Closed-loop cooling starts at Fukushima Daiichi

29 June 2011

TEPCO has begun injecting decontaminated water into units 1, 2 and 3 of the Fukushima Daiichi nuclear power plant. The start of closed-loop cooling water circulation is a major breakthrough for the site, which has been producing hundreds of tons of contaminated water per week to compensate for the reactors' decay heat.

The decontaminated water comes from a newly installed treatment system. Currently, a total of 13 m3/hr of treated water is being injected: 3.5m3 for each of units 1 and 2, and 9m3 for unit 3. (In addition, 3m3/hr of water is pumped from the existing water source, the filtrate water tank). Both units 2 and 3 (though not unit 1), are returning water to the system via the centralised radioactive waste treatment facility tanks.

Unit 3 is receiving more cooling water because it is likely the hottest reactor. According to new temperature data, unit 3 feedwater nozzle temperature was 154.6°C, RPV temperature of 129.1°C as of 27 June. Unit 1 readings were 115.9°C and 100.8°C; unit 2 readings were 109.7°C and 119.5°C, respectively.

Tepco has confirmed that since the treatment system started up on 17 June it has been removing more than 99.99% of caesium; it expects it has a decontamination factor of 10,000-100,000. Over its first three days of operation, it filtered 2500 tons of water.

A sample of unit 1 spent fuel pond water has found higher-than-normal levels of activity - it contains about 1.8 million times as much caesium 137 as before the incident. However, TEPCO concludes that the majority of the fuel in unit 1 is undamaged, and the radioactivity came from external sources, such as contaminated steam and dust.

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