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.
Sampling sites of black substances in Fukushima Prefecture and inventory of radioactive Cs-137 (Bq/m2) released from Fukushima Dai-ichi Nuclear Power Plant in color contour showing numbered locations of samples analyzed
The black substance was found to contain high activities of both 137-Cs (half life ~30 yr) and 134-Cs (half life ~2 yr) with values between 0.43 and 17.7 MBq/kg (MBq = 1,000,000 Bq). For a primer on units used by scientists to report radioactivity please consult the following
diary. Isotopes of Pu 239 and 240 (reported as the sum of or 239,240-Pu) were detected in the black substance at much lower activities of 0.15-1.14 Bq/kg. By comparing the ratio of uranium (U) to 239,240-Pu the authors were able to determine that the Pu was more likely from the nuclear power plant than from the legacy of atmospheric weapons testing in middle of the 20th century.
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:
Table summarizing the % of fuel inventory of 137-Cs and 239,240-Pu released to the environment as determined by Sakaguchi et al. (2014)
Roughly 2% of 137-Cs fuel inventory at the Fukushima plant was released compared to 0.00004% of 239,240-Pu. The total mass of 239-240-Pu released based on this analysis is < 600 milligrams or 0.6 grams of these long lived isotopes.
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.