ML052150404
| ML052150404 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 08/25/2005 |
| From: | Hiland P NRC/NRR/DIPM/IROB |
| To: | |
| Rich Laura, NRR/DIPM 301-415-1837 | |
| References | |
| RIS-05-025 | |
| Download: ML052150404 (5) | |
See also: RIS 2005-25
Text
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
August 25, 2005
NRC INFORMATION NOTICE 2005-25:
INADVERTENT REACTOR TRIP AND
PARTIAL SAFETY INJECTION ACTUATION DUE
TO TIN WHISKER
ADDRESSEES
All holders of operating licenses for pressurized-water reactors (PWRs) and boiling-water
reactors (BWRs) except those who have permanently ceased operations and have certified that
fuel has been permanently removed from the reactor vessel.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform
addressees about recent operating experience related to the growth of tin whiskers in
electronic circuits at nuclear power stations. Recipients are expected to review the information
for applicability to their facilities and consider appropriate actions to avoid similar problems.
However, the measures suggested in this information notice are not NRC requirements and no
specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
On April 17, 2005, Millstone Nuclear Generating Station, Unit 3, experienced an unexpected
safety injection actuation and reactor trip caused by a fault on a solid state protection system
(SSPS) circuit card. The fault generated a false low steamline pressure signal, bypassing the
2-out-of-3 SSPS logic and causing the A safety train actuation and reactor trip. The licensee
examined the failed circuit card using a magnifying glass and found a microscopic tin filament
(approximately 2 mm long). The filament created a bridge between the affected diode and the
output trace on the card. This microscopic filament of tin called tin whisker, had grown out of
the tin coating covering the leads of the diode.
The licensee inspected all circuit cards in the SSPS and discovered tin whiskers on other circuit
cards. In each case, the whisker appeared to originate at the tin coating on diode leads.
Suspect cards were either replaced or cleaned before being placed back in service. The
licensee sampled additional circuit cards from other important plant systems but found no other
evidence of tin whiskers.
BACKGROUND
Tin whiskers are electrically conductive crystalline structures of tin that sometimes grow from
surfaces where pure tin (especially electroplated tin) is used as a final finish. Tin whiskers have
Page 2 of 3
been observed to grow to lengths of several millimeters (mm) and in rare instances up to
10 mm. Electronic system failures have been attributed to short circuits caused by tin whiskers
that bridge closely spaced circuit elements maintained at different electrical potentials.
Tin whiskers appear to have increased following international efforts to remove alloying metals
such as lead from solder and other circuit card manufacturing materials to reduce
environmental and health hazards. With the move toward lead-free electronics, tin has become
a drop-in replacement for the tin-lead finish currently used for electrical component
terminations. The move to lead-free electronics means that failures of some high-reliability
components may continue to increase until a solution to the tin whiskers problem is found. Tin
whiskers have been cited as the cause for various minor component failures in the nuclear
industry and significant failures in the aerospace industry.
DISCUSSION
Some of the failures due to whiskers are documented in licensee event reports (LERs):
Plant
LER No.
NUDOCS Accession No.
Dresden Unit 2
50-237/1987-22
8709230145
Duane Arnold
50-331/1990-04
9005010072
Dresden Unit 2
50-237/1997-19
9801270112
South Texas Unit
50-499/1999-06
9910080186
In most of the events, metallic whiskers caused a short of the local power range monitors
(LPRM) detectors resulting in a momentary spike on the average power range monitors
(APRMs). In other cases, whiskers resulted in a failure of a channel input relay to the
engineered safety features (ESF) actuation logic. In most cases, failure of the channel inputs in
to the reactor protection system (RPS) or the ESF actuation did not result in a full RPS or ESF
actuation. Only half of the RPS or ESF logic was met.
The incident at Millstone Unit 3 demonstrates that a single tin whisker can cause a protective
feature to actuate. It is reasonable to assume that the same phenomenon could also prevent a
protective system actuation. The extent-of-condition review performed at Millstone also showed
that circuit cards need not be in service to be susceptible to whiskering. Research available
from NASAs Goddard Space Center (http://nepp.nasa.gov/whisker) and Computer Aided Life
Cycle Engineering (CALCE) at the University of Maryland supports this discovery and provides
other valuable information on prevention techniques and growth mechanisms. While the
information provided directly states that the exact mechanism for growth is unknown, common
growth conditions and theories are discussed.
The data from the extent-of-condition review at Millstone Unit 3, NASA and CALCE information
indicate that more than one manufacturer makes high-reliability circuit cards susceptible to tin
whiskering. The data also indicates that tin whiskering is not significantly influenced by the
environment in which the cards are used. Therefore, if one card procured from a specific
vendor shows evidence of whiskering, all cards of that type from the same manufacturer can be
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expected to show signs of whiskering. In general, components containing 3% or greater lead
concentration in the solder and/or manufactured with conformal coatings appear to be less
susceptible to tin whiskering.
CONTACTS
This information notice requires no specific action or written response. Please direct any
questions about this matter to the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
/RA/
Patrick L. Hiland, Chief
Reactor Operations Branch
Division of Inspection Program Management
Office of Nuclear Reactor Regulation
Technical Contacts:
Thomas Sicola, R-I/DRS
301-415-3295
610-337-5109
E-mail: pjr1@nrc.gov
E-mail: tps1@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site,
http://www.nrc.gov, under Electronic Reading Room/Document Collections.
OFFICE EMEB:DE
Tech Editor
DE:EE
RI
NAME
RLaura
PKleene
AGHowe
JRogge (E-mail)
DATE
08/04/2005
08/04/2005
08/09/2005
08/08/2005
OFFICE RES
TL:C:IROB:DIPM
SC:OES:IROB:DIPM
C:IROB:DIPM
NAME
WKemper
IJung
MJRoss-Lee (IJung for)
PLHiland
DATE
08/10/2005
08/19/2005
08/19/2005
08/25/2005