The Preparatory Commission for Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) announced on April 13, 2011 that the radioactive materials from the Fukushima I Nuclear Power Plant had reached the southern hemisphere:
- Update 13 April: radioactivity also measured in the southern hemisphere -Since the double disaster of the 9.0 magnitude earthquake and tsunami that affected hundreds of thousands of people and seriously damaged the Fukushima Daichi power plant in Japan on 11 March 2011, minute traces of radioactive emissions from Fukushima have spread across the entire northern hemisphere. A monitoring network designed to detect signs of nuclear explosions picked up these traces from the stricken power plant.
To date, more than 35 radionuclide stations that are part of the International Monitoring System (IMS) have provided information on the spread of radioactive particles and noble gases from the Fukushima accident. The IMS is a global network that will comprise 337 facilities when complete. Sixty-three of the 80 planned IMS radionuclide stations are already operational and able to detect airborne radioactivity.
The first analysis results of the monitoring data became available a few days after the accident. A clear picture quickly emerged. Initial detections of radioactive materials were made on 12 March at the Takasaki monitoring station in Japan just 250 km away from the troubled power plant. The dispersion of the radioactive isotopes could then be followed to eastern Russia on 14 March and to the west coast of the United States two days later.
Spreading across the entire globe
Nine days after the accident, the radioactive cloud had crossed Northern America. Three days later when a station in Iceland picked up radioactive materials, it was clear that the cloud had reached Europe. By day 15, traces from the accident in Fukushima were detectable all across the northern hemisphere. For the first four weeks, the radioactive materials remained confined to the northern hemisphere, with the equator initially acting as a dividing line between the northern and southern air masses. As of 13 April, radioactivity had spread to the southern hemisphere of the Asia-Pacific region and had been detected at stations located for example in Australia, Fiji, Malaysia and Papua New Guinea.
(The full announcement at the link.)