United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-45: Overspeed of the Turbine-Driven Auxiliary Feedwater Pumps and Overpressurization of the Associated Piping Systems

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

                                July 6, 1990


Information Notice No. 90-45:  OVERSPEED OF THE TURBINE-DRIVEN AUXILIARY
                                   FEEDWATER PUMPS AND OVERPRESSURIZATION OF 
                                   THE ASSOCIATED PIPING SYSTEMS 


Addressees:

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose:

This information notice is intended to alert addressees to potential 
problems resulting from the combined failures of the governor and the 
overspeed trip mechanism (OTM) of turbine-driven auxiliary feedwater (AFW) 
pumps that can result in turbine overspeed and overpressurization of the AFW 
system.  Because similar turbine-driven pumps are also used in the high 
pressure core injection (HPCI) and reactor core isolation cooling (RCIC) 
systems, they are also subject to the same failure mechanism.  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.

Description of Circumstances:

Two events involving overpressurization of the AFW system as a result of AFW 
turbine overspeed are discussed in the following paragraphs.  The turbine 
overspeeds were caused by the combined failures of the turbine governor and 
the OTM.  The OTM did not function to close the steam trip valve when the 
governor had failed to respond properly to turbine speed increases.

Rancho Seco Nuclear Generating Station

On January 31, 1989, the AFW turbine oversped and overpressurized both 
trains of the AFW system.  Both trains of the AFW system were declared 
inoperable, and the plant was shutdown.  The turbine overspeed occurred 
while the licensee was testing a newly modified governor.  The turbine speed 
reached 6020 rpm in a few seconds after start and remained at that level for 
three minutes.  The governor failed to limit the turbine at the rated speed 
of 3600 rpm and the turbine OTM also failed to trip the turbine at its 
setting of 4500 rpm.  An operator stopped the turbine by closing the steam 
inlet valve.  The AFW system pressure corresponding to the overspeed was 
calculated to be 3850 psig.  This pressure 


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exceeded the AFW system design pressure of 1325 psig.  Although only the 
turbine-driven train was operating, the overpressurization also affected the 
motor-driven train because of the system configuration which tied the two 
trains together.  The licensee's maintenance test program provided for no 
specific valve lineup requirement, which could have limited the amount of 
piping that was subject to overpressurization in the event of a turbine 
overspeed.  

The licensee determined that inadequate preventative maintenance and 
inadequate periodic testing of the OTM were the root causes of the OTM 
failure to function properly.  As a result, the licensee included monthly 
testing of the OTM and trip valve in the licensee corrective actions to 
prevent recurrence.  Maintenance test procedures for the AFW pump were also 
modified.  The discharge valve of the turbine-driven pump would be closed 
before the licensee conducted a turbine surveillance test.  In the case of 
turbine overspeed, this closed valve would limit the quantity of piping that 
is subject to overpressurization.  In addition, the turbine overspeed 
surveillance test would be performed with the turbine decoupled from the AFW 
pump shaft to avoid any overpressurization of the AFW system in the event of 
a turbine overspeed.

San Onofre Nuclear Generating Station Unit 2

On July 3, 1988, the AFW turbine oversped while personnel were performing 
low-voltage testing of the steam inlet valve to the AFW turbine.  Based on 
the test procedures, when the licensee removed power from the inlet valve, 
power was also removed from the governor valve.  The governor valve failed 
open and resulted in the turbine reaching overspeed conditions.  The OTM did 
not function properly to trip the turbine.  An attempt to close the inlet 
valve from the control room failed because the power had been removed.  The 
operator in the control room then closed the steam inlet isolation valve, 
terminating the event.  Control room indications showed that the 
turbine-driven pump reached 5000 rpm (the overspeed trip setpoint is 3500 
rpm).  The licensee later calculated that the AFW system had been 
pressurized to 2420 psig, which exceeded the system design pressure.

The licensee determined that the failure of the OTM to trip the turbine was 
due to tappet ball damage.  In addition to the replacement of the tappet 
ball assembly, the licensee's corrective actions included performing annual 
visual inspections of the tappet assembly and tests of the OTM every 
refueling outage.

The isolation valves in the discharge interconnection line to the two motor-
driven trains were closed during the test.  Therefore, the 
overpressurization was limited to the discharge pipe section from the 
turbine-driven pump up to the isolation valves and the motor-driven trains 
were not affected.  However, had the overspeed occurred during normal 
operation of the AFW system, the overpressurization would have extended to 
all of the AFW system piping because the isolation valve would have been in 
its normally open position.

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Discussion of Safety Significance:

Failure of both the governor valve and OTM, as occurred in these events, 
could result in severe damage to the turbine and pump due to overspeed.  
Without prompt operator action to control the speed of this equipment, the 
internal rotating elements could fail, possibly resulting in debris 
missiles.  

These events also reveal the vulnerability of the AFW system to 
overpressurization caused by turbine-driven pump overspeed.  The 
vulnerability is inherent to the piping configuration and normal valve 
lineup which allows for cross-connection between trains on the discharge 
side of the pump.  Although isolation valves are installed in the cross 
connections, they are kept open during normal system operation to allow the 
trains to share the pumps.  With this system configuration, 
overpressurization of the turbine-driven train immediately affects the other 
trains and causes the entire AFW system to be vulnerable to 
overpressurization.  With the AFW system isolated from the steam generators, 
there is no pressure relief device with sufficient capacity to limit 
overpressure.  Also, the pump recirculation lines do not have sufficient 
capacity to discharge enough pump flow to limit the overspeed pressure.  The 
Woodward governor, the OTM, and the Terry turbine that experienced the 
overpressure events described herein are used in the AFW systems of nearly 
all pressurized water reactors.

Similar Woodward governors, OTMs, and Terry turbines are used in the HPCI 
and RCIC systems of boiling water reactors.  Therefore, these systems are 
also vulnerable to overpressurization from the simultaneous failure of the 
governor and OTM.  

These events also highlight the importance of adequate maintenance and 
testing of this equipment to assure the reliability of these safety systems.

Related Generic Communications:

The turbine overspeeds described herein were caused by the combined failures 
of the governor and the OTM.  Although the OTM is installed to provide 
protection for the pump and turbine, in the event of governor failure, both 
the governor and OTM may fail simultaneously due to various problems 
affecting their operation and reliability.  Failure of the governor and OTM 
have been previously described in the following information notices:

1.   NRC Information Notice 88-67, "PWR Auxiliary Feedwater Pump Turbine 
     Overspeed Trip Failure."

2.   NRC Information Notice 88-09, "Reduced Reliability of Steam-Driven 
     Auxiliary Feedwater Pumps Caused by Instability of Woodward PG-PL Type 
     Governors."

3.   Information Notice No. 86-14, "PWR Auxiliary Feedwater Pump Turbine 
     Control Problems."

4.   Information Notice No. 86-14, Supplement 1, "Overspeed Trips of AFW, 
     HPCI, and RCIC Turbines."
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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 NRR project 
manager.




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contacts:  Chuck Hsu, AEOD
                     (301) 492-4443

                     John B. MacKinnon, AEOD
                     (301) 492-9097

                     Charles G. Hammer, NRR
                     (301) 492-0791

Attachment:  List of Recently Issued NRC Information Notices
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