WASHINGTON - The Nuclear Regulatory Commission told the nation's nuclear power plants to start more stringent inspections this week in response to acid corrosion found on the reactor cap at an Ohio plant.

All 69 plants with pressurized water reactors must ensure the reactor caps do not have cracks through visual inspections and either chemical or ultrasonic tests. Plants identified as being at high risk of acid corrosion must perform the tests more often.

Previously, plant operators had to perform only the visual inspection.

The order arose from the agency's internal investigation of how it handled corrosion at the Davis-Besse plant, NRC Chairman Richard Meserve said in prepared testimony for a congressional hearing on Thursday.

Years of visual inspections at the plant in Oak Harbor, Ohio, east of Toledo, missed boric acid leaks that nearly ate through the 6-inch-thick steel cap that covers the reactor vessel. That part of the reactor was difficult to access and observe because of metal insulation inches above it.

The nuclear plant, operated by Akron-based FirstEnergy Corp., has been shut down since February 2002, when it was closed for maintenance. The corrosion was discovered last March.

Ohio Sen. George Voinovich, chairman of the Senate Subcommittee on Clean Air, Climate Change and Nuclear Safety, scheduled a hearing Thursday on how the NRC handled the Davis-Besse situation.

Meserve said in prepared testimony that the agency has "focused on mistakes the NRC made in dealing with boric acid corrosion issues in the 1990s."

"This degradation was preventable, and the licensee's actions leading up to the discovery of the corrosion in March 2002 are unacceptable," Meserve's testimony said.

Federal regulators have admitted they suspected the Davis-Besse reactor had cracks but haven't explained why they didn't shut the plant down sooner.

A report released recently by the NRC's inspector general said agency officials could have shut the plant down several months before the hole was discovered but wanted to avoid hurting the owner financially.

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Nuclear Regulatory Commission: http://www.nrc.gov/

FirstEnergy Corp.: http://www.firstenergycorp.com/

Senate Subcommittee on Clean Air, Climate Change and Nuclear Safety: http://epw.senate.gov/cleanair_108.htm