Plutonium mass balance released from the Nagasaki A-Bomb and the applicability for future environmental research

The existence of plutonium was publicly revealed on 9 August 1945 by the nuclear explosion of the Nagasaki A-Bomb whose Pu critical mass has been considered to be between 10–15 kg (still classified). Approximately 1.2 kg was fissioned during the detonation and the remaining mass was released into the environment. The height of the explosion was 503 m and the conditions were cloudy with a humidity of 71%. The ground temperature was 28.8°C with a light west-south-west wind of 3.7 m/s. The amount of both local and global fallout was investigated by measuring a fission product, 137Cs, and unfissioned Pu. Deposition rates were determined for surface soils, reservoir sediments, and the concentration in vegetation, fish and other living materials in the region, up to 100 km from the hypocentre. Only 0.0375 kg of Pu (or 0.3–0.4% of the total Pu in the bomb) was deposited as local fallout from the release. The highest concentration (64.5 mBq/g or 199 mBq/cm2 of 239+240Pu in the surface soils was found at 2.8 km east from the hypocentre where “black rain” precipitated 25 min after the detonation. The concentration rapidly decreased in both directions from this point. At 100 km east from the hypocentre, Pu fallout values were at background levels while only background levels were observed west of the centre. Average global fallout from weapon tests was 5.9 mBq/cm2 in this region, 32.5° latitude and 130° longitude. The reservoir sediments contained a Pu concentration of 142 mBq/cm2. This average was obtained by integrating 90 cm of the core layers (1910–1981 AD). Biological materials contained insignificant amount of Pu because their mass was small compared to the surface soils and the sediments. Only background levels of Pu were found on the west side of the hypocentre. The ratio of unfissioned 239+240Pu and fission product 137Cs can supply valuable information to aid in understanding the mechanism of Pu interaction with the environment. The ratio was as high as 45% at the centre of the “black rain” compared to the northern hemisphere average of 2.3%, as of 1990. At the time of “black rain” droplet formation 137Cs was not fully formed from the mass 137 fission chain, 137Te → 137I → 137Xe → 137Cs. Calculations showed that it would take nearly 1 h to form the total cumulative yield for 137Cs. The time of “black rain” formation was estimated to be 195 s for a 15 kg Pu bomb and 406 s for a 10 kg Pu-bomb. These results suggested that 13.8 kg or 92% (or 0.0064 mBq/cm2) of Pu was deposited throughout the northern hemisphere as global fallout.