The Nuclear Regulatory Commission on Tuesday announced that Watts Bar Nuclear Plant's Unit 1 reactor shut down unexpectedly early Saturday morning.

"The reactor tripped due to a trip [malfunction] signal from the turbine or non-nuclear side of the plant," said NRC spokesman Roger Hannah.

"The cause of the turbine trip is under investigation at this time," said an NRC statement. "All systems functioned as designed with the exception of pressurizer backup heaters which failed to energize."

TVA spokesman Ray Golden termed the unplanned outage "an uneventful shutdown caused by a non-nuclear piece of equipment with no implications from the reactor safety side."

The reactor was restarted early Tuesday morning with no radiological problems, he said.

In the meantime, TVA lost three days worth of it cheapest power generation - 1,150 megawatts a day - and had to replace it with higher-cost energy, such as coal, and perhaps had to purchase power from other utilities to fill the gap.

Golden said the problem was a malfunction in a circuit card on a newly installed computer system that assists the operation of the plant's steam-driven turbine that makes electricity. That malfunction caused the turbine to "trip" or quit, much as a malfunction in a home appliance would blow a fuse.

When the steam turbine shut down, a similar fuselike trip caused the reactor to automatically shut down.

A similar event occurred at Browns Ferry Nuclear Plant's three reactors last month when tornadoes destroyed TVA grid lines in Alabama.

With nowhere for the energy of the reactors to go, they are designed to stop producing that energy. To stop producing nuclear energy, they must shut down.

The Watts Bar reactor stopped working automatically as it was designed to, "but the goal of no automatic shutdowns is one of the ways we monitor our performance, so I'm sure it will be seen as a negative event," Golden said.

He said the malfunctioning pressurizer backup heater also will be reviewed. Its job is to keep pressure up in the steam generator, but that wasn't necessary once the reactor had tripped, he said.

"It is a moot point, but I guess NRC felt compelled to report it," Golden said.

During the unplanned outage, the reactor was cooled with pumped water from the plant's normal system. Meanwhile, backup diesel generators were on standby, but not needed, according to Golden and the NRC statement.