Information Notice No. 84-66: Undetected Unavailability of the Turbine-Driven Auxiliary Feedwater Train
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
August 17, 1984
Information Notice No. 84-66: UNDETECTED UNAVAILABILITY OF THE TURBINE-
DRIVEN AUXILIARY FEEDWATER TRAIN
All pressurized water power reactor facilities holding an operating license
(OL) or construction permit (CP).
This information notice is provided to alert recipients of a potentially
significant problem pertaining to undetected unavailability of the turbine-
driven auxiliary feedwater (AFW) train. It is expected that recipients will
review the information for applicability to their facilities and consider
actions, if appropriate, to preclude a similar problem occurring at their
facilities. However, suggestions contained in this information notice do not
constitute NRC requirements and, therefore, no specific action or written
response is required.
On January 12, 1982, the turbine-driven AFW pump at Sequoyah 2 failed to
start on a safety injection actuation. Investigation revealed the electronic
overspeed trip latch function for the stop valve had not been relatched
following an earlier overspeed trip. The valve is normally relatched from
the control room. The operator had apparently not held the valve hand switch
for the necessary 10 seconds, and there was a design deficiency in that no
indication was provided in the control room to determine that the valve had
been reset. As corrective actions, the licensee has modified procedures to
require local visual verification of latching, improved valve checklists,
and posted precautionary labels or signs both in the control room and
locally at the stop valve.
On August 11, 1983, the turbine-driven AFW pump failed to start following a
low-low steam generator level signal because the pump turbine trip valve was
in the tripped position. Although it could not be substantiated, the valve
had apparently been tripped and left in that position following maintenance
and testing activities on August 3, 1983. It appeared that when the pump was
removed from service, the control room "trip" pushbutton was inadvertently
depressed instead of the "stop" pushbutton, resulting in activation of the
trip valve. In addition, the valve trip position limit switch was out of
adjustment causing the "trip" indication in the control room to be
inoperable. A design
August 17, 1984
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change request was initiated to provide a positive control room indication
of a trip valve latched condition. Until this change is complete, the trip
valve will be verified to be in the latched position by daily observation.
At approximately 1800 hours, October 5, 1983, during routine full power
operation, the control room operator noticed that the trip indication light
for the turbine-driven AFW pump was illuminated. Subsequent investigation
revealed that the valve was in the tripped position and had apparently been
left in that position following completion of routine pump surveillance
testing at 1700 hours, October 5, 1983. When the problem was identified, the
AFW pump trip valve was latched. The redundant electrically driven pumps
were operable throughout the occurrence and the turbine-driven pump was
restored to operability within the time period specified in the action
Investigation of the incident revealed that the operator who relatched the
pump trip valve at the time of the surveillance had turned the valve
handwheel to set the valve linkage but had not completed the latching
operation by turning the handwheel back to the starting position. This
resulted in the valve remaining in the tripped position. No local mechanical
position indication existed on the valve, thus detailed knowledge of the
valve operating linkage was required to ensure that the valve was in the
proper position. As a corrective action, the licensee installed local valve
position indication, and the licensee is planning to install control room
indication of the "latched" condition.
San Onofre 3
On October 31, 1983, at 1925 hours, Unit 3 was manually tripped from 62%
reactor power in response to a loss of main feedwater. An emergency
feedwater actuation signal (EFAS) was received when the unit was tripped.
However, the turbine-driven AFW pump failed to start. Both electrically
driven AFW pumps started and remained operable during this event. The
operator investigated the failure of the turbine-driven pump, found the pump
turbine in a tripped condition, and manually reset the pump turbine's steam
admission valve. The cause of the pump trip was unknown.
The pump had previously been satisfactorily tested on October 30, 1983, at
1820 hours. Although control room instrumentation was available to signal
when the pump turbine tripped on turbine overspeed, troubleshooting of the
instrumentation subsequent to the plant trip on October 31, 1983, indicated
that there were intermittent failures of the instrumentation to signal when
the pump turbine was tripped. These intermittent failures were investigated
The San Onofre AFW pump turbine steam admission valve is normally closed and
opens when a EFAS signal is received. When the AFW pump turbine trips on
overspeed, the condition is alarmed in the control room. However, it is
possible to relatch the trip mechanism in such a way that the overspeed trip
is reset but the valve is not and the condition is not indicated in the
August 17, 1984
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On December 28, 1983, with the reactor at 100% power, the plant entered a
7-day Limiting Condition for Operation (LCO) Action Statement when it was
discovered that the turbine driven AFW pump was inoperable. The cause was
that the throttle on the pump had been tripped. The licensee felt that a
contract person, working in the area of the pump, bumped the lever into the
tripped position earlier in the day. The limit switch did not make contact;
as a result, there was no indication in the control room of the pump being
The valve was reset. The licensee investigation found that the valve
operated sluggishly because of insulation debris in the external valve
linkages. The valve limit switch had not been activated because of the
limited valve stroke.
In five events at operating reactors (1982-1983), the turbine-driven AFW
pumps were unavailable because the steam supply was isolated (trip and
throttle valve was not latched). The condition was noted either during
routine inspections or as the result of an investigation of failure of the
turbine-driven AFW pump to respond to an engineered safety features
actuation signal. Three of these five events were failures to return a
safety system to an operable condition, including failure to verify, and
maintenance was a significant contributing factor in four of the events.
On the basis of AEOD evaluations, each of these events were found to have
limited safety significance because (1) the motor-driven AFW pumps were
operable and available in all cases, and (2) the LCO time period was
generally not exceeded. However, because events involving undetected
unavailability of the turbine-driven AFW train could be significant at other
plants or under other circumstances, the NRC recommends that recipients of
this notice review these events and the various preventive actions
summarized in Table 1 for applicability to their plants.
No written response to this information notice is required. If you need
additional information about 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
Technical Contact: Ray Smith, IE
1. Table 1: Summary of Various Actions Taken by Facilities to Prevent
Undetected Unavailability of the Turbine Driven AFW Train
2. List of Recently Issued IE Information Notices
Summary of Various Actions Taken By Facilities to Prevent
Undetected Unavailability of the Turbine-driven AFW Pump
1. Design change to provide a positive control room indication of a trip
valve "latched" condition.
2. Regular adjustment and testing of the limit switches to ensure
operability (where limit switches are used to provide the control room
with the status of trip valves).
3. Local verification of position after resetting the trip valve.
4. Visual verification daily or once per shift to see that the valve is
5. Local mechanical valve position indication installed and permanent tags
attached to the valve providing instructions for operation.
6. On-shift training in operation of the trip valve for all personnel who
are required to operate the valve.
7. Improved housekeeping to prevent fouling external valve linkages.
8. Warning sign installed near the trip lever.
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