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.