[Lewis Thomson (OP)]

Donald has presented this question to find answers.

"After the tsunami-Fukashima nuclear reactor accident, a large area of Japan was abandoned.  How long was Hiroshima and Nagasaki was abandoned after destruction by atomic bombs?  I can't bring any documentation. Certainly there must have been radioactive contamination."

Place your findings in the comments below...

[Mckenzie]

Not an expert but most probably it depends on amount of nuclear material and radioactivity. Hiroshima, Nagasaki are alive and vibrant now and units used on japan were 15 ktonns. It would be interesting to know exact estimate based on elements present.

[alancalverd]

It depends on a number of factors which boil down to three principal parameters: the initial level of contamination, the half-life of the contaminant, and the acceptable level of contamination for re-occupation of the territory. 

Nuclear bombs distribute most of the debris into the stratosphere, whence it is widely dispersed and thus the local contamination activity may be fairly low, with an initial halflife of just a few days. Chernobyl, on the other hand, scattered a lot of long-lived radionuclides over a small area, though stratospheric contamination was indeed measurable a thousand miles away.

You also need to consider the bioavailability of longlived contaminants. Uranium and plutonium are very unpleasant: difficult to detect in drinking water, but dangerous alpha emitters, whilst shorter halflife gamma emitters like cesium and iodine are easy to detect and control. Whilst you can assign a crude halflife to the contamination for emergency purposes (how long can you allow fire and ambulance crews to work in the zone), you have to do some clever radiochemistry to predict the longterm activity of what may be a very complicated mixture of radionuclides from a real accident, and you may declare the aquifer and crops unusable  for thousands of years..   

My understanding of Fukushima is that the plume chemistry was relatively simple compared with Chernobyl.

[wolfekeeper]

The main radioactive elements that are a concern after a nuclear accident are cesium-137, strontium-90, iodine-131.

They are of a concern for many times their half life. Iodine-131 has a half life of 8 days, so is gone in a few months. The real nasties are Cesium-137 and strontium-90 which have half lives of about 30 years.

They're both solid at normal temperatures, and much heavier than air. If they were lying on the ground around you they are not a super big problem, but if you get them inside you they tend to bioaccumulate, and then you're being irradiated for the rest of your life. Which would be bad. So you wouldn't want to farm the area because your crops and animals may suck up the radioactive elements.

So these areas are likely to be unusably radioactive for at least a few hundred years or so until the cesium and strontium decay.

[alancalverd]

until the cesium and strontium decay.

And there's the rub. They never completely disappear. Fortunately we evolved on a radioactive planet so can tolerate a  quantity of ingested radionuclides and external radiation. There are standard tables of tolerable contamination of foodstuffs and people do inhabit areas where the natural background doserate is high (up to 40 millisievert/year - UK average is 2.5) so the required fallow time is calculable in principle.

[wolfekeeper]

until the cesium and strontium decay.

And there's the rub. They never completely disappear.

I mean.. they will eventually. There's either an atom of cesium-137 left or there isn't. But below a certain amount, it's below background radiation and nobody cares. Right now, it's far above, and it's going to take hundreds of years, unless it washes off into the sea or something.