in a month starting March 20. It is a conclusion from the simulation done by the international team of scientists including those from the USRA (Universities Space Research Association), Nagoya University, and Tokyo University. Their paper will be published in the electronic version of the Proceedings of the National Academy of Sciences of the United States of America.
It is assumed that most of these areas do not have the level of contamination that may affect the health or may require decontamination. However, there may be "hot spots" with locally high radiation, and a detailed investigation may be necessary, according to the scientists.
Teppei Yasunari of USRA, Tetsuzo Yasunari of Nagoya University, and Ryugo Hayano of Tokyo University combined the global atmospheric transfer model (20-kilometer grid) developed in Norway, the meteorological data from European Centre for Medium-Range Weather Forecasts (ECMWF) [in Redding, UK] and radiation fallout measurement data from Japan's Ministry of Education and Science to do the simulation.
その結果、日本列島へのセシウム137沈着量は1カ月間で1000テラ（テラは1兆）ベクレル以上と推定された。福島県を中心に東北、関東の太平洋側の 沈着量が多いのは文科省の航空機モニタリング結果などと一致したが、中部、中国、四国の山岳地帯や北海道でも、低気圧が通過した際に同原発からセシウム 137の微粒子を含む風が流入し、雨で沈着した可能性が示された。
According to their simulation, the cesium-137 deposition in Japan was estimated to be more than 1,000 terabecquerels in one month [starting March 20]. The simulation matched the result of the aerial survey by the Ministry of Education that showed the Pacific Ocean side of Tohoku and Kanto had a larger amount of cesium deposition. However, the simulation also shows that when the low pressures passed over the mountain areas in Chubu, Chugoku and Shikoku regions and in Hokkaido, wind that contained cesium-137 particles from Fukushima I Nuclear Power Plant may have blown and the particles may have fallen with the rain and deposited in the soil.
The paper is here: http://www.pnas.org/content/early/2011/11/11/1112058108.full.pdf
From the "Discussion" section of the paper (page 4, left column)
...we expect the true soil contamination across Japan to be considerably more variable than in our estimate. Even in regions where we find relatively low soil contamination levels, hot spots with high concentrations (e.g., due to convective rain fall, orographic enhancement of rainfall, or fine-grain soil flow by rainwater on the ground) may be possible. In contrast, relatively clean patches may also be present in areas with high overall contamination levels.
Supplementary information and data: http://www.pnas.org/content/suppl/2011/11/14/1112058108.DCSupplemental/pnas.1112058108_SI.pdf
From Page 5 of the supplementary data above, "Estimated Cs137 Concentration in Soil" (screen shot from a movie)
"Movie S4. Estimated 137Cs concentration range in soil using DRT of 0.001 and conversion coefficient (CC) of 38, 53, and 68 kgm−2 over Japan. CCs of 38, 53, and 68 kgm−2 correspond to −1σ, mean value, and þ1σ in Fig. S5, respectively. Outputs 0.2° × 0.2° were interpolated into finer resolution using cubic interpolation. The Merged IBCAO/ETOPO5 Global Topographic Data Product (1) was used to mask out ocean area below 0 m above sea level (a.s.l.)."