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NRC reports Davis-Besse safety pump design flaw

06/25/03

John Funk and John Mangels
Plain Dealer Reporters

Federal inspectors have identified a second design flaw in a vital pair of emergency pumps at the Davis-Besse nuclear plant that must be fixed before the rust-damaged reactor can be tested and restarted.

The plant's two high-pressure injection pumps - critical to keeping the reactor's radioactive core from overheating and melting after a coolant-loss accident - might themselves overheat and fail in certain conditions.

Plant owner FirstEnergy Corp. already is spending between $3 million and $4 million to correct a separate design problem that could have allowed debris from a burst pipe to damage the pumps' bearings, causing the equipment to jam and fail.

Davis-Besse engineers caught that potential vulnerability, but their analysis missed the pump overheating problem.

A Nuclear Regulatory Commission inspector raised the concern during a special inspection last fall.

A subsequent study by FirstEnergy and its engineering contractor verified that pump overheating, although unlikely, could occur and that the design flaw had been present since Davis-Besse's construction in the mid-1970s. FirstEnergy reported its findings to the NRC June 10. The agency made them public Friday.

About two-thirds of the nation's commercial nuclear reactors, including Davis-Besse, operate under extremely high pressure - more than a ton per square inch - to keep the mix of water and boron that bathes the reactor's core from boiling away.

If one of the pipes carrying coolant in and out of the reactor were to rupture, emergency pumps would shoot reserve supplies of water into the core. If the rupture were small, the two high-pressure pumps would be needed to counteract the residual pressure.

The accident scenario that prompted the extra scrutiny involves a very small rupture, the size of a fingertip or less. With such a small hole, the reactor's pressure could rapidly cycle down and then back up as cool water injected by the pumps interacts with the continuing heat of radioactive decay in the core.

If this "re-pressurization" goes high enough, it could overwhelm the high-pressure pumps' ability to work against it, a condition called "deadheading." The pumps themselves generate heat, and without enough water flowing through them, their components could be damaged and not work properly.

FirstEnergy's analysis argues that even if such an improbable chain of circumstances occurred, Davis-Besse's reactor operators could use other valves and pumps, though not designed to rigorous emergency standards, to prevent the core from melting.

That thinking is "a little screwball," said nuclear engineer David Lochbaum of the Union of Concerned Scientists. "The high-pressure injection system is supposed to be the safety net."

NRC and Davis-Besse officials have discussed what to do about the pump problem, said Jack Grobe, chair of the special agency panel overseeing FirstEnergy's 16-month effort to renovate and restart the power plant. The company's failure to catch the design flaw was an NRC concern, and the agency pressed Davis-Besse to do additional analytical work, Grobe said.

The company finally decided on a fairly simple fix that is not expected to delay what FirstEnergy hopes will be an August restart, said spokesman Todd Schneider.

After a Pennsylvania company has finished installing special filtering screens to keep out debris, the pumps will be trucked back to Davis-Besse. There, workers will install a loop of stainless steel tubing between the pumps and the emergency sump, where spilled coolant collects. The loop will keep sufficient water flowing through the pumps to prevent overheating, Schneider said.

The company's engineers missed the potential problem, he said, because their focus had been on replacing the pumps rather than modifying them.

That's still surprising, said Lochbaum, "given the fishbowl that Davis-Besse has been in."

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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