Dr. Z – Someone told me that trees can be radioactive – especially in New England. No way!
Well…way. Sorry, but it’s true – there’s an interesting part, and a boring part to this. Let’s do the boring part first so we can finish with a bang.
The boring part starts with cosmic rays slamming into the atmosphere and creating radioactive carbon-14 (C-14) and tritium (H-3) – these are called cosmogenic radionuclides. Carbon and hydrogen are both used by living organisms (including trees) so they’re taken up by the trees and become incorporated into the wood, the trunk, the leaves, and the roots. And, since they’re both radioactive, the tree is very slightly radioactive. But this isn’t specific to New England – pretty much every tree in the world has C-14 and H-3, so there’s nothing special about them. Special – that’s where the cesium-137 (Cs-137) comes in!
Since 1945 the nuclear nations have set off 507 known atmospheric nuclear explosions, in addition to a number of underground tests that vented to the atmosphere. On top of that, Chernobyl and Fukushima discharged radioactive materials into the atmosphere and these were spread throughout the Northern Hemisphere, and likely reached south of the equator as well. When uranium and plutonium fission, one of the most common elements formed is cesium and, with a half-life of about 30 years, the longest-lived cesium isotopes formed is Cs-137. These tests spread Cs-137 throughout the world.
Chemically, cesium is very similar to sodium and potassium and, as such, it travels everywhere in an organism that the lighter elements go – pretty much everywhere. And, although cesium atoms are somewhat larger than their lighter counterparts, they’re still taken up by the roots of plants and trees. On top of that, cesium is a volatile element – it melts at 28.5 C (83 F) and it vaporizes at a paltry 671 C. So the cesium that’s formed in nuclear fission disperses easily into the environment and, once there, it’s easily taken up into the tissues of plants and animals. So what happened is that atmospheric nuclear weapons testing and a few nuclear reactor accidents releases Cs-137 into the atmosphere. The winds carried the Cs-137 around the world, leaving a faint trail on the ground beneath.
Once deposited on the ground, the cesium was taken up by the plant roots, circulated through the plants’ tissues, and some became fixed in the plant. In addition some of the Cs-137 fell onto the soil where it became bound to organic molecules and to the clay minerals in the soil; from there it could be taken up by the roots of plants and trees. Since Cs-137 has such a long half-life this means that trees growing today that were never exposed to nuclear fallout can still push their roots into layers of soil that contain Cs-137, bringing it into the trees where it can become fixed in the wood. Oh – and the New England part – that’s because the fallout plumes from atmospheric nuclear testing in Nevada tended to drift to the northeast, depositing minor amounts of radioactivity in New England.
One more thing to mention is that I heard a presentation a few years ago about the amount of Cs-137 in wood ash. It turns out that whatever Cs-137 that was in the tree becomes concentrated in the ash because, when the organic parts burn away they take about 90% of the material with them. Since the cesium doesn’t go up the chimney, it remains in the ash – with its activity concentration increased by a factor of 10. So some of the ash from trees that were exposed to Cs-137 turns out to have enough of the nuclide to measure fairly easily (if, that is, you have the right lab equipment).
And that’s how firewood becomes radioactive!