United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-75, Supplement 1: Disabling of Diesel Generator Output Circuit Breakers by Anti-Pump Circuitry

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                 April 17, 1989 

Information Notice No. 88-75, SUPPLEMENT 1:  DISABLING OF DIESEL
                                                 GENERATOR OUTPUT CIRCUIT
                                                 BREAKERS BY ANTI-PUMP 


All holders of operating licenses or construction permits for nuclear power 


This notice supplements information contained in Information Notice No. 
88-75 concerning design problems with anti-pump circuitry used by circuit 
breakers with automatic closing logic.  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. 


Information Notice No. 88-75 described two events at nuclear power 
reactors during which the capability to either automatically or manually (from
the control room) close diesel generator output circuit breakers was lost 
because of design problems involving the breaker anti-pump circuitry.  It was 
noted that anti-pump circuit problems could apply not only to diesel generator
output breakers, but also to other safety-related breakers with automatic 
closing logic. 

Since issuance of Information Notice No. 88-75, design deficiencies have 
been reported involving the anti-pump circuitry for the auxiliary feedwater 
pump breakers, component cooling water pump breakers, and service water pump 
breakers at Zion Units 1 and 2. The anti-pump circuitry is designed to prevent
cycling of the circuit breaker between the closed and tripped (open) positions
when both automatic close and automatic trip signals exist concurrently.  The 
anti-pump circuitry prevents repeated attempts to close the breaker under 
valid trip (fault) conditions.  However, it was discovered that certain 
loss-of-power scenarios could result in concurrent automatic close and 
automatic trip signals that would cause the breaker to trip and lock out in 
the tripped position and would prevent reclosure (because of seal-in of the 
anti-pump circuit) although a valid standing closure signal is present and no 
fault condition exists.  

.                                                       IN 88-75, Supplement 1
                                                       April 17, 1989
                                                       Page 2 of 3

Description of Circumstances: 

On October 25, 1988, as a result of a review of tests performed to verify 
proper sequencing of emergency loads onto the emergency buses during a loss of
offsite power, the engineering department at Zion Nuclear Power Station 
determined that the component cooling water (CCW) pumps and the auxiliary 
feedwater (AFW) pumps may not start on emergency power as designed following a
loss of offsite power because of the design of the pump circuit breaker 
anti-pump circuitry.  Further investigation revealed that a similar problem 
could occur with the service water (SW) pumps. 

Specifically, before the test, the train A CCW pump was in standby and the 
train B CCW pump was running.  When power was removed from the emergency bus 
providing power to the train B CCW pump as part of the test, the pump stopped,
creating a low-pressure condition in the discharge header, which in turn 
caused an automatic start signal for both the train A and B CCW pumps.  
Therefore, the circuit breaker for the train B CCW pump received an automatic 
trip signal (from the simulated loss of offsite power condition to allow 
proper sequencing) concurrent with an automatic close signal (from the 
initiation circuitry on low header pressure).  This caused the anti-pump 
circuitry for the breaker to energize and seal in, preventing further breaker 
closure attempts, and left the pump deenergized with its breaker locked out by
the anti-pump circuitry.  When the low header pressure signal was cleared by 
automatic initiation of the train A CCW pump, the anti-pump circuitry for the 
train B pump reset (deenergized) and allowed the train B pump to start upon 
receipt of the loss of offsite power sequence timer signal. 

After reviewing the above test sequence and related circuit breaker traces, 
the licensee determined that during an actual loss of offsite power, in which 
power would be lost to all buses providing power to the CCW pumps, the 
breakers for each pump would have received start signals on low header 
pressure concurrent with undervoltage trip signals and would have been locked 
out as a result of the anti-pump feature, resulting in the loss of all CCW.  
This condition would not clear even after the emergency diesel generator 
attained full speed and voltage, and the sequenced breaker closure signals 
were generated.  To start a CCW pump would require that the pump control 
switch be placed in the "AFTER-TRIP" position to reset (deenergize) the 
anti-pump circuit, thereby allowing subsequent breaker closure attempts.  
Further investigation revealed that the service water pump breakers could 
similarly be locked out on a standing low header pressure automatic closure 
signal coincident with a second level undervoltage trip signal. 

The licensee also concluded that a loss of offsite power condition, occurring 
at power, could result in a reactor/turbine trip accompanied by an immediate 
shrinking of steam generator inventory to below the low-low level setpoint for 
automatic initiation of auxiliary feedwater.  In this case, the auxiliary 
feedwater pump breakers would receive concurrent close and trip signals, and 
the anti-pump circuitry would cause the same lockout condition as mentioned 
above (for the CCW and SW pumps), preventing automatic initiation of auxiliary
.                                                       IN 88-75, Supplement 1 
                                                       April 17, 1989 
                                                       Page 3 of 3 

The Zion licensee modified the breaker control logic for the CCW and AFW pumps
so that the automatic closure signals (CCW header low pressure and steam 
generator low-low level) are blocked following a loss of offsite power 
condition until the diesel generators attain full speed and voltage and are 
supplying power to the emergency buses, and the pump breakers have received 
automatic close signals via the load sequencer.  Similarly, the SW pump 
breaker control logic was modified so that the closure signal on SW header low
pressure is blocked until emergency bus voltage has been restored and the pump
has been sequenced following a second level under-voltage condition. 

The circuit breaker anti-pump problem was not discovered during previous loss-
of-power tests because the tests are performed on a bus-by-bus basis with the 
plant at hot shutdown.  Redundant CCW and SW pumps powered from the bus not 
under test would maintain header pressure above the automatic initiation 
setpoint.  Also, the magnitude of the steam generator level shrink at hot 
shutdown was not sufficient to cause an AFW pump automatic start signal on 
low-low level.  Therefore, the anti-pump circuitry did not actuate because 
breaker close signals were not received concurrent with the breaker trip 
signals.  This event demonstrates that when complete system integrated testing
cannot be performed under actual conditions, it is important to carefully 
analyze all differences between plant conditions during testing and conditions
expected to exist when the equipment under test is required to perform its 
safety function in order to verify the acceptability of the test. 

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the techni-
cal contacts 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:  F. Burrows, NRR 
                     (301) 492-0833 

                     R. Kendall, NRR 
                     (301) 492-3140

Attachment:  List of Recently Issued NRC Information Notices
.                                                       Attachment 
                                                       IN 88-75, Supplement 1 
                                                       April 17, 1989 
                                                       Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

89-40          Unsatisfactory Operator Test  4/14/89        All holders of OLs
               Results and Their Effect on                  or CPs for nuclear
               the Requalification Program                  power reactors. 

89-39          List of Parties Excluded      4/5/89         All holders of OLs
               from Federal Procurement                     or CPs for nuclear
               or Non-Procurement Programs                  power reactors. 

89-38          Atmospheric Dump Valve        4/5/89         All holders of OLs
               Failures at Palo Verde                       or CPs for nuclear
               Units 1, 2, and 3                            power reactors. 

89-37          Proposed Amendments to        4/4/89         All U.S. NRC 
               40 CFR Part 61, Air                          licensees. 
               Emission Standards 
               for Radionuclides 

89-36          Excessive Temperatures        4/4/89         All holders of OLs
               in Emergency Core Cooling                    or CPs for nuclear
               System Piping Located                        power reactors. 
               Outside Containment 

88-86,         Operating with Multiple       3/31/89        All holders of OLs
Supp. 1        Grounds in Direct Current                    or CPs for nuclear
               Distribution Systems                         power reactors. 

89-35          Loss and Theft of Un-         3/30/89        All U.S. NRC 
               secured Licensed Material                    byproduct, source 
                                                            and special 
                                                            nuclear material 

89-34          Disposal of Americium         3/30/89        All holders of an 
               Well-Logging Sources                         NRC specific 
                                                            authorizing well-

89-33          Potential Failure of          3/23/89        All holders of OLs
               Westinghouse Steam                           or CPs for PWRs. 
               Generator Tube 
               Mechanical Plugs 
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Friday, May 22, 2015