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.
One of the goals of the InFORM project is to make measurements of radionuclides in the North Pacific Ocean to determine maximum activities that will determine impacts on the marine ecosystem and residents of the west coast. The purpose of this post is to bring to the attention of readers a recently published correction to a prominent model that predicts the activity of Fukushima derived Cesium-137 (137-Cs, half life ~30 years) in seawater of the North Pacific. The diary is part of an ongoing series aimed at discussing research addressing the impact of the Fukushima nuclear disaster on the health of the North Pacific Ocean and inhabitants of North America’s west coast. Predictions of a model by Rossi and colleagues published in Deep-Sea Research in 2013 of the evolution of the plume of seawater contaminated by the Fukushima triple meltdowns are an order of magnitude too high. Rather than a range of ~1-30 Bq/m^3 reported previously maximum activities off the west coast of North America are likely to be ~3 Bq/m^3 or about more than 25 times lower than maximum activities measured in the Pacific in the mid-20th century resulting from atmospheric weapons tests. These activities are not likely to represent significant radiological health risks to the North Pacific ecosystem or residents of the North American west coast.
A paper by Rossi et al. (2013) used a Lagrangian model to predict the temporal and spatial evolution of the seawater plume contaminated by the Fukushima nuclear disaster beginning in March 2011. The model predicted a range of 10-30 Bq/m^3 137-Cs in waters off the coast of North America at 49 degrees North latitude as demonstrated in the figure shown below:
This model predicted higher maximum 137-Cs activities in seawater in the North Pacific compared with a similar model published by Behrens et al. (2012) that had maximum activities off of North America reaching only ~1-2 Bq/m^3.
Recently, after comments from Professor Michio Aoyama of Japan, Rossi and colleagues recognized an error in their model and have published a correction to their 2013 study here. The error resulted in a factor of 10 overestimation of maximum activities of 137-Cs in the Pacific such that maximum 137-Cs off N. America will likely be between 1 and 3 Bq/m^3. The corrections to the model do not affect the conclusions of the study and results from the 2013 study are easy scaled to the more accurate values given the Langrangian approach used by the authors in the original work.
The figure below shows the time evolution of the plume at various latitudes along the international date line and compares the model output with measurements made by Aoyama et al. (2013) along the international dateline at about 40 degrees N in 2012.
The factor of 10 lower activity correction better agrees with the Behrens et al. (2012) modeling study and measurements of 137-Cs in seawater made by Japanese and North American scientists.
As part of an ongoing series documenting the impacts of the Fukushima disaster on the North Pacific and west coast, this post summarizes a recently published study by Delvan Nevilleand colleagues in the peer-reviewed journal Environmental Science and Technology. The paper reports measurements of Cesium-134 and Cesium-137 in 26 albacore tuna caught off the west coast of North America between 2008 and summer 2012. Because of its relatively short half-life (~2 years) 134-Cs is an unambiguous tracer of radionuclides released from the Fukushima Dai-ichi disaster which began in March 2011. Fish collected in 2011 and 2012 had higher 134-Cs and 137-Cs that was due to Fukushima sourced cesium in the Pacific. Fish collected in 2008-2009 had lower 137-Cs activities that largely reflected historic releases of the isotope from atmospheric nuclear weapons testing in the 20th century. The authors conclude that given the highest levels of Cs isotopes measured in albacore tuna, human consumption of the fish would not not represent a significant increase in annual radiation dose. The corresponding radiological health risk due to Fukushima derived radiocesium in these tuna is, therefore, very small. Continue reading Fukushima Radionuclides in Pacific Albacore Tuna Off the US Coast→
The purpose of this blog is to bring to the attention of interested readers a recent peer-reviewed, open-access study published in the Journal or Radiological Protection . The investigators describe the design and manufacture of a whole body sensor whose purpose is the detection of 137-Cs (half-life ~30 years) in children who were proximate to radionuclide releases after the triple meltdowns at the Fukushima-Daiichi nuclear power plant which began in March 2011. Health impacts of the disaster are likely to be most acute in Japan given that levels of radionuclides in air, soil and water resulting from the disaster were higher compared to levels measured and expected on the west coast of North America. The detector in question (called BABYSCAN) is demonstrated to have a detection limit of better than 50 Bq/body and has been installed in a hospital in Fukushima. Because children are most vulnerable to the impacts of ionizing radiation, 100 Fukushima children were scanned for the presence of 137-Cs and none were found to have detectable levels of the isotope in their bodies. Larger scale measurements of the population will be reported as the long term impacts of low levels of ionizing radiation present owing to the Fukushima disaster warrant further study.
As a result of the radionuclides released to the atmosphere after the meltdowns in March 2011 at the Fukushima-Daiichi nuclear power plant, Fukushima Prefecture was contaminated with radioactive cesium (134-Cs and 137-Cs) and other isotopes, which pose radiological health risks to the resident population. As would be expected for parents around the world, parents of small children in Fukushima Prefecture have great concern about the internal exposure experienced by their offspring.
The paper by Hayano and colleagues describes the design and manufacture of a whole body scanner designed specifically for the purpose of detecting gamma-emitting isotopes that have been internalized by children. The instrument was first installed at the Hirata Central Hospital in Fukushima Prefecture in December 2013. The design principles, implementation and the initial operating experience are reported in the paper.
The age distribution of the first 100 children who were selected for isotope counting are given in the figure below:
Radioactive 134-Cs and 137-Cs was not detected in any of the 100 subjects. Naturally occurring radioactive 40-Potassium (40-K) was detected in all subjects. Typical gamma-ray energy spectra are are shown in the figure below with black dots indicating data collected with a subject in the instrument (4 min of counting), and those shown in grey dots were taken without subject (blank measurement taken over 5 hours and normalized to 4 min).
Given the experimental conditions the minimum detectable activity (MDA) for 137-Cs (Bq/body), was calculated for each subject and plotted against weight for each child as shown below.
The detection limit of BABYSCAN for 137-Cs, one of the most significant isotopes with respect to radiological health risks released from Fukushima, is better than 50 Bq/body. Despite this low detection limit, 137-Cs was not detected in any of the first 100 children scanned from the most contaminated areas of Japan in Fukushima Prefecture. Ongoing analyses will be carried out on a larger-scale with BABYSCAN and reported in publications by the investigators in the future. I will report on data as it becomes available.