Morning Report for May 14, 2007

Headquarters Daily Report

• REGION IV

REGION IV

PARTIAL LOSS OF OFFSITE POWER AT DIABLO CANYON

Discussion:

On May 12, 2007, at 10:25 a.m. PDT, offsite electrical startup power (230 kV) was lost to Diablo Canyon, Units 1 and 2.
Unit 1 was in a refueling outage with the reactor core off-loaded to the spent fuel pool. One of three diesel generators was also unavailable due to maintenance. On the loss of startup power the remaining two diesel generators auto-started and powered their respective loads. Control room operators restarted the spent fuel pool cooling pump and restored spent fuel pool cooling. Spent fuel pool cooling was interrupted for approximately 4 minutes and spent fuel pool temperature remained unchanged. No emergency declaration was required since Unit 1 was defueled.

Unit 2 remained at 100 percent with auxiliary power supplying Unit 2 equipment. The loss of startup power caused all diesel generators to auto-start; however, the electrical loads remained on the normal electrical power supply which was not affected by the loss of offsite startup power.

The loss of the startup power to the plant was due to a fault on a 230 kV transmission line at Morro Bay substation. As per design, protective relaying sensed the fault and locked out startup power to the Diablo Canyon switchyard.

At approximately 2:30 p.m. PDT, repairs were completed and electrical startup power was restored to both units.

This Morning Report is being submitted for information only. The NRC resident inspectors monitored licensee response to the event.

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