Recent news stories are suggesting that TEPCO is considering evaporating the 280,000 tonnes of highly radioactive water held in the 1,500 tanks at Fukushima.
See http://www.reuters.com/article/2015/04/08/us-japan-fukushima-water-idUSKBN0MZ0WC20150408 and
Disposing large volumes of highly tritiated water is a serious problem for TEPCO but its evaporation proposal is quite dangerous. It is based on several misconceptions.
First, evaporating this radioactive water will NOT isolate the radioactivity: all it would do is convert liquid tritiated water to tritiated water vapour which would be emitted to the air at Fukushima and result in high exposures to those downwind of the plumes.
Second, evaporation would make the problem worse as, contrary to what many people assume, aerial emissions are more hazardous than liquid discharges to sea. Briefly, this is because you can avoid drinking tritiated water and food to a large degree, but you can’t avoid breathing in tritiated water vapour or absorbing it through skin.
Third, tritium is not “relatively harmless” as alleged. This is a common misconception: in fact, tritium is a relatively dangerous nuclide. For example, its beta particle inside the body is more harmful than most X-rays and gamma rays. But that’s just one aspect: tritiated water vapour has several other dangerous properties, and organically bound tritium (ie attached to lipids, carbohydrates and proteins) inside us is even more dangerous. See “Tritium- risks not properly assessed” in http://www.ianfairlie.org/lectures/
A complicating factor is the very high tritium levels in the tanks. From Japanese Government Meti files http://www.meti.go.jp/earthquake/nuclear/pdf/140424/140424_02_008.pdf
– see slides 5 and 21- it can be seen that the tritium concentration in March 2014 was about 500,000 Bq per litre.
This is a very high level. As far as I’m aware, no internationally agreed limits exist for discharging tritium to water. But as a yardstick, the limit used by Ontario Power Generation (a nuclear utility in Canada) is 4,000 Bq/L.
TEPCO is facing a storage problem with its tanks on site now full, and no space to build more. But neither evaporating the tank contents nor discharging them to sea appears to be a solution.