United States Nuclear Regulatory Commission - Protecting People and the Environment

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
                           WASHINGTON, D.C. 20555

                              November 5, 1985

                         TYPE REACTOR TRIP BREAKERS 


All power reactor licensees and applicants. 


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. 


                                                            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. 


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 

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  

                                                            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. 


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 

     (c)  Plant operation with either RTB INOPERABLE shall continue only as 
          provided for in the plant Technical Specifications. 

                                                            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. 

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 

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 


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

5. Sequoyah 1                      Currently S/D; will install before

6. North Anna 1                    Currently S/D; will install before 

7. Byron 1                         Currently S/D; will install before

8. Zion 2                          Currently S/D; will install before 

9. Cook 2                          Will install prior to startup 
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