Information Notice No. 87-35:Reactor Trip Breaker, Westinghouse Model DS-416, Failed to Open on Manual Initiation from the Control Room
SSINS No.: 6835
IN 87-35
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
July 30, 1987
Information Notice No. 87-35: REACTOR TRIP BREAKER, WESTINGHOUSE MODEL
DS-416, FAILED TO OPEN ON MANUAL
INITIATION FROM THE CONTROL ROOM
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or
construction permit (CP) employing Westinghouse DS-416 reactor trip breakers.
Purpose:
This notice is provided to alert recipients to a potentially significant
safety problem associated with a reactor trip breaker (RTB). The NRC expects
that recipients will review this notice for applicability to their facilities
and consider actions, if appropriate, to preclude a similar problem. However,
suggestions contained in this notice do not constitute NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances:
On July 2, 1987, McGuire Nuclear Station Unit 2 was performing control rod
drop tests after its recent refueling outage. This test was in progress with
the plant in mode 3 (hot shutdown). With all control rods inserted and the
RTBs closed for testing the next bank of control rods, station personnel
smelled smoke in the vicinity of the RTBs. A manual trip of A and B train
RTBs was initiated from the control room. Only the A train RTB opened. The B
train RTB was eventually tripped manually at the breaker panel. The smoke had
come from the B train breaker shunt trip coil, which had burned and shorted
while trying to open the breaker. The coil is designed for intermittent duty
and to carry current only until the breaker opens. Failure of the breaker to
open resulted in a prolonged and damaging current. Operators in the control
room stated that open indications for both the A and B train redundant RTBs
were observed for all attempted breaker opening evolutions during the control
rod drop testing process. However, the event recorder indicated that the B
train RTB failed to open on a previous manual trip attempt (approximately 4
minutes before) when operators were setting up for the control rod drop test
on the last bank of rods.
8707270073
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July 30, 1987
Page 2 of 3
An NRC Augmented Inspection Team (AIT) evaluated the licensee's investigation
into the reactor trip breaker problem. Abnormal wear and a broken weld were
found in this early vintage of Westinghouse DS-416 breaker (see Westinghouse
Figure, Attachment 1). The broken weld was on the main drive link between the
center pole lever and the pole shaft. Except for the shunt trip coil that had
burned and shorted while trying to open the breaker, the breaker's electrical
controls and auxiliary contacts were verified to be properly wired and operat-
ing as designed. The cause for the anomalous breaker status indication is
still under investigation.
Attempts to repeat the condition, where the breaker was mechanically binding
in the closed position, were minimally successful. Preliminary conclusions of
the AIT are that the breaker's mechanical binding was caused by a combination
of wear (greater than 2000 cycles of operation), manufacturing tolerances in
this early vintage breaker, and the broken weld. These factors may have
combined to allow sufficient lateral movement of the main linkage to cause it
to jam at or near full breaker closure and thus prevent the breaker from
opening. Since the control room operating personnel stated that they observed
the open indication on the closed B train RTB, the field wiring is being
verified by the licensee to ensure that wiring is as designed. The shorted
shunt trip coil had allowed 125 volts dc between the positive terminal and the
chassis; a "sneak" circuit is possible.
Discussion:
Final conclusions for the cause of this event have not been reached. Further
investigation and dismantling of the breaker will be conducted in Westinghouse
laboratory facilities. The licensee and NRC will participate in this investi-
gation. If the results indicate findings different than the above preliminary
conclusions, a supplement to this notice will be issued.
The licensee is inspecting all of the RTBs for signs of abnormal wear, cracks
in welds, and excessive lateral play (greater than 1/8 inch) in the roller end
of the main drive link where it contacts the close cam. This measurement had
not previously been part of the periodic preventive maintenance for the RTB.
Moreover, following any reactor trip, the licensee is ensuring the open posi-
tion of both RTBs by inspecting the breaker before reclosure. These are
short-term corrective actions until the detailed analysis of the deficiencies
is completed.
A significant number of generic communications have been issued with regard to
reactor trip breakers (RTBs) and similar circuit breakers used in safety-
related systems. Such communications that may be related to the matter in
this information notice are listed in Attachment 2.
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July 30, 1987
Page 3 of 3
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the Regional
Administrator of the appropriate NRC regional office or this office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: C. Vernon Hodge, NRR
(301) 492-8196
T. Peebles, RII
(404) 331-4196
A. Ruff, RII
(404) 331-5540
Attachments:
1. Power-Operated (Stored-Energy) Mechanism Graphic Details
2. Generic Communications on Reactor Trip Breakers and Similar Circuit
Breakers
3. List of Recently Issued NRC Information Notices
. Attachment 2
IN 87-35
July 30, 1987
Page 1 of 1
GENERIC COMMUNICATIONS ON REACTOR TRIP BREAKERS
AND SIMILAR CIRCUIT BREAKERS
Information Notice 86-62, "Potential Problems in Westinghouse Molded Case
Circuit Breakers Equipped With a Shunt Trip," July 31, 1986.
Information Notice 85-93, "Westinghouse Type DS Circuit Breakers, Potential
Failure of Electric Closing Feature Because of Broken Spring Release Latch
Lever," December 6, 1985.
Bulletin 85-02, "Undervoltage Trip Attachments of Westinghouse DB-50 Type
Reactor Trip Breakers," November 5, 1985.
Information Notice 85-58, "Failure of a General Electric Type AK-2-25 Reactor
Trip Breaker," July 17, 1985.
-----Supplement 1, November 19, 1985.
Information Notice No. 83-76, "Reactor Trip Breaker Malfunctions (Undervoltage
Trip Devices on GE Type AK-2-25 Breakers)," November 2, 1983.
Generic Letter 83-28, "Required Actions Based on Generic Implications of Salem
ATWS Events," July 8, 1983.
Information Notice 83-18, "Failures of the Undervoltage Trip Function of
Reactor Trip System Breakers," April 1, 1983.
Bulletin 83-04, "Failure of the Undervoltage Trip Function of Reactor Trip
Breakers," March 11, 1983.
Bulletin 83-01, "Failure of Reactor Trip Breakers (Westinghouse DB-50) to Open
on Automatic Trip Signal," February 25, 1983.
Circular 81-12, "Inadequate Periodic Test Procedure of PWR Protection System,"
July 22, 1981.
Bulletin 79-09, "Failures of GE Type Circuit Breaker in Safety Related
System," April 17, 1979.
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