by Jay T Cullen
The purpose of this post is to bring to the attention of readers here a review of the available measurements and science based investigations of the Fukushima Daiichi Nuclear Power Plant (FDNPP) and its impact on the Pacific Ocean ecosystem and public health. This post is part of an ongoing effort to summarize scientifically rigorous information about the disaster for interested readers. The new paper is a product of a working group on radioactivity in the ocean convened by the Scientific Committee on Ocean Research (SCOR) an international non-governmental non-profit organization. I highly recommend this paper for anyone who wishes to better understand what the international scientific community has found about the marine release, fate and impact of FDNPP-derived radionuclides in the marine environment. The working group was made up of 10 experts from 9 different countries, including Japan, and published the open access paper in Annual Reviews. The main findings of the review were as follows:
- The amount of 137Cs released from the plant was ~50-fold less than the fall out from nuclear weapons testing in the 20th century and ~5-fold lower than that released from Chernobyl in 1986. Total releases from Fukushima are similar to the discharges of 137Cs from the nuclear fuel reprocessing plant Sellafield in the UK
- Initial releases in the weeks to months after the disaster which began on March 11, 2011 dwarf those from aggregated ongoing releases from the plant site
- The majority of radionuclide releases ended up in the Pacific Ocean with most deposition and input occurring close to the FDNPP
- Current range of estimates of the total 137Cs ocean source term are 15-25 PBq (PBq = 1015 Becquerel where a Bq is one nuclear decay event per second). While many other radionuclides were released from FDNPP, the most likely isotopes to represent a health risk to the marine ecosystem and public are those of Cs given their longer half-lives for radioactive decay (134Cs = ~2 yrs; 137Cs = ~30 yrs) and higher relative abundance compared to other isotopes of concern in the FDNPP source term
- Because Cs is very soluble it rapidly dispersed in the ocean after the disaster given mixing, transport and dilution by ocean currents. Peak levels of 137Cs occurred close to the plant in 2011 where activity concentrations near FDNPP was tens of millions of times higher than before the accident. By 2014 137Cs concentrations in the central North Pacific was about six times the remaining weapons testing fallout and about 2-3 times higher than prior fallout levels in the northeast Pacific near to North America. Most of the fallout remains concentrated in the top few hundred meters of the ocean. Measurements being made by the Fukushima InFORM project indicate that maximum 137Cs levels off the North American coast are likely to occur this year before declining to levels associated with background nuclear weapon testing before the accident by about the end of this decade
- There are unlikely to be measurable effects on marine life with the exception of coastal areas very close to FDNPP immediately after the disaster. Monitoring of fish species in Fukushima Prefecture show that about 50% of samples in coastal waters had radiocesium levels above the Japanese 100 Bq kg-1 limit, but that by 2015 this had dropped to less than 1% measuring over the limit. High levels continue to be found in fish around and in the FDNPP port
- Given levels in seawater and marine organisms measurable impacts to human health through contact with the ocean and the consumption of seafoods are very unlikely
There are many informative graphics and moderately technical summaries of available studies found in the new paper. The authors highlight the difficulty of monitoring radionuclides in the ocean given the dynamic nature of the sea and logistical challenges presented by the temporal and spatial scales and low levels of FDNPP derived contamination going forward. In addition to providing ongoing assessments of risk to the environment from the disaster it is likely that useful information about ocean circulation will be obtained through continued monitoring efforts.
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.
Maximum activities of ~1-3 Bq/m^3 as the heart of the contaminated plume reaches the North American coast in the coming 2 year period are roughly 25-fold lower than 137-Cs activities in the North Pacific circa 1960 resulting from atmospheric weapons testing. Therefore, it is unlikely that 137-Cs activities of 3 Bq/m^3 or associated radionuclides released at lower total activities from Fukushima will represent significant health risks to the North Pacific ecosystem.
Ongoing monitoring of radionuclide activities in the North Pacific is required to ground-truth models of Pacific Ocean circulation and plume evolution and to provide the best information to determine likely impact to residents of North America.
The purpose of this post is to compare the concentrations of Sr-90 and Cs-137 in the North Pacific Ocean over the last 50 years to the concentrations predicted to arrive on the west coast associated with waters affected by release of radionuclides from the Fukushima-Daiichi Nuclear Power Plant. Given present levels that are being measured in the eastern Pacific and barring release rates that significantly exceed past rates in March-April 2011, when release rates were 10,000-100,000 times greater than ongoing releases at the plant, the impact on marine organisms and the marine environment is likely to be less significant than impacts owing to radioactivity in the 20th century. What follows is a comparison of the concentrations measured and predicted over much of the Pacific owing to Fukushima to the concentrations that were present in the mid-1960s from the fallout of atmospheric weapons testing that is free from any discussion of safe doses or models of radiation exposure to organisms.
This post is part of an ongoing series that endeavors to provide useful and accurate information about: 1) the fate of Fukushima derived radionuclides in the Pacific Ocean, and, 2) the impact of these radionuclides on the marine ecosystem and the west coast of North America. The purpose of this diary is to draw attention to a number of poorly researched posts about a recently published study (unfortunately this study is behind a publisher pay-wall) in a Chinese journal that predicts a concentrated plume of radioactive elements from Fukushima arriving on the west coast. It is an unfortunate but common example of how news aggregation sites can misinterpret the results of a scientific study and misinform the public.
What models can and cannot say about the Fukushima plume
The study in question by Fu and co-workers published in the Journal of Ocean University of China in 2014 (behind pay-wall unfortunately) is wholly incapable of describing the behavior of dissolved radionuclides in the plume that is now arriving on the west coast of North America.
From the paper the authors themselves state in the methods that:
“In the study, the radioactive pollutant in the ocean is treated as a mixture of multiple Lagrangian particulates, and each particulate represents a radioactive element. The particulates can move in both horizontal and vertical directions, but cannot diffuse and mix with surrounding seawater.”
What this means is that rather than being allowed to mix and diffuse (or decay or sink after becoming associated with particles) the radionuclides are treated as neutrally buoyant drifters. The model, therefore, greatly overestimates the concentrations of radionuclides reaching the west coast of North America in the plume.
For those interested in models using accurate physics that will allow for an accurate prediction of radionuclide concentrations consult the following studies:
The Behrens et al. study is open-access while the Rossi et al. study is not. Measurements taken in the North Pacific by Canada’s Department of Fisheries and Oceans and InFORM team member Dr. John Smith indicate that the Rossi et al. study predicts the arrival time of the plume on the west coast but overestimates the activity of the Fukushima derived radionuclide 137-Cs. Behrens et al. predict a too late time of arrival but with lower activities that appear to more realistic. It important to note that these models carry the own simplifications and assumptions (e.g. see section 3.4 Caveats of the Behrens et al. (2012) study) and that recent measurements suggest that some of the Fukushima plume is being dispersed to the south rather than to the east in the Pacific (e.g. Kumamoto et al. (2014) open-access; more on this study in a forthcoming post).
Articles that confuse the conclusions of the Chinese study are a good example of poor reporting on an important subject. The example here was originally spawned by Energy News who have a history of inaccurate reporting on Fukushima and then propagated through the web by uncritical followers of the site.