Bulletin 85-02: Undervoltage Trip Attachments of Westinghouse DB-50 Type Reactor Trip Breakers
SSIN No.: 6820
OMB No.: 3150-0011
IEB 85-02
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
November 5, 1985
IE BULLETIN NO. 85-02: UNDERVOLTAGE TRIP ATTACHMENTS OF WESTINGHOUSE DB-50
TYPE REACTOR TRIP BREAKERS
Addressees:
All power reactor licensees and applicants.
Purpose:
The purposes of this bulletin are: to inform all power reactor licensees and
applicants of recent reactor trip breaker reliability problems, to assure,
that licensees of currently operating Westinghouse power reactor facilities
that have not yet installed the automatic shunt trip modification are
performing undervoltage trip attachment (UVTA) force margin testing of the
reactor trip breakers (RTBs), and to clarify the "OPERABILITY" requirements
related to RTBs.
Description of Circumstances
On October 29, 1985, at 13:57 EST the D. C. Cook Nuclear Station Unit No. 2
tripped from approximately 80% of full power. The reactor trip was triggered
by a spurious indicated low flow condition in loop 2. The trip signal
originated from the reactor coolant pump breaker contacts. Following the
reactor trip condition it was immediately discovered that the reactor trip
breaker associated with Train "A" reactor protection system (RPS) did not
open, and the trip function was accomplished by only the "B" train RTB
(which subsequently on November 3, 1985 failed to pass the UVTA force margin
test). Two of four safety-related feedwater isolation valves did not close
because the actuation signal to close should have originated from contacts
in the failed Train "A" circuit breaker. The remaining two feedwater
isolation valves did operate properly because the train "B" reactor trip
circuit breaker opened and triggered the isolation action to these two
valves. Also, the feedwater regulating valve in each loop was closed by the
"B" train RTB signal.
Subsequent action by the licensee included quarantining the failed Train "A"
reactor trip breaker. This was facilitated by opening the circuit breakers
to the motor-generators.
Investigation into the event by the licensee and Westinghouse attributed the
failure of the Train "A" reactor trip breaker to a faulty undervoltage trip
attachment (UVTA) which did not provide enough lifting force to the breaker
trip bar. The breaker is identified as the Westinghouse Type DB-50.
8511050230
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IEB 85-02
November 5, 1985
Page 2 of 4
Subsequent to the October 29, 1985 incident, all RTBs at the DC Cook station
were tested. UVTA trip force margin testing of the Unit 2 "B" breaker on
November 3, 1985 identified degradation of the margin below the 20 ounce
specified margin (i.e., the requirement for the (UVTA is to exert 50 ounces
of force, 30 ounces to move the trip bar and 20 ounces for margin). The
significance of the degradation of the UVTA attachment of the "B" breaker,
in conjunction with the failure of the "A" breaker to trip on October 29,
1985 renews concern for common mode failures that could potentially lead to
an ATWS event.
Background:
As a result of the February 22 and 25, 1983 Salem anticipated transients
without scram (ATWS) events, the NRC issued IE Bulletin 83-01 and formed a
task force to assess the generic implications of these events. On March 11
1983, Southern California Edison reported that three GE-manufactured RTBs on
San Onofre Unit 2 and one on Unit 3 failed to open during testing of the
UVTA. As a result of these failures, the NRC issued IE Bulletin 83-04.
The task force's actions resulted in the issuance of NUREG-1000, "Generic
Implications of the ATWS Events at the Salem Nuclear Power Plant" and
Generic Letter 83-28, delineating the procedural and plant changes required.
Findings in NUREG-1000 were based, in part, on assurances that improved
maintenance of the breakers would improve the reliability of the UVTAs to an
acceptable level for the short term, after which the longer term corrective
actions would be implemented.
The longer term corrective actions were directed by NRC Generic Letter
83-28. These actions had two major aspects related to the RTB/UVTAs: (1) the
installation of a plant modification that provides for the automatic
activation of the shunt trip coil of the RTB for any automatic reactor trip
signal, and (2) reliability improvements in the RTB/UVTA based upon UVTA
life testing by the vendor. The life test formed the basis for a UVTA
replacement interval of 1250 cycles and a lubrication interval of 200
cycles.
The recent failures at D. C. Cook Unit 2 involved RTBs that were refurbished
by the vendor only 4 months previously, including the installation of new
UVTAs. The Unit 2 RTB "A" (that failed on October 29, 1985) had experienced
only about 75 operations on the UVTA. The Unit 2 RTB "B" (that failed on
November 3, 1985) had experienced about half as many operations on the UVTA.
In this short time, both UVTAs suffered a serious reduction of their trip
output force.
The Westinghouse specification for the DB-50 breaker specifies a minimum
total force output of the UVTA to be 50 ounces. This total force output
provides a force margin of 20 ounces above the maximum force required to
trip the breaker. The UVTA trip force margin is measured by temporarily
hanging a weight on the trip bar of the breaker and actuating (i.e.,
de-energizing) the UVTA. Many plants include a force margin test as part of
the 18-month surveillance actions. If the weight is less than the UVTA
margin, the UVTA will consistently trip the breaker. If the weight is
greater than the UVTA margin, the UVTA will consistently not trip the
breaker. If the weight approximates the UVTA margin, the UVTA
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IEB 85-02
November 5, 1985
Page 3 of 4
will not be consistent in its ability to trip the breaker during the test.
At D.C. Cook, three tests at each weight were used to assure valid test
results. Testing of RTB "A" indicated that the UVTA force margin had fallen
to 5 ounces or less. Testing of the RTB "B" indicated that the UVTA force
margin was no longer as great as 20 ounces. If the UVTA does not have
adequate margin, it is unacceptable and requires replacement or maintenance.
The root cause of the lost force margin is not known at this time;
laboratory failure analysis of the failed UVTAs is being arranged. It is
believed that re-lubrication may temporarily restore some of the lost force.
NRC preliminary information is that 9 Westinghouse PWRs have not yet
implemented the automatic shunt trip modification. As shown in the
attachment, only 3 of these plants are currently operating; namely:
Kewaunee, Beaver Valley Unit 1, and Turkey Point Unit 4.
REQUIRED ACTIONS FOR LICENSEES OF CURRENTLY OPERATING WESTINGHOUSE PWR
FACILITIES THAT HAVE NOT YET IMPLEMENTED THE ACTUATION OF
RTB SHUNT TRIB COIL ON ALL AUTOMATIC REACTOR-TRIP SIGNALS
1. Perform a test of the UVTA of each RTB that is in service to determine
that adequate force margin exists, as soon as practical, but not to
exceed 7 days of receipt of this bulletin. With a weight of 20 ounces
attached to the trip bar, the UVTA by itself must trip the RTB three
times in succession. This test shall be performed with the UVTA and
breaker in "as found" conditions, prior to any lubrication or other
maintenance. Any occurrence of a force margin less than 20 ounces
defines an unacceptable UVTA.
2. Modify the monthly test procedure for the Reactor Protection System to
add conducting the UVTA force margin test, prior to any lubrication or
adjustment of the UVTA, for each RTB that is in service. This test is
normally conducted on one RTB each month, i.e., two months between
tests on the same RTB. If an RTB fails the UVTA force margin test, the
redundant RTB is to be similarly tested within 8 hours. This procedural
modification should be in place for the next such test after receipt of
this bulletin and remain in force, until the automatic shunt trip
modification is fully implemented.
3. Provide written instructions to the plant operating staff, within 7
days of receipt of this bulletin, requiring that:
(a) The content of this bulletin be reviewed by each licensed operator
at the start of his/her next duty shift.
(b) A reactor trip breaker shall be declared INOPERABLE, if the UVTA
either does not successfully pass the force margin test or
otherwise may not be capable of performing its intended safety
function.
(c) Plant operation with either RTB INOPERABLE shall continue only as
provided for in the plant Technical Specifications.
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IEB 85-02
November 5, 1985
Page 4 of 4
(d) The NRC is to be notified via the emergency notification system
within 4 hours of any RTB being declared inoperable.
4. Provide a written report to the NRC within 30 days of receipt of this
bulletin, which discusses the specific actions taken pursuant to this
bulletin and the results thereof.
Should a licensee determine that any action requested by this bulletin
jeopardizes overall plant safety, the NRC should be notified of that fact
and provided with appropriate justification for not implementing the
requested action. Such notification shall be made within 7 days of receipt
of this bulletin.
The written reports shall be submitted to the appropriate Regional
Administrator under oath or affirmation under provisions of Section 182a,
Atomic Energy Act of 1954, as amended. Also, the original copy of the cover
letters and a copy of the reports shall be transmitted to the U.S. Nuclear
Regulatory Commission, Document Control Desk, Washington, DC 20555 for
reproduction and distribution.
This request for information was approved by the Office of Management and
Budget under a blanket clearance number 3150-0011. Comments on burden and
duplication may be directed to the Office of Management and Budget, Reports
Management, Room 3208, New Executive Office Building, Washington, D. C.
20503.
If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office of one of the technical
contacts listed below.
James Taylor, Director
Office of Inspection and Enforcement
Technical Contacts: J. T. Beard, NRR
(301) 492-7465
V. Thomas, IE
(301) 492-4755
Attachments:
1. Facilities Without Automatic Shunt
Trip Installed
2. List of Recently Issued IE Bulletins
.
Attachment 1
IEB 85-02
November 5, 1985
Page 1 of 1
FACILITIES WITHOUT AUTOMATIC SHUNT TRIP INSTALLED
FACILITY REMARKS
1. Kewaunee Install in February 1986
2. Beaver Valley Install in May 1986
3. Turkey Point 4 Install in January 1986
4. Sequoyah 2 Currently S/D; install Spring 1986
startup
5. Sequoyah 1 Currently S/D; will install before
startup
6. North Anna 1 Currently S/D; will install before
startup
7. Byron 1 Currently S/D; will install before
startup
8. Zion 2 Currently S/D; will install before
startup
9. Cook 2 Will install prior to startup
Page Last Reviewed/Updated Tuesday, March 09, 2021
Page Last Reviewed/Updated Tuesday, March 09, 2021