Air concentration of radiocaesium in Tsukuba, Japan following the release from the Tokai waste treatment plant: comparisons of observations with predictions

On March 11, 1997 a fire and explosion accident occurred at the bituminization facility of the Power Reactor and Nuclear Fuel Development, Tokai, Japan. As a result of this accident, 134,137Cs was detected in an air filter sample collected at the Meteorological Research Institute, Tsukuba during March 10 to 12. The 134,137Cs air concentration was about 100 and 10 μBq m−3, respectively. This result suggests that there was little radiation exposure of the residents in the area. The average 137Cs air concentration during this period was about two orders of magnitude higher than “baseline” air (sub-μBq m−3) during February to April, 1997, measured by ultra-low background γ-spectrometry. By a simple calculation using a Gaussian plume model with the measured data, we estimated the minimum emission of the radioactivity by the PNC accident to be in the range 60 MBq to around 600 MBq. The meteorological condition during the week of the accident are also described.