OAK HARBOR - When you need to move an 84-ton, 18-foot-wide steel dome -- the critical part to rebuilding the damaged Davis-Besse nuclear power plant in Oak Harbor -- you don't send it by FedEx, UPS or load it in the back of a pickup truck.

Instead, you gingerly lift the $55 million-plus part, called a reactor vessel head, onto the middle of a 187-foot-long tractor trailer riding on 74 tires, cover it with a tarp, strap it down with chains, throw another tarp on it and then crawl along for 250 miles at no more than 45 miles an hour.

With an armed escort.

With the arrival Thursday of the never-used, nonradioactive replacement vessel head from Midland, Mich., FirstEnergy completed a crucial phase in its quest to repair Davis-Besse by year's end.

The plant, along the Lake Erie shoreline about 25 miles east of Toledo, has been shut down since mid-February for refueling and a safety inspection that in March unexpectedly found that boric acid ate two cavities nearly through the reactor's old vessel head. While no radiation was released into the environment, nuclear power critics say poor maintenance and mismanagement at Davis-Besse could have created a catastrophic accident.

FirstEnergy had hoped to keep the new vessel head's arrival time and date secret for security reasons.

But the Ohio Department of Transportation late Wednesday issued a press release outlining the massive domed-shaped object's travel itinerary through the state. Because the vessel head isn't radioactive, it was classified as a super-wide commercial shipment -- something ODOT issues traffic alerts about.

In any case, it would have been difficult hiding the fact that a 187-foot-long truck and its state highway patrol escorts were moving slowly along major highways in Michigan and Ohio. It left Midland about 9:30 a.m. Wednesday.

The entourage arrived outside Davis-Besse along state Route 2 about 12:15 p.m. Thursday. The tractor-trailer finally worked its way through the contractor entrance gates about 10 minutes later, helped greatly by the fact the rear portion of the tractor-trailer had its own steering mechanism.

As workers prepared the vessel head to be taken off the tractor-trailer, guards armed with automatic weapons stood watch.

The entire operation went smoothly, FirstEnergy spokesman Todd Schneider said. The vessel head was finally taken off of the tractor-trailer about 3:40 p.m., and moved to another location on the property.

FirstEnergy will have to cut a 20-foot by 20-foot hole in the containment building, which houses the reactor, to get the vessel head inside. While containment chambers have been cut open at other nuclear plants around the country, this will be the first time that a high-pressure water jet will be used to slice into the thick concrete walls, Schneider said.

``That will give us a nice, clean cut,'' he said.

The radioactive fuel that powers the reactor has been removed from the containment chamber and stored elsewhere.

Also, the damaged portions of the old vessel head have been cut out and are being analyzed.

There is some doubt the Nuclear Regulatory Commission will allow the plant to power up again this year. The NRC says FirstEnergy first needs to prove that there's no other damage inside the plant and that company management is up to the task of preventing further damage.

The cost to repair Davis-Besse could be as high as $300 million, based on the purchase price of the replacement head, other repairs, maintenance and upgrades, and the need to buy electricity during the peak usage months of July and August, when air conditioners typically are used the most.