This diary serves as an update to a previously published diary summarizing efforts to determine how much plutonium (Pu) was released to the environment as a result of the Fukushima Dai-Ichi nuclear disaster. Plutonium is an alpha radiation emitting isotope that, if internalized, can represent a significant radiological health risk. Previous measurements of Pu in air, soil, plants and seawater following the 2011 disaster suggest that Fukushima released about 100,000 times less Pu to the environment than the Chernobyl disaster did in 1986. New measurements of "black substances" found along roadsides in high radiation areas in Fukushima Prefecture support previous work showing that Pu was released from the Fukushima plant. Based on the relative activity of Pu to radioactive cesium (137-Cs) the study determined that 2.3x10^9 Bq of 239,240-Pu (580 mg) was released or about 0.00004% of the Pu core inventories. This release from Fukushima is roughly 40,000 times lower than Chernobyl and 5,000,000 times lower than 239,240-Pu released during atmospheric weapons testing in the 20th century.
Sakaguchi and co-workers collected black substances along roadsides in high radiation locations in Fukushima Prefecture in May-September 2012.
Using the total activity137-Cs released to the atmosphere during the early stages of the disaster in 2011 and the 239,240-Pu/137-Cs ratio measured in the black substances Sakaguchi and colleagues calculate the total release of 239,240-Pu to the environment. Results of these calculations are summarized in the table below:
In comparison to the 2.3 x 10^9 Bq of 239,240-Pu released from Fukushima, about 9 x 10^13 Bq was released during the Chernobyl disaster in 1986 and about 1.1 x 10^16 Bq was released to the environment due to atmospheric weapons testing largely in the 1950's and 1960's.
More studies of the distribution of Pu isotopes in soil, plants and especially seawater are required to improve the release estimates of these alpha emitting fuel components. Release of Pu directly to the ocean from Fukushima is poorly constrained at present. I will provide updates to these estimates as new peer-reviewed studies are published.