(Reuters) - The Nuclear Regulatory Commission said a June breaker fire at the 478-megawatt Fort Calhoun nuclear plant was of "high safety significance," increasing work the Omaha Public Power District (OPPD) must complete before the troubled unit can restart.

The NRC's preliminary "red" safety violation, the agency's most serious classification, is the second for a U.S. reactor in as many years. In 2011, the NRC slapped the Tennessee Valley Authority with a red violation at its Browns Ferry 1 reactor in Alabama.

Fort Calhoun shut 11 months ago for a month-long refueling that had to be extended when rising flood water from the Missouri River surrounded the site, forcing OPPD to install water-filled berms to keep water away from key safety equipment.

On June 7, while the unit was shut, a fire broke out in one of two switchgear rooms. The fire knocked out power to systems that cool the reactor's spent fuel for about 90 minutes, the NRC said.

The NRC said a special inspection after the fire concluded that a deficient design modification and maintenance of replacement electrical equipment contributed to the fire which could have occurred when the plant was operating at full power.

"The inspection team determined that prior to the fire, your staff failed to adequately investigate the source of an acrid odor in the west switchgear room that had been present for three days," the NRC said in a letter to David Bannister, OPPD's chief nuclear officer. "A proper investigation may have prevented the fire."

The latest action follows the NRC's decision late last year to put Fort Calhoun in a special inspection category due to multiple safety problems.

The plant is subject to an NRC monitoring program reserved for stations with significant performance issues that have been shut for extended periods of time.

Scrutiny of operations at all 104 U.S. nuclear reactors has been increased in the wake of the March 2011 earthquake and tsunami that destroyed the Fukushima nuclear plant in Japan.

OPPD said it has completed much of the work necessary to address the NRC's concerns stemming from the breaker fire.

Earlier this month, operators began testing equipment installed to replace the fire-damaged breaker.

"We are redesigning our electrical system to provide additional isolation of the equipment so that a problem in one system does not spread into the backup system," OPPD said on its website.

Workers have also restored equipment at a meteorological tower at the site which was knocked out by floodwater, the utility said.

OPPD gave no target date for Fort Calhoun's restart. The unit cannot restart without NRC approval.

"OPPD leadership and our board of directors are committed to doing everything possible to return Fort Calhoun Station to a high level of performance in a safe, efficient and timely manner," said OPPD Chief Executive Gary Gates in a statement.

(Reporting By Eileen O'Grady; editing by Carol Bishopric)