TINY FILAMENT SEEN AS CULPRIT IN APRIL SHUTDOWN AT MILLSTONE
July 5, 2005

WATERFORD, Conn. --A thin filament of metal, barely visible to the eye,
was the culprit in an electrical short that forced the Millstone Power
Station to shut in April, technicians have found.

The presence of the filament, known as a "tin whisker," is the focus of
a study by engineering experts at the federal Nuclear Regulatory
Commission that could result in a notice alerting the industry, The Day
of New London reported Monday.

Dominion, the plant's owner, has notified reactor owners in the United
States and abroad.

During the first 24 hours of the nuclear reactor shutdown at Millstone
on April 17, technicians zeroing in on a computer malfunction were
stumped.

Two technicians for Dominion and their supervisor, Timothy Reyher,
figured out that a computer circuit card had signaled an unsafe drop in
pressure in the reactor's steam system, as if a break in a steam line
occurred. The condition led safety systems to automatically shut down
the reactor as intended and brought the electric generator to a halt.
The plant was not restarted for two weeks.

Reyher said the pressure was not low. The card, also known as a digital
logic card, had no obvious signs of wear or damage such as burn spots or
discoloration. Still, the card failed tests aimed at replicating the
correct electrical signal.

Reyher and lead engineer Keith Deslandes said a technician took a closer
look through a magnifying glass.

"They saw something different," Reyher said. "And they asked themselves,
'What can this be? A piece of solder? Something's there. Let's take a
picture.' "

Under a high-powered microscope, they spotted the filament.

The tin whisker can disrupt electrical flow and disable satellites and
interrupt service, according to the Goddard Space Flight Center.

The tin whisker that shorted out at Millstone's Unit 3 reactor triggered
an automatic shutdown designed to protect the reactor, but that is not
what worries the Nuclear Regulatory Commission. Instead, the tin whisker
could prevent a safety system from working properly, said NRC spokesman
Neil Sheehan.

Dominion officials have removed, photographed, cleaned and inspected 103
computer monitoring circuit cards at Unit 3 and replaced four that
showed signs of tin whiskers, said Dominion spokesman Pete Hyde.

Kevin Pelletier, a sales and marketing manager at Massachusetts
Materials Research Inc., which examined Millstone's circuit card, said
scientists have seen tin whiskers before, but "never related to a
nuclear power plant."

The April 17 shutdown induced a variety of systems to shut down and
protected the reactor.

"You want the system to detect problems at the very initial stages
rather than later so the system functioned as it should," Sheehan said.



? Copyright 2005 The New York Times Company



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