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July 14, 2002

 



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Other | Article published Sunday, July 14, 2002
Davis-Besse engineers lulled into false sense of security
Boric-acid leaks suggested less-serious problems
Picture

Boric-acid deposits like these on the vessel flange at Davis-Besse were considered fairly harmless.
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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 didnít 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-Besseís 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 hadnít 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 plantís 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 itís OK to operate with rust on the reactor vessel head," said Dr. William Corcoran, head of Nuclear Safety Review Concepts in Windsor, Conn. "Thereís 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.

    Itís 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. Thatís 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 acidís dangers since at least the early 1980ís:

  • 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. Thatís because the boric acid in these other incidents was dry by the time it hit the reactor head - largely because of the heat - and wasnít 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. Thatís not much considering there are a half-million gallons flowing through the pipes every minute, Mr. Lochbaum said, adding that Davis-Besseís leakage rate was often one-tenth-gallon per minute.

    "My understanding is the limits are about as low as you can reliably detect. You canít 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 couldnít 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 thereís a lesson to be learned there," said Mr. Wilkins.



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