OAK HARBOR, Ohio -- FirstEnergy's nuclear company has restarted its Davis-Besse nuclear reactor following a biennial shutdown for refueling, inspections and maintenance.

The 908-megawatt plant was operating at 60 percent of power this afternoon and expected to reach full power during the week. One megawatt, or 1 million watts, is enough electricity to power 800 to 1,000 homes.

During the 44-day shutdown, the company completed construction of a new emergency water and power supply building designed to meet federal and industry emergency standards that have been developed since the disaster at Fukushima, Japan, in March 2011.

Constructed next to the reactor containment building, the hardened three story structure has been built on bedrock under the plant and contains more than 130,000 square feet of space.

The building houses storage tanks containing 290,000 gallons of water and two large gas-turbine generators to power pumps that would push that water into the reactor if a major disaster knocked out all of the plants redundant emergency systems, which is what happened in Japan following the tsunami.

The 290,000 gallons of stored water would be sufficient to cool the plant for up to 24 hours while crews set up emergency pumps and lines to draw water from Lake Erie.

There is also sufficient space in the new building to store those additional portable pumps and large generators, said Jennifer Young, FirstEnergy spokeswoman.

Davis-Besse already has two very large emergency diesel generators, also in hardened buildings, but the Nuclear Regulatory Commission's post Fukushima standards require plant owners to assume the diesels have been knocked out as well as all other vital systems, as they were at Fukushima, Young said.

During the ongoing disaster at Fukushima, emergency crews lost each reactor's emergency diesel generators, power from transmission lines, and the ability to pump water into the reactor cores to prevent them from melting.

Since the Fukushima disasters, the NRC and the U.S. nuclear industry have developed standards that power plant owners must meet in order to deal with such extreme emergencies.

"It's fair to say the building and equipment cost millions of dollars," said Young. "It's part of our strategy to protect the plant from unexpected events. It's a facility we hope we never have to use."

The equipment and hardened building will also enable the company meet new National Fire Protection Association standards that the Nuclear Regulatory Commission has approved.

Complying with the NFPA standard is voluntary, said Young. "But it's something we wanted to do."

Also during the shutdown, other crews replaced about a third of the reactor's 177 fuel rod assemblies as well as two of the plant's four 50-ton electric motors that drive the reactor's very large coolant pumps.