Friday, August 14, 2015

What's more radioactive than a nuclear power plant?

A lot of things, it turns out. But the one you'd probably least expect? Waste from a non-nuclear power plant, by a factor of 100.

Would we feel different about fossil fuels if this 
warning were mandatory on coal-fired power plants? 
Image courtesy Torsten Henning, Public Domain
On Wednesday, we published a Physics+ article about radiation, written in memory of the bombing of Hiroshima, 70 years prior. While the author did a fantastic job in describing the state of the art on low-dose radiation research, I was troubled by a line where he cited "widespread deployment of nuclear power" along with medical scans and air travel as a potential contributor to chronic low-dose radiation. I took issue with the line because, counterintuitive as it might be, widespread deployment of nuclear power is acting to decrease the radiation burden of the average individual. To understand how, we'll need a smidge of radiation biophysics knowledge, along with a touch of nuclear engineering. If that sounds scary, don't worry; I promise to keep it simple.

The first thing you need to know is that "radiation", in the strictest sense, is everywhere. Light, heat, radio waves, microwaves, all of it is radiation. In practical contexts, even protons and electrons are referred to as radiation, if they're moving fast enough. The kind we need to worry about, ionizing radiation, is anything with enough momentum to knock electrons free of their home molecules or, in some cases, knock entire molecules out of place. Ionizing radiation creates breaks in the strands of our DNA, and when our bodies try—and fail—to repair that damage, things can go wrong.

These strand breaks come in two varieties: single and double. A single strand break isn't usually a huge deal; the double-helix nature of DNA means that every strand has a backup copy in the form of complementary bases "across the aisle" (A goes to T and C goes to G, if you recall freshman biology). However, double strand breaks, which tend to be caused by higher-energy, more massive particles, leave you without that backup. Since we all have two copies of each chromosome, one from each parent, the body will sometimes go and check what's in that spot on your other chromosome and use that information to repair itself, but this is a complicated process; if it goes wrong, your cell has no choice but to effectively duct tape the broken strands together and hope the genes it lost weren't too important. Ordinarily, after too much damage, a cell will undergo a sort of suicide known as apoptosis; it cranks out enzymes that "chew up" its DNA, and sends out signaling molecules that let the body know the cell's remains need to be cleaned up. But if the DNA which encodes the instructions for this process is missing or damaged, the cell can become "immortal": the first step toward cancer.

We're exposed to a good amount of ionizing radiation in our everyday lives. The most obvious source is the sun; the sun's emissions extend well beyond the visible range of light in both directions, and contain high-energy charged particles, the most dangerous kind of radiation. However, between the magnetosphere and the 12 km of atmosphere above our heads, we're pretty well-shielded from the worst of this; most of what makes it through is UV photons, which can't be deflected by the magnetosphere because they're uncharged (higher-energy photons like x-rays and gamma rays are almost entirely blocked by the atmosphere, although flight attendants and others who spend long periods of time at high altitudes are at increased cancer risk). Fortunately, UV radiation can be blocked by glass, sunscreen, or the melanin in your skin before it does any damage to the DNA.

A much less intuitively obvious source of radiation is the earth. While the sun's light is created through fusion, a much more subdued process of nuclear fission is occurring directly beneath our feet at all times, as the radioactive elements in the earth's crust undergo their slow decay. These decay reactions give off particle radiation as well as gamma rays, and can pose a serious danger to human health; the NAS estimates that radon gas, which seeps out of the earth into improperly-insulated subterranean structures, is responsible for at least 15,000 deaths by lung cancer each year in the US alone. When radon decays into polonium, it gives off alpha particles, bundles of two protons and two neutrons, otherwise known as helium nuclei. These particles can easily cause the double strand breaks I mentioned earlier; their high mass and charge cause them to transfer practically all of their momentum to the first thing they hit. As a result, when an alpha source is outside your body, it generally isn't a huge deal; the same property that makes them so dangerous to your DNA also makes them harmless under ordinary circumstances; virtually all environmental alpha particles never make it further than the dead skin cells of the epidermis.

But when an alpha-emitter like radon is inhaled or ingested, it becomes a problem. The cells within our lungs are exposed directly to the environment, so a radon molecule that decays while it's in your lungs is almost certainly going to damage your DNA. If you need another reason to quit smoking, you'll be pleased to know that smokers have a higher body burden of radiation than average citizens, as alpha-emitters like polonium-210 make it from the soil to the cigarette, leaving radioactive elements trapped in the tar on the inside of your lungs.

While you may be asking yourself what this all has to do with power plants, the connection should become clear when you consider that coal and other fossil fuels are "dirty" in every sense of the word. Fossil fuels are a sedimented mix of minerals and organic hydrocarbons, and contain significant quantities of heavy elements, some of which are radioactive. Oil and natural gas contain significant amounts of radon, which is difficult to separate out; it has a boiling point very close to propane's and, being a noble gas, is resistant to electrical or chemical filtration. The ash that escapes when coal is burnt also contains heavy radioactive elements; if coal fly ash isn't properly filtered when it's burned or processed, those elements escape into the air, where they can be inhaled and ingested.

The US has regulations mandating high-voltage filters to capture coal fly ash, but the process is less than perfect, and disposal of the captured ash can be problematic; just this year, the US Attorney's Office charged Duke Energy with illegal dumping of coal ash, and states across the country have reported contamination of water systems as ash leaches or spills from inadequate containment facilities. (It's worth noting that, in all these cases, the danger from toxic metals like selenium and arsenic finding their way into our food and water is much greater than that of the associated radiation.)

So what about the runoff from a nuclear power plant? To give you an idea of how radiation is contained, let's take a look inside the workings of a pressurized water reactor, the most common type of nuclear plant:
Image courtesy NRC, Public Domain.
The self-sustaining fission reaction of Uranium-235, which occurs in the reactor vessel at the left, produces fast-moving bundles of nucleons*, which heat high-pressure water that's constantly circulating through the orange loop. This water, which has been exposed to radiation but isn't radioactive itself, is used to heat other water, which is at a lower pressure and can boil off as steam, which is then used to turn a turbine before being captured and re-condensed with cold water from an outside source like a river. (Steampunk fans rejoice; for all our space-age technology, 20% of the US's energy comes from nuclear steam.)

The only exchange with the environment here, the condenser's coolant, is three steps removed from the actual radioactive material, meaning that the system adds almost no appreciable radiation to the environment under ordinary conditions of operation. When the fuel is spent, it's stored in closely-watched pools of water until it's "cooled off"; these pools do a spectacular job of containing that radiation, to the point that you could swim in one quite safely.

Ultimately, the relative ease of containing nuclear energy's waste products comes down to a question of scale and efficiency. Worldwide, there's roughly a metric ton of coal burned per person, per year. With that kind of volume, pollution is inevitable no matter how good the filtration system is. Nuclear fuel rods, on the other hand, are so much more efficient by weight that containment after they're used up isn't a problem. On top of that, their storage and disposal is much more tightly regulated, simply because it's feasible to regulate the disposal of that quantity of material.

In any discussion of the relative risks associated with various energy technologies, the possibility of meltdowns is an inevitable talking point. However, the risk of a possible meltdown has to be weighed against the collective health impact of constant emissions, oil spills, pipeline leaks, and the myriad other environmental hazards which accompany fossil fuel use (to say nothing of the geopolitical ramifications associated with fossil fuel dependence). It's also important to consider that, with proper emergency management protocols, the risk of civilian radiation exposure from a nuclear containment failure can be mitigated; two of the most famous incidents in recent memory, Three Mile Island and Fukushima Daiichi, didn't result in a single death, contrary to popular belief.

While environmental advocacy groups like greenpeace press for a "nuke-free" world, it seems they've given fossil fuels something of a pass, or else are tired of repeating themselves about the dangers and have moved on to a topic where they're more likely to affect change. Unfortunately, given the alternative, that change may not be for the better.


  1. What you fail to go into with any depth is the way that the waste product is dealt with. In the US it has been found that nuclear waste has been disposed of by use of filling up cemented discharge into hydraulic fracturing wells.

    This is a well documented fact. What we don't know is how many other countries practice this.

    Articles like this are wonderful but act as a reassurance that is not possible when it comes to how market forces dictate the actions of the corporates and how the profits always come before safety considerations.

    1. Your comment about the fracking wells alarmed me!
      I read up on it and, I have to say, I get why that sounds scary.
      But when you think about it, wouldn't mixing it with cement and burying it deep underground keep it from going anywhere? It's not going to seep into the groundwater, because it's mixed into concrete (and I can almost guarantee you they don't do that within a few miles of drinking water wells).
      It actually seems like a pretty good way to get rid of it, you know? I would be more concerned about actual fracking, if I were you; that's definitely been linked to contaminated drinking water, whereas I've never heard of radioactive water being a problem.

      I think our society's failure to utilize nuclear fuel to its full potential is less of an infrastructure and disposal problem and more, as you said, an example of profits being put before safety considerations; in China, air pollution kills about 1.6 million people—far more than the collective death toll of the Chernobyl accident—every year.
      But a transition to nuclear means a lot of coal miners, and coal mine owners, and oil barons would see a serious hit to their wages. A lot of the latter have influence over the energy policies of their countries, so it's logical that they would try to discourage that transition, even if that ultimately acts against the best interest of the general population. Market forces, as you said, seem to dictate the way things go. :)

    2. Equivocation does not serve the cause of discourse.
      Spent reactor fuel has absolutely not been disposed of in this way. In fact, in the USA, most of it is sitting in pools of water or in casks on concrete pads at the plants where it was used, because we haven't reprocessed civilian reactor fuel since 1974 (not a choice I endorse). On the other hand, it's hard to say why the sizable quantities of radioactive waste that results from oil & gas extraction (water-soluble radium salts leached out of the insoluble uranium & thorium minerals ubiquitous in the Earth's crust are the major source), especially by hydrofracking, should not be put down the same wells it came out of. See my link for a little background.

    3. Link could use fixing, but thanks.
      If you're looking for the article that inspired the parent comment, you can find about 100 words of it here, just so you know it exists.

  2. Some quantity comparison would have been helpful.

  3. I'm a little shocked that the writer, who far the most part, writes accurately, and whose general point of comparison is correct, could make such a gross error about the nature of nuclear fission. No alpha particle is released directly from fission of a U-235 nucleus. Fission of a U-235 atom produces an average of just under 2.5 neutrons, and usually two fission fragments - the nuclei of two smaller atoms, one usually a little less than half, the other a little more than half the mass of the original U-235 nucleus. The vast majority of the energy released is in the form of the kinetic energy of the two relatively massive (compared to an alpha) fission fragments. The rest of the released energy is in the kinetic energy of the neutrons, and also in the eventual radioactive decay of those fission fragments, which could be either gamma, beta, or yes - alpha decay. However, very few of the fission fragments decay by alpha decay (they're too small, generally), and in any case are not a direct result of fission.

    1. Mark,
      Thanks for the correction! I knew something didn't feel right about alphas heating water.
      I'll be checking my decay chains carefully in the future.

    2. To be very strict about it, a 4He nucleus is a possible product of ternary fission (when the uranium nucleus splits into three parts rather than two, with the third being much smaller than the other two), but it's a very low probability. Tritium is rather more common, & it's this (rather than two successive neutron captures) which is the major source of tritium in light-water-reactor coolant.

  4. DNA damage resulting from ionizing radiation is a worry, but because DNA is subject to frequent copy errors and breaks from other causes, there are active repair mechanisms. This new understanding of DNA repair is amazing. This lecture video shows some remarkable details of the process.

    1. Virgil,
      I don't mention them by name, but the repair mechanisa got a shoutout above.
      Those repair guys take time to work, though; this is why ~dose rate~ is just as important as total dose; smoking five cigarettes in one night is much worse for you than smoking one cigarette every night for five nights; if you give your body time to recover, you're less likely to lose both copies of a base pair.
      Thanks for reading!

  5. In my comment, I gave the url for part 2 of the you tube video. This is for part 1. Sorry.

  6. "A lot of things, it turns out. But the one you'd probably least expect? Waste from a non-nuclear power plant, by a factor of 100."

    Except that's not what the Scientific American Article, which references an study written in 1978 (How timely!) says. It does say that the risk of being struck by lightning is higher than the risk of radioactive particles from a coal plant:

    "Other risks like being hit by lightning," he adds, "are three or four times greater than radiation-induced health effects from coal plants."

    Why do you lie in your article right from the beginning? Do you ever plan on correcting your lies?

    1. Direct quote from the article:

      "In fact, the fly ash emitted by a power plant—a by-product from burning coal for electricity—carries into the surrounding environment 100 times more radiation than a nuclear power plant producing the same amount of energy."

      Did you miss that bit?