Fidel MD
05-20-2015, 12:02 AM
The short answer is that a half-life doesn't mean the time it takes for something to become safe, it's the time it takes for half of something to become different.
Picture a jar with 100 red marbles in it, and each marble has a half-life of 1 day, and the marbles convert (spontaneously) to green marbles.
Tomorrow (1 day later), the jar will have 50 red marbles, and some number of green marbles. We don't know at all which individual marbles will turn from red to green, or exactly when in the last day they will turn: We just know that after a day, half of them turned.
Now, let’s say the green marbles have a half-life of 1 minute, and turn to blue marbles. Our jar, after 1 day, has 50 red marbles, some green marbles, and some blue marbles. We can calculate statistically how many there should be of green and blue, but it will change with time, and we can't tell when any particular marble will change.
Lets say that once they turn blue, they quit changing...eventually, all the marbles will turn blue.
Thats how radionuclides decay. Call the red marbles tellurium, (which actually started as 235-Uranium in a reactor), and after some time turns into 131-Iodine (the green marbles). 8 days and a little change (the half-life of 131-I is 8.02 days) turns into 131-Xenon (the blue marbles).
So, we have some 131-Iodine in the jar. We take all the marbles out, and fill the jar up again with JUST green marbles. 8 days later, half are blue, right?
Well, in the process of turning from red to green to blue, they give off energy, in the form of ionizing radiation. That is the kind of radiation we worry about.
So, we have our jar giving off energy. Some of it happens very quickly, some not so quickly, but on average, half of the 131-Iodine will turn to 131-Xenon every 8 days.
After 8 days (1 half life), we have 50 green marbles, and 50 blue ones, right? But the jar is still giving off energy, same as last week, just not quite as much of it, right (because after another 8 days another half-life), we have 25 green marbles (half of 50). They're still giving off energy, though. So, another half life, another 8 days, and we have 12 or 13 green marbles, still giving off ionizing radiation. Another 8 days or half life and we're down to (lets say) 6. Still giving off ionizing radiation. Thats been 4 half lives, so far, and while the total amount of ionizing energy is lower, it's still giving off some, right? Lets go another half-life, another 8 days, and we're down to 3 green marbles. Quite a step down after 5 half-lives (call it 40 days), but still a little bit of radiation.
And with radiation, its' all a matter of chance. How much is safe? How much is none at all? We could wait 1000 years, and not be conclusively sure that there was no radiation at all, because there will probably still be an atom or two (a marble or two) of 131-I in there, because there aren't 100 marbles in the jar, there are literally trillions and trillions in the kilogram (theres a number we can use to calculate about how many, trust me: Its a LOT).
We like at least 5 half-lives reduction in intensity to call something kind of safe. Some say 10 half-lives.
Some radionuclides have half-lives that are measured in microseconds (I wrote a paper in college, for chemistry class, on one of the shortest lived, 215-Francium with a half-life of 85 nanoseconds), to (again) 128-Tellurium, the longest at 2.2x1024 years (160 trillion times longer than the existence of the universe). Quite a spread.
Which ones give off their energy fastest? The ones that 'pop' at 85 nanoseconds, or the ones that 'pop' at 160 trillion times the existence of the universe? Generally, the shorter-lived ones are more worrisome.
But, that isn't all there is to it - the types of energy they give off is an issue as well. Is it an alpha particle? Relatively low energy, not able to penetrate a sheet of paper, or unbroken skin, but able to damage skin or tissue in near proximity or contact? A beta particle, more energetic, able to penetrate skin? Gamma rays, very energetic, able to penetrate lots of things? Some or all of these together?
And what happens is important, too. 131-Iodine turns into 131-Xenon, which is stable (doesn't undergo any decay, or turn into anything else). Other products of fission inside a nuclear reactor (or detonation) turn into other isotopes, some of which are very bad news, some of which are not so bad (relatively speaking), and a few of which are stable. Ultimately, 239-Plutonium decays (through Uranium, Thorium, Proctactinium, Actinum, and others to lead, and that is the end of the line....after a long, long time over all, but some of the intermediate steps are quite short (a few milliseconds). Each of the intermediates has to be analyzed and considered separately for its risk.
So, it's not a matter of waiting a single half-life period for radiation to become 'safe'. Personally, for radioactive Iodine from nuclear reactors or detonations, I plan on taking KI for at least 41 days (5*8.02 half lives). Of course, I'm old enough that I don't really have to worry about thyroid cancer anyway, but why take chances?
Hope this helps.
Picture a jar with 100 red marbles in it, and each marble has a half-life of 1 day, and the marbles convert (spontaneously) to green marbles.
Tomorrow (1 day later), the jar will have 50 red marbles, and some number of green marbles. We don't know at all which individual marbles will turn from red to green, or exactly when in the last day they will turn: We just know that after a day, half of them turned.
Now, let’s say the green marbles have a half-life of 1 minute, and turn to blue marbles. Our jar, after 1 day, has 50 red marbles, some green marbles, and some blue marbles. We can calculate statistically how many there should be of green and blue, but it will change with time, and we can't tell when any particular marble will change.
Lets say that once they turn blue, they quit changing...eventually, all the marbles will turn blue.
Thats how radionuclides decay. Call the red marbles tellurium, (which actually started as 235-Uranium in a reactor), and after some time turns into 131-Iodine (the green marbles). 8 days and a little change (the half-life of 131-I is 8.02 days) turns into 131-Xenon (the blue marbles).
So, we have some 131-Iodine in the jar. We take all the marbles out, and fill the jar up again with JUST green marbles. 8 days later, half are blue, right?
Well, in the process of turning from red to green to blue, they give off energy, in the form of ionizing radiation. That is the kind of radiation we worry about.
So, we have our jar giving off energy. Some of it happens very quickly, some not so quickly, but on average, half of the 131-Iodine will turn to 131-Xenon every 8 days.
After 8 days (1 half life), we have 50 green marbles, and 50 blue ones, right? But the jar is still giving off energy, same as last week, just not quite as much of it, right (because after another 8 days another half-life), we have 25 green marbles (half of 50). They're still giving off energy, though. So, another half life, another 8 days, and we have 12 or 13 green marbles, still giving off ionizing radiation. Another 8 days or half life and we're down to (lets say) 6. Still giving off ionizing radiation. Thats been 4 half lives, so far, and while the total amount of ionizing energy is lower, it's still giving off some, right? Lets go another half-life, another 8 days, and we're down to 3 green marbles. Quite a step down after 5 half-lives (call it 40 days), but still a little bit of radiation.
And with radiation, its' all a matter of chance. How much is safe? How much is none at all? We could wait 1000 years, and not be conclusively sure that there was no radiation at all, because there will probably still be an atom or two (a marble or two) of 131-I in there, because there aren't 100 marbles in the jar, there are literally trillions and trillions in the kilogram (theres a number we can use to calculate about how many, trust me: Its a LOT).
We like at least 5 half-lives reduction in intensity to call something kind of safe. Some say 10 half-lives.
Some radionuclides have half-lives that are measured in microseconds (I wrote a paper in college, for chemistry class, on one of the shortest lived, 215-Francium with a half-life of 85 nanoseconds), to (again) 128-Tellurium, the longest at 2.2x1024 years (160 trillion times longer than the existence of the universe). Quite a spread.
Which ones give off their energy fastest? The ones that 'pop' at 85 nanoseconds, or the ones that 'pop' at 160 trillion times the existence of the universe? Generally, the shorter-lived ones are more worrisome.
But, that isn't all there is to it - the types of energy they give off is an issue as well. Is it an alpha particle? Relatively low energy, not able to penetrate a sheet of paper, or unbroken skin, but able to damage skin or tissue in near proximity or contact? A beta particle, more energetic, able to penetrate skin? Gamma rays, very energetic, able to penetrate lots of things? Some or all of these together?
And what happens is important, too. 131-Iodine turns into 131-Xenon, which is stable (doesn't undergo any decay, or turn into anything else). Other products of fission inside a nuclear reactor (or detonation) turn into other isotopes, some of which are very bad news, some of which are not so bad (relatively speaking), and a few of which are stable. Ultimately, 239-Plutonium decays (through Uranium, Thorium, Proctactinium, Actinum, and others to lead, and that is the end of the line....after a long, long time over all, but some of the intermediate steps are quite short (a few milliseconds). Each of the intermediates has to be analyzed and considered separately for its risk.
So, it's not a matter of waiting a single half-life period for radiation to become 'safe'. Personally, for radioactive Iodine from nuclear reactors or detonations, I plan on taking KI for at least 41 days (5*8.02 half lives). Of course, I'm old enough that I don't really have to worry about thyroid cancer anyway, but why take chances?
Hope this helps.