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Armchair Engineering: Radiation
Mar. 18th, 2011 04:33 pmThere are plenty of panel discussion various universities have put on the web regarding the Fukushima disaster, and how reactors work and all that but I figured I'd go back to basics a bit.
Q: So, what's radiation?
A: It's one of these three:
-An atomic nucleus moving really fast. This is called α-radiation (alpha-radiation)
-An electron (or positron), moving really fast. This is called β-radiation (beta-radiation)
-Electromagnetic radiation (like light, X-rays etc.) This is called γ-radiation (gamma-radiation)
-Neutron radiation, which is a neutron moving really fast (essential for the operation of a nuclear reactor or neutron bomb, but inconsequential here since fission was stopped a week ago and the free neutrons vanished with that.)
Note that γ-radiation is a ray, like light, while α-, β- and neutron radiation are fast moving particles. (Yeah, I know, but I'm trying to generalize here.)
The uranium in a nuclear reactor is turned into various other radioactive substances as it undergoes fission. It's those things other than uranium that are the problem.
The radioactive materials at Fukushima produce all three kinds of radiation. In fact, it's possible that a radioactive atom decays into multiple radioactive atoms and releases one or more kinds of radiation, and then those resulting radioactive atoms decay further producing different kinds of radiation.
Only γ-radiation (γ-rays) travel any significant distance, and only they will penetrate any significant barrier. They're presumably what makes the current power plant site so radioactive and working there so hard, since there's no way to shield the workers against it. However, the effects fall of exponentially with distance, and a really radioactive γ-source a good distance away doesn't really harm you.
While α- and β-radiation can easily be blocked by either a thin layer of metal, or even topmost layer of dead skin cells, those are the ones that are of most concern as far as human health is concerned.
Radioactivity isn't like smoke; you can't take a jar, scoop up radiation from the power plant and take it with you. Instead, the problem is that dust or other small particles of radioactive materials such as Caesium-137 (Cesium-137), Iodine-131 and Strontium-90 that exist in the reactor fuel get spread around and are then ingested or inhaled by humans. When those substances enter the body, they get very close to various organs and the electrons and nuclei they emit wreak havoc with DNA (and other molecules). In that sense, contamination by radioactive materials is not different from an industrial release of mercury or dioxins, just the mechanism of toxicity in the body is different.
All three substances behave differently, as far as their propensity to flow with the wind, rain down, dissolve in water etc, so once they're released into the environment things become really complicated.
Iodine is easy to defend against by policing meat and milk from grazing animals, washing produce, and taking non-radioactive iodine products to prevent the absorption of the radioactive variety. Its half-life is about 8 days, so in a few months most of it will have decayed into harmless stable Xenon, some γ and β. As long as it does its decaying outside of the body, it's pretty much harmless. In the body it concentrates in the thyroid, and if it does its decaying there, there's an elevated risk of thyroid cancer. Most of the statistically inferred deaths from the Chernobyl accident were thyroid cancers in children, and could have been largely prevented by safe iodine and government control for of the food supply for a few months after the accident.
Caesium-137 has a half-life of 30 years, so it'll remain dangerous for hundreds of years. It's the primary reason the area surrounding the Chernobyl reactor is uninhabitable, and will be the reason why the Fukushima site and its environs will be uninhabitable (though, so far, at a far lesser scale than Chernobyl.) It spreads in the body into all kinds of tissue, and hangs around in nature, and concentrates along the food chain. Reindeer, salmon etc. end up with significant Caesium contamination. The decay is a bit more complicated, Wikipedia goes into more detail.
Strontium-90 is dangerous because it accumulates in bone, where it damages the bone marrow and causes leukemia and other cancers. More details in Wikipedia.
The effect on a human body, then, depends on what the radioactive substance is, and whether it has been introduced into the body. The weather and precautionary measures will dramatically change those factors.
To further complicate matters, effect of each of these three kinds of radiation are different. While γ-rays go through all kinds of shielding, they are far less damaging to the human body than α- or β-radiation that has been introduced inside the body.
There are all kinds of units being bandied around in the news. The one that you should watch for is the Sievert (Sv). It tries to take some of these variables into account to be an accurate measure of the effect of radiation on the human body, so a dose of one Sievert of one kind of radiation should be as harmful as one Sievert of radiation from another kind. Usual SI prefixes apply, so usually we're talking about μSv (micro-Sieverts) or mSv (milliSieverts = 1000 micro-Sieverts = 0.001 Sieverts).
Exposure also isn't linear. A first responder dosed with 100 mSv in a minute will have different effect from a civilian who gets 100 mSv over the period of a month, but aside from very short exposures this shouldn't come into play too much.
Note that Sv measures total dose. It's like a unit of distance. When you want to talk about how dangerous or radioactive a location is, you use the rate, Sv/time, for example Sv/hour (Sv/h), much as you would use miles/h to indicate speed. You may know it's 735 km from Tampa to Atlanta, and that's the distance you have to cover, but you need speed (80 km/h) to get an indication how fast you get there.
Similarly, when radiation readings are being bandied about, if the population gets a small dose of a relatively high rate of radiation it sounds dramatic, but is no worse (and often better) than a much, much smaller rate of radiation that contaminates the area and subjects the population to a higher dose over a longer period of time.
Fundamentally, considering the information coming from the various sources, count your Sieverts and mind your SI prefixes. Typical background is 2.4 mSv/year, but radiation workers, first responders etc. can get up to around 50 mSv/year under normal working conditions, and may go into doses in the low hundreds of mSv under emergency conditions. (Obviously, if you respond to an emergency and get 100 mSv in one shift, you'll be doing desk duty away from radiation for the rest of the year.) All sources I've read suggest that even doses of a few hundred mSv do not produce statistically significant increases in cancer rates, above that you're talking about things like increasing your risk of cancer from 40% to 41%.
Q: So, what's radiation?
A: It's one of these three:
-An atomic nucleus moving really fast. This is called α-radiation (alpha-radiation)
-An electron (or positron), moving really fast. This is called β-radiation (beta-radiation)
-Electromagnetic radiation (like light, X-rays etc.) This is called γ-radiation (gamma-radiation)
-Neutron radiation, which is a neutron moving really fast (essential for the operation of a nuclear reactor or neutron bomb, but inconsequential here since fission was stopped a week ago and the free neutrons vanished with that.)
Note that γ-radiation is a ray, like light, while α-, β- and neutron radiation are fast moving particles. (Yeah, I know, but I'm trying to generalize here.)
The uranium in a nuclear reactor is turned into various other radioactive substances as it undergoes fission. It's those things other than uranium that are the problem.
The radioactive materials at Fukushima produce all three kinds of radiation. In fact, it's possible that a radioactive atom decays into multiple radioactive atoms and releases one or more kinds of radiation, and then those resulting radioactive atoms decay further producing different kinds of radiation.
Only γ-radiation (γ-rays) travel any significant distance, and only they will penetrate any significant barrier. They're presumably what makes the current power plant site so radioactive and working there so hard, since there's no way to shield the workers against it. However, the effects fall of exponentially with distance, and a really radioactive γ-source a good distance away doesn't really harm you.
While α- and β-radiation can easily be blocked by either a thin layer of metal, or even topmost layer of dead skin cells, those are the ones that are of most concern as far as human health is concerned.
Radioactivity isn't like smoke; you can't take a jar, scoop up radiation from the power plant and take it with you. Instead, the problem is that dust or other small particles of radioactive materials such as Caesium-137 (Cesium-137), Iodine-131 and Strontium-90 that exist in the reactor fuel get spread around and are then ingested or inhaled by humans. When those substances enter the body, they get very close to various organs and the electrons and nuclei they emit wreak havoc with DNA (and other molecules). In that sense, contamination by radioactive materials is not different from an industrial release of mercury or dioxins, just the mechanism of toxicity in the body is different.
All three substances behave differently, as far as their propensity to flow with the wind, rain down, dissolve in water etc, so once they're released into the environment things become really complicated.
Iodine is easy to defend against by policing meat and milk from grazing animals, washing produce, and taking non-radioactive iodine products to prevent the absorption of the radioactive variety. Its half-life is about 8 days, so in a few months most of it will have decayed into harmless stable Xenon, some γ and β. As long as it does its decaying outside of the body, it's pretty much harmless. In the body it concentrates in the thyroid, and if it does its decaying there, there's an elevated risk of thyroid cancer. Most of the statistically inferred deaths from the Chernobyl accident were thyroid cancers in children, and could have been largely prevented by safe iodine and government control for of the food supply for a few months after the accident.
Caesium-137 has a half-life of 30 years, so it'll remain dangerous for hundreds of years. It's the primary reason the area surrounding the Chernobyl reactor is uninhabitable, and will be the reason why the Fukushima site and its environs will be uninhabitable (though, so far, at a far lesser scale than Chernobyl.) It spreads in the body into all kinds of tissue, and hangs around in nature, and concentrates along the food chain. Reindeer, salmon etc. end up with significant Caesium contamination. The decay is a bit more complicated, Wikipedia goes into more detail.
Strontium-90 is dangerous because it accumulates in bone, where it damages the bone marrow and causes leukemia and other cancers. More details in Wikipedia.
The effect on a human body, then, depends on what the radioactive substance is, and whether it has been introduced into the body. The weather and precautionary measures will dramatically change those factors.
To further complicate matters, effect of each of these three kinds of radiation are different. While γ-rays go through all kinds of shielding, they are far less damaging to the human body than α- or β-radiation that has been introduced inside the body.
There are all kinds of units being bandied around in the news. The one that you should watch for is the Sievert (Sv). It tries to take some of these variables into account to be an accurate measure of the effect of radiation on the human body, so a dose of one Sievert of one kind of radiation should be as harmful as one Sievert of radiation from another kind. Usual SI prefixes apply, so usually we're talking about μSv (micro-Sieverts) or mSv (milliSieverts = 1000 micro-Sieverts = 0.001 Sieverts).
Exposure also isn't linear. A first responder dosed with 100 mSv in a minute will have different effect from a civilian who gets 100 mSv over the period of a month, but aside from very short exposures this shouldn't come into play too much.
Note that Sv measures total dose. It's like a unit of distance. When you want to talk about how dangerous or radioactive a location is, you use the rate, Sv/time, for example Sv/hour (Sv/h), much as you would use miles/h to indicate speed. You may know it's 735 km from Tampa to Atlanta, and that's the distance you have to cover, but you need speed (80 km/h) to get an indication how fast you get there.
Similarly, when radiation readings are being bandied about, if the population gets a small dose of a relatively high rate of radiation it sounds dramatic, but is no worse (and often better) than a much, much smaller rate of radiation that contaminates the area and subjects the population to a higher dose over a longer period of time.
Fundamentally, considering the information coming from the various sources, count your Sieverts and mind your SI prefixes. Typical background is 2.4 mSv/year, but radiation workers, first responders etc. can get up to around 50 mSv/year under normal working conditions, and may go into doses in the low hundreds of mSv under emergency conditions. (Obviously, if you respond to an emergency and get 100 mSv in one shift, you'll be doing desk duty away from radiation for the rest of the year.) All sources I've read suggest that even doses of a few hundred mSv do not produce statistically significant increases in cancer rates, above that you're talking about things like increasing your risk of cancer from 40% to 41%.
