FirstEnergy finds cracks at Ohio nuclear reactor

* Davis-Besse shut to replace reactor vessel head

* Small cracks found in concrete shield building

* Cracks not like Progress Florida Crystal River reactor

NEW YORK, Oct 17 (Reuters) - FirstEnergy said it discovered small cracks in the concrete shield building surrounding the containment structure at the Davis-Besse nuclear power plant in Ohio, which was shut for another reactor vessel head replacement, a spokesman for the Ohio-based energy company said Monday.

"We don't believe there will be a problem with the schedule to replace the vessel head. Engineers are conducting a thorough investigation of the cracks. We should have an answer later this week," FirstEnergy spokesman Todd Schneider told Reuters.

Schneider was quick to point out that these cracks were different than the concrete problems with the containment dome at Progress Energy's Crystal River nuclear plant in Florida.

Crystal River has been shut since September 2009 after workers discovered a gap in the concrete containment dome after they cut through the structure to replace the plant's aging steam generators. The plant is not expected to return until at least 2014.

Schneider said the "microcracks" at Davis-Besse were barely visible. He said they were discovered while workers were cutting away the concrete with pressurized water to create an opening for the vessel head.

The shield building is a 250-foot (76.2 meters) tall, two and a half foot thick concrete structure that surrounds the steel containment structure, which is about an inch and a half thick. The steel containment and the concrete shield building surround the pressurized water reactor's vessel and steam generators and are designed to keep radioactive materials within the reactor in case of an accident.

The containment structures do not have a door big enough for the reactor vessel head, which is nearly 17 feet in diameter, eight feet tall and weighs more than 82 tons (74,389 kilograms), FirstEnergy said.

THIRD VESSEL HEAD

This is Davis-Besse's third reactor vessel head.

It cost the company about $600 million to replace the first vessel head ($300 million) and buy replacement power ($300 million) after workers in 2002 discovered borated water, which acts as the reactor coolant, leaked from a control rod drive mechanism and ate a six inch hole in the first vessel head. The plant did not return to service until 2004.

In 2010 during a scheduled refueling outage, the company found small cracks in the control rod nozzels and decided to replace the second vessel head. Schneider said this head replacement project would cost about $115 million.

Schneider said the company was still moving forward with the head replacement work and would next cut the steel rebar that supports the concrete in the shield building before cutting through the inner steel containment structure.

He said the discovery of the cracks has not impacted the work schedule but he could not say when the reactor was expected to return to service. He did note the outage was expected to last longer than a typical refueling outage.

The reactor shut on Oct. 1. A typical refuel lasts about four weeks. Electricity traders guessed Davis-Besse would return after about six weeks in the middle of November.

Schneider said FirstEnergy was not refueling the reactor at this time. According to Reuters data, the reactor is on a 24-month refueling cycle and will likely shut in the spring of 2012. It last shut for refueling during the spring or 2010.