Consistency is good – there’s a sense of security in knowing that some things will generally remain constant over time. We can always count on gravity, for example, to hold us firmly to the ground; politicians are typically pandering and self-serving; I can count on radioactivity to consistently decay away; and so forth. Of course, not all consistency is good – as Emerson noted, “A foolish consistency is the hobgoblin of little minds, adored by little statesmen and philosophers and divines.” We can also count on the American public to consistently question whether or not evolution actually occurs; many of us know that our perfectionist boss will always insist on yet another round of reviews and edits before letting a document go out the door; we will always find people who are apparently proud of their lack of knowledge; and we can expect that a certain category of blogger will continue to see the end of the world on the near horizon. It is this latter category I’d like to talk about this time – particularly the batch that continues to insist that the reactor accident at the Fukushima Dai’ichi site is going to kill millions.
Before launching into this piece I’d like to point you to a wonderful counter-example of what I just said – a blog posting by oceanographer and University of Washington professor Kim Martini. I have been accused of being part of the pro-nuclear and/or pro-radiation lobby because of my long years of experience as a radiation safety professional – Dr. Martini told me that she became interested in this topic, researched it herself, and came to her conclusions independently of the nuclear energy and radiation safety professionals. In short, she is scientifically competent, intelligent, and has no reason to be biased either pro- or anti-nuclear.
The latest round of Fukushima silliness is the contention that Americans need to evacuate the West Coast because of an apparently imminent release from one or more of the affected reactors and/or the Reactor 4 spent fuel pool. There are also those who blame the Fukushima accident for massive starfish die-offs, for sick animals along the Alaskan coast, and more – all of which (according to the good Dr. Martini) are far from accurate. And anti-nuclear activist Helen Caldicott has gone as far as to state that the entire Northern Hemisphere might need to be evacuated if things get as bad as she fears and the Unit 4 spent fuel pool collapses. So let’s see what the facts are, what the science can tell us, and what the real story might be.
Can the melted reactors go critical?
There have been predictions that the ruined reactor cores will somehow achieve criticality, producing more fission products and spreading more contamination into the water. While this is not strictly speaking impossible it is highly unlikely – sort of like saying that it is remotely possible that Bill Gates will leave me his fortune, but I’m still contributing to my 401(k) account. To achieve criticality (to a nuclear engineer or a reactor operator, “criticality” simply means that the reactor is operating at a constant power) requires reactor fuel that’s enriched to the right percentage of U-235, a critical mass of the uranium (enough to sustain a chain reaction), and it has to be in a configuration (the critical geometry) that will permit fission to occur. Also important in most reactors is a moderator – a substance such as water that will slow neutrons down to the point where they can be absorbed and cause the U-235 atoms to fission. In reactors such as the ones destroyed in Fukushima require all of these components to achieve criticality – take away any one of them and there will be no fission chain reaction.
The ruined reactor cores meet some of these requirements – since they’d been operating at the time of the accident we know that they had a critical mass of sufficiently enriched uranium present. Surrounded by water (either seawater or groundwater), they are likely also immersed in a moderator. But absent a critical geometry the cores cannot sustain a fission chain reaction. So the question is whether or not these cores can, by chance, end up in a critical geometry. And the answer to this is that it is highly improbable.
Consider, for example, the engineering and design that goes into making a nuclear reactor core. Granted, much of this design goes into making the reactors as efficient and as cost-effective to operate as possible, but the fact is that we can’t just slap some uranium together in any configuration and expect it to operate at all, let alone in a sustained fashion. In addition, reactors keep their fuel in an array of fuel rods that are immersed in water – the water helps slow the neutrons down as they travel from one fuel element to the next. A solid lump of low-enriched uranium has no moderator to slow down these neutrons; the only moderated neutrons are those that escape into the surrounding water and bounce back into the uranium; the lumps in a widely dispersed field of uranium will be too far apart to sustain a chain reaction. Only a relatively compact mass of uranium that is riddled with holes and channels is likely to achieve criticality – the likelihood that a melted core falling to the bottom of the reactor vessel (or the floor of the containment) would come together in a configuration that could sustain criticality is vanishingly low.
How much radioactivity is there?
First, let’s start off with the amount of radioactivity that might be available to release into the ocean. Where it comes from is the uranium fission that was taking place in the core until the reactors were shut down – the uranium itself is slightly radioactive, but each uranium atom that’s split produces two radioactive atoms (fission fragments). The materials of the reactor itself become radioactive when they’re bombarded with neutrons but these metals are very corrosion-resistant and aren’t likely to dissolve into the seawater. And then there are transuranic elements such as plutonium and americium formed in the reactor core when the non-fissioning U-238 captures neutrons. Some of these transuranics have long half-lives, but a long half-life means that a nuclide is only weakly radioactive – it takes 15 grams of Pu-239 to hold as much radioactivity as a single gram of radium-226 (about 1 Ci or 37 GBq in a gram of Ra-226), and the one gram of Cs-137 has about as much radioactivity as over a kilogram of Pu-239. So the majority of radioactivity available to be released comes from the fission products with activation and neutron capture products contributing in a more minor fashion.
This part is basic physics and simply isn’t open to much interpretation – decades of careful measurements have shown us how many of which fission products are formed during sustained uranium fission. From there, the basic physics of radioactive decay can tell us what’s left after any period of decay. So if we assume the worst case – that somehow all of the fission products are going to leak into the ocean – the logical starting place is to figure out how much radioactivity is even present at this point in time.
In January 2012 the Department of Energy’s Pacific Northwest National Laboratory (PNNL) used a sophisticated computer program to calculate the fission product inventory of the #1 and #3 reactors at the Fukushima Dai’ichi site – they calculated that each reactor held about 6.2 million curies (about 230 billion mega-becquerels) of radioactivity 100 days after shut-down. The amount of radioactivity present today can be calculated (albeit not easily due to the number of radionuclides present) – the amount of radioactivity present today reflects what there was nearly three years ago minus what has decayed away since the reactors shut down. After 1000 days (nearly 3 years) the amount of radioactivity is about 1% of what was present at shutdown (give or take a little) and about a tenth what was present after 100 days. Put all of this together and accounting for what was present in the spent fuel pools (the reactor in Unit 4 was empty but the spent fuel pool still contains decaying fuel rods) and it seems that the total amount of radioactivity present in all of the affected reactors and their spent fuel pools is in the vicinity of 20-30 million curies at this time.
By comparison, the National Academies of Science calculated in 1971 (in a report titled Radioactivity in the Marine Environment) that the Pacific Ocean holds over 200 billion curies of natural potassium (about 0.01% of all potassium is radioactive K-40), 19 billion curies of rubidium-87, 600 million curies of dissolved uranium, 80 million curies of carbon-14, and 10 million curies of tritium (both C-14 and H-3 are formed by cosmic ray interactions in the atmosphere).
How much radioactivity might be in the water?
A fair amount of radioactivity has already escaped from Units 1, 2, and 3 – many of the volatile and soluble radionuclides have been released to the environment. The remaining radionuclides are in the fuel precisely because they are either not very mobile in the environment or because they are locked inside the remaining fuel. Thus, it’s unlikely that a high fraction of this radioactivity will be released. But let’s assume for the sake of argument that 30 million curies of radioactivity are released into the Pacific Ocean to make their way to the West Coast – how much radioactivity will be in the water?
The Pacific Ocean has a volume of about 7×1023 ml or about 7×1020 liters and the North Pacific has about half that volume (it’s likely that not much water has crossed the equator in the last few years). If we ignore circulation from the Pacific into other oceans and across the equator the math is simple – 30 million curies dissolved into 3×1020 liters comes out to about 10-13 curies per liter of water, or about 0.1 picocuries (pCi) per liter (1 curie is a million million pCi). Natural radioactivity (according to the National Academy of Sciences) from uranium and potassium in seawater is about 300 pCi/liter, so this is a small fraction of the natural radioactivity in the water. If we make a simplifying assumption that all of this dissolved radioactivity is Cs-137 (the worst case) then we can use dose conversion factors published by the US EPA in Federal Guidance Report #12 to calculate that spending an entire year immersed in this water would give you a radiation dose of much less than 1 mrem – a fraction of the dose you’d get from natural background radiation in a single day (natural radiation exposure from all sources – cosmic radiation, radon, internal radionuclides, and radioactivity in the rocks and soils – is slightly less than 1 mrem daily). This is as close as we can come to zero risk.
This is the worst case – assuming that all of the radioactivity in all of the reactors and spent fuel pools dissolves into the sea. Any realistic case is going to be far lower. The bottom line is that, barring an unrealistic scenario that would concentrate all of the radioactivity into a narrow stream, there simply is too little radioactivity and too much water for there to be a high dose to anyone in the US. Or to put it another way – we don’t have to evacuate California, Alaska, or Hawaii; and Caldicott’s suggestion to evacuate the entire Northern Hemisphere is without any credible scientific basis. And this also makes it very clear that – barring some bizarre oceanographic conditions – radioactivity from Fukushima is incapable of causing any impact at all on the sea life around Hawaii or Alaska let alone along California.
There’s no doubt that enough radiation can be harmful, but the World Health Organization has concluded that Fukushima will not produce any widespread health effects in Japan (or anywhere else) – just as Chernobyl failed to do nearly three decades ago. And it seems that as more time goes by without the predicted massive environmental and health effects they’ve predicted, the doom-sayers become increasingly strident as though shouting ever-more dire predictions at increasing volume will somehow compensate for the fact that their predictions have come to naught.
In spite of all of the rhetoric, the facts remain the same as they were in March 2011 when this whole saga began – the tsunami and earthquake killed over 20,000 people to date while radiation has killed none and (according to the World Health Organization) is likely to kill none in coming years. The science is consistent on this point as is the judgment of the world’s scientific community (those who specialize in radiation and its health effects). Sadly, the anti-nuclear movement also remains consistent in trying to use the tragedy of 2011 to stir up baseless fears. I’m not sure which of Emerson’s categories they would fall into, but I have to acknowledge their consistency, even when the facts continue to oppose them.