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Davis-Besse finds new pump problem

03/29/03

John Funk and John Mangels
Plain Dealer Reporters

The high-pressure emergency pumps that are supposed to keep the Davis-Besse nuclear reactor's fuel from melting in an accident might fail in some situations, plant engineers have found.

The two vital pumps are vulnerable to fouling from debris.

The pumps' vulnerability came to light several weeks ago during plant owner FirstEnergy Corp.'s intensive review of Davis-Besse's design documents. The review is part of the company's efforts to convince federal regulators that Davis-Besse can be operated safely again.

The Toledo-area plant has been idle 13 months for repairs and inspections after workers found a pineapple-size rust hole in the reactor's lid last spring.

A smaller-scale design review by the plant and the Nuclear Regulatory Commission in the 1990s failed to identify the emergency pumps' weakness. The flaw was unintentionally created in the mid-1980s while Davis-Besse engineers were trying to solve a different problem.

The pumps' potential to seize up in a crisis is the second long-standing safety defect in the emergency cooling system that plant engineers recently have identified. In December, they reported that Davis-Besse's undersized sump could have been clogged with debris during an accident, choking off the flow of water to cooling pumps.

FirstEnergy, which hopes to have Davis-Besse ready to seek NRC approval for restart in April or May, is considering whether to modify the pumps or replace them.

The latter prospect would take several weeks and could stretch out the already tight restart time table, said Davis-Besse spokesman Richard Wilkins. The company will decide "within a week or so."

The NRC is aware of the pump matter and will review the appropriateness of whatever action FirstEnergy proposes, said agency spokesman Jan Strasma.

While it took nearly two decades to identify the pumps' Achilles heel, it is better to know late than not at all, said David Lochbaum, a nuclear safety engineer with the Union of Concerned Scientists, a watchdog group.

Lochbaum praised the Davis-Besse engineers' ability to spot such a subtle problem, and their willingness to bring it to the NRC's attention regardless of cost or schedule pressures.

"This is a good catch," he said. Davis-Besse "ought to get more credit for catching it than blame for missing it earlier. Catching this so late in the game, there had to be incredible pressure to 'pencil-whip' it away, but none of that seems to have occurred."

The emergency pumps are part of a system to keep the hot, radioactive reactor core supplied with coolant in case of a rupture. The reactor operates at high pressure. If the rupture is large, that pressure will fall quickly, triggering two low-pressure, high-volume pumps that can replace large amounts of coolant.

If the rupture is small, two high-pressure, low-volume pumps kick in to counteract the reactor's own high operating pressure and force coolant back in. It is those high-pressure pumps that are vulnerable at Davis-Besse. The plant's engineers unknowingly created the problem in the mid-1980s when they were trying to compensate for another issue plaguing reactors at the time.

The high-pressure pumps originally were supplied only by a water tank that eventually would run dry. By then, though, emergency relief valves should have popped, depressurizing the reactor enough for the other pumps to operate. Those low-pressure pumps could suck spilled coolant from the plant's emergency sump, in theory recirculating as long as necessary.

The emergency valves at many plants tended to stick closed, however, meaning that reactor pressure might remain high and the high-pressure pumps would have to stay on longer.

To get around the pumps' water supply problem, Davis-Besse engineers connected them to the emergency sump system. But they didn't realize the implication of what they had done.

At Davis-Besse, the bearings that keep the high-pressure pumps spinning are water-cooled, using the same liquid the pumps are circulating. If that water is from the storage tank, it is free of impurities. But water collected from the sump may have debris from the crippled reactor or the plant floor.

As a new team of Davis-Besse engineers has only recently realized, it's possible that some particles might be small enough to pass through the high-pressure pumps' filters, damage their bearings and cause the pumps to fail. There is no backup.

Davis-Besse is the only plant in the nation whose emergency high-pressure pumps are designed this way, the NRC's Strasma said. Its low-pressure pumps aren't affected by the debris problem, he said.

The new engineering review that turned up the pumps' vulnerability was more rigorous than the company's and the NRC's original work and benefited from modern analytical methods and years of additional experience, Wilkins said.

"We didn't do as thorough a job of the analysis as we do now," he said. "The issue we're talking about is theoretical. The way we ask the question is 'Is this possible?' If it's possible, we've got to keep pursuing it."

For full coverage of Davis-Besse, go to www.cleveland.com/davisbesse/

To reach these Plain Dealer reporters:

jfunk@plaind.com, 216-999-4138

jmangels@plaind.com, 216-999-4842


2003 The Plain Dealer. Used with permission.
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