The purpose of this post is to communicate why the more you know about radiation the less you worry about nuclear radiation – even the consequences of the terrible accident at Fukushima Daiichi.
To get your skeptical circuits warmed up, let's begin with the above graphic, an excerpt from Randall Munroe's What-If XKCD where Randall “answers your hypothetical questions with physics, every Tuesday”.
What if I took a swim in a typical spent nuclear fuel pool? Would I need to dive to actually experience a fatal amount of radiation? How long could I stay safely at the surface?
Randall's exploration of the question is a useful introduction to how to think about risk and radiation dose – in relation to intensity, exposure time and mediation medium (water in this example). Randall begins
Assuming you’re a reasonably good swimmer, you could probably survive treading water anywhere from 10 to 40 hours. At that point, you would black out from fatigue and drown. This is also true for a pool without nuclear fuel in the bottom.
After you've enjoyed “Spent Fuel Pool“, I recommend Randall's Radiation Dose Chart, which has become a frequently-cited resource for an introduction to radiation dose and risk. The chart is useful for an overview of relative magnitudes. In addition to Randall's chart I recommend that you download for your archive Natural Radioactivity, published by the physics department of Idaho State University. That is “ground truth” on the details of background radiation in the oceans, or land – lots of numbers and units.
With that gentle introduction I hope you are ready to read some resources that go into Fukushima monitoring in a bit more detail. Are you worried about contamination from the Fukushima Daiichi reactors? E.g., turning the Pacific Ocean into a place too dangerous to swim? Too dangerous to eat the Blue Fin Tuna?
First you will find your hard data at Monitoring environmental radiation Nuclear Regulation Authority (NRA), Japan. In particular, you can find the weekly Sea Area Monitoring reports. As I write the latest report is for 10 December, 2013 (PDF).
To make sense out of all the Becquerels/Litre in the NRA tabulations I recommend Putting Fukushima in Perspective: A primer on radioactivity in the Ocean written by University of Victoria marine chemist Jay T. Cullen (@JayTCullen). Dr. Cullen is investing his personal time in science communication to inform the public about the real risks associated with contamination from the Fukushima site. From his primer article:
Talk of plumes of radioactivity being broadcast across the Pacific must take into account that the background radioactivity of seawater is about 14 Bq/L. It is important that although one can detect isotopes from the reactor in the environment the absolute levels are very low and will be lower as the ocean mixes, and the isotope decays.
Dr. Cullen is using 14 Bq/L as the global ocean radioactivity – what does that mean? Well, one Becquerel is that quantity of a radioactive material that will have 1 transformations in one second. So the unit Bq/L tells us there is a concentration of radioactive elements in each litre of ocean that emits at the rate of 1 count per second (cps). We don't know what the material is, but we know a Geiger counter would detect 14 counts/second from a typical litre of sea water. And we know empirically (by swimming in the stuff, eating the Tuna, etc.) that 14 Bq/L is perfectly safe. Even if we don't know exactly what the number means.
Click the thumbnail for full size graphic
So let's examine some of the extensive NRA monitoring, which publishes weekly sampling results from sites immediately around the Fukushima Daiichi breakwater, out to open ocean. The thumbnail to the left shows the worst/highest sample values for Cs-134 and Cs-137 that I could find in the open sea zone (full size).
In the next table I have compared the worst samples to typical ocean background radiation. What we see is that dilution and decay of the cesium isotopes has already reduced the radiation to levels that are insignificant in relation to normal. That indicates that US Pacific coast residents do not need to be alarmed.
Some like to use the radioactivity of a banana to make these units more familiar. A typical banana emits about 15 Bq due to the potassium isotope K-40. So radiation-wise eating a banana is similar to drinking a litre of typical ocean, ignoring retention rates. If you are comfortable with bananas and seawater, but are still concerned about the Fukushima contribution, think of it this way. Equivalent to eating that banana, you would have to drink between 3 and 6 cubic meters of pure water contaminated with the measured concentrations of Fukushima cesium. I think I prefer to get my radiation dose from the banana, but I appreciate they are equivalent.
But what about concentration of the insignificant levels by fish and mollusks into dangerous levels if consumed? Good question. I asked the same question, which led me back to Dr. Cullen again for the analysis of that issue, titled What Controls Levels of Fukushima Radioisotopes in Marine Organisms?
Scientists normally report the amount of a radioactive element in an organism in units of concentration where the mass or activity of the radionuclide is given relative to the weight of the organism or its tissue. The units of these measurements are, therefore, either kilogram (kg) or activity in Becquerel (Bq = disintegrations per second) divided by the mass of the organism or tissue (kg/kg or Bq/kg). We want to understand how much radionuclide ends up in the organism relative to the isotopes concentration in seawater which can be reported in either kg per liter of seawater or Bq per liter of seawater (kg/L or Bq/L). By determining the ratio of the concentration of a radionuclide in an organism to the concentration of the isotope in seawater we define the Concentration Factor (CF) which has units of L/kg:
So if the CF for an element in a given organism is a very high number then that radioisotope tends to bioaccumulate and is found at higher concentrations in the organism than in the surrounding marine environment. Conversely, if the CF is low there is little risk of bioaccumulation in the organism.
So what is the bottom line on seafood?
What can we expect on the west coast of North America?
Beginning in the new year we can expect seawater affected by the Fukushima disaster to arrive at our coast in the Pacific northwest. Peak concentrations in the heart of the plume of affected seawater are expected to be on the order of 0.001 to 0.020 Bq/L based on measurements and physical models of ocean circulation. The much lower radionuclide concentrations are the result of mixing and the decay of shorter lived isotopes. Given known CFs for marine organisms these seawater concentrations will result in much lower concentrations of radionuclides in organisms residing on the west coast compared to their Japanese cousins. The radioactive dose to these organisms or consumers of these organisms will be dominated by the naturally occurring radionuclide Po-210.
A confirming evaluation of the food chain question was published in the June 25, 2013 issue of the Proceedings of the National Academy of Sciences Evaluation of radiation doses and associated risk from the Fukushima nuclear accident to marine biota and human consumers of seafood [open access]. Excerpt from the abstract:
To link the radioactivity to possible health impairments, we calculated doses, attributable to the Fukushima-derived and the naturally occurring radionuclides, to both the marine biota and human fish consumers. We showed that doses in all cases were dominated by the naturally occurring alpha-emitter 210Po and that Fukushima-derived doses were three to four orders of magnitude below 210Po-derived doses. Doses to marine biota were about two orders of magnitude below the lowest benchmark protection level proposed for ecosystems (10 µGy⋅h−1).
My bottom line is — if you wish to monitor for any dangers developing when Fukushima seaborne contamination reaches California, then I suggest you subscribe to Dr. Cullen's blog MarineChemist. That's what we do (we subscribe to his RSS feed). If there is anything to worry about then Dr. Cullen will let you know. Or you can just subscribe to Seekerblog!
I promised to also discuss “what is scary?” My answer is the post-antibiotic world where antibiotics don't work any more. That is really, really scary, especially if you are a geezer like me. Climate change is very scary – but antibiotic resistance is spreading as I write. The big hurts from climate change will probably be after-death experiences for me.