Fukushima unit 1 spent fuel pool cooling under control

18 August 2011

Power station operator Tepco has started an alternative two-loop cooling system at the Fukushima Daiichi unit 1 spent fuel pool, 10 days after starting a similar system in unit 4. With this system it has been able to determine the spent fuel water temperature for the first time since the incident (debris had blocked a thermometer suspended from the concrete pump). Two days after the cooling system was switched on, the pool’s water temperature was measured to be 39.5°C. All four units’ spent fuel pools are now cooled by an active circulating cooling system.

In addition the Fukushima Daiichi water treatment system has been augmented with five evaporative desalination systems (three from Areva and two from Toshiba), with a total combined capacity of 250 m3/day. The fresh water produced is injected back into the reactors and the most concentrated salt water is stored in a concentrated liquid storage tank. Another 750m3/day of desalination capacity is expected to start up in October.

•Tepco workers have sampled gas inside the unit 2 primary containment vessel. The nuclides found in greatest quantities were relatively long half-life fission products such as Kr-85, (half-life 10.76 years) and Xe-131m, (12 days), suggesting that the core had not gone critical again recently. Other short half-life nuclides such as Xe-133 (5 days), Xe-135 (9 hours) and Xe-138 (14 mins) should have been detected, but were not. Levels of Cs-134 and Cs-137 were 20-100 times lower than those detected in air sampling of unit 1, depending on the exact sample.

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