Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
IN 87-35, Supplement 1
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 16, 1987
Information Notice No. 87-35, SUPPLEMENT 1: REACTOR TRIP BREAKER,
WESTINGHOUSE MODEL DS-416,
FAILED TO OPEN ON MANUAL
INITIATION FROM THE CONTROL
ROOM
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is being provided to alert addressees to the determi-
nation of the cause of the mechanical binding that resulted in the failed
reactor trip breaker (RTB) described in Information Notice No. 87-35. The NRC
is considering the need to request action by licensees using Westinghouse DS
series breakers in Class IE applications. This supplemental notice also
discusses other concerns that arose during investigations of the RTB failure
but that did not contribute to the binding of the RTB. It is expected that
recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems. However,
suggestions contained in this information notice do not constitute NRC
requirements; therefore, no specific action or written response is required.
Background:
Information Notice 87-35, dated July 30, 1987, discussed the July 2, 1987,
event at McGuire 2 in which an RTB would not open upon receipt of an
electrical command signal. The RTB had bound mechanically. The shunt trip
coil burned and shorted during the attempt to open the breaker. Operators in
the control room stated that they observed open indications for both redundant
RTBs, but the event recorder showed that only one had opened. The licensee's
investigation, observed by an NRC Augmented Inspection Team, revealed abnormal
wear of the pole shaft assembly and a broken weld joining the center pole
lever and the pole shaft, but did not identify the specific cause of binding.
Because the licensee's facilities for further investigation were limited,
further investigation was to be conducted in a Westinghouse laboratory.
8712100417
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December 16, 1987
Page 2 of 4
Failure Mode:
The McGuire 2 RTB failed to open because the main roller was wedged between
the raised edge of the close cam and the right-hand side frame plate (viewing
the RTB from the rear). A labeled view of the RTB mechanism is shown in
Figure 1. When the RTB is closed, the position of the mechanism is as shown in
Figure 2. A conceptual sketch showing the main roller wedged between the right
side frame plate and the left cam segment, as viewed from the rear of the RTB,
is shown in Figure 3. Similar wedging could occur between the left side frame
plate and the right cam segment.
In the Westinghouse DS series breakers, the close cam (item 2 in Figure 1) is
composed of four steel segments that are sandwiched together and held by three
rivets. The two outer segments are heat-treated steel; the two inner segments
are non-hardened steel. The surface of the segments is supposed to be of
uniform shape. However, on the McGuire RTB, the two outer segments are
slightly larger than the inner segments, providing the edge to catch the main
roller (item 15). Only a slightly raised cam edge is necessary to allow
binding. In addition, the distance between the inner surface of the close cam
edge and the side frame plate (item 19) must be close to the width of the main
roller.
The main roller can become wedged during the closing action of the breaker.
As the close cam rotates, the edge of the main roller is caught, as shown in
Figure 4. Continued rotation of the close cam causes the main roller axis to
straighten. This action causes the edges of the main roller to attempt to
separate the close cam and the side frame plate. However, the close cam and
side frame plate are not free to move and, therefore, they wedge the main
roller in place. When an attempt is made to trip the breaker, the wedging of
the main roller prevents the main roller from rolling down the close cam face
to allow the circuit breaker to open. The wedging of the main roller also
prevents full discharge of the closing springs (not shown in Figure 1),
leaving the close cam 18 degrees from a fully rotated position.
Both lateral displacement of the main roller end of the main drive link (item
14) and a small rotation (3 to 5 degrees) of the main roller axis are
necessary to allow wedging. If the weld joining the center pole lever (item
9) to the pole shaft (item 8) is sound, the main roller end of the main drive
link could still move laterally and even allow the main roller to strike the
side plate. However, a sound weld would not allow sufficient rotation of the
axis for wedging to occur. A large number of cycles of operation (3000 or
more), however, could cause wear that would allow the necessary rotation of
the axis.
Additional details on the failure mode and the Westinghouse tests are
contained in References 1, 2, and 3.
Other Concerns:
Stop Roller Bindinq. The inspection of the McGuire 2 RTB components at
Westinghouse revealed that the close cam surface had been peened. The peening
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December 16, 1987
Page 3 of 4
flattened and laterally expanded the surface of the outer cam segments, creat-
ing a mushroom shape. Of key concern was mushrooming in the area of the stop
roller (item 1), which holds the mechanism in readiness for release of the
spring release latch (item 16). The extreme mushrooming impeded rotation of
the stop roller. It is possible that sufficient mushrooming could totally
prevent operation of the stop roller, which could prevent closure of the
circuit breaker upon demand. While not of safety concern for an RTB, this
failure to close condition would be of concern for a breaker in a Class IE
application requiring energization of the connected loads.
RTB Position Indicating Light. At McGuire, red and green lights placed below
each of the two RTB spring-loaded manual control switches in the control room
serve to indicate whether the associated RTB is closed or open, respectively.
Operators are trained to operate both control switches simultaneously, one
with each hand, and to interpret each set of lights as representing the actual
status of each breaker. However, the red lights serve the additional purpose
of indicating continuity of the shunt trip circuit for each breaker.
The design circuitry is such that the absence of the red light can mean either
that the breaker has opened as intended or that the associated shunt trip
circuit has been interrupted. Determining which is the case cannot be done
from the red light behavior alone; rather, the red light behavior must be
interpreted in combination with the green light behavior and other control
room indications, such as rod position displays. Operators need to understand
that a "malfunction" of the red light may in reality be a valid indication
that the associated shunt trip circuit is inoperable. Additionally, it is
important that operators understand that the combined absence of the red and
green lights after release of the spring-loaded manual control switch may mean
that the associated breaker has failed to open in response to the electrical
demand and that immediate local verification or trip action is needed. At
McGuire, pressing the manual trip plate at the RTB did not open the breaker,
but manipulating the manual spring-charging handle did open it.
Since the McGuire event, the licensee has modified the requalification
training program for operators to ensure that they understand the potential
meaning of the various combinations of RTB indications and that they follow
appropriate verification procedures for suspected "malfunctions" of these
indications.
Trip Latch Pivot Pin. During an NRC inspection at Braidwood 2 in late Septem-
ber 1987, the licensee reported that a Westinghouse DS-416 RTB at Braidwood 1
failed to close because improper brazing of the pivot pin (item 4 in Figure 1)
to the trip latch (item 5) resulted in disengagement of the two. While not of
safety concern for an RTB, this failure to close condition would be of concern
for a breaker in a Class IE application requiring energization of the
connected loads.
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No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical
contact listed below or the Regional Administrator of the appropriate regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: Vern Hodge, NRR
(301) 492-8196
Darl Hood, NRR
(301) 492-8961
K. R. Naidu, NRR
(301) 492-9659
References:
1. "Interim Report on McGuire 2B Reactor Trip Breaker Failure
Evaluation and Recommended Corrective Actions from Westinghouse,"
Franklin Research Center, September 30, 1987, enclosure to letter
from D. S. Hood, NRC, to H. B. Tucker, Duke Power Company, October 16,
1987, NRC Docket No. 50-370
2. NRC Inspection Report Nos. 50-369/87-22 and 50-370/87-22,
August 31, 1987
3. "Reactor Trip Breaker Failure Due to Mechanical Failure,"
Licensee Event Report 50-370/87-009, Duke Power Co., August.3, 1987
Attachments:
1. Figure 1. Linkages of DS-416 Breaker Mechanism
2. Figure 2. Position of Mechanism with RTB Closed
3. Figure 3. Roller Wedged Between Left Cam Segment
and Right Side Frame Plate
4. Figure 4. Binding of Roller
5. List of Recently Issued NRC Information Notices
