Sept 20 (Reuters) - Small cracks in the shield building at the Davis-Besse nuclear power plant in Ohio pose no danger to the integrity or ability of the structure to operate, the plant's owner FirstEnergy Corp said on Friday.
During routine inspections, the company said it identified three "very tight, subsurface pre-existing cracks" in the building that were invisible to previous inspection technology.
The company also said cracks initially identified in 2011 appear to have grown slightly in three locations.
"Analysis of all inspection results to date confirms the 2011 conclusion that the shield building's structural integrity is not impacted by the presence of these tight cracks," Davis-Besse Site Vice President Ray Lieb said in a statement.
The 894-megawatt plant, located in Oak Harbor, about 120 miles (193 km) north of Columbus, was the site of what the U.S. Nuclear Regulatory Commission (NRC) has called one of the most dangerous nuclear incidents in the United States.
In 2002 workers discovered corrosion had eaten a hole in the plant's reactor vessel head, the repair of which kept the unit shut until 2004.
The NRC fined FirstEnergy more than $5 million for the actions that led to the corrosion.
The initial cracks in the shield building were discovered in October 2011 when the building was opened to install a new reactor head.
The reinforced concrete shield building provides protection from threats such as wind and tornadoes. The building surrounds a carbon steel vessel containing the reactor, FirstEnergy said.
In a move supported by the NRC, the company applied a protective coating to the shield building and established a long-term inspection program.
In the most recent inspection, FirstEnergy said it used high-definition cameras to examine the interior of the building's walls through a series of inspection ports - or core bores.
The company said it has examined 43 of the 82 core bores and expects to complete the checks in the next several weeks.