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Measuring Fukushima Contamination in Fish Caught in Hawaii

Yellowfin tuna, Thunnus albacares leaping from the water

By Jay T. Cullen

The purpose of this post is to summarize a recently published, peer reviewed, scientific study that investigated levels of Fukushima derived contamination in fish caught in the North Pacific and sold at market in Hawai’i.  This post is part of an ongoing series dedicated to bringing quality scientifically derived information to readers so that they can form an evidence based opinion regarding the environmental impact of the Fukushima Daiichi Nuclear Power Plant meltdowns. The paper by Azouz and Dulai (both at the University of Hawai’i at Manoa) summarizes levels of human made 134-Cesium (134Cs half life ~2 years) and 137-Cesium (137Cs half life ~30 years) and naturally occurring 40-Potassium (40K half life 1.25 billion years) in 13 different fish purchased in Hawai’i in 2015.  The findings of the study were that:

  1. 3 of the 13 fish had detectable levels (above the 95% confidence interval) of 134Cs which can be linked to the Fukushima disaster
  2. Highest levels of radiocesium were found in ‘ahi tuna with 134Cs and 137Cs of 0.10 ± 0.04 Bq kg-1 and 0.62 ± 0.05 Bq kg-1 respectively
  3. Most of the fish carried no fingerprint of the Fukushima disaster
  4. Levels of radiocesium were well below intervention levels of 1,200 Bq kg-1 set by the United States Food and Drug Administration
  5. Doses to fish consumers from human made radioisotopes were 30-1,000 fold lower than the dose experienced because of naturally occurring 40K in the fish
  6. Neither the effective dose from the natural nor the human made radioisotopes represent a significant health risk to consumers of the fish given scientifically established dose-response relationships

These results agree with the results of the Integrated Fukushima Ocean Radionuclide Monitoring Project (InFORM) I head up at the University of Victoria which has been making similar measurements on North Pacific fish returning to rivers in North America.

The Azouz and Dulai paper was published recently in the journal Pacific Science and can be found here.  The authors obtained 13 different species (Ahi, Albacore Tuna, King Salmon, Cod, Dover Sole, Halibut, Mahi Mahi, Monchong, Onaga, Opah, Opakapaka, Swordfish and Yellowfin Tuna) of fish that were caught in the North Pacific (>20oN) and commonly consumed in Hawai’i at local markets.  Information about the range and size of the fish are given in Table 1:

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Levels of Radiocesium in Fish From Hawai’i

Samples of the fish tissue were freeze dried and homogenized before gamma emitting radioisotopes were measured using a gamma spectrometer by counting samples for a period of 7 days. Levels of 134Cs, because of its short half life, serve as a fingerprint of Fukushima in samples as previous sources of this human made isotope (e.g. 20th century nuclear weapons testing and the Chernobyl disaster) are sufficiently far in the past that all of the isotope has decayed away and is no longer present in the environment.  Results of the analyses are summarized in the following figure:

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Fig. 1 Cesium activities in fish collected in the North Pacific in 2015 and available for consumption in Hawai’i

In 3 fish statistically significant (>95% confidence interval) but trace levels of 134Cs was detected.  Given that 137Cs/134Cs ratio in vast majority of the release from the Fukushima site was ~1 the authors were able to determine the fraction of radiocesium present in these fish owing to Fukushima versus legacy sources like atmospheric weapons testing.  Maximum radiocesium levels in the fish approached 0.7-0.8 Bq kg-1 which is more than 1,500 fold lower than conservative levels thought be a health risk set by the FDA (1,200 Bq kg-1).  Most fish had radiocesium attributable to weapons testing fallout. Fukushima radiocesium accounted for ~60% of the radiocesium detected in an Ahi measured by the authors.

Levels of Naturally Occurring 40-Potassium in Fish

Naturally occurring 40K decays with a half life of 1.25 billion years and in taken up into the tissue of marine fish.  The levels of 40K in the fish measured by the authors are summarized in the table below:

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Levels of artificial radiocesium and naturally occurring 40-K in fish from Hawai’i

Activity of 40K (Bq kg-1) tended be ~100 fold higher in the fish tissue than radiocesium activities.

Effective Dose of Ionizing Radiation and Health Impact to Fish Consumers

The authors determined the impact of fish consumption on the ionizing radiation dose experienced by individuals consuming an average amount of fish per year (24.1 kg per year or 53.1 pounds per year).  The table below compares the dose in nanoSieverts per year (10-9 Sv yr-1) owing to historic and Fukushima sourced radiocesium and naturally occurring 40K in seafood.

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Committed effective dose to fish consumers from artificial (human made) and naturally occurring 40-K

Converting isotope activities in the fish to dose demonstrates that 40K is responsible for ~100 times higher dose than 134Cs + 137Cs. Doses to humans from consuming the fish owing to radiocesium were 0.02–0.2 µ Sv yr-1, while doses of 6–20 µ Sv yr-1 were contributed by the natural 40K present in the same fish. These levels of radioisotopes and the calculated doses to consumers are similar to those reported by the InFORM project who have looked at Pacific salmon returning to rivers and streams in North America over the last 3-4 years. It is important to note that the bulk of ionizing radiation dose to fish consumers normally results from 210-Polonium (210Po half life 138 days) naturally present in the fish but this isotope was not measured in the Azouz and Dulai study.

Conclusion

Fukushima derived radioisotopes 134Cs and 137Cs were detected (at 95% confidence interval) in 3 of 13 fish caught in the North Pacific and commonly consumed by people living in the Hawaiian islands.  The radiocesium in most fish reflected contamination largely present in the North Pacific Ocean owing to atmospheric weapons testing during the last century.  The levels of radiocesium in the fish were a small fraction of the levels of naturally occurring radioisotopes like 40K.  The committed effective dose of ionizing radiation to fish consumers is dominated by the naturally occurring isotopes and do not remotely approach levels known to represent a significant or measurable health risk to human beings.  The results of this study agree with previously published research and results of the InFORM project which focuses on the impact of the Fukushima disaster on the marine ecosystem and public health in North America.

The fate of atmospheric Fukushima radiation

Did you enjoy your trip? If you were alive during the Fukushima meltdown in 2011, you received an extra dose of radiation equal to that received on a roundtrip flight from Vancouver to Tokyo. This is the result according to research presented by Nikolaos Evangeliou of the Norwegian Institute for Air Research at the annual meeting of the European Geophysical Union earlier this year.

Continue reading The fate of atmospheric Fukushima radiation

It’s safe to return to some parts of Fukushima, study suggests

Bags of contaminated topsoil are collected at a temporary storage site in Fukushima prefecture. (Kyodo/AP Images)

By Katherine Kornei
Originally Published in Science
Mar. 10, 2017

Six years after Japan’s Fukushima Daiichi Nuclear Power Plant disaster, thousands of former residents evacuated from the region are returning home. But is it safe? A new study says yes, finding that inhabitants of a never-evacuated village just 60 kilometers away don’t have to worry about dangerous levels of radiation over their lifetime. The team also found that natural radioactive decay and weathering from rain deserve far more credit for reducing radiation levels than do expensive decontamination efforts, like topsoil removal. Continue reading It’s safe to return to some parts of Fukushima, study suggests

Fukushima Six Years On

In the last year, TEPCO has continued its efforts to reduce groundwater flow though the site, has capped the port sediments to reduce the risk of resuspending radionuclides, and has begun evaluating the current state within the primary containment vessels of reactors 1 – 3 while also working on construction of the cranes that will be necessary for eventual removal of spent fuels and decommissioning.

Groundwater flow at the site has been reduced by two-thirds since freezing of the ice wall began at the end of March 2016. The wall has been frozen in a phased approach and currently roughly five sections of the wall on the mountain side of the site have yet to be frozen which should further reduce water flow.

Two layers of capping sediments have been laid down within the Fukushima marine port in an effort to limit resuspension of radionuclides in the sediments through water or animal movements of sediments. The areas immediately adjacent to the reactors were covered in 2012, but the cap over the rest of the port was completed in December 2016. Water samples within the port show that contamination has decreased substantially since 2013 and that levels of 134Cs have dropped below the TEPCO detection limit (280 – 640 Bq m-3) in most locations. Levels of 137Cs remain between 480 and 3,100 Bq m-3 within the port. Immediately outside the port, levels of both 134Cs and 137Cs are below detection limits (420 – 800 Bq m-3). While these levels far exceed what we are measuring on the west coast of British Columbia, and are far higher than InFORM detection limits, they are a significant drop compared to what was seen even in 2013 and are below the Canadian action level for drinking water of 10,000 Bq m-3, though drinking seawater is never a good idea for humans.

At units 1 and 3, construction of building covers and cranes to remove debris and materials from the spent fuel pools is ongoing. Gaining more press attention were the results and images from two robotic incursions into the primary containment vessel of reactor 2 in January. Not unexpectedly, as the robots approached to the reactor pressure vessel, radiation levels increased to the highest levels yet measured at the site, potentially fatal to humans if exposed for only a few seconds. While the robots were disabled by the conditions, the images are vital for developing an initial plan later this year for the lengthy decommisioning process ahead. Robotic investigations of the primary containment vessel are planned for reactor 3 in the coming weeks. Additional robotic imaging missions will be necessary to obtain a better picture of the damages sustained and the current fuel placement after the meltdowns in 2011.

Source: TEPCO

Additional recent reporting from Science.

As evacuees move back, Fukushima cleanup faces daunting obstacles

Workers decontaminate a forest near Fukushima in Japan. Jeremy Sutton-Hibbert/Alamy Stock Photo
Workers decontaminate a forest near Fukushima in Japan. Jeremy Sutton-Hibbert/Alamy Stock Photo

By Dennis Normile
Originally published in Science
Mar. 2, 2017

TOKYO—Six years into a decommissioning effort expected to last into the 2050s, an official leading the work on the stricken Fukushima Daiichi Nuclear Power Plant claims that cleanup crews are making “steadfast progress.” But thorny technical obstacles must be overcome.

The 9.0-magnitude earthquake off Japan’s northeast coast on 11 March 2011 triggered one of history’s most devastating tsunamis. The one-two punch killed nearly 16,000 people, left more than 2500 missing, and wiped out infrastructure in coastal communities.

The tsunami also knocked out Fukushima’s systems for cooling its nuclear reactors, causing core meltdowns in three of the plant’s six reactors. Hydrogen explosions blew out the walls and roofs of the buildings housing units 1, 2, and 3, releasing massive amounts of radiation [editors note: explosions were in units 1, 3, and 4]. Much of the contamination was swept into the Pacific Ocean, but winds deposited fallout over parts of northeastern Japan. Some 160,000 people living near the plant were evacuated or fled on their own.

On the eve of the sixth anniversary of the disaster, officials took pride in what they view as successful efforts to minimize the health threat to surrounding communities. Radiation from the crippled reactors is no longer having an impact outside the plant, Naohiro Masuda, head of decommissioning for Fukushima owner Tokyo Electric Power Co. (TEPCO), said today at a briefing here. He noted that evacuated residents are returning to their homes as decontamination work reduces exposure levels below thresholds. At the power plant, radiation levels are now so low that the 6000 workers slowly demolishing the damaged reactor halls need only wear typical construction site safety gear except when working near the three reactors that suffered meltdowns. And radiation levels just offshore remain below the limit for drinking water set by the World Health Organization, Masuda said. Given the progress, he reiterated that TEPCO is confident they can stick to a previously set roadmap that envisions completing the decommissioning 30 to 40 years after the accident. But doing so won’t be cheap. Last December, Japan’s Ministry of Economy, Trade and Industry revised its estimate of the total cost of decommissioning up to $188 billion.

Stemming ocean contamination has been a thorny challenge. Since early in the crisis, crews have circulated water through the damaged reactors to prevent overheating that could lead to further fuel melting. That water, and groundwater flowing through the site, is heavily contaminated and TEPCO has struggled to keep it from seeping into the Pacific. Schemes to divert groundwater away from the plant and freeze a wall of soil around the reactors down to bedrock—to contain contaminated water—have minimized leaks, Masuda said.

In the meantime, TEPCO has accumulated 960,000 tons of contaminated water stored in 1000 10-meter-tall tanks at the site. TEPCO has removed cesium, strontium, and more than 50 other radionuclides from that water. But they have been stymied by tritium, a radioactive hydrogen isotope in the water. Several experimental approaches to removing the tritium “were judged to be impractical,” Masuda said.

Tritium occurs naturally in water but in minuscule concentrations. Simply releasing the tritium-laden water, perhaps after further dilution, is one disposal option, Masuda said. Another would be to evaporate the water, releasing some tritium into the atmosphere, as was done at the Three Mile Island nuclear plant in Pennsylvania after its 1979 accident. An advisory committee is now studying the problem and will hold discussions with local communities “so TEPCO will be able to act in a responsible manner in dealing with the tritium,” Masuda said.

Another major hurdle is determining the condition and location of the melted fuel, much of which is believed to have dropped to the bottoms of the containment vessels where high radiation levels preclude human entry. Robotic investigations are proving problematic. In January, the camera on a robotic probe sent into the Unit 2 containment vessel was fried by radiation, though it did return important images before its demise. Then last month, a small robot on tanklike treads was sent through a 10-centimeter-diameter pipe into the vessel to investigate the presumed location of the damaged fuel. But it got tangled up in debris and was abandoned.

TEPCO is now thinking it might need a robot able to jump over debris. And they are planning robotic investigations of the units 1 and 3 containment vessels in preparation for a planning session this summer to set a policy for recovering the melted fuel.

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DOI: 10.1126/science.aal0849