Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
December 14, 1993
NRC INFORMATION NOTICE 93-96: IMPROPER RESET CAUSES EMERGENCY DIESEL
GENERATOR FAILURES
Addressees
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for an emergency diesel generator
(EDG) to fail to start on demand or to trip during operation due to improper
execution of the reset operation at the conclusion of the previous run. 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 are not
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances
H.B. Robinson
In August of 1987 at the H.B. Robinson plant, startup trips occurred on the
"A" EDG. With the assistance of a vendor representative, several steps were
taken in an effort to preclude such trips, including replacement of the
governor, replacement of the governor oil, replacement of sticky fuel
injection pumps, checking of injection pump timing, and rebalancing fuel rack
settings. In February of 1988, startup trips were again experienced. It was
concluded, after detailed inspection of the latch mechanism parts, that there
was a possibility of wear on the tip of the latch, which may have been a
contributing factor to the trips. These parts were replaced with new parts.
On June 3, 1992, during the performance of a test activity, the "A" EDG
tripped on mechanical overspeed after the engine had been operating
approximately one half hour. After detailed consideration of possible
scenarios for the trips and considering prior actions taken, it was determined
that the most probable cause was a failure to properly reset the trip
mechanism after the previous run. It was concluded that operations personnel
apparently were unaware of the importance of moving the reset lever through
its full travel. In addition, resetting was difficult for shorter individuals
due to the location of the reset lever, which is positioned over the exhaust
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IN 93-96
December 14, 1993
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manifold. Operations personnel were subsequently trained on the proper method
for actuating the reset lever. Also, a step was provided to permit easier
access to the lever.
Duane Arnold
On June 11, 1993, while the Duane Arnold plant was operating at 85 percent of
full power, the "A" EDG tripped approximately 3 seconds after the initial
start signal during an operability surveillance test. An evaluation was
conducted by the licensee of possible EDG trip causes, including occurrence of
an actual overspeed condition, failure of the overspeed mechanism, actuation
of the emergency stop pushbutton, failure of the emergency stop pushbutton,
latch assembly not fully latched, latch assembly failure, mechanical trip
mechanism failure, and annunciator circuit or trip lever microswitch failure.
It was concluded that the most probable cause of the trip was that the latch
assembly was not fully engaged from the previous reset operation and engine
startup vibration unlatched the trip mechanism. Less than complete engagement
of the latch assembly could be caused by an inadequate reset operation.
Corrective actions adopted by the licensee included procedure revisions to
ensure a more positive latching operation, improved physical access to the
reset lever, and operator training.
The procedure revision directs operators to perform the reset operation a
second time following the initial resetting of the trip. The second reset is
somewhat easier to perform and should ensure that the latch is fully engaged
in the event that the initial latching sequence resulted in a partially
latched condition. Verification that the latch is fully seated is
accomplished by checking that the reset lever subsequently moves freely.
Discussion
In response to experience with previous trips during fast starts, the diesel
manufacturer for both plants, (Coltec/Fairbanks Morse Engine Division), had
previously issued a service information letter (SIL, Volume A, Issue 25, dated
August 1, 1991) addressing a revised overspeed trip adjustment. The SIL
recommended that the overspeed trip setting be raised from the range of 112 to
115 percent to the range of 115 to 117 percent. In light of events that
occurred at H.B. Robinson and Duane Arnold, the manufacturer has now advised
these licensees that proper resetting of the trip mechanism would be enhanced
by slowly moving the reset lever fully to the reset position and back, twice.
The inadvertent partial reset latching operations described above may also
occur on other units having similar mechanical trip mechanisms.
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IN 93-96
December 14, 1993
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This notice requires no specific action or written response. If you have any
questions about the information in this notice, please contact one of the
technical contacts listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.
/s/'d by BKGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: Larry Garner, RII
(404) 331-4663
Chris G. Miller, RIII
(319) 851-5111
Walter P. Haass, NRR
(301) 504-3219
Attachment:
List of Recently Issued NRC Information Notices
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