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.
This post is part of an ongoing series that represents an effort to communicate peer-reviewed scientific studies of the impact of the Fukushima nuclear disaster on the North Pacific Ocean and residents of the west coast of North America. 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 (239-Pu half-life 24,100 years; 240-Pu 6,570 years) are not being made given the potential for these isotopes to pose radiological health risks. Measurements of air, soil and water indicate that Pu was released and broadcast into the environment as a result of the triple reactor meltdowns with estimates of the source on the order of 2.3×10^9 Bq of 239,240-Pu or 580 milligrams of the isotopes. Measurements of isotope composition and activity of Pu in seawater and sediments off the coast of Japan indicate that there was no detectable change resulting from the nuclear disaster (behind pay wall). Given that the Fukushima signal is not detectable in the ocean off Japan relative to legacy sources from atmospheric weapons testing in the 20th century there is likely little information in making the same measurements in the eastern Pacific off of North America.
Members of the public are concerned about the presence of the alpha-emitting isotopes of Pu and have been asking why measuring for these elements in seawater and marine biota is not a priority of the InFORM network. The purpose of this diary is to explain why such measurements are less likely to provide information about the plume and its impacts.
A recently published paper by Bu and colleagues in the peer-reviewed Journal of Chromotography A reports the development of a new method to determine Pu isotopes in small (20 – 60 liters) samples of seawater and measurements made of these isotopes off the coast of Japan from July 2011 to January 2013 until the present. Locations where samples were collected are shown in the figure below:
For all the seawater samples analyzed by Bu and colleagues, the 239-,240-Pu activities and 240-Pu/239-Pu atom ratios where found to be 0.00043 to 0.0056 Bq m^-3 and from 0.227 to 0.284, respectively. The results are summarized in Table 4 of the paper and are shown below:
Before the Fukushima accident in March 2011, Pu isotopes were being monitored off the coast of Japan to assess the radiological impact of the nuclear plants on the marine environment. The 239-,240-Pu activities before the meltdowns were below 0.0083 Bq m^−3 and 0.022 Bq m^−3 respectively, with 240-Pu/239-Pu atom ratios between 0.173 and 0.322. These ratios represent the influence of the Pacific Proving Ground nuclear weapon test site, which was characterized by a high 240-Pu/239-Pu atom ratio (0.30–0.36). Results after the Fukushima disaster were typically in the background data range, suggesting no detectable Pu contamination from the accident in the marine environment ~30 km offshore of the Fukushima Dai-ichi reactor complex. This conclusion is consistent with findings from previous studies of Pu isotopes in marine sediments in the western North Pacific after the Fukushima accident.
Given the absence of isotopic and concentration anomalies thus far in the western Pacific resulting from the Fukushima meltdowns there is not very much information to be gained about the evolution of the contaminated seawater plume in time and space. Similarly, the impact of the Fukushima disaster on the health of marine ecosystem with respect to Pu isotopes will be difficult to quantify relative to weapons testing background levels that persist in the environment.
On the Methodology Used to Make the Measurements (If You are Interested, IYI)
The approach used by Bu and colleagues to measure Pu isotopes at such low concentrations and activities involves applying sector field high resolution inductively coupled mass spectrometry. The instrument is able to separate chemical species by their respective mass to charge ratios using a strong electromagnetic field downstream of the plasma ionization source. Great pains were taken maximize the instruments sensitivity to measure the isotopes of interest 238-U, 239-Pu, 240-Pu, and 242-Pu. To remove the seawater matrix (cations and anions that would reduce instrument sensitivity) and elements with mass to charge ratios that would interfere with Pu detection like 238-U the seawater samples were purified using ion selective resins held in columns by passing them through successive loading and elution steps. This process is summarized in the following flow diagram from the paper:
The preconcentration and sensitivity of SF-ICP-MS allows for the very low detection limits required to quantify Pu in relatively small (20 – 60 L) volumes of seawater.