TEPCO has restored cooling of the spent fuel pools affected by last week's loss of cooling. Cooling systems of the unit 1, unit 3, unit 4 and common spent fuel pools lost power for about a day, without any real risk of boil-off.
In 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 blamed 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 switchgears, and the system's circuits were cut. The rat's charred remains were found in a temporary transformer housing.
It said that 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 the power cut happening again.
In other news, TEPCO has moved the first of nine exsisting dry storage casks from the cask storage building to the common fuel pool building for inspection. The casks, which are about 100 tons and 5.6m long, hold up to 50 fuel assemblies. Their movement requires sequential operation of transport dollies, overhead cranes and dedicated vehicles. The cask will be inspected and three fuel assemblies removed for inspection. Gas sampling within the cask will aim to detect the presence of krypton gas, which indicates damaged cladding. Damaged fuels will be unloaded from the cask and loaded into the common fuel pool, and the cask would be refilled with undamaged fuel from the common spent fuel pool.
The latest status of Fukushima Daiichi decommissioning, as monitored by the Japan Atomic Information Forum's Atoms in Japan service, is available via this link: