Most Recent Measurements of Plutonium in Pacific: Fukushima Fallout Undetectable

By Jay T. Cullen

@JayTCullen and @FukushimaInFORM

The purpose of this post is to report results from two recently published studies on plutonium releases from Fukushima to the Pacific Ocean. The post contributes to an ongoing series where results from 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 are reported. A frequently asked question of those involved in monitoring the health of the North Pacific is why more measurements of the long lived, alpha-emitting isotopes of plutonium (239Pu half-life 24,100 years; 240Pu 6,570 years) are not being made given the potential for these isotopes to pose radiological health risks. Previous work indicates that 239+240Pu 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. These two most recent studies monitored the activity and isotopic composition of Pu in seawater and marine sediments off of Japan from 2008-2013. Similar to earlier work these studies find that the release of Pu isotopes by the Fukushima accident to the Pacific Ocean has been negligible. The Fukushima signal is not detectable in the ocean off Japan relative to legacy sources from atmospheric weapons testing in the 20th century. Given these accumulating results 239+240Pu from Fukushima is unlikely to negatively impact the health of the Pacific Ocean ecosystem and levels in the environment from Fukushima will not pose a danger to the population of North America.

The two peer-reviewed studies in question are papers published by Oikawa and colleagues in the Journal of Environmental Radioactivity and Bu and colleagues published in the Journal of Radioanalytical and Nuclear Chemistry.

Oikawa and colleagues

Oikawa and colleagues collected sediment cores and seawater samples near nuclear power plants (NPPs) operating in Japan from April 2008 to June 2011. Locations of sampling sites and NPPs are shown in the following figure from the study:

Location of sampling points off the NPPs around the Japanese archipelago before and soon after the Fukushima Dai-ichi NPP accident (2008–2011).


The authors found that neither the isotopic ratio 239+240Pu nor the activities of the isotopes in marine sediments or seawater were affected by the Fukushima NPP disaster. Rather these properties reflected the presence of Pu in the environment from atmospheric weapons testing in the 20th century falling on a mixing line between global fallout and Pacific Proving Ground testing isotope ratios. Shown below is the activity of the isotopes and their ratios at sampling sites over the length of the study for ocean surface waters:

The box plot chart of the 239+240Pu concentrations found in the surface waters at the 37 sampling locations during 2008–2011.

 

Relationship between 239+240Pu concentration and 240Pu/239Pu atom ratio found in the surface waters at the 37 sampling locations during 2008–2011.

Bu and colleagues study

Similar findings were reported by Bu and colleagues who have continued their marine monitoring work looking for Fukushima derived Pu in the Pacific. Their previous work is summarized here and here. In their most recent paper they report on isotope measurements in a sediment core collected in January 2013, 100 km off the coast of the Fukushima NPP identified as K06 on the map below:

Sampling stations of the sediments collected in the western North Pacific off the FDNPP site after the nuclear accident. Stations of ES4, ES5, MC5, FS1, ES2, FS5, MC1 and F1 are redrawn from their previous papers.

 

They found that compared to cores collected from nearby locations in July 2011 – July 2012 there was no change in the isotopic composition or activities of 239+240Pu. Both the activities and isotopic composition reflected the presence of global fallout and Pacific Proving Ground close-in fallout rather than Pu from the Fukushima Dai-ichi NPP. Two years on from the disaster initial and ongoing releases of Pu from the site can’t be detected in the marine environment.

Conclusion

Given the absence of both isotope ratio and activity anomalies thus far in the western Pacific traceable to the Fukushima meltdowns it is unlikely that any impact on organisms or the North Pacific ecosystem should be expected. This is consistent with the amount of Pu released from the Fukushima site being very small as air, soil and water measurements have so far indicated (source on the order of 2.3×109 Bq of 239+240Pu or 580 milligrams of the isotopes). Ongoing monitoring of Pu isotopes and other radionuclides released in much greater amounts like 137Cs is an important service by the scientific community that will help to determine the impact of the Fukushima NPP disaster on the marine environment. Our project, InFORM, will continue to make measurements of Fukushima derived radionuclides in seawater and marine organisms over the next 3 years to help assess the potential for negative health impacts in the eastern Pacific.

Results of ongoing research by the scientific community will be communicated as it becomes available.

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