Complex repairs to Unit 2 pressurizer are about to be put to the ultimate test

February 10, 1991|By Luther Young

It represents the longest, costliest, most complex repair ever made at Calvert Cliffs, a $20 million project to fix leaks on the Unit 2 pressurizer that have kept the nuclear reactor shut down since May 1989.

And the results of the 14-month project -- a major technical challenge involving innovations in automated welding, non-destructive testing and radiation decontamination -- will be put to the ultimate test when Unit 2 is brought back up to full power in March.

"This was one of the most complicated repairs ever undertaken in the nuclear industry," said Bernard Rudell, the Baltimore Gas and Electric Co. engineer who directed the project. "We took our time, and we did it right."

But the repair foreshadows the kind of difficult, expensive fix -- 10 times the original cost of the pressurizer -- that could become increasingly necessary as nuclear plants like Calvert Cliffs show their age. Unit 2 began operating in 1977, Unit 1 in 1975.

And BG&E isn't ruling out the future need for similar repairs on Unit 1's pressurizer, although testing has revealed no sign of the stress cracks that apparently caused the leaks. As a hedge, the company has purchased a spare pressurizer from a utility in Washington state.

"It was a real mystery why we had cracking on Unit 2 and not Unit 1," said Christian Poindexter, vice chairman of BG&E's board, referring to initial puzzlement over the difference between the twin, 825-megawatt reactors by the Chesapeake Bay near Lusby in Calvert County.

The problem was discovered in May 1989, six weeks into a routine refueling outage of Unit 2. Workers found evidence of radioactive water leaking from welds around tubelike sleeves penetrating the bottom of the pressurizer to permit installation of electric heating elements.

A tank 8 feet in diameter and 35 feet tall, with 4-inch-thick steel walls, the pressurizer is a critical component in the operation of a nuclear reactor, as heat from the decay of radioactive fuel makes steam to power a turbine and generate electricity.

The pressurizer "acts like a surge protector," said Mr. Rudell, keeping the superheated water under pressure to prevent it from turning to steam and boiling away before it reaches the steam generator. The 7-foot-long electric heating rods help regulate the temperature and pressure of the water at a constant 653 degrees and 2,250 pounds per square inch.

As part of the primary cooling loop of the reactor, the pressurizer water contains boron, a radiation-absorbing material. White streaks of encrusted boron around 23 of the 120 heater sleeves on the vessel's rounded bottom alerted BG&E to the leaks on Unit 2.

"Metallurgists have known for 30 years about something called 'primary water stress corrosion cracking,' " said Donald Graf, manager of nuclear outage and project management. The process exploits minute flaws in the molecular structure of metal alloys to form cracks.

They have since learned that one of the metals most susceptible to the stress cracking is a nickel/chromium/iron alloy known as Inconel 600, widely used in nuclear power plants at the time Calvert Cliffs was built.

And, after months of testing that included X-ray analysis and removal of a leaking sleeve by cutting through the steel, investigators found that metal had been weakened by heat during a machining operation, or reaming, that widened holes in the vessel prior to welding the sleeves in place.

Why did Unit 1's pressurizer escape the problem? "They didn't do that extra reaming on Unit 1, and they had trouble fitting the heater sleeves," Mr. Graf said. "When they got to Unit 2, they thought they were being smart by making the holes a little bigger ahead of time."

That unfortunate decision set a clock ticking in the Unit 2 pressurizer, and it took $20 million and more than a year of intense effort by BG&E and 40 contractors to devise a repair, build mock-ups of the vessel for practice and accomplish it.

A month was spent laboriously decontaminating the radioactive pressurizer, tucked close to the reactor inside the huge concrete containment structure, so that six workers at a time could safely maneuver into the cramped space directly beneath the vessel.

The 120 old sleeves were cut out one by one -- and new sleeves of a resistant alloy welded in place -- by a semiautomatic robot moving on a system of tracks. Because the metal had to be heat-treated to 500 degrees, huge air conditioners and water-cooled suits were required.

"It's better than new," Mr. Rudell insists.

And BG&E may recoup some of its costs by licensing the complex repair process, co-owned with Babcock and Wilcox Co., to other nuclear plants that experience similar pressurizer leaks.

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