VERNON - Vermont Yankee nuclear power reactor went into an emergency shutdown Wednesday afternoon as it was increasing power production after a planned month-long refueling and maintenance outage.

Entergy Nuclear spokesman Larry Smith said the plant was at about 70 percent power when it went into an automatic shutdown at about 3:30 p.m. on what was the hottest day of the year.

He said preliminary indications are that there was a problem in the plant's new switchyard, which was connected to the 650-megawatt reactor during the recent refueling outage.

"We think it's in the switchyard," Smith said late Wednesday afternoon. "The plant protects itself when it senses a problem."

Smith said there was no release of radiation, and the reactor will be restarted after the problem has been identified and repairs have been completed.

Kerrick Johnson, vice president for external affairs for Vermont Electric Power Co., or VELCO as it is known, said the company was working with Entergy to determine whether the problem occurred in its new $54 million switchyard, which was built north of the plant.

"They may be right," he said of Entergy's initial diagnosis that the problem was in VELCO's switchyard.

VELCO had purchased the old 345-kilovolt switchyard from Entergy a few years ago and built the new switchyard as part of the $219 million Southern Loop/Coolidge Connector project, which is on track to be completed in October, Johnson said. The project is designed to bring more stability to the New England power grid.

Plant personnel are looking at three pieces of equipment, he said, called "current transformers." There are two of the transformers in the new switchyard and one in Vermont Yankee's footprint, he said.

He said plant personnel were investigating to see whether all three current transformers had been readjusted properly for the reactor's startup.

The shut down of Yankee came on the hottest day of the year so far: the temperature reached 94 degrees in Vernon, according to the National Weather Service.

At ISO-New England in Holyoke, Mass., spokeswoman Marcia Blomberg said preliminary figures showed that New England electrical demand hit 22,700 megawatts of power at about 4:30 p.m. Wednesday afternoon. She said that Wednesday's peak was below the New England peak, which occurred in August 2006, when demand hit 28,130 megawatts.

Blomberg said ISO-New England doesn't comment on individual power generators - such as Vermont Yankee - but she said around 4 p.m., the system sensed that demand was equaling available power so the system's stand-by generators kicked in.

She said spring is considered the "shoulder season" in the power business, and traditionally many power generators, not just nuclear power plants, go off-line to do routine maintenance.

There are several key plants in New England currently doing maintenance, she said.

"We have these procedures in place so we can operate the system," she said, noting there were more than 350 different power generators in New England.

susan.smallheer@rutlandherald.com