Boric acid is used in the primary coolant bath surrounding uranium rods in the reactor core.

FirstEnergy's latest repair plan modifies its initial plan for the crippled unit and raises the estimated cost of the repair to $25 million from $16 million earlier.

The repair plan calls for cutting out the corroded area of the reactor head and welding in a 6-inch thick, corrosion-resistant nickel alloy plate.

The plate will be about 17.5 inches in diameter and cover the control rod drive mechanism nozzle opening adjacent to the corroded area, as well as another nozzle opening near the corrosion.

At one of the nozzles, the acid had eaten all the way through the 6-inch thick vessel head.

The corrosion was so severe that a three-eighths inch thick stainless steel liner inside the reactor was the only barrier left between the reactor core, which operates under enormous pressure, and the metal shroud surrounding the reactor vessel.

On March 12, the NRC sent metallurgy experts and engineers to Davis-Besse to observe FirstEnergy inspectors.

FirstEnergy's preliminary repair plan, submitted earlier this month, called for cutting out a 13-inch diameter area around the corrosion damage and repairing the nozzle near the corrosion.

In the current plan, the diameter of the repair was enlarged to encompass the other nearby nozzle to make the overall area stronger.

With NRC approval of the repair plan and completion of the repairs, the company hopes to have Davis-Besse in service during the third quarter.

Based on this timetable, the company has secured on-peak energy to replace generation from Davis-Besse through the end of August.

Net replacement power costs are expected to be between $10 million and $15 million per month through June, and between $20 million and $25 million per month for July and August, the company said.

Davis-Besse is owned by Akron, Ohio-based FirstEnergy Corp. and operated by its subsidiary FirstEnergy Nuclear Operating Company.