From the PNAS paper, beginning with the, um, scary stuff:
On March 11, 2011, an earthquake and subsequent tsunami led to flooding of the Fukushima Dai-ichi nuclear power plants in Japan. Coolant pumps failed to operate and the power plant reactors overheated, leading to a release of radionuclides directly into the ocean exceeding that from any previous accident (3). The release of radionuclides produced a 1–2 wk pulse that peaked on April 6, 2011 with ocean concentrations of 68 MBq m−3 (3) and an estimated total release of up to 22 × 1015 Bq of 137Cs (4) (1 Bq = 1 disintegration s−1).
Some remarks, hopefully not too trenchant, on the concentrations of radionuclides in seawater: The conversion factor from "Bq" (Becquerel) to millicurie (mCi) is 3.7 X 1010
Bq/Ci. This means that the concentration of radionuclides in the ocean outside the plant peaked at about 1.8 milli
curies (thousandths of a Curie) per cubic meter on April 6 of 2011. The density of seawater is roughly 1030 kg m-3
, or roughly 1.03 metric tons per cubic meter. The Density of Seawater.
This means that if you were to drink a ton or so of seawater outside the Dai-ichi nuclear plant on April 6, 2011 you might have received a dose comparable to to the low end of some typical radiochemical doses received in radionuclide based diagnostics and treament.
This suggests that it might not have been a good idea to drink one or two tons of seawater outside the Fukushima nuclear plants on April 6, 2011, or if one did, that one should not do it often.
The fish however are not in a position in which the ingestion of seawater is optional.
Let's see what we can learn about radioactive fish from the paper.
First some book keeping. Radioactive cesium has been in the oceans since the late 1940's owing to nuclear weapons testing and, in two instances, events in a nuclear war, which, as luck would have it, took place in Japan in the only nuclear war ever observed.
(Interestingly, despite having been the site of the only nuclear war, Japan has the highest life expectancy in the world, 80.2 years although it seems likely that this long life expectancy is most likely in spite of its victimhood in nuclear war and not because of it.)
This radioactive cesium-137, 137Cs, - which has a half-life of 30.08 years - has long been tracked around the world and the background levels are generally well understood, despite showing some distributional effects related to intentional and unintentional release of this isotope. I wrote a diary in this space about the international distribution of cesium-137: Every Cloud Has A Silver Lining, Even Mushroom Clouds: Cs-137 and Watching the Soil Die.
(By the way, if you are interested in the general scientific level of rote, dogmatic anti-nukes, you may peruse an anti-nuke's list of half-lives of radioactive nuclei, a list that is notable inasmuch as there are zero half-lives for which a half life is correct: Zero correct half lives.)
Three other radioactive isotopes and one non-radioactive isotope are found in used and active nuclear fuel: The non-radioactive isotope of cesium that is formed as a fission product in nuclear reactors is cesium-133 (133Cs), which is the only non-radioactive isotope of cesium. Mined cesium's chief use is to lubricate oil and gas drilling wells, including those that are used for all that wonderful fracking that is going on around the country. The radioactive isotopes, besides 137Cs, are Cs-134 (134Cs), Cs-135 (135Cs), and Cs-136 (136Cs).
The latter, 136Cs has a half-life of 13 days, and the amount of it that remains since April 6, 2011 is approximately 1.4 ten billionths of any amount that leached out of the reactor remains. It wasn't very much to begin with, because of most of decayed in the reactor during operation, where its decay heat helped to run the turbines that the reactors drove until they were struck by a tsunami, whereupon instead of generating electricity, they began to generate paroxysms of stupidity from anti-nukes. Everyone who is alive today obviously survived the release of highly radioactive 136Cs.
134Cs is not directly formed in nuclear reactors from the fission of uranium or plutonium, since the mass number 134 among fission products ends up as 134Xe, a non-radioactive (and naturally occurring) isotope of the gas xenon. Living things have been breathing 134Xe for the entire history of life on this planet. (Interestingly, some of this 134Xe is believed to have formed from the spontaneous fission of plutonium that may have accreted with the earth or may have been present in the smaller rocky bodies from which the earth formed.)
However, although it does not form directly from fission, 134Cs nevertheless does accumulate in reactors. The mechanism is this: 133Cs is directly formed in these reactors and initially is not radioactive. However all reactors require a flux of free neutrons to keep them going and one of the ways that neutrons can interact with matter is to be absorbed into an atomic nucleus, generally referred to as neutron capture. When a non-radioactive isotope absorbs a neutron, depending on which isotope it is, it can become a radioactive isotope in a process known as "induced radioactivity." We write 133Cs[n,ɤ]134Cs which is a way of saying that 133Cs absorbs a neutron, releases a ɤ ray and becomes 134Cs.
The quantity of 134Cs depends on the nature of the fuel employed, the nature of the reactor employed, and most sensitively, on the amount of time and the level of power that the fuel has sustained.
In general, the physical quantity of 134Cs that forms in a nuclear reactor is always much lower than the quantities of 137Cs and 133Cs formed, again, because it is not a fission product. However, since 134Cs has a much shorter half life than 137Cs, 2.0652 years as opposed to 30.08 years, gram for gram, 134Cs is much more radioactive than 137Cs, but also doesn't stay around as long. For instance, only 1 in every 14 million atoms of 134Cs formed in thermonuclear tests in 1963 survives today, whereas 32.33% of the 137Cs formed in those same tests is still here. The difference in the half-lives means that gram for gram, the radioactivity of 134Cs - what we call the "specific activity" - is about 14.5 times as great as the activity of 137Cs.
Thus if 134Cs is present, it is of relatively recent origin. By the way, a diarist appeared on this site claiming that the presence of 134Cs in the seas around Fukushima was evidence that the reactors were still critical well after the accident in a so called "China Syndrome" fantasy. This is arrant nonsense, and can only be made by a person who is totally ignorant of nuclear science.
As it turns out, shortly after the 9.0 earthquake and 14 meter tsunami struck the reactors, the ratio of radioactivity between the two isotopes was close to 1 to 1. A paper published in October of 2011 (Environ. Sci. Technol. 2011, 45, 9931–9935), cited as a reference in the PNAS paper had this to say:
During the first month of release data, 134Cs/137Cs activity ratios were one (0.99 (+ / - 0.03)) for Dai-ichi north and south discharge channels) and extremely uniform (Supporting Information Figure S1). This makes the tracking of Fukushima derived radionuclides in the ocean quite straightforward, since given its relatively short 2 year half-life, the only source of 134Cs in the North Pacific at this time would be the Dai-ichi NPPs. Hence in addition to the elevated Cs activities, the presence of 134Cs is a unique isotopic signature for tracking these waters and calculating mixing ratios. This ratio of Cs isotopes is determined by reactor design and fuel cycle and age. Interestingly a 134Cs/137Cs ratio of 1.0 here is considerably higher than 25 years ago when a ratio of 0.54 (+ / - 0.04) was reported in Chernobyl fallout.6
The same paper has this to say:
Finally though, while the Dai-ichi NPP releases must be considered “significant” relative to prior sources off Japan, we should not assume that dose effects on humans or marine biota are necessarily harmful or even will be measurable...
...With respect to dose effects on humans, at a level approaching 100 000 Bq m-3 for 134Cs and 137Cs found at the Dai-ichi discharge channels in June the dose due to direct exposure during human immersion in the ocean can be calculated to be 1 μSv d-1, and would be at least a factor of 10 lower if on a ship above and not in direct contact with the water. This is insignificant relative to the average dose from all sources to the Japanese population of about 1.5 mSv yr-1. This low dose should not be surprising, as levels of the most abundant naturally occurring radionuclide in the oceans, potassium-40, are comparable to 137Cs offshore, with a typical ocean value of 12 000 Bq m-3. Levels of 137Cs in June and July at Dai-ni and Iwasawa Beach of 4000 to 10 000 Bq m-3 were comparable to permissible drinking water limits for 137Cs , which in the US are 7400 Bq m-3 (EPA limit of 40 μSv yr-1calculated here for 1 L per day consumption) and 10 000 Bq m-3recommended by the World Health Organization. Thus even at the observed concentrations at the discharge channels in June and July there will be no significant direct dose effect on humans, and only a short distance away, 137Cs concentrations would be below drinking water limits for Cs isotopes.
This would a good time for some rote scientifically illiterate anti-nuke to pipe in and tell us all that "there's no safe level of radioactivity," ignoring of course that you would die
without having some radioactive material in you, in particular, potassium.
I made some remarks about potassium-40 (40K) in the ocean in this space where I showed that it contains about 530 billion curies of this radioactive isotope, an amount that easily dwarfs all the radioactive cesium of all the reactors that have ever operated for all time. Here's the link: How Radioactive Is the Ocean?
So let's get to the point. We were talking about tuna.
The PNAS paper says this about tuna:
The Pacific bluefin tuna (PBFT), Thunnus orientalis, is a highly migratory fish that inhabits the western and eastern North Pacific Ocean at various life stages (5) (Fig. 1A). Mature PBFT spawn in the western Pacific, and some juveniles remain in Japanese waters while others migrate eastward to the California Current Large Marine Ecosystem (CCLME) (Fig. 1A), with most migrating late in their first year or early in their second (5). Thus, all Bluefin between years 1–2 (here, 2-y-old PBFT) caught during summer in the eastern Pacific must have migrated from the western Pacific within several months of capture. Waters north of the Kuroshio Current (Fig. 1A) showed high radionuclide concentrations in spring 2011 (3), and juveniles make extensive use of this region before their eastward migration to the CCLME (6).
We tested the possibility that juvenile PBFT served as biological vectors of radionuclides between two distant ecoregions: the waters off Japan and the CCLME. We analyzed 2-y-old PBFT caught off San Diego,CA, in August 2011, known from size to be recent Japan migrants, for the presence of Fukushima-derived radionuclides. Because Cs accumulates in themuscle tissue of fish (7),we analyzed the white muscle tissue of PBFT in 2011 for concentrations of 134Cs, 137Cs, and various naturally occurring γ-emitting radionuclides. To rule out non-Fukushima sources of radiocesium in fish muscle, we also measured radionuclide concentrations in PBFT collected in California waters before the Fukushima discharge (2008) and in yellowfin tuna (YFT), T. albacares (August 2011), in the CCLME where they are highly residential (8, 9).
Here's what they found out:
Inferences about the safety of consuming radioactivity-contaminated seafood can be complicated due to complexities in translating food concentration to actual dose to humans (12), but it is important to put the anthropogenic radioactivity levels in the context of naturally occurring radioactivity. Total radiocesium concentrations of post-Fukushima PBFT were approximately thirty times less than concentrations of naturally occurring 40K in post-Fukushima PBFT and YFT and pre-Fukushima PBFT (Table 1). Furthermore, before the Fukushima release the dose to human consumers of fish from 137Cs was estimated to be 0.5% of that from the α-emitting 210Po (derived from the decay of 238U, naturally occurring, ubiquitous and relatively nonvarying in the oceans and its biota (13); not measured here) in those same fish (12). Thus, even though 2011 PBFT showed a 10-fold increase in radiocesium concentrations, 134Cs and 137Cs would still likely provide low doses of radioactivity relative to naturally occurring radionuclides, particularly 210Po and 40K.
One may review the data in table 1 by accessing the paper directly.
In comparison to the naturally occurring 40K the radiocesium in the tuna is well, um, trivial.
It is not my intention however to prevent terrified scientifically illiterate anti-nukes from being scared of radioactive tuna.
Like I said at the outset, tuna may actually be a species in trouble, and anything that serves to protect these organisms from severe stress to their populations is probably desirable.
Eat tofu instead.
Have a nice day tomorrow.
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