The Tennessee Valley Authority (TVA) has reached another milestone in its effort to bring Watts Bar Unit 2 into commercial operation: The unit completed its final power ascension test—a 50% load rejection from full power—and safely returned to full power on September 30.

The testing had been halted on August 30 due to a switchyard transformer fire. Although the fire did not affect Unit 2’s systems, which responded to the event as designed, the affected transformer had to be replaced before testing could resume. The TVA said a rigorous and detailed inspection, repair, and testing process was performed before the transformer was returned to service on September 26.

Power ascension testing included more than 40 required tests at various power levels up to 100% output, with pauses at 30%, 50%, and 75% (Figure 1). The gradual increase in power provided data, which was used to verify that the unit was operating as designed.

1. Watts Bar Unit 2 power ascension testing plan.
More than 40 tests were conducted at various power levels to ensure the unit responded as designed. Courtesy: TVA

The testing also included intentional trips of both the reactor and the turbine generator. The deliberate excursions are designed to demonstrate that safety systems work as intended and would respond adequately during an emergency.

“When we decided to complete Unit 2, we made a commitment to the people we serve that we would take the time necessary to do so safely and with high quality,” said Joe Grimes, TVA executive vice president of generation and chief nuclear officer. “The Watts Bar team has now passed another important milestone in fulfilling that commitment to reliably provide additional low-cost, clean energy to the Tennessee Valley.”

The last hurdle for Watts Bar 2 is a pre-commercial reliability run. The unit must operate at 100% power for a minimum of 20 days before commercial operation can be declared.

Aaron Larson, associate editor (@AaronL_Power, @POWERmagazine)