Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Dai-ichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition

On 11 March 2011, an earthquake occurred about 130 km off the Pacific coast of Japan's main island Honshu, followed by a large tsunami. The resulting loss of electric power at the Fukushima Dai-ichi nuclear power plant de- veloped into a disaster causing massive release of radioac- tivity into the atmosphere. In this study, we determine the emissions into the atmosphere of two isotopes, the noble gas xenon-133 ( 133 Xe) and the aerosol-bound caesium-137 ( 137 Cs), which have very different release characteristics as well as behavior in the atmosphere. To determine radionu- clide emissions as a function of height and time until 20 April, we made a first guess of release rates based on fuel inventories and documented accident events at the site. This first guess was subsequently improved by inverse modeling, which combined it with the results of an atmospheric trans- port model, FLEXPART, and measurement data from several dozen stations in Japan, North America and other regions. We used both atmospheric activity concentration measure- ments as well as, for 137 Cs, measurements of bulk deposi- tion. Regarding 133 Xe, we find a total release of 15.3 (un- certainty range 12.2-18.3) EBq, which is more than twice as high as the total release from Chernobyl and likely the largest radioactive noble gas release in history. The entire noble gas inventory of reactor units 1-3 was set free into the atmo- sphere between 11 and 15 March 2011. In fact, our release estimate is higher than the entire estimated 133 Xe inventory