>I'm guessing that this is wrong, though, and that the radioactivity from longer half-life radionuclides will eventually start to dominate and the "effective half-life" will be much longer.
Exactly. From a report on Chernobyl [0]: "Most of the decrease [in radiation] in the coming years will be at only the rate of the physical half-life of 137Cs." Cesium-137 has a half-life of 30 years [1], so if we use the number given in the article (10,000 roentgens/hour) the dose is apparently 434 million times background radiation, 23 microroentgens/hour [2]. It would take over a millennium to reduce Cesium-137 by a factor of 434 million[3]
That's the Elephant's Foot, though. Randall Munroe cites Chernobyl as having 6 millisieverts/hour on average[4]. That's 3000 times background radiation [2], or a 350-year wait before Chernobyl emits the same amount [5].
This is obviously a simplification, though. Plutonium isotopes were released too [0], which have a much longer half-life (thousands of years). It's possible those small amounts of plutonium will emit enough radiation to make Chernobyl have noticeably higher radiation levels than the background dose for much longer.
A final caveat: I'm not an expert or even a devoted amateur. Your question just sparked my curiosity. Although, this New York Times article [6] says scientists say it takes about 10-13 half lives for an area to recover, which is close to my 350 year figure.
Exactly. From a report on Chernobyl [0]: "Most of the decrease [in radiation] in the coming years will be at only the rate of the physical half-life of 137Cs." Cesium-137 has a half-life of 30 years [1], so if we use the number given in the article (10,000 roentgens/hour) the dose is apparently 434 million times background radiation, 23 microroentgens/hour [2]. It would take over a millennium to reduce Cesium-137 by a factor of 434 million[3]
That's the Elephant's Foot, though. Randall Munroe cites Chernobyl as having 6 millisieverts/hour on average[4]. That's 3000 times background radiation [2], or a 350-year wait before Chernobyl emits the same amount [5].
This is obviously a simplification, though. Plutonium isotopes were released too [0], which have a much longer half-life (thousands of years). It's possible those small amounts of plutonium will emit enough radiation to make Chernobyl have noticeably higher radiation levels than the background dose for much longer.
A final caveat: I'm not an expert or even a devoted amateur. Your question just sparked my curiosity. Although, this New York Times article [6] says scientists say it takes about 10-13 half lives for an area to recover, which is close to my 350 year figure.
[0]: https://www.oecd-nea.org/rp/chernobyl/c02.html. Search for 239Pu, not plutonium, to find the plutonium reference.
[1]: http://en.wikipedia.org/wiki/Caesium-137
[2]: http://en.wikipedia.org/wiki/Roentgen_%28unit%29#Significanc...
[3]: http://po.st/3ckuGC
[4]: https://xkcd.com/radiation/
[5]: http://po.st/T2qoni
[6]: http://www.nytimes.com/2011/03/20/weekinreview/20chernobyl.h...
Edit: Fixed math and added New York Times link. (Previously I based the Elephant's Foot calculations on 1,000 roentgens/hour.)