Fukushima Daiichi NPS Prompt Report 2013

This is follow-up information regarding the "overflow of water from the top of a temporary notch tank for groundwater at the bank protection", which was found today (on October 17).

Based on our on-site investigation, this incident was found to have been caused by stopping of a pump used for transfer from the temporary notch tank to a water tank due to shutdown of power supply (a diesel generator) for the pump.

Further, the shutdown of the diesel generator was found to have been caused by activation of an earth leakage circuit breaker due to an insulation failure that occurred to a backup pump connected to the same power supply as the foregoing transfer pump.

Accordingly, we took off the backup pump having an insulation failure, and restarted the diesel generator. Then, at around 1:20 PM, we restarted the operation of the transfer pump.

Meanwhile, we took preventive action. We connected a pump (a suction pump) for pumping up at the well point and a pump for transfer from a temporary notch tank with the same power supply (a diesel generator), and thereby enabled pumping up at the well point and transfer from the temporary notch tank to stop at the same time following power supply shutdown.

Please refer to the following material (released on August 15, 2013) for a schematic view of the pumping mechanism at the well point.
http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2013/images/handouts_130815_03-e.pdf