Fukushima Daiichi NPS Prompt Report 2013

Today, concerning the turning on of the standby failure indicator of the Unit 5 emergency diesel power generator (hereinafter, D/G) (B), we conducted a detailed investigation on the position detection circuit of the fuel handle (a device to block fuel supply to the D/G) and the fuel handle position.

We found out that the fuel handle of Unit 5 D/G (B) had been displaced from the normal position as compare to the fuel handle of Unit 5 D/G (A) which has been maintained in the standby state.

Based on this result, we considered that this trouble was caused in the following manner: due to the displacement of the Unit 5 D/G (B) fuel handle, pressing of the fuel handle against the position detection circuit (a limit switch) became insufficient (indicated the off state); and the standby failure indicator was turned on as a result. We will implement the following measures to prevent such turning on of the standby failure indicator from reoccurring:
·Near a D/G fuel handle, place a sign that indicates the normal position where the handle should be secured.
·In the operational procedure manual, add a clear description of the normal position where a D/G fuel handle should be secured.
·Provide information on this incident to the operators.

Since the cause was made clear, we returned the fuel handle to the normal position, and confirmed that the standby failure indicator was turned off. At 4:23 PM today (on July 5), a verification run was started. Then, during a halt operation, we noticed that the air pressure of the air reservoir was low. We decided to later investigate the cause of the reduced air pressure of the air reservoir. For this reason, we are to maintain the cancellation of the standby state of Unit 5 D/G (B), which started at 3:45 PM today.