A main bank transformer fire has put a halt to power ascension testing at the Tennessee Valley Authority’s (TVA’s) Watts Bar Unit 2 nuclear plant.

The incident occurred at about 9:10 p.m., on August 30, when an electrical fault on the 2B main bank transformer caused the Unit 2 main turbine to trip. The reactor, by design, also tripped, inserting all control rods fully.

Quick and Appropriate Response

The main bank transformers are located in an exterior switchyard (Figure 1). They are filled with oil, and in this instance, the electrical fault resulted in an oil fire, which was reported about a minute after the unit tripped. TVA’s onsite emergency response team answered the call to put out the fire.

1. Watts Bar site overview.
The main bank transformers are located in the switchyard near the turbine building. Courtesy: TVA

“We are very proud of our onsite team who responded appropriately to this type of incident,” Jim Hopson, a TVA spokesperson, told POWER. “Obviously, our teams drill and practice responses to these types of occurrences, so they acted very appropriately in stabilizing the situation and extinguishing the fire before there could be any other significant issues associated with it.”

By 10:30 p.m., the fire was reported to be out. There were no injuries as a result of the incident, and the general public was not in any danger, according to the TVA. Watts Bar Unit 1 was unaffected and remains online.

Testing Temporarily Halted

The incident occurred while Unit 2 was being brought up to full power for the first time. The reactor had reached 99% power the previous day and was being allowed to stabilize prior to commencing a series of tests.

“There are several full-power, power ascension tests,” Hopson said. “Some of those tests require us to make fairly substantial changes in power, including a 10% and a 50% load reduction test, and then the last major milestone is a 100% trip.”

The incident is likely to keep Watts Bar Unit 2 from reaching commercial operation by the end of summer, which had been the TVA’s most recent target.

Hopson noted that the unit must operate at 100% power for a minimum of 20 days before commercial operation could be declared. He said the declaration is not automatic, however; it depends upon a few other factors as well, but the 100% load rejection test is the last significant milestone.

The TVA has a spare transformer onsite, if needed, but at this point, it hasn’t fully diagnosed the damage or established whether to repair or replace the existing transformer. The installed transformer is not new; it has been onsite since the 1990s when Unit 1 entered commercial operations, but it has been extensively modified, updated, and maintained. It has even been cycled into operation on Unit 1, “so it’s not like this is a brand new unit that has never been used before,” Hopson said.

Although no timetable was offered on when Unit 2 might reach commercial operation, the transformer repairs will take some time, and Unit 2 testing cannot continue until the transformer is back in service or another path is established to get power from the generator to the switchyard. The TVA has also made it clear that it won’t rush the process.

“We will declare commercial operations when we feel that the unit is safe and reliable enough to do so,” Hopson said.

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