Damage at the Davis-Besse nuclear power plant was caused by acid that started leaking as long as eight years ago onto the reactor vessel head, a preliminary report from FirstEnergy Corp. says.

In addition, power plant operators didn't recognize a number of small signs that, when taken collectively, would have alerted them to the fact that boric acid was eating through the 6 inches of carbon steel that makes up the 150-ton safety device that covers the radioactive fuel core, the report shows.

But those signs now may help the nation's 68 other nuclear power plants of similar design tell if they also have acid corrosion problems. The Davis-Besse damage has drawn widespread industry attention.

Plant staff and management did not understand the significance of dry boric acid deposits on top of the Davis-Besse vessel head or realize it was the result of significant corrosion. Those were among the findings in the five-page report by FirstEnergy scientists and engineers.

Plant operators also were hindered by the design of the vessel head, which made it difficult to do thorough cleaning and inspections. Radiation exposure, high temperatures and insulation that covers the vessel head also hid the problem, the report said.

FirstEnergy yesterday released a draft of a report showing what a team of experts believes to be the root cause of the damage at the now shut down power plant in Oak Harbor, about 25 miles east of Toledo. The plant was shut down on Feb. 16 for refueling and a safety inspection that subsequently led to the discovery of two acid-caused cavities.

``It doesn't appear it will have an impact on making a repair,'' plant spokesman Richard Wilkins said of the report. ``I'm saying that with a caveat, that this is a preliminary report.''

The Akron utility is making plans to repair the vessel head at a cost of between $5 million and $10 million, and hopes to get the plant restarted no later than the end of June. The Nuclear Regulatory Commission has to approve any repairs, and could instead order that the damaged part be replaced, a process that would delay restarting Davis- Besse by two years.

Should the NRC allow the vessel head to be repaired, FirstEnergy would modify its inspection and maintenance programs for the reactor vessel head, taking into account the new information in the report, Wilkins said.

FirstEnergy decided several years ago not to make modifications to the vessel head that would have made cleaning and inspections easier, knowing that it was going to replace the vessel head in 2004, Wilkins said. The report said FirstEnergy's decision to defer making those modifications contributed to the damage.

The report said evidence shows boric acid began leaking out of a device called a control rod nozzle between four and eight years ago. Five of the vessel head's 69 nozzles, which run through the top of the reactor vessel head, were found to be cracked. Three of those nozzles had cracks that went all the way through the walls.

Boric acid, which makes up the reactor coolant, leaked through the nozzles and began to corrode the vessel head's 6-inch-thick carbon steel exterior along two of the cracked nozzles.

Acid ate through all 6 inches of carbon steel at what is called Nozzle #3, stopping only when it hit a thin layer of stainless-steel cladding that lines the vessel head's interior. It's possible that the nozzle began developing hairline cracks as early as 1987, and that the cracks went all the way through the wall of the nozzle between 1994 and 1996, the investigation shows. Significant acid corrosion was happening by 1998, the investigators conclude.

Acid also created a much smaller cavity at Nozzle # 2.

Boric acid has been known to leak from other parts on the reactor vessel called flanges -- it's an industrywide problem -- and kept them from suspecting additional acid leaks from the cracked nozzles, the report says.

Monitoring equipment in the containment building that holds the reactor detected such things as boric acid, iron oxide (rust) and moisture, ``but no collective significance was recognized. However, it is not clear if these could have led to the discovery of the problem on the (reactor) head in time to prevent significant damage,'' the report says.

It will be at least a week before a final root cause analysis report is done and submitted to the NRC, Wilkins said.

Investigators need to pull out and examine Nozzle #2 where the smaller cavity was found, he said. That won't happen for another week or two, he said.

If the acid had eaten all the way through the reactor vessel head, a ``loss of coolant'' accident would have occurred. That's when coolant spews out into the large containment chamber that surrounds the reactor. Safety systems would have shut down the reactor immediately, with no leak of radioactivity into the environment, officials have said.