The temperature of the water in the used fuel pool of unit 4 at the damaged Fukushima Daiichi plant has dropped significantly with the start-up of an alternative cooling system. Meanwhile construction of a supporting structure beneath the pool has been completed.
Fukushima Daiichi unit 4 was offline for regular maintenance when the 11 March earthquake and tsunami struck, meaning that its full inventory of nuclear fuel was stored in the used fuel pool at the time. The pool contains 1535 fuel rods, many more than held in the used fuel pools of the other units at the site and therefore generating more heat.
Tokyo Electric Power Company (Tepco) started setting up an alternative circulation system, utilizing a heat exchanger, at unit 4 in mid-July. The system is designed to pump warm water out of the pool, cool it, and then return the water to the pool. The company said that it began a test run of the system at 0:44 am on 31 July. The temperature of the pool at that time was between 86°C and 87°C. Tepco said that following the successful trial run of the system, it had now put it into full operation.
By 5:00 pm on 31 July, the temperature of the pool had fallen by a few degrees, but by 8:00 am on 1 August the temperature had reportedly dropped to 63°C.
Tepco said that it intends to reduce the temperature of the pool of unit 4 to between 30°C and 40°C within one month.
The company has already installed similar alternative cooling systems at the used fuel pools in Fukushima Daiichi units 2 and 3. The temperature of those pools on 31 July was 34°C and 31°C, respectively.
Tepco is also setting up an alternative fuel pool cooling system at unit 1 and aims to run a test operation of that system within the next couple of weeks. The temperature of that fuel pool is unknown due to a failure of the instrumentation.