Palo Verde Generating Station, located 45 miles west of Phoenix, Arizona, is the nation’s largest power producer. Just a few days ago on April 8, 2023, Palo Verde Unit 1 had a rather complicated reactor trip that is being investigated by the NRC. 

The sequence of events are as follows:

• The main turbine tripped due to a loss of hydraulic pressure.
• When this occurs, the output breakers from the turbine’s generator should automatically open, but they did not.  Therefore, the operators in the control room took the appropriate manual action to open these breakers.
• When these breakers are opened, power for the reactor coolant pumps should automatically transfer to their alternate source. This did not occur and the reactor protective system automatically initiated a reactor trip.
• Operators in the control room manually initiated a main steam isolation signal, in accordance with their procedures, and the atmospheric ateam dump valves opened as the plant was operating at 100% power prior to the turbine trip.    

According to the NRC, “Following the reactor trip, all control element assemblies inserted fully into the core. No automatic specified system actuation was required or occurred. No emergency plan classification was required per the Emergency Plan. Safety related buses remained powered from offsite power during the event and the offsite power grid is stable.”

The loss of hydraulic pressure, the main generator output breakers failing to automatically open, and failure of the fast bus transfer actuation are being investigated. The unit reconnected to the grid on April 14, 2023.

picture credit: U.S. Department of Energy