A team of two TEPCO employees and four contractors explored the unit 4 spent fuel pond at the damaged Fukushima Daiichi nuclear power plant on 26 June.

Unit 4 spent fuel pool on reactor building 5th floor on 28 June

Pool water temperature was between 84-93° as of 27 June; 200 tons of cooling water was injected. An alternative cooling arrangement is planned for installation by the end of July.

The utility has plans to install an alternative fuel pool cooling system as it has at unit 2 and 3. Radiation dose was found to be relatively low: 3.7 mSv/hr at surface of pool water. Dose at the side of the pool, 50cm above the water level, was 0.5 mSv/hr.

Because of relatively high dose rates inside the unit 3 reactor building, TEPCO has used a robot to vacuum dust from the ground floor. Radiation was reduced. Also, some 50 steel plates have been laid on the floor to reduce radiation by a third. TEPCO plans to install instrumentation and apparatus to inject nitrogen into containment.

In other news, TEPCO halted water injection operations into units 1, 2 and 3 for four days (1-4 July) to install a 1000 m3-capacity buffer tank. The tank improves reliability of water injection. The tank is inserted between the treated water tank and the (freshwater supply) filtered water tank, and pumps to units 1, 2 and 3. Water treatment, however, was planned to continue during the stoppage.

TEPCO has also announced that a 362m-long breakwater ranging from 2.4-4.2m high has been constructed at 10m high elevation as a protection against a magnitude 8.0 tsunami.

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