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This was the fourth unexpected shutdown at Palo Verde this year. Three of those involved radiation leaks.
In addition to those shutdowns, Unit 3 last month had a non-radioactive vent-line leak.
ALERT DECLARED - REACTOR TRIP DUE TO LOSS OF OFF SITE POWER
On June 14, 2004, at approximately 07:44 Mountain Standard Time (MST) all three units at the Palo Verde Nuclear Generating Station experienced automatic reactor trips coincident with a grid disturbance and loss of offsite power in the Palo Verde Switchyard. Unit 2 declared an ALERT Emergency Plan classification at approximately 07:54 due to a loss of AC power to essential buses reduced to a single power source for greater than 15 minutes such that any additional single failure would result in a station blackout. Subsequently, at 09:51 Unit 2 downgraded the Emergency Plan classification to a NOTIFICATION OF UNUSUAL EVENT when AC power was restored from a single essential bus to both essential buses. Units 1 and 3 declared a NOTIFICATION OF UNUSUAL EVENT at 07:53 MST due to a loss of offsite power to essential buses for greater than 15 minutes. The NOTIFICATION OF UNUSUAL EVENT was terminated for all 3 units at 12:07 MST.
Unit 1 and 2 manually initiated a Main Steam Isolation System ESF actuation by procedure. Unit 3 received an automatic Main Steam Isolation System ESF actuation.
Due to the loss of offsite power, the Emergency Plan Technical Support Center (TSC) was unavailable. The Unit 2 Satellite TSC was to be staffed by the Emergency Response Organization in response to the loss of assessment capability. Power to the TSC has since been restored.
The Emergency Plan ALERT declaration includes staffing of the Joint Emergency New Center to address expected media interest.
All three units were at normal operating temperature and pressure prior to the trip. All CEAs inserted fully into the reactor cores. All Emergency Diesel Generators (EDGs) (2 per unit) associated with each of the 3 units started as expected in response to the loss of offsite power to their safety buses. Unit 2's train "A" EDG started, but did not indicate volts or amps and was manually shutdown.
The offsite power grid had several perturbations for approximately one hour following the event but has been stable since. LCO 3.8.1, AC Sources - Operating, was entered in each unit as a result of this event. Heat removal is to atmosphere via atmospheric dump valves in natural circulation. Main steam safety valves may have lifted for a brief time. Restoration of forced reactor coolant circulation is pending assurance that the offsite power grid can reliably support the load. No major equipment was inoperable prior to the event that contributed to the event.
All 3 units are stable at normal operating temperature and pressure in Mode 3. The event did not result in any challenges to fission product barriers and there were no adverse safety consequences as a result of this event. The event did not adversely affect the safe operation of the plant.