First emission estimates
In the phase of March 12 to 13, the Fukushima emissions were mostly transported to the Pacific, eventually hitting the CTBTO station in Sacramento/California. In the phase March 14 to 15, on the other hand, most of the emissions were transported inland, hitting the CTBTO station in Takasaki, Japan. Based on simulated dilution factors and measurements, we were able to have a first rough source estimate.
Regarding Iodine-131, the picture is relatively homogeneous. A source term of 1017 Bq per day would explain the measurements in Takasaki as well as Sacramento. The total 4-day emission of 4 1017 Bq is on the order of 20% of the total emissions of Iodine-131 that occurred during the Chernobyl accident. Regarding Cesium-137, the situation is a bit different. In the cloud eventually propagating to the United States, the ratio of Iodine-131 to Cesium-137 was about 30. This is similar to the Chernobyl accident. In Takasaki, however, this ratio was four. This would indicate a much larger Cesium-137 release in the second two-day period after the accident. Taking this together, the source terms would be about 3 1015 Bq during the first two days, and 3 1016 during the second two-day period. In sum, this could amount to about 50% of the Chernobyl source term of Cesium-137.
