Operator Tepco has restored cooling at the spent fuel pools affected by loss of cooling systems during the week of 18 March. Cooling systems of the unit 1, unit 3, unit 4 and common spent fuel pools lost power for about a day, but without, says Tepco, any real risk of boil-off.
During that time, temperature increases ranged from 2°C (unit 1) to 6.6°C (for the common spent fuel pool). However, final temperatures were still relatively low (maximum 32°C). Actual temperature rises in all four cases were below predicted rises. TEPCO estimated it would take at least four days for the fastest-rising spent fuel pool, unit 4, to reach 65°C from about 25°C.
Tepco says that the cause was a rat climbing into the unit 3-4 temporary switchgear and causing a short circuit. The short circuit created an overcurrent in it and other switchgear, and the system's circuits were cut. The rat's remains were found in a temporary transformer housing.
The delay in restoring power was caused by difficulty finding the location of the failure; as a result, the entire unit 3-4 switchgear unit was detached. Hooking up to an alternative power line 'took a substantial amount of time'.
Tepco is planning to build in extra redundancy in the spent fuel pool cooling circuits to reduce the risk of such a power cut being repeated.
The latest status of Fukushima Daiichi decommissioning, as monitored by the Japan Atomic Information Forum's Atoms in Japan service, is available via this website:
http://www.jaif.or.jp/english/news_images/pdf/ENGNEWS01_1363330317P.pdf
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