Category Archives: Peer Reviewed

Possible Melted Fuel Seen for First Time at Fukushima Plant

by Mari Yamaguchi
July 21, 2017
Originally published by the Associated Press

TOKYO (AP) — An underwater robot captured images of solidified lava-like rocks Friday inside a damaged reactor at Japan’s crippled Fukushima nuclear plant, spotting for the first time what is believed to be nuclear fuel that melted six years ago. Continue reading Possible Melted Fuel Seen for First Time at Fukushima Plant

Swimming robot probes Fukushima reactor to find melted fuel

by Mari Yamaguchi
July 19, 2017
Originally published by the Associated Press

TOKYO (AP) — An underwater robot entered a badly damaged reactor at Japan’s crippled Fukushima nuclear plant Wednesday, capturing images of the harsh impact of its meltdown, including key structures that were torn and knocked out of place. Continue reading Swimming robot probes Fukushima reactor to find melted fuel

What is the optimum level of ionizing radiation exposure for Life?

By Jay T. Cullen

High energy cosmic rays from deep space lead to a cascade of energetic particles and ionizing radiation in our atmosphere that contribute to the dose experienced by living organisms on Earth. (Swordy, UChicago/NASA)



An interesting open access, peer-reviewed study was published earlier this year in Frontiers in Microbiology that examined how lower than background doses of ionizing radiation affected the growth of bacteria.  This post is part of an ongoing series dedicated to communicating scientifically derived information related to the impacts of ionizing radiation in the environment largely in response to the Fukushima Daiichi nuclear power plant meltdowns in 2011.  Life emerged on our planet billions of years ago when levels of environmental radioactivity were about 5-fold higher than they are today. On average living organisms experience a background ionizing radiation dose of ~1-2 milliSievert (mSv) although there is significant geographical variation across the globe given local geology (radioisotope content of rocks and minerals) and altitude (exposure to cosmic radiation).  Deviations from background occur due to proximity to medical exposure or nuclear energy or weapon related events that only act to increase the dose livings things must tolerate.  Castillo and Smith (2017) conducted experiments to understand how bacteria responded when they were grown in lower than background ionizing radiation dose conditions.  How did they do this and what did they find?

Experimental Conditions

How exactly do you get lower than background ionizing radiation dose conditions for an experiment?  Castillo and Smith were given access to the Waste Isolation Pilot Plant (WIPP) in Carlsbad, NM which you may be aware of given an accidental release of artificial radionuclides that occurred there in 2014. The low background radiation experiment (LBRE) used the geological conditions at WIPP, radiation shielding and radiation sources to test how lower than background ionizing radiation doses affected the growth and gene expression of radiation tolerant bacteria Shewanella oneidensis and Deinococcus radiodurans. The LBRE laboratory is located at a depth of 660 m (~1/3rd of mile) inside a 610 m thick salt deposit that is naturally low in naturally occurring radioisotopes and emits significantly less radiation than other rock formations. To further lower ionizing radiation exposure experiments can be conducted in a 15 cm-thick vault made from pre-World War II (and therefore not exposed to nuclear weapons testing artificial radionuclides), low-activity steel.

This pre-World War II, 15 cm thick steel chamber used to incubate the below background treated cells in the WIPP underground. (WIPP)

Castillo and Smith incubated cells inside the vault to achieve lower than background doses of ionizing radiation (WIPP formation + metal shielding) and control cells grown in the presence of 11.5 kg of a potassium-rich salt (KCl) to generate an energy field of gamma radiation close to aboveground background levels. The WIPP facility, local geology and experimental setup with radiation doses experienced by the bacteria are shown in the figure below.

All organisms on earth grow under the influence of a natural and relatively constant dose of ionizing radiation referred to as background radiation, and so cells have different mechanisms to prevent the accumulation of damage caused by its different components.
LBRE at the WIPP. Numbers in red indicate dose rate in nGy hr-1 =  nSv hr-1. (A) The WIPP site, located near Carlsbad, NM, designed for the permanent disposal of artificial radionculide wastes 660 m below ground in the middle of a 610-m-thick Psalt deposit. (B) LBRE pre-WWII steel vault showing the location of the treatment (below-background) and control (background) incubators, with their respective estimated radiation dose. (C) Side view of the LBRE underground laboratory housed in portable laboratories in the WIPP. (D) Comparison of measured and modeled ionizing radiation dose rates.

Cells were grown in the presence of lower than and at natural background radiation doses and their growth and gene expression measured.

What did they find?

The two organisms responded differently to the radiation treatments.  S. oneidensis cultures did not show a significant difference in growth in response to the reduced radiation dose while D. radiodurans growth was inhibited at the beginning of its exponential growth phase and remained significantly lower than the control with normal background radiation levels. When D. radiodurans was taken from lower than normal radiation and returned to normal background ionizing radiation doses its growth returned to normal again.

Lines indicate bacterial growth under radiation-sufficient (background) and radiation-deprived (below background) conditions with p-values indicating whether differences are significant or not above the datapoints for (A) S. oneidensis and (B) D. radiodurans. The dotted line is the reciprocal control where D. radiodurans was placed back in background from below background conditions where growth returned and was identical to the background treatment.

So D. radiodurans was not able to grow as fast in low radiation conditions while S. oneidensis grew equally well at lower than background and background levels of ionizing radiation.  The authors found that gene expression between the two species was significantly different as well. During mid-exponential phase (8 h in S. oneidensis), six genes related to oxidative stress response, DNA repai, protein folding, and a putative efflux pump that pushes metals out of the cells were turned on (blue bars in graph above).  Poor growth under low radiation for D. radiodurans became clear (p < 0.05) at 34 h . The difference in gene expression for D. radiodurans was that genes related to DNA repair and protein folding activities were turned on, while genes necessary for dealing with oxidative stress and energy production were turned off. The regulation of these genes by D. radiodurans was reversed when the cells were returned to normal levels of radiation suggesting that difference was driven by the reduced amount of ionizing radiation they were exposed to in the lower than normal treatment.

What is the explanation and what does this mean?

The authors thought that S. oneidensis responded to the lack of ionizing radiation as an environmental stress and mounted a classic stress-response to the reduction of natural levels of environmental radiation allowing it to grow at its maximum rate. In contrast, D. radiodurans did not sense this stress, did not mount a stress response, and was therefore limited in its ability to grow (was less fit). Specifically, under radiation-reduced conditions, S. oneidensis increased its ability to deal with oxidative damage, repair DNA damage, and repair damaged proteins, which allowed it to continue to grow normally. In the case of D. radiodurans, it did not respond by expressing enough of these critical genes and suffered as a consequence. The authors are continuing their work to test:

  1. why radiation deprivation may increase oxidative stress and levels of reactive oxygen species (hydrogen peroxide, hydroxyl radical and superoxide) inside the cells.
  2. whether or not the ability to sense and respond to the absence of normal levels of radiation is a trait that both prokaryotic (bacteria and archaea) and eukaryotic (e.g. plants and animals) possess.

This study adds to a growing body of scientific literature that suggest that some level of ionizing radiation may be required for cells to appropriately regulate their internal function and be maximally fit.


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:


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:

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:

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.

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.


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