NRC and TVA officials said the Browns Ferry network involved is an internal-only network and — when operated as designed — cannot accept data from outside sources. TVA spokesman Terry Johnson said the utility believes the failure may have been caused by an unexplained glitch in the computer system.

But when pressed, the officials would not categorically rule out the possibility of outside access.

“We have reasonable assurance that there is no external access to this system,” said Eva Brown, the NRC’s project manager at Browns Ferry. “We did an independent assessment to convince ourselves that (TVA’s) conclusions were acceptable, and there was no evidence of an external source.”

Shutdowns at nuclear plants are somewhat rare; Browns Ferry had two shutdowns in all of 2006, and has had two so far this year.

NRC spokesman Scott Burnell said the agency’s public notice on the August 2006 incident should serve to warn other operators of the potential problem, although the NRC is not requiring any action.

“At this point there isn’t any regulatory reason to,” he said. “Sometimes it does take small events like this to bring issues to the attention of the staff at the plant and the NRC. That’s why we issued this informational notice.”

Joe Weiss, managing partner at Applied Control Solutions and an expert on industrial computer security, said he doubted that anyone intentionally caused the Browns Ferry network to fail. But, he said, it raises concerns regardless.

“The whole area of cyber security in industrial facilities is effectively in its infancy,” he said. “There needs to be a greater appreciation within the nuclear community that these systems truly are connected.”

Since the Sept. 11, 2001, terrorist attacks, security experts have warned of vulnerabilities in the computer networks of the nation’s “critical infrastructure,” including emergency response agencies, electricity providers and water treatment plants.

A 2005 report from the Environmental Protection Agency’s inspector general, for example, found that water utilities had installed computer-based remote controls “with little attention paid to security,” leaving valves, pumps and chemical mixers open to cyber attack.

In 2003, a computer virus temporarily disabled the safety monitoring system at the Davis-Besse nuclear station in Ohio, even though the utility thought the network was protected from such a breach.