From March to June 2011, the global radionuclide network of the CTBTO, detected radionuclides emitted from the Fukushima nuclear power plant for a period of more than 6 weeks at all of the monitoring stations in the northern hemisphere. Very high concentrations were observed which in some cases even exceeded the functional capabilities of the high sensitivity monitoring systems
Radioactivity is monitored on a global scale by the International Monitoring system (IMS) radionuclide network, which is being built for the Verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). This network consists of 80 particulate stations. In the period from April to July 2011, more than 40 stations detected radionuclides released from Fukushima NPP and more than 1600 samples contained radiation originating from this event. The data obtained from all stations of the IMS network in the northern hemisphere showed an almost constant ratio of Cs-137 and Cs-134.
Data from the CTBTO network showed that radioactivity from Fukushima was first detected on 12 March at the Takasaki CTBTO monitoring station in Japan indicating that the radioactive plume initially travelled in a south westerly direction. Several gaseous fission products like Cs-134, Cs-136, Cs-137, I-131, I-131, I-133, Te-132, Ba-136m and Xe-133 were detected. Due to the early observation of Te-132 it can be concluded that the fuel in the reactor core was damaged shortly after the earthquake and tsunami
Within 2 weeks the whole northern atmosphere was affected. The radioactive plume next travelled to eastern Russia (14 March) and then crossed the Pacific towards the North American continent to Europe and to Central Asia. The dominant radionuclides were xenon isotopes and especially Xe-133 together with I-131, Cs-134, and Cs-137, and further short-lived radionuclides like Te-132 and I-132 were also detected.
Radioactive material released from Fukushima was detectable all across the northern hemisphere 12 to 15 days after the accident. A CTBTO monitoring station in Iceland detected radioactive isotopes indicating that the plume had reached Europe on 20 March. This was confirmed by European monitoring networks (Masson, et al., 2011). For the first four weeks, the radioactive materials remained confined to the northern hemisphere but by 13 April was detected at stations located in Australia, Fiji, Malaysia and Papua New Guinea indicating that it had reached the southern hemisphere (CTBTO, 2011).