Fission Products & Half-Lives April 13, 2011Posted by Lofty Ambitions in Science.
Tags: Nobel Prize, Nuclear Weapons, Physics, Radioactivity
Two weeks ago, we wrote about the distinction between radiation and radioactivity (click HERE for post). Last week, we wrote about the two radioactive elements used in nuclear power plants (click HERE for post). As we pointed out, for the most part, it’s not uranium and plutonium that are getting into the environment as a result of the nuclear accident at Fukushima Daiichi (click HERE for latest IAEA updates).
No, we’re hearing about radioactive iodine and radioactive cesium, which are the byproducts or leftovers of the nuclear reactions that produce energy. In other words, what starts out as uranium goes through the process of fission, and we end up with a bunch of isotopes—or variants of elements—including iodine-131, cesium-137, and strontium-90. Because the cladding that seals the nuclear fuel rods has melted, iodine-131 and cesium-137 are what engineers found in the tunnels underneath the reactors. That’s what’s been leaking into the sea and the air. Those are the substances that are being found in tap water and on spinach. The dissemination of these isotopes is one reason that the nuclear accident was redefined yesterday as a 7—the highest severity—on the International Nuclear Event Scale.
Radioactive iodine is especially dangerous because it accumulates in the thyroid. Japanese officials shamefully admitted that they delayed doling out iodine tablets in the first few days of the nuclear accident. Those pills of non-radioactive iodine-127 would have helped keep the thyroid busy with safer iodine in hopes that the radioactive isotope wouldn’t build up. People exposed to radioactive iodine are at greater risk for developing thyroid cancer in years to come.
Glenn Seaborg, whom we mentioned last week as the discoverer or creator of plutonium, also discovered cesium-137. Atmospheric nuclear weapons testing in the 1950s dispersed this isotope liberally and richly around the world. Radioactive cesium is metabolized like potassium, so it gets distributed throughout the body, especially in muscle tissue. That’s the way it’s processed by the human body, but also by the body of a cow producing milk or the body of a pig that will someday become part of someone’s breakfast. Even plants—like the marine plants off the coast of Japan or the grass that the milk cows eat—absorb cesium-137 as they would potassium. In other words, radioactive cesium is especially problematic because it gets taken up into the food chain easily and because lingers longer.
Lingering—that’s a measurement called half-life. Iodine-131 has a half-life that roughly matches the shelf-life of spinach at the grocery store: just over eight days. But cesium-137 has a half-life of just over 30 years. That means radioactive cesium hangs around as generations grow up. Once it gets into the food chain, it remains for our children. But at least it’s not plutonium, which has a half-life of roughly 80 million years.
So, what does half-life really mean? If iodine-131 has a half-life of just eight days, does that mean we’re safe eight days after it gets into the environment? No.
Half-life refers to the rate of decay of a radioactive isotope. After eight days, a given amount of iodine-131 will give off half as much radioactivity. Another eight days, and it’s halved again, leaving only one-quarter, and so on. This isotope decays—releases beta particles and gamma rays—and becomes xenon. Radioactive elements are continually remaking themselves, and iodine-131 does it relatively quickly.
By comparison, cesium-137 is that unwanted dinner guest who just won’t leave after the meal is over. It’s an isotope that had disappeared from Earth for billions of years, until we started doing controlled fission reactions with uranium and brought it back into existence here. And once we produce it, it remains half as radioactive thirty years later.
Just yesterday, Reuters reported that strontium-90, another fission byproduct, was detected near the Fukushima Daiichi plant. Like cesium-137, radioactive strontium has a long half-life, at almost 29 years. As much as 80% of the strontium-90 a person takes in is excreted, but almost all the rest that remains in the body is processed like calcium and gets into the bones. It’s the isotope linked to increases in leukemia in a population years later.
What’s especially interesting and disturbing about an isotope’s half-life, in relation to other concepts of scale and measurement about we wrote recently (click HERE for post), is that half-life doesn’t depend on the amount or quantity of material, nor does it depend on the environment surrounding it. Cesium-137 takes 30 years to halve its level of radioactivity, whether there’s a huge pile or a few atoms, whether it’s hot or cold. If we have a mixture of radioactive iodine and radioactive cesium, the half-life isn’t the average of the two isotopes’ half-lives; each decays at its own rate. We can’t do much of anything to change a radioactive isotope’s rate of decay.
This decay is not only seemingly independent, but is also an example of probability. After 30 years, we know that cesium-137 will be half as radioactive. But we don’t know which atoms will remake themselves. If we pick one cesium-137 atom, we would be flipping a coin; when we check back in 30 years, there’s a 50% chance it’ll be barium.
Probability points us in the direction of risk assessment, which is an aim of this series of recent regular posts (starting March 16 and continuing on March 28, March 30, April 6, and today). We thought we’d get to risk more quickly, but working through concepts related to the nuclear accident in Japan has been a larger undertaking than we expected. Keep reading Lofty Ambitions—we’ll get to risk, though it’s 50/50 whether it’ll be next week.