The Nuclear Regulatory Commission (search) proposed a record $5.45 million fine Thursday against the operator of a nuclear plant where leaking acid ate nearly all the way through a 6-inch-thick steel cap on the reactor vessel.

The NRC said FirstEnergy Nuclear Operating Co. (search) restarted the Davis-Besse plant in 2000 without completing a cleaning and inspection of the reactor vessel head, then misled the agency about what it had done.

The leaking boric acid (search) was found two years later during a routine inspection — the most extensive corrosion ever seen at a U.S. nuclear reactor.

"This substantial fine emphasizes the very high safety and regulatory significance of FirstEnergy's failure to comply with NRC requirements," said Luis Reyes, NRC executive director for operations.

The NRC also said it is banning one of the company's former engineers from working in the nuclear industry for five years. The agency said that Andrew Siemaszko was responsible for making sure the reactor vessel head was cleaned and inspected and that he deliberately provided false information.

The damage at the plant along the Lake Erie shore, 30 miles east of Toledo, ranks among the nation's worst nuclear problems since the accident at Three Mile Island (search) in Pennsylvania in 1979. It led to a review of 68 similar plants nationwide.

The Davis-Besse plant was closed for two years but returned to full power last April.

The fine more than doubles the previous record of $2.1 million handed down by the NRC in 1997 against the operators of the Millstone nuclear plant in Connecticut for safety violations.

FirstEnergy spokesman Richard Wilkins said the company had no immediate comment. The plant operator and Siemaszko have 90 days to appeal.

A federal grand jury is investigating whether the company provided false statements to the NRC, and the utility said in December that it probably would face charges.

Siemaszko has said he was wrongly fired and that since 1999 he had told supervisors the reactor needed to be cleaned.

"The allegations made by the NRC reveal the agency's serious misunderstanding of the facts, at best, or deliberate misrepresentation, at worst," said Billie Garde, Siemaszko's attorney.

NRC spokesman Scott Burnell said Siemaszko was singled out because "his name was on the paper that said the reactor head had been properly cleaned and no damage had been found." He said others who were involved may soon face NRC sanctions.

FirstEnergy, based in Akron, spent $600 million making repairs and buying replacement power because of the two-year shutdown.