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Tepco workers have dealt with a temporary loss of cooling systems at Fukushima Daiichi unit 4'sÌýused fuel pool. Meanwhile, Japan's first reactor to restart after the Fukushima accident has reached criticality.

Ohi 3 back in action The first Japanese nuclear powerÌýreactor to be allowed to restart following the Fukushima accident, Ohi 3, has restarted and has now reached criticality, Kansai Electric has announced.

Alarms sounded at unit 4's fuel pool on the morning of Saturday 30 June, when a problem with the pool's alternative cooling system caused the system to shut down automatically.

After confirming that there was no leakage from the pool, Tokyo Electric Power Co (Tepco) investigations suggested that the problem lay in the system's uninterruptible power supply (UPS). After the problematic UPS was isolated, cooling was restarted in the afternoon of 1 July.

In total, the pool's cooling system was out of action for nearly 33 hours. During that time, the pool's temperature rose from 31°C to 42.9°C, well within its design parameters - the normal operating temperature of theÌýused fuel pond would be around 30°C when circulation is maintained, although such pools are designed to be safe up to about 85°C in the absence of pumped circulation.

At the time of the March 2011 earthquake and tsunami, Fukushima 4's pool was unusually full as it was being used to hold a full core load of 548 fuel assemblies while the reactor was undergoing maintenance, as well as used fuel assemblies. WorkÌýwas recently completed to install a protective steel cover over the pool, which was exposed by the hydrogen explosion that tore the roof off the building.

Researched and written
by ¶¶Òõ´«Ã½ÔÚÏß Nuclear News