This is the next update from Charlotta Sanders. Charlotta Sanders is the chairperson of the American Nuclear Society’s Radiation and Protection Shielding Division. ( And my visiting teacher. On Friday we get to hang out. ) Anyway... This is the most recent email she sent regarding radiation and Fukishima. Read on if this is something you care about. If not simply wait for my next fabulous installment of my own thoughts.
As a nuclear worker, if someone ran into my office and said “There’s a radiation problem out at the facility!” the first things I would want to know (pretty much in order) are: 1) what’s radioactive, 2) how much, and 3) what form.
What’s radioactive?
When I’m asking “what’s radioactive”, I mean “what specific material is being radioactive”. This tells me three things: what kind of radiation it is, how strong each particle of radiation is, and how long the radiation level is going to stay that high.
When most people talk about materials, we typically talk about elements (the things on the periodic table: hydrogen, oxygen, uranium, iodine, etc). When a nuclear engineer talks about materials, we also usually talk about isotopes. Each element has multiple isotopes. Isotopes are just atoms of the same element that have slightly different weight, and therefore different nuclear properties. An example is hydrogen: we have normal hydrogen (hydrogen-1), and then deuterium (hydrogen-2), and then tritium (hydrogen-3), and so on. [*] (Not all isotopes get a special name, like the hydrogen ones. Usually we just say “uranium-235” and “iodine-131”.)
[*] The number after the name refers to the number of protons and neutrons in the isotope – we don’t need to go into that here, but you can think of it as a measure of the weight. Hydrogen-2 is heavier than hydrogen-1. When you hear someone in the nuclear field talk about “heavy water”, they are referring to water molecules that have one hydrogen-2 atom and one hydrogen-1 atom instead of water molecules made with two hydrogen-1 atoms (“light water”, aka the stuff we drink).
Each radioactive isotope has a specific radiation signature. Physicists have mapped these out for us into very handy tables: there is the Chart of the Nuclides (sometimes called the table of isotopes), and there are lots of charts and tables that provide detailed radiation data. The charts tell me what kinds of radiation particles are being given off, what their energy is, and how often particles are being given off (the half-life). So when that person in my office says “It’s cesium-137”, I can pull out my book and know what that means. [**]
[**] The chart this information comes from looks like this. Cesium-137 is a simple one; for example iodine-131 looks like this. (More complicated does not mean more dangerous, it just means more math for me.) Different charts published by different people will sometimes give numbers that are different in the last few decimals; this is because physicists are still trying to measure these things to the last decimal place. The differences don’t change our results very much.
Kinds of radiation particles
So when an isotope emits radiation, it’s actually spitting out parts of itself. When it does this, it changes into a different isotope. That’s why we call the process of an isotope emitting radiation “decay” – it’s not the same isotope anymore. (For example, if you were to remove all of the chicken from a chicken Caesar salad, it wouldn’t be a chicken Caesar salad anymore – it would just be a Caesar salad.) And the parts it can spit out come in only a few main types: alpha, beta, gamma, and neutron. [***] (I call them by their names here, but for our purposes it doesn’t make much difference; you could name them after colors or ice cream flavors or whatever you wanted.)
[***] This is another simplification: there are also fission fragments, which is when an atom splits into parts that are big enough to be new atoms all on their own. Like if you took your chicken Caesar salad and put all the chicken in one bowl, the dressing in another bowl, and split up the lettuce: you’d have a Caesar salad in one bowl and a grilled chicken salad in the other. But the other types of radiation are much smaller, like individual pieces of shredded Parmesan. And there are X-rays, which are pretty much exactly like gamma rays except gamma rays come from the nucleus and X-rays come from the outer electron shell.
So why does this matter? The different types of radiation have different weights and different electrical charges. For our purposes, this means: the different types of radiation interact differently with the world they are moving through. Different things will protect against them (“shield” them), and they have different effects on the human body.
Alpha radiation is big and carries an electrical charge, and usually slow. (I like to think of these particles like fat lazy bumblebees.) This means it doesn’t move through material very well – it gets stopped right near the surface. If you were standing right next to a source of alpha radiation, holding up a piece of paper would be enough to protect you. If you were to pick it up, the top layers of dead skin on your hands would be enough to protect you. Because of this, we don’t worry about alpha radiation much – only if you eat or inhale something that gives off alpha radiation. More on this later.
Beta radiation is small and carries an electrical charge. Beta radiation can be stopped by thin sheets of plastic, or wood, or plexiglass.
Gamma radiation is the smallest and fastest, and does not carry an electrical charge. It goes through materials easier than any other kind of radiation; at high energies these particles might require many inches of steel or lead to stop them. (I think of these like those little biting midges (sand flies, no-see-‘ums) that are small enough to get through your window screen – only faster.)
Neutron radiation is big (way bigger than betas but not as big as alphas), and does not carry an electrical charge. It’s a bit harder to stop than betas, too; typically you’ll see a thick concrete wall used as a shield. Metal shields like are used for gamma radiation will also work.
How strong each particle of radiation is
Here “strong” = how much energy the radiation particle has, which you can think of as how fast it’s moving. [+] This matters because radiation does damage to things (human tissue, other materials) when it bumps into atoms and leaves some of its energy behind. The more energy a particle has, the more damage it could potentially do.
[+] Again, a simplification. Energy is a measure of both speed and weight. So if a large particle like an alpha particle and a small particle like a neutron had the same energy, the alpha particle would be moving slower. But a neutron with a low energy is always moving slower than a neutron with a high energy.
How long the radiation level is going to stay that high
Because isotopes turn into other isotopes when they emit radiation (that is, when they decay), a pile of radioactive isotopes won’t have the same radiation level forever. (Think of it like AA batteries: if you put them in the closet and don’t use them for four years, they won’t have full charge when you finally do try to run that flashlight.) But it’s not a linear relationship: that is, they don’t have strength 10 on the first day and 9 the next and 8 the next until they run out on day 10. It’s exponential: more like strength 8 the first day and 4 the next day and 2 the day after and 1 the day after that, and then on day 10 it’s strength 0.0156 and on day 20 it’s strength 0.000015 and somewhere in there it’s so low we stop counting. Each isotope has a different rate at which the radiation level dies away. We have a number to help us compare the rates, called the “half-life”. The half-life is the amount of time it takes for the radiation level to drop by half. So in the example I just gave (8-4-2-1), the half-life is one day. Half-lives can be as small as seconds or as long as hundreds of thousands of years.
We can do the reverse math, too. Using the half-life, I can tell you how many times per minute (or hour, or day, or year) that pile of isotopes is decaying (giving off radiation particles). A short half-life means that pile is decaying very often. [++]
[++] I have to be careful here not to say it’s giving off more radiation particles, because sometimes a decay can spit out more than one particle. So an isotope that spit out two particles at each decay might be giving off more particles than another isotope that spit out only one at a time, even if the one-particle isotope had a shorter half-life.
So to recap: short half-lives mean a lot of very frequent decay, and as a result the radiation levels drop much, much faster. Long half-lives mean less-frequent decay, so the radiation levels stick around longer.
How much?
So! I have found out what kind of radiation we’ve got (what our radioactive materials are). This tells me three things: what kind of radiation it is, how strong each particle of radiation is, and how long the radiation level is going to stay that high. Now I need to know how much.
But how do you measure this? The person in my office could tell me a lot of things. They could tell me how much of the original material we’re dealing with by weight: say, 50 grams (or 1.76 ounces, if you prefer imperial units). But that’s only really helpful if there’s a good way to measure the weight of material. What if we don’t know the mass? What if a radiation detector just told us that some of the Cs-137 we keep behind that wall has gotten out of its container?
So we could measure in decays per time. This is a thing we can measure with instruments, because our detectors can count particles, and since we know what kind of material is back there [+++], we know how many particles per decay, and we can do the math. We call this kind of unit the “activity”. In SI units, activity is measured in becquerels (Bq) – one Bq is one decay per second. In imperial units, it’s measured in curies (Ci). These units are a rate (they measure decays per unit time). (Curies are based on the number of decays per second of 1 gram of radium-226, and 1 Ci = 37,000,000,000 Bq. If you think the conversions from imperial to SI are hard, trust me, they are. We always always always have to double-check our math and our units. I wish the United States would just switch to SI already.)
[+++] Cool thing: If we *didn’t* know what kind of material was back there, we also have detectors that can tell us the energy of the particles and what kind they are in addition to how many particles there are. We could use this like a fingerprint to determine what radioactive isotopes are present, and in what amounts. It’s not always perfect – if two people both pick up a water glass, their fingerprints might overlap and then it’s hard to separate them – but we can often do a pretty good job.
But running into my office and telling me there are 0.5 curies isn’t very informative, either. What if there was more than one kind of radioactive isotope, one that spat out very energetic gamma particles and one that spat out slow ones? I would have to sit down and do math to figure out if there was a problem. It would be nice if there was some sort of unit that took the energy of the particles into account… And there is – it’s called “absorbed dose”.
In SI units absorbed dose is measured in a unit called the gray (Gy), and in imperial units it’s the rad (never abbreviated, just “rad”). (1 Gy = 100 rad.) These units both measure the amount of energy that the radiation particles deposit in some set mass of material. These units are not a rate, so when we talk about how strong a radiation level is, we would say “grays per hour” or “rad per hour”, but when we talk about the dose someone got, we would talk about “grays” or “rads”. (Just like if you’re talking about a bad rainstorm, you would say “a half-inch per hour”, but if you’re talking about how much your garden got soaked during yesterday’s all-day steady rain, it would be “one inch”.) And they also don’t assume what material we’re talking about, so we would have to say “grays per hour in human tissue” or “rads per hour in air”. [^] If someone just says “rads”, I don’t know if they’re talking about rads in air or rads in people or rads in marshmallows, and it makes a difference! (Usually, but not always, engineers are talking about rads or grays in human tissue, so I would probably assume this is what the person in my office meant if they left that part out.)
[^] There is also a special unit for energy absorbed in air, that always refers to energy absorbed in air and not in some other substance. It’s called the Roentgen, and is abbreviated as R, and it is *only* used for gamma radiation and not for the others. Confused yet? I don’t blame you. We use the Roentgen because it’s an easy quantity to measure with a radiation detector. But it turns out that as a rule of thumb, a radiation field of 1 R (which is in air, remember) would give you close to but not exactly 1 rad in human tissue (which is what we usually care about). So a lot of people in engineering use R and rad interchangeably.
Okay, so we’re getting there. If the person in my office tells me “25 millirad per hour in tissue”, I have a pretty good idea of what we’re talking about. That’s a level that would be totally fine and normal in a radiation area where people only had to go every so often, but not normal for a general-occupancy area like an office where people would sit for long periods of time. It’s also low enough that workers could most likely spend a week straight with no breaks trying to fix it without going over their allowed annual dose – but to confirm this I would need one other piece of information.
You see, the last thing we need to measure isn’t just how much energy the radiation could dump into a human body. It’s how much damage that energy could do. And here’s one place where it becomes important what kind of radiation we’re talking about. Because the different kinds of radiation deposit their energy in different ways, and they are not equally harmful. Alpha particles depositing some set amount of energy in a human body will do more harm than neutrons depositing that same amount of energy. Neutrons, in turn, are more harmful than betas and gammas for the same amount of energy deposited. So we have one last kind of unit to measure this: “dose equivalent”. The dose equivalent is the absorbed dose times a number that accounts for how some particles are more dangerous than others. The dose equivalent is measured in Sieverts (Sv) in the SI system, and in rem in imperial units (rem has no abbreviation; it actually *is* abbreviation). (1 SV = 100 rem.) For gammas and betas, an absorbed dose of 1 rad in tissue will give you a dose equivalent of 1 rem. For neutrons, 1 rad = anywhere from 2 to 11 rem, depending on the energy of the neutrons. For alpha particles, 1 rad = 20 rem.
So if that person in my office is talking about beta or gamma radiation, 25 millirad per hour in tissue is the same as 25 millirem per hour. And a worker could stand there for 200 hours before reaching their normal yearly allowed dose. But if the person in my office is talking about neutron radiation, we might be looking at more like 250 milirem per hour, which gives a worker only 20 hours to stand there before they hit their normal yearly allowed dose.
So that very longwinded explanation is why we try to always answer the question “how much radiation” with some quantity given in rem per hour or in Sieverts per hour. Almost all of our regulatory limits in the US are specified in terms of rem, rem in a year, or rem per hour. (Internationally they use Sieverts the same way.) If the person in my office starts with “Cesium-137 at 25 millirem per hour”, I don’t have to do any math, I don’t have to remember what kind of radiation, I don’t have to pull out my book to see how energetic the particles are – I know right away where that is in relation to our design basis and our allowable limits.
But of course you probably don’t, because you don’t deal with this every day. So before we move on to the bits, I want to leave you with some references. I haven’t talked at all about the effects of dose, or how much dose is “harmful”, or where various limits are set for dose to the public, to workers, for normal situations, for accidents, and so on. But tons and tons and tons of people have done that for me. When you hear a number given in rem or Sieverts, you can look it up somewhere like these places:
http://www.nytimes.com/interactive/2011/03/16/world/asia/20110316-japan-quake-radiation.html – Dose levels at Fukushima during the accident.
http://xkcd.com/radiation/ – Best chart I have seen in a very long time. Puts all the doses into perspective by listing amount of radiation received from common everyday activities and common medical procedures, limits for various situations (public, workers, emergency workers, doses at edges of facilities), and doses from Chernobyl, Three Mile Island, and Fukushima.
http://www.ornl.gov/sci/env_rpt/aser95/tb-a-2.pdf – More activities and reference points, like the chart above.
http://www.new.ans.org/pi/resources/dosechart/ – An interactive dose chart to calculate what dose you are receiving from your usual routine.
http://hps.org/documents/risk_ps010-2.pdf – Lots of technical language, but this position statement by the American Health Physics Society basically says that we don’t have any evidence that doses below about 5 rem in a year (50 milliSieverts, or 50,000 microSieverts) cause any health effects at all (even cancer in later life). This is part of why our limits to the public are set this low. (Note that some people disagree; they think there’s still a small risk from doses under this level.)
http://people.reed.edu/~emcmanis/radiation.html – Another set of charts. Note that doses under 400 milliSieverts, even if received all at once, don’t have any clinical symptoms and are not even considered radiation sickness (although they still may increase the chances of cancer in later life by less than 0.1%). [^^]
http://en.wikipedia.org/wiki/Radiation_poisoning – Effects of various large doses of radiation. Remember that these effects are all for doses received in a sudden single large exposure; spread out over time they are much less harmful or not harmful at all.
[^^] Compare “less than 0.1%” to the current lifetime risk of cancer in the United States today from all sources: 42%. [1]
What form?
I also ask about what form the radioactive material is in (solid metal? powder? liquid? gas?). This is so I can figure out how likely the radioactive material is to move around. Why is this important?
If there’s a source of radiation near you, you can reduce your dose in three ways. First, time – don’t spend a lot of time near it (no-brainer). The longer you’re around it, the more of a dose you will get. Second, distance – stay farther away from it. Because the radiation is moving out from the radioactive thing, the further out the radiation gets, the fewer particles there are per unit volume. (Think of blowing up a balloon – the bigger you make the balloon, the thinner the balloon stretches.) So just staying further away will mean a smaller dose. Third, radiation can be slowed down or stopped by materials in between you and the radioactive thing. Depending on the specific kind of radiation, good materials might be water, or steel, or concrete, or something else. We call this “shielding”. But air also makes a pretty good shield, especially if you are talking about alpha particles, or about long distances.
So now is maybe a good time to mention that there are a couple of ways to get a radiation dose. There is “direct radiation”, which is like having a flashlight shine on you – the radioactive material is over there, you are over here, and the radiation (alphas, betas, gammas, or neutrons) is crossing the space between you. But the other way to get a dose is when you are over here and then the radioactive material is also over here, typically on your skin or in your lungs or in your stomach. We call radioactive material where it’s not supposed to be “contamination”.
When you are talking about direct radiation, you can control your time, distance and shielding. When you are talking about contamination on the skin, you can’t control distance or shielding; you just have to try to get it off you as quickly as possible. (Usually this means showers, or washing with special chemicals that will help remove the material, sometimes along with a couple of layers of dead skin.) When you are talking about something you inhaled or ate, that’s when we have the least control: you can’t change the distance, you can’t change the shielding, and you don’t really have much you can do to influence time. In most cases, the radioactive material is going to leave your body pretty much in its own good time.[^^^] We have a term for how fast the body does this called the “biological half-life”. It works exactly the same as the half-life we discussed earlier: the biological half-life is the amount of time that has to go by before there is half as much of the radioactive stuff left in your body.
[^^^] There are some exceptions, depending on what you got into you and how. For example, if you were to somehow eat or drink something that contained significant amounts of tritium-in-water (which remember is just one of the isotopes of hydrogen, so this would be radioactive water), the treatment is to speed up how fast your body process water. Methods for this include consuming many more liquids (non-tritiated, of course), or using dialysis machines. [2]
So as long as the radioactive material is staying in one place, we’re basically good – we can control time, distance, and shielding. But if it can move, we’re going to have a harder time protecting people from it. The worst form – the one you never want to deal with – are particulates (powders). These are little bitty pieces of the radioactive material, that can get picked up by wind, or washed around by water, or stuck to people’s skin and transferred to some other surface. (Think about the last time you were in contact with glitter –maybe a classroom, someone’s makeup, Mardi Gras. Think about how long you kept finding glitter afterwards. It’s like that.) That’s part of why Chernobyl was so bad; when the fuel burned, it turned into radioactive ash, which went way up into the air on the thermal drafts from the fire. And then continued to fall out of the air all over the countryside for months.
The easiest form is a nice solid that doesn’t break apart into little chunks – like a metal. Easy to deal with; stays in one place; unlikely to eat it or breathe it. As my colleague Dr. H. L. Dodds once said, about the worst danger from holding a chunk of uranium metal would be accidentally dropping it on your foot.
In between we have liquids and gases. (I lump them together here, because most things that are liquids have a habit of turning into gases at some point through evaporation.) Gases are not actually as bad as particulates in most cases – even though they can move around a lot more – because gases tend to spread way out in air. That means in most cases by the time a plume of radioactive gases has reached you, it’s so thin that the dose has gone way down. Gases are mostly a problem if you are either close to the source and standing in the path of the plume, or if you are in an enclosed space with the gas. (One of the more famous radioactive gases is radon, which gives off alpha radiation. Remember how we said alpha radiation isn’t really a problem unless you are breathing it? Now you know why radon in your basement is bad.)
So a brief note on Fukushima, then:
The radiation levels onsite at the Fukushima plant are being caused by direct radiation from the spent fuel, as well as radioactive gases released from the coolant and from inside of the spent fuel that failed. Of the radioactive material that is currently able to move around, the specific isotopes we are mostly concerned with are krypton-85, cesium-137, and iodine-131, which are the isotopes you will probably hear mentioned the most in the news. They are all present in gas form (iodine and cesium are present as compounds that easily turn into gases, but can turn back to liquids or solids while being transported offsite).
Radiation levels continue to change, and you can check various news sites for the latest numbers. The levels are much higher onsite at the plant because the workers are basically standing in the cloud of gases. Levels offsite are much lower (usually at least 1000 times lower) for many reasons. One, there’s enough distance even at the gate of the plant that direct radiation from the spent fuel isn’t detectable. Two, as the cloud of gases moves downwind it gets weaker because it spreads out. Three, as the cloud of gases moves downwind, there is time for the radiation levels in the cloud to decrease (iodine-131 has only an 8-day half-life).
By the time the plume makes it outside the evacuation zone, the plume has dissipated enough that standing in it while it blows by won’t give you much of a radiation dose. But breathing in the iodine will get it into your body, where it will stay until your body naturally gets rid of it. (Iodine is a particular problem because your body likes to store it all in one place (your thyroid) instead of distributed through the body, and so people exposed to enough radioactive iodine sometimes get thyroid cancer.) And cesium gets bound up into plants, so vegetables grown in the area will contain a little extra radioactive material for a little while. (Most plants already contain potassium, which is naturally radioactive.) That’s why they are telling residents just outside the evacuation zone to stay indoors and to be careful what they eat.
They have also distributed potassium iodide tablets to residents in the immediate area. Potassium iodide contains non-radioactive isotopes of iodine. The idea is that by taking the tablet, you flood your thyroid with “good” iodine, so that when the “bad” iodine gets into your body, your thyroid doesn’t need to grab it and hold on to it, so most of it goes right back out again much faster. The potassium iodine tablets will help Japanese residents near the Fukushima plants avoid thyroid cancer, but they don’t protect other parts of the body besides the thyroid, and they don’t protect against anything other than iodine. Health effects from these sources are expected to be negligible to the residents around Fukushima.
References
[1] http://hps.org/physicians/documents/Radiation_Effects.ppt
[2] http://www.physics.isu.edu/radinf/tritium.htm
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