United States Nuclear Regulatory Commission - Protecting People and the Environment

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
.                                                            IN 87-35
                                                            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.  
.                                                            IN 87-35
                                                            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.

Page Last Reviewed/Updated Tuesday, November 12, 2013