Nuclear

Power loss forces Palo Verde nuclear unit offline

Arizona Public Service is investigating a series of developments that led to the shutoff at the plant: A loss of hydraulic pressure, the main generator output breakers failing to automatically open and the fast bus transfer not actuating.
Power loss forces Palo Verde nuclear unit offline
The Palo Verde Generating Station in Tonopah, Arizona (Source: U.S. Department of Energy).

Editor’s update: Unit 1 re-connected to the grid at 11:40 a.m. MST on April 14.

Unit 1 of the Palo Verde Nuclear Plant in Arizona automatically tripped offline April 8 due to a loss of power to reactor coolant pumps, according to a report filed with the U.S. Nuclear Regulatory Commission (NRC).

The notification to regulators from operator Arizona Public Service said prior to the reactor trip, the main turbine tripped due to a loss of hydraulic pressure. The main generator output breakers did not automatically open on the turbine trip as expected, so control room operators opened the breakers.

Once the breakers were opened, the two 13.8 kV electrical distribution buses failed to complete a fast bus transfer, which resulted in the loss of power to the reactor coolant pumps, initiating the reactor trip.

According to the NRC filing, control room operators manually actuated a main steam isolation signal per procedure, requiring use of the atmospheric dump valves. The unit had been operating at 100% power.

“Following the reactor trip, all control element assemblies inserted fully into the core,” reads the event narrative. “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 the fast bus transfer not actuating are being investigated.

“As soon as the plant (Unit 1) safely shut down, we started an investigation into what caused this to happen and why,” said an Arizona Public Service spokesperson in an email. “That investigation is still ongoing.”

The spokesperson was not able to comment further.

Palo Verde is the largest nuclear plant in the U.S., with a total net summer generating capacity of about 3,937 MW.

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