A recent leak of 300 tons of highly radioactive water at Fukushima No. 1 has highlighted the long term problems that Tokyo Electric Power Co. (Tepco) is facing in its struggle to manage the crisis at the wrecked nuclear power station (see Japan Times, 2013-08-21). One massive steel tank had been leaking as much as 10 tons of water a day for a month before the leak was noticed. The water level in the tank dropped by 3 m before anyone noticed. It is not clear yet how the water is escaping.
The water in the tank has been used for cooling the melted reactor cores. Consequently it is highly radioactive from strontium, cesium and tritium. At a distance of 50 cm, as much as 100 millisieverts per hour (mSv/h) were measured. That means a nuclear worker there would absorb as much radiation in one hour as is legally permitted over a total of 5 years.
You might think that with a witches’ brew like that on its hands, Tepco would take every possible precaution to prevent leaks and to monitor fluid levels. Tepco uses both welded steel tanks and temporary tanks for storing contaminated water at Fukushima No. 1. Welded tanks are supposed to be stronger and more leak proof, whereas temporary tanks can be bolted together quickly from sheet metal and plastic. About one third of the over 1000 tanks at Fukushima No. 1 are temporary tanks, including the one that recently leaked. Tanks of this type have been used at the site since December 2011 and they are supposed to last five years before needing repair or replacement. So far 4 of these tanks have leaked, yet Tepco is planning to install even more temporary tanks for storing water. I guess they must be cheaper.
I am curious why the leaks were not detected sooner. Are there no monitoring devices installed that can automatically report water levels?
Tepco is planning to treat the water in the tanks with its ALPS filtering system, which can remove radioactive cesium and strontium from the water, but not tritium. It was meant to start operating this month, but after problems it is now expected to not resume operation until December.
Even after treatment, Tepco will have a water problem. Any water pumped from the turbine halls that has been in contact with the reactor basements has elevated levels of radioactive tritium. No chemical removal system exists for tritium, as it’s an isotope of hydrogen, one of the two elements that make up water. Tepco can not simply evaporate water from those tanks to reduce volume and concentrate contaminants into a smaller volume, as the tritium would be released with water vapour and come down as rain again elsewhere. So what is it going to do? Release it into the atmosphere slowly? Dilute it with sea water? Or store hundreds of thousands of tons of water for hundreds of years? Neither alternative seems very appealing.