|
|
Other |
Article published Sunday, July 14, 2002
Davis-Besse engineers lulled into false sense of
security
Boric-acid leaks suggested
less-serious problems
Boric-acid deposits like these on the vessel flange at
Davis-Besse were considered fairly harmless.
|
By KELLY
LECKER
BLADE STAFF WRITER
OAK HARBOR, Ohio - For years, corrosive acid
dripped from flanges above the reactor vessel at the Davis-Besse
nuclear plant onto its protective, carbon-steel vessel
head.
These leaks were not new to the nuclear industry - they
have been documented for nearly two decades - and because the acid
was dry when it hit the vessel head, it was considered fairly
harmless. Much of the boric acid that leaked onto the head was left
there.
But according to nuclear power experts, the leaks were
causing a much more serious condition: a false sense of security
about how destructive boric acid could be.
"The sociologists
would say it leads you down a trap where abnormal becomes normal,"
said David Lochbaum, a nuclear safety engineer with the watchdog
group Union of Concerned Scientists. "It led Davis-Besse and the
Nuclear Regulatory Commission to tolerate the condition instead of
fixing it."
So when Davis-Besse engineers saw boric acid on
the reactor vessel head at their plant here about 25 miles east of
Toledo, they assumed it was relatively harmless, coming from the
same old flanges that are like connectors between a pipe and another
component, according to a Davis-Besse spokesman and nuclear industry
experts. They didn’t realize the leak was from cracking control-rod
drive-mechanism nozzles - a leak that was much more corrosive
because the boric acid would be moist. The nozzles allow control
rods into the reactor, which allow operators to start and shut down
the plant.
Boric acid is used to slow down the nuclear
process and control the heat.
By the time Davis-Besse
officials realized what was happening, boric acid had eaten a
milk-jug-sized hole through six-inches of carbon steel on the
reactor vessel head. All that was left was a 3/8-inch stainless
steel liner to keep the radioactive water from escaping the reactor
and going into a containment building that protects the public from
radioactive leaks.
A former NRC commissioner called the event
the worst threat to safety by a nuclear plant since the Three Mile
Island accident.
Federal records indicate why Davis-Besse
personnel may not have suspected the nozzles were a problem and why
they did not completely clean the boric acid from the reactor vessel
head:
Nozzle cracking was a byproduct of aging, and the older Oconee
nuclear plant in South Carolina had not reported problems. This was
in 1997. Last year Oconee did find nozzle cracking, which led to NRC
action and the discovery of Davis-Besse’s problems. The same company
built the Oconee and Davis-Besse plants.
Buildup of boric acid on the reactor vessel head kept plant
operators from seeing cracks in the control rod nozzles.
Boric acid deposits from leaking flanges had been left on the
vessel head before, and the head hadn’t corroded. An industry boric
acid corrosion guidebook said in some cases a layer of boric acid
can protect the head from further corrosion by keeping water
away.
While leaking flanges often were blamed for the boric
acid gathering on the reactor vessel head, there were no flange
leaks found in an inspection during the plant’s 1996 refueling
outage, and very limited leakage was found in 1998. Still, the
amount of boric acid on the head kept growing, according to an
inspection report filed in May by the NRC.
"The licensee
apparently did not deduce that it then must have come from pressure
boundary leakage, such as nozzle cracking," the report
states.
Davis-Besse said that because of the way the plant
was constructed, the reactor vessel head was difficult to see and
even harder to reach.
"It was difficult if not impossible to
determine when you have this buildup of boron on the reactor head
whether the source was the flanges or whether it was from one of
these nozzles," FirstEnergy spokesman Richard Wilkins said. "What
the issue was in the industry, because of these flanges leaks, was
seeing boric acid buildup on the reactor head was not seen as a
major problem."
During this current refueling outage, the
company is looking at its inspection and maintenance programs and
how they can be made more stringent, Mr. Wilkins said. It also is
cutting bigger access holes in the service structure, which allow
engineers to look at the reactor vessel head and to reach
it.
Nuclear watchdogs say better ways are needed to reach and
inspect the reactor head, and that Davis-Besse and other plants
should be forced to follow their boric acid control plans, which
call for cleaning up boric acid residue and finding the source of
every leak. And they argue that every leaking flange - no matter how
common or insignificant - should be fixed so it can be ruled out if
something more serious cracks or leaks.
"They should have
been cleaning the reactor vessel head down to bare metal with every
outage. There is nothing in any analysis report that says that it’s
OK to operate with rust on the reactor vessel head," said Dr.
William Corcoran, head of Nuclear Safety Review Concepts in Windsor,
Conn. "There’s probably 15 or 20 opportunities they had to prevent
this without changing any regulation."
----------
To
understand how nuclear power is used in pressurized water reactors,
think back to your days in high school chemistry and the chapter on
atoms.
Nuclear power plants such as Davis-Besse use uranium
atoms as fuel. An atom is the strongest force of nature, but it can
be broken, or split.
The nucleus, or center, of an atom is
made up of neutrons and protons. If a neutron from one atom hits
another atom, it splits that atom apart. Neutrons break away,
hitting more atoms, which split and hit even more atoms, and so on
in the chain reaction.
The whole time these atoms are
breaking apart, they are creating heat. The heat is used to convert
water to steam. The pressure of the steam turns a turbine that
generates electricity.
It’s easy to see how this chain
reaction of atoms hitting atoms can speed up - and heat up - too
quickly. So engineers add boric acid to the water in the reactor.
Boric acid absorbs some of the rogue neutrons - meaning they are not
splitting any more atoms - and allows engineers to slow down the
heat process.
Unfortunately, boric acid acts like a piranha
when it meets carbon steel, eating through the metal. That’s why
many components of the reactor vessel are lined with stainless
steel, which is not eaten by boric acid. The inside of the reactor
head has that stainless steel lining; the outside does
not.
The nuclear industry has been aware of boric acid’s
dangers since at least the early 1980’s:
In 1987, more than 500 pounds of boric acid crystals were found
on the reactor vessel head at Turkey Point 4 in Florida after
reactor coolant leaked from a seal in an instrument tube.
A pile of rust-colored boric acid accumulated on the reactor
vessel head at Salem Unit 2 in 1987. The pile was 3-feet by 5-feet
and one foot high. Reactor coolant leaked through three pinholes in
the instrumentation.
These and other incidents documented in
NRC records did not cause the extent of damage seen at Davis-Besse;
in fact it is the only plant with such extensive corrosion. That’s
because the boric acid in these other incidents was dry by the time
it hit the reactor head - largely because of the heat - and wasn’t
very corrosive. The boric acid leaking from the nozzles at Davis
Besse was in a moist area, and hot jet of boric acid steam corroded
the steel, Mr. Corcoran said.
The leaks that caused the
corrosion were not gushing at a rate far more than the NRC allowed.
In fact, the leakage rate for the water that contained the boric
acid was within the limitation set by the agency of one gallon per
minute. That’s not much considering there are a half-million gallons
flowing through the pipes every minute, Mr. Lochbaum said, adding
that Davis-Besse’s leakage rate was often one-tenth-gallon per
minute.
"My understanding is the limits are about as low as
you can reliably detect. You can’t lower those limits," Mr. Corcoran
said. Jan Strasma, an NRC spokesman agreed. He said anytime you have
pumps and valves and flanges, there will be some
leaking.
"The leakage limits are limits for leakage of water.
... Those are reasonable and valid for leakage of water," he said.
"What the limit does is allow you to take action before something
becomes a severe problem."
Pressurized water reactors like
Davis-Besse, which use boric acid, have boric acid corrosion control
plans that call for engineers to inspect the leakage or boric acid
accumulation and to look for the source of the leak, Mr. Strasma
said.
In 1996, a Davis-Besse engineer initiated a report
detailing the fact that the corrosion control plan had not been
followed during the last outage, and it could not be fully done this
time - meaning the reactor vessel head couldn’t be cleaned
completely - because engineers had limited access to the head,
according to an NRC report. The report said the boric acid buildup
made it difficult to tell if the leak was from the flanges or the
nozzles. But it concluded that the probability of it being from a
nozzle was "relatively low."
The incident at Davis-Besse has
caused engineers there and at other plants to look at boric acid in
a new way, even the dry boric acid crystals thought to be pretty
benign.
"We do not think that now. Obviously there’s a lesson
to be learned there," said Mr. Wilkins.
| |
|
