Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 83-17
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
March 31, 1983
INFORMATION NOTICE NO. 83-17: ELECTRICAL CONTROL LOGIC PROBLEM RESULTING IN
INOPERABLE AUTO-START OF EMERGENCY DIESEL
GENERATOR UNITS
Addressees:
All nuclear power reactor facilities holding an operating licensee (OL) or
construction permit (CP).
Purpose:
This information notice is provided as a notification of a potential problem
in the control logic circuitry which could adversely affect the auto-start
feature provided for diesel generators at nuclear facilities. It is expected
that recipients will review the information for applicability to their
facilities. No specific action or reference is required.
Description of Circumstances:
Recently the licensee performed a loss of offsite power test at the North
Anna Unit 2 Station. The test is conducted periodically and is a technical
specification requirement. Specifically, this test is performed to verify
the capability of the emergency diesel generators (EMDs) to load-shed and
reload the essential emergency busses following loss of an EMD when offsite
power is unavailable.
Briefly, the selected emergency bus was "set up" to simultaneously receive
electrical power from the offsite power source and the associated EMD. A
safety injection test signal was then initiated and the diesel generator
unit was manually shut down. These actions were followed by opening the
offsite power supply feeder breaker to the selected emergency bus. Given
these conditions, the associated EMD did not respond to an auto-start call
upon resetting its shutdown relay.
When the licensee investigated the event, he found that the periodic test
procedure used to conduct this test did not specifically address a 60-second
time delay feature in the diesel generator's restart circuitry. The purpose
of this time delay is to allow the diesel generator to come to a complete
stop before attempting a restart. This time delay prevents fuel from. being
supplied to the diesel generator for 60 seconds after the shutdown relay has
been manually reset by a remote pushbutton located in the control room.
Further review of this event determined that when the shutdown relay was
manually reset (because of the presence of the safety injection signal
mentioned above), the compressed starting air was admitted to start the
diesel even though no fuel was admitted during the 60-second delay period.
At the end of this delay,
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IN 83-17
March 31, 1983
Page 2 of 2
fuel was admitted but the supply of compressed air used to rotate the engine
for a restart attempt had been consumed. Thus, the diesel unit effectively
became inoperable until the air storage tanks were repressurized.
This licensee's action taken to preclude this event from recurring included
modifying the test procedures so as to require the control selector switch
for the diesel generator being tested to be placed in the "local" position
before the shutdown relay is reset. This action, in effect, blocks the
auto-start diesel signal from activating the air start solenoid for the
diesel generator being tested. Following this action, the shutdown relay may
be manually reset and the time delay permitted to run out. Subsequently, the
control selector switch may be returned to its normal "remote" position at
which time the diesel generator unit being tested should auto-start and
reload its associated emergency busses in accordance with design
requirements.
During an in-depth study of the control logic circuitry for the diesel
generator units at Hatch Unit 1 Station, a potential control logic problem
was identified which is identical to the problem at North Anna Unit 2
Station. Subsequently, the licensee implemented a design change which now
allows fuel to be supplied into the diesel unit concurrent with the logic
signal calling for diesel generator "re-start."
Since North Anna Unit 2 is a pressurized water reactor power plant and Hatch
Unit 1 is a boiling water reactor power plant, the control logic problem, as
discussed, has strong generic implications ,and may affect many of the
addressees of this information notice. Licensees should be aware that since
the local/remote control switch is located in the control room and is placed
in the "remote" position during normal plant operation, it will require
direct immediate operator corrective action to preclude failure of the
diesel generator to re-start under the set of conditions discussed above. On
this basis, we suggest these addressees review this information for
applicability to the control logic circuitry of diesel generator units in
use at their plant.
No written response to this notice is required. If you have questions
regarding this matter, please contact the Regional Administrator of the
appropriate NRC Regional Office, or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
CONTACT: V. D. Thomas
301-492-4755
Attachment:
List of Recently Issued Information Notices
