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.

radiation dispersion map
© German Federal Institute for Geosciences and Natural Resources
Dispersion of radioactivity after the damage to the Fukushima Daichi power plant in Japan on 11 March 2011.

To date, more than 30 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.

Initial findings

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 300 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 northern hemisphere

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. The radioactive materials remain confined to the northern hemisphere as the equator acts as a dividing line between the northern and southern air masses.

Findings confirm Fukushima release

The CTBTO's monitoring system can detect a range of radioactive isotopes, among them Iodine-131 and Caesium-137. Looking at the ratios between the various radioactive isotopes -- in particular Caesium-137 -- enables the source of the emission to be identified. In the case of the current readings, findings clearly indicate radionuclide releases from a damaged nuclear power plant, which is consistent with the recent accident at Fukushima in Japan.

Unparalleled sensitivity

The CTBTO's radionuclide stations are designed to register minuscule amounts of radioactive particles and noble gases -- down to a number of a few atoms. The system's sensitivity is second-to-none -- it can detect a concentration of 0.1 g of radioactive Xenon evenly distributed within the entire atmosphere of Earth. A rooftop detector at the CTBTO's headquarters in Vienna still catches traces of emissions from the 1986 Chernobyl disaster.