On Friday, 17 Jun 2011 TEPCO started up the water cleanup plant built for it by Areva SA in France, initially using only two of the four processing lines. Its objective is to decontaminate an estimated 105,100 t of radioactive water to make it safe to use it for reactor cooling or for shipping it to a nuclear waste processing site. Each processing line is designed to handle 300 t of water per day. Within five hours the processing had to be halted, as radioactivity built up much more quickly inside the system than expected.
TEPCO is under tremendous time pressure. Another 500 t of cooling water are pumped into the reactors from the Sakashita dam about 8 km to the wets of the plant every day. After it has cooled what’s left of the reactor cores it has nowhere to go.
Based on numbers published by the company on June 3, there were already 16,200 t of water in unit 1, 24,600 t in unit 2, 28,100 t in unit 3, 22,900 t in unit 4 and 13,300 t in the central radiation waste treatment building, which had mostly pumped been pumped there for from unit 2. In each unit, the basement of the turbine hall accounts for about half the water, with about another quarter in the reactor building and the rest split between the unit’s adjacent radioactive waste treatment building and a underground trench.
Radiation levels near the accumulated water in the basements are as high as 1000 mSv/h. The current legal limit any emergency worker can be exposed to is 250 mSv in total, which they would get in 15 minutes. It’s not safe to work anywhere near this water. If enough accumulated radioactive water can not be decontaminated far enough to be able to reuse it for cooling then TEPCO needs to keep pumping fresh water while bringing online ever increasing storage capacities to prevent the radioactive water from flooding the plant and spilling into the Pacific ocean.
Between them the buildings hold about 142,000 Terabecquerels (TBq) of Cesium-134 (half life 2 years) and 141,000 TBq of Cesium-137 (half life 30 years). Unit 2 holds about half of the total, and almost all of the rest evenly split between unit 3 and the central radiation waste building. That leaves the water in the basements of unit 1 and unit 4, which taken together only contribute a little over 1 percent of the total radioactivity according to TEPCO data. Units 2 and 3 each not only contain 50-60% more water than unit 1, it also is on average about 30 times more radioactive than the unit 1 water, which in turn is 10 times more radioactive than the water in unit 4. The latter had been shut down for maintenance since late November last year and had no fuel in the core at the time of the accident.
Judging solely by the water in the basements, unit 2 was the source of roughly 3/4 of the radioactive release in Fukushima, with most of the rest coming from unit 3. Even though on photographs unit 2 looks the least damaged of the four blocks, internally it is assumed to be in the worst shape, as it suffered an explosion in its pressure suppression chamber (torus) and it may also be damaged elsewhere.
An even greater percentage of the contamination of land is linked to unit 2 versus unit 3 than indicated by their basement radioactivity levels: When unit 3 was being vented and subsequently suffered the worst hydrogen explosion of all four units, the wind was blowing from the west, carrying most of the released radioactivity out over the Pacific. When things went badly wrong at unit 2 however, the wind was blowing from the South-East, carrying huge amounts of contamination over land to Namie and Iitate in the North-West. This is where contamination levels as high as in the most polluted portions of the Chernobyl exclusion zone have been measured.
The nuclear soup in the reactor basements contains about 3 kg of Cs-134 and 44 kg of Cs-137. If the water decontamination system works, most of this will eventually end up in sludge and filter elements that will be stored as highly radioactive waste.
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