United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-48: Failure of Turbine-Driven Main Feedwater Pump to Trip because of Contaminated Oil

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                                July 6, 1993


NRC INFORMATION NOTICE 93-48:  FAILURE OF TURBINE-DRIVEN MAIN FEEDWATER PUMP
                               TO TRIP BECAUSE OF CONTAMINATED OIL


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 potential problems resulting from suspended
particles in the control oil in the trip system for turbine-driven main
feedwater pumps.  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 

On August 27, 1992, while at 80-percent power, LaSalle County Station, Unit 2,
experienced a reactor scram when the main turbine tripped.  Several unexpected
control system responses occurred.  The focus of this information notice is on
the failure of both turbine-driven main feedwater pumps to trip on both
automatic and manual signals.  This failure resulted in uncontrollable filling
of the reactor vessel above the coolant level setpoint (Level 8) for trips of
the motor-driven main feedwater pump, the reactor core isolation cooling
system, and the high-pressure core spray system.  When the reactor coolant
level reached the administrative limit for coolant level (above Level 8), 
operators manually closed the outboard main steam isolation valves to prevent
the downstream main steamlines from being filled with water.  This action also
terminated steam flow to the turbine-driven main feedwater pump.  Closure of
the turbine stop valves by local mechanical trip actions for both turbine-
driven main feedwater pumps was successful only after repeated attempts.

Discussion

At LaSalle, a control oil system is used to trip closed the turbine stop valve
to shut down the turbine for each of the two turbine-driven main feedwater
pumps.  Control oil, as well as bearing lubrication oil, for both pumps is
supplied from the main turbine lubrication oil system reservoir.  The main
turbine lubrication oil system does not contain an oil filter in the 


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                                                           IN 93-48
                                                           July 6, 1993
                                                           Page 2 of 2


control oil line to the turbine-driven feedwater pump stop valves.  The
licensee determined that the root cause of the failure of the turbine-driven
main feedwater pumps to trip was suspended particles in the main turbine oil
system.  Suspended particles in the oil created flow blockages in the control
oil dump valves.  This prevented proper drainage of the trip system.  Most of
the accumulated particles were small, but some were up to 6 mm (1/4 in.) long. 
The licensee believes this condition resulted from an accumulation of crud in
the low flow areas of the control oil ports over a period of time.  The
maintenance program had not included flushing of these ports in previous
refueling outages.  Operators flushed the control oil system, replaced several
parts, and tested the system before returning it to service.  The licensee
plans to include flushing these ports in preventive maintenance of these trip
mechanisms.

In Generic Letter 89-19, "Request for Action Related to Resolution of
Unresolved Safety Issue A-47, `Safety Implication of Control Systems in LWR
Nuclear Power Plants' Pursuant to 10 CFR 50.54(f)," the NRC identified a
number of safety concerns resulting from coolant overfill events.  In boiling
water reactors, reactor vessel overfill events can affect the safety of the
plant in several ways.  In pressurized water reactors, steam generator
overfill events produce similar concerns.  The safety concerns arise from
potential increased loads on steamline supports from increased deadweight or
seismic forces, potential water hammer loads, potential release of coolant or
excessive cooling caused by stuck-open secondary safety valves, potential
inoperability of important valves in the secondary system, and potential
weakening of steam generator tubes from thermally induced tensile loads
(mainly applicable to once-through steam generator designs).  

This information 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.


                                          ORIGINAL SIGNED BY


                                       Brian K. Grimes, Director
                                       Division of Operating Reactor Support
                                       Office of Nuclear Reactor Regulation

Technical contacts:  T. Greene, NRR
                     (301) 504-1175

                     V. Hodge, NRR
                     (301) 504-1861

Attachment:  
List of Recently Issued NRC Information Notices


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