The fire, which occurred in the area where energy is harnessed onto power lines about one-quarter of a mile away from the reactors, affected one of the two reactors at the site. Dominion Resources, which owns the reactors, hasn't figured out why it happened, but no workers were harmed and no radiation was released.

"This happened on our property, but away from the power plant," said Rick Zuercher, spokesman for Richmond-based Dominion.

The reactor was going to be restarted, but it remains shut down because the company then discovered the system that provides backup water to cool the plant had leaks in the pipes. That means Dominion couldn't guarantee it could get enough water to cool down the plant under certain emergency conditions.

"That system was one of the key systems that didn't work at Three Mile Island," said David Lochbaum, a nuclear expert at the Union of Concerned Scientists, referring to the worst nuclear accident in the United States. "It's an important backup system."

The shutdown of a nuclear plant for multiple days for anything other than routine maintenance means the problem is fairly significant. Dominion's nuclear plants provide a large amount of cheap electricity that must be replaced from more expensive sources when a reactor is not running.

The federal Nuclear Regulatory Commission said in April that the same nuclear unit that is out at Surry had too many shutdowns last year. The average nuclear plant had less than one unplanned shutdown in 2003, said Lochbaum.

In a nuclear power system, a nuclear reactor heats water in steam generator tubes that are surrounded by cooler water.

The hot water in the tubes turns the surrounding cold water into steam, which powers a turbine that creates electricity through a generator.

That steam then drops down, gets cooled and condensed, and turns back into water before heading back to the steam generator through a system of pumps. But the water circulation system shuts down when the reactor does, so the backup supply of water is important.

As soon as the fire started, it caused the steam generator and the nuclear reactor to automatically trip and shut off just after 9 p.m. Rods that cool the nuclear reactor automatically dipped into the core to cool it down.

"The station is built to sense anomalies and as soon as the units sensed the anomalies they shut down," said Zuercher.

Three backup water pumps started immediately and supplied water to cool the steam generator, which in turn helps cool the reactor.

One minute after the reactor shut off, the Surry control room technicians found out there was a fire in the switchyard, the area where power levels are adjusted and then sent over the electric grid.

The fire, which the plant's fire squad put out in just over a half hour, affected only one reactor because the other one has separate electric lines.

"It basically destroyed the transformer," said Zuercher.

People shouldn't be concerned that the plant was going to be restarted before discovering the source of the fire, said Lochbaum, because it was likely related to the transformer itself or something unpreventable like lightning.

"It's unlikely the new transformer will have the same fault," said Lochbaum.

After midnight Saturday, workers realized there was a problem with the backup water pumps. The Nuclear Regulatory Commission's resident inspector, who works full time at the site to provide guidance, came to the plant.

Dominion told the NRC late Saturday night that the backup water pipes that run underground to the steam generator were degraded and leaking and were "declared inoperable."

The level in the emergency water storage tank was decreasing at a rate of about eight gallons a minute.

Workers discovered that was leaking in the basement of the building that houses the backup water supply.

There was plenty of water to cool the steam generator, but it wasn't clear the system could perform its intended cooling function under certain conditions, such as an earthquake. Dominion employees are using ultrasonic testing to find out where the leaks are and will replace those pipes.

By Sunday morning, Dominion was informing regulators that because of the leak, the reactor was going from a temporary "hot shutdown" to a longer "cold shutdown" -- which takes much longer to start the reactor again.

Once a reactor goes into a cold shutdown, it typically will perform some other maintenance, further extending the shutdown, said Lochbaum. Zuercher would not say when the unit will be turned back on.