Fukushima operator Tepco has put into operation a radiological decontamination system for the unit 2 spent fuel pool. Tepco said that the system needed to start in advance of a desalination system that is also planned, since radiologically concentrated seawater is difficult to treat.

The system takes water from the circulating cooling system installed earlier this year (after the heat exchanger), and passes it through a truck-mounted valve unit, a truck-mounted caesium adsorption tower, and returns the water to the cooling system line before heat exchange. The system is truck-mounted to save space. It is expected to reduce the level of caesium by a factor between 100 and 1000.

Tepco chose unit 2 because it exhibits the highest radiological contamination level; 120,000 Bq/cc of caesium-134 and 110,000 Bq/cc of caesium-137. That level is about two-thirds higher than unit 3’s spent fuel pool, and 5-10 times the level at the unit 1 pool, according to sampling data extracted in August and September. The unit 4 spent fuel pool water had much lower radiation levels: 120 Bq/cc and 8.2 Bq/cc.

An investigation team has ventured into the unit 4 reactor building to gather evidence to determine the cause of the 15 March explosion in unit 4. The favoured hypothesis at the moment is that hydrogen generated in unit 3 flowed back up unit 4 air-conditioning ducts and into the unit 4 reactor building. The team drew one firm conclusion: that the explosion likely occurred mainly on the fourth floor of the building, for two reasons. First, the fifth floor deck was pushed up while the third floor deck was pushed down, and secondly that wire mesh attached to fifth floor air conditioning inlets was bent in the reverse direction of air flow. On the fourth floor itself, all of the air-conditioning ducts had been pulverised and scattered over the floor, suggesting that the explosion occurred around them.

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