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
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
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
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
``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,
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.