Global dispersion and transport

The spread of radioactive pollutants was not confined to Japan. Due to the prevailing westerlies during the accident, the radioactive nuclides had the potential to be transported offshore, across the Pacific Ocean, and further to the North American continent. Monitoring of seawater, soil and atmos­phere was being done at 25 locations on the plant site, 12 locations on the boundary, and other locations further afield [1]. Trace amounts of radiation, including iodine-131, cesium-134 and cesium-137, were being observed around the world (New York State, Alaska, Hawaii, Oregon, California, Montreal, and Austria) [13]. Radioactive isotopes originating from Fukushima were picked up by over 40 CTBTO radionuclide monitoring stations [15] .

On March 17-18, 2011, the first arrival of the airborne fission products, iodine-131, iodine-132, tellurium-132, cesium-134, and cesium-137, was detected in Seattle, Washington (USA) by their characteristic gamma rays. Leon et al. [16] used a NOAA HYSPLIT model to assess their transport time and possible trajectories across the Pacific. Plate XII (between pages 448 and 449) shows three trajectories of the radioactive nuclides, which indicate the range of transport pathways. The start time was set to March 12, 2011 at 10 UTC (Coordinated Universal Time), which was approxi­mately 3 hours after the reported explosion from unit 1. The trajectories were calculated for three heights in the atmospheric boundary layer: 500, 1,000, and 1,500 m above ground level. The 500 m trajectory was found to be caught up in and raised by a cyclonic system over the Bering Sea. The trajectories started at 1,000 m and 1,500 m were also partially lofted but did not get involved in the cyclonic pattern. Instead, they were found to be rapidly transported across the Pacific. Upon arrival at the west coast of the US, the transport again split, with one arm transported to the north in a cyclonic direction around a low pressure system located off the coast of Washington state. There were rain showers and cool weather in western Washington at the arrival time of the plume, and the strong divergence and precipitation associated with these weather systems most likely significantly reduced the concentrations of radionuclides that were transported. The trajectory initially started at 1,500 m was transported in the boundary layer towards California. Overall, the trajectories support the notion of transport of the radionuclides from the Japanese boundary layer to the US boundary layer in only 5-6 days. This result is significantly faster than the other previ­ous work which examined the trans-Pacific transport times [17], especially considering the radionuclides were released in the boundary layer over Japan and measured in the boundary layer along the US west coast.

After crossing the North American continent, the contaminated air masses were anticipated to continue to move towards the North Atlantic and reach Europe. The first sign of the radioactive material in Europe was detected on March 19, 2011 in Iceland, 7 days after the explosion of the unit 1 reactor. On March 23-24, most European countries had detected the radiation. Around March 28-30, the first radioactive peak was observed. The time — and spatially-averaged values from March 20 to April 13 in Europe were 0.076 and 0.072 mBq/m3 for cesium-137 and cesium-134, respectively [ 18]. Cesium-137 airborne activity levels reported after the Fukushima Daiichi NPP incident were at least 10,000 to 100,000 times lower than those observed after the Chernobyl accident. Regardless of the radio­nuclide considered, airborne activity levels remained sufficiently low as to be of no concern to public health in Europe. Although the prevailing wind during the accident was westerly, the radiation effect in Hong Kong, more than 2,000 miles southeast from Fukushima, was also detected before April 14, 2011. An activity of iodine-131 of 62.5 pBq/m3 was first detected on March 26, 2011, and a maximum value of 828 pBq/m3 was observed on March 29, 2011, in Hong Kong [19].

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