Category Archives: NW Pacific

Open Access Review of Fukushima Radionuclide Source Term, Fate and Impact in Pacific

Schematic of Fukushima Daiichi sources of 137-Cs from Buesseler et al. (in press). Atmospheric fallout (1) and direct ocean discharges (2) represent total petabecquerels (PBq = 10^15 Bq) released in the first month of the meltdowns. Groundwater fluxes (3) and river runoff (4) are approximate ranges for the first year in terabecquerels (TBq = 10^12 Bq), a unit 1,000 times smaller than the PBq used for fallout and direct discharge. Details on source term estimates can be found in the paper ( (Buesseler et al. 2017)

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.

Dramatic Decrease of Fukushima Derived Radionuclides in the Northwest Pacific Ocean 2011-2012

By Jay T. Cullen

A schematic view of the formation and subduction of mode waters in the North Pacific

The purpose of this diary is to report on a recently published (Jan 2015) open-access, peer reviewed study which examined the activities of 137Cs (half life 30.2 yr), 134Cs (half life ~2.1 yr) and 90Sr (half life ~28.8 yr) in the northwest Pacific off the coasts of Japan and China. The diary is part of a ongoing effort to communicate the results of scientific research into the impact of the Fukushima Dai-ichi nuclear disaster on environmental and public health. Men and colleagues report on how activities of these fission produced isotopes changed between three research expeditions in June 2011, December 2011 and June 2012. Activities in seawater decreased dramatically through time for all three isotopes consistent with very high release rates measured from the Fukushima site in March-April 2011 followed by ongoing but many orders of magnitude (10,000 – 100,000 fold) lower releases from the site thereafter. By 2012 the impact of the Fukushima releases could be still be detected in most samples for Cs isotopes however 90Sr distributions were much more uniform with the highest measured activity only slightly above the pre-Fukushima background. These results are consistent with:

  1. the relatively small source term for 90Sr from compared with the Cs isotopes from Fukushima as determined by measurements of air, soil and water after the disaster
  2. the much lower Fukushima derived activities for these isotopes in the eastern Pacific off of North America being measured given decay and mixing of the contamination as it is transported by ocean currents

Continue reading Dramatic Decrease of Fukushima Derived Radionuclides in the Northwest Pacific Ocean 2011-2012

Release, Dispersion and Fate of Radioactive Strontium From Fukushima in the Northwest Pacific Ocean

By Jay T. Cullen

The purpose of this diary is to summarize recent models and measurements of the release of strontium-90 (90-Sr, half life 28.8 yr) to the ocean resulting from the triple meltdowns at the Fukushima-Daiichi nuclear power plant in March 2011. This post is part of an ongoing series aimed at understanding the impact of the disaster on the North Pacific Ocean and residents of the west coast of North America. 90-Sr is a beta-emitting element that is a radiological health concern given its relatively long half life and similar chemistry to the nutrient calcium (Ca). Previous peer-reviewed work indicate that releases of 90-Sr were about 30-10,000 fold less than 137-Cs and similar to the release of 90-Sr from the Chernobyl disaster in 1986 and about 600-fold lower than the releases from atmospheric weapons tests that peaked in the mid-1960’s. Given maximal release rates after the disaster, modeled activities of 90-Sr in the marine foodweb and in fish that accounts for bioconcentration and accumulation predict maximal dose rates from Fukushima to human consumers three orders of magnitude less than doses owing to the presence of 137-Cs in marine products and thus well below maximum dose limits thought to be detrimental to public health. Continue reading Release, Dispersion and Fate of Radioactive Strontium From Fukushima in the Northwest Pacific Ocean

Authors Lower Fukushima Cesium in North Pacific By Order of Magnitude

By Jay T. Cullen


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:

Activities of 137-Cs predicted by the Rossi et al. (2013) model on the continental shelves of North America at two latitudes and off Hawaii over time.

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.

Activities of 137-Cs predicted by the Rossi et al. model along the international dateline in the N. Pacific over time at various latitudes.

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.

Plutonium in the Pacific Ocean From Fukushima

By Jay T. Cullen


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:

Map showing seawater collection stations from the western North Pacific and Tokyo Bay since the FDNPP accident.

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:

Flow chart of the analytical procedure for the determination of Pu isotopes in seawater by anion-exchange chromatography and SF-ICP-MS.

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.