By Jay T. Cullen

This post reports on the most recent study of plutonium releases from Fukushima to the Pacific Ocean. The post contributes to an ongoing effort to report peer-reviewed studies on the impact of the triple meltdowns at the Fukushima Dai-ichii nuclear power plant on the health of the Pacific ecosystem and residents of the west coast of North America. Plutonium is an alpha-emitting isotope that carries significant radiological health risks if internalized with risk of exposure increasing with the activity of Pu isotopes in the environment. Previous work indicates that 239,240-Pu releases from Fukushima were about 100,000 and 5,000,000 times lower than releases from the Chernobyl disaster in 1986 and 20th century weapons testing respectively. Initial measurements of Pu isotopes in seawater and marine sediments off the coast from Fukushima indicated no detectable change occurred in Pu inventories in the western Pacific after the disaster. More recent and more expansive work supports earlier studies drawing the conclusion that up to two years after the accident the release of Pu isotopes by the Fukushima accident to the Pacific Ocean has been negligible.

A paper by Bu and colleagues was recently published in the peer-reviewed journal Environmental Science and Technology which investigated the activity of Pu isotopes marine sediments collected within 30 km of the Fukushima reactor sites. 239,240,241-Pu and radiocesium isotopes (134-Cs and 137-Cs) were measured. Given that Pu is a particle reactive element that would tend to be concentrated in sediments such measurements should help to determine the extent and degree of Fukushima derived Pu in the marine environment. Sample collection sites are indicated in the map below.

Relatively high activities of 134-Cs and 137-Cs and a decay corrected ratio near 1 indicated that the sediments were indeed contaminated with Fukushima derived radionuclides.

In contrast to the clear imprint of Fukushima derived Cs on the marine sediments the activities of 239,240-Pu and 241-Pu were low compared with the background level before the accident. The Pu activity ratios (240-Pu/239-Pu and 241-Pu/239-Pu) suggested that the Pu detected was the result of global fallout and the pacific proving ground (PPG) close-in fallout resulting from atmospheric weapons testing in the 20th century. The following figure is a mixing diagram that helps to determine the relative contributions to the observed Pu contamination of marine sediments off the Japanese coast.

The mixing diagram indicates that the isotopic ratio of Pu in marine sediments is inconsistent with a significant release of Fukushima Pu to the marine environment. The isotopic composition of Pu in marine sediments is consistent with Pu deposited during atmospheric weapons testing in the last century.

While initial releases from the plant and ongoing releases due to groundwater infiltration and terrestrial runoff have been negligible thus far according the authors they rightly point out that significant inventories of Pu are insecurely stored at the Fukushima site. So far estimates suggest that about 2.3×10^9 Bq of 239,240-Pu or 580 milligrams of the isotopes have been broadcast to the environment from Fukushima. Bu et al. (2014) estimate that contained within the roughly 270,000 tons of radioactive liquid waste stored in large tanks at Fukushima there exists approximately a further 1×10^8 Bq of 239,240-Pu. Given that future earthquakes or other events could mobilize this Pu, continued monitoring of Pu isotopes in the marine environment is necessary and prudent.