Information Notice No. 88-75, Supplement 1: Disabling of Diesel Generator Output Circuit Breakers by Anti-Pump Circuitry
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
April 17, 1989
Information Notice No. 88-75, SUPPLEMENT 1: DISABLING OF DIESEL
GENERATOR OUTPUT CIRCUIT
BREAKERS BY ANTI-PUMP
CIRCUITRY
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
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.
Background:
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.
8904110183
. 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
feedwater.
. 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
licensees.
89-34 Disposal of Americium 3/30/89 All holders of an
Well-Logging Sources NRC specific
license
authorizing well-
logging
activities.
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
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