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HomeNRC LibraryDocument CollectionsGeneral CommunicationsInformation Notices1994

# 

```
                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                            WASHINGTON, D.C.  20555

                               October 26, 1994


NRC INFORMATION NOTICE 94-76:  RECENT FAILURES OF CHARGING/SAFETY INJECTION    
                               PUMP SHAFTS 


Addressees 

All holders of operating licenses or construction permits for pressurized
water reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice (IN) to alert addressees to recent failures of charging/safety
injection pump shafts at facilities designed by the Westinghouse Electric
Corporation (Westinghouse).  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.

Background

At Westinghouse-designed facilities, charging/safety injection pumps have
three functions:  (1) to deliver seal injection flow to the reactor coolant
pumps, (2) to deliver makeup water flow to the reactor coolant system, and  
(3) to deliver high head safety injection and recirculation flow to the
reactor coolant system during and after a loss-of-coolant accident.  During
normal operation, one of two, or in some cases, one of three pumps is always
in operation to deliver seal injection and makeup water flow.  The loss of an
inservice charging/safety injection pump creates a condition that may
challenge reactor coolant pump seal integrity and, if makeup water flow is not
restored in a timely manner, can result in a loss of coolant inventory. 

On February 29, 1980, the NRC issued IN 80-07, "Pump Shaft Fatigue Cracking," 
to alert recipients to failures of charging/safety injection pump shafts that
occurred during the 1970s.  All of the charging/safety injection pump shafts
addressed in that notice were procured by Westinghouse from the Pacific Pump
Division of Dresser Industries (now Ingersoll-Dresser Pump Co).  Actions taken
at that time to correct the problem included design modifications, changes in
the heat treatment of the shaft material, and the use of formed cutting tools
during fabrication.  Also, abnormal operation of the pumps such as operation
with a partial or complete loss of fluid or with high vibration present was
found to be a significant contributor to the shaft failures.  The Westinghouse
Nuclear Service Division issued Technical Bulletins TB-77-9, TB-78-1, and
TB-79-6, to provide guidance on vibration monitoring, operation and
maintenance of the pumps, and allowable vibration amplitude limits.

9410200153.                                                              IN 94-76
                                                              October 26, 1994
                                                              Page 2 of 5


Recent events indicate that failures of charging/safety injection pump shafts
continue to be a problem.  

Description of Circumstances
  
Sequoyah Unit 1

On February 18, 1991, plant operators for Sequoyah Unit 1 received indications
of decreasing flow and increasing motor current on charging pump 1B-B.  When
efforts to restore full flow failed, they declared the pump inoperable and
began shutting down the reactor as required by plant technical specifications. 
Charging pump 1B-B had been in operation for several months and had shown no
previous signs of degradation.  The licensee disassembled the pump, found
heavy wear on the impeller shoulders and the balance drum, and found a         
280-degree crack in the shaft near the 11th stage impeller.  Westinghouse
analyzed the shaft and determined that the crack had been in the shaft for
several months (possibly years) and that the rotating element was of the
improved type referred to in IN 80-07.  The licensee replaced the rotating
element and, after testing the pump successfully, returned the pump to
service. [Licensee Event Report (LER) 50-327/91-003] 

Callaway

On February 2, 1992, the Union Electric Company Callaway Plant was at power
and charging pump B was in service to support operations.  Plant operators
received indications of zero flow in the charging header and at the reactor
coolant pump seal.  The operators placed charging pump A in service to restore
normal flow, and pump B tripped.  Plant personnel inspected pump B and found
that the shaft had sheared on the outboard end between the balance drum lock
nut and balance drum mating area.  The licensee documented the failure in
Suggestion Occurrence Solution 92-105 and replaced the failed shaft.

Shearon Harris

On March 18, 1993, operators at the Shearon Harris plant received indications
of a shaft failure on charging/safety injection pump B.  The indications were
high motor current, low charging flow, and low pump discharge pressure.  The
operators secured pump B from service and placed pump A in service.  Plant
personnel uncoupled the pump from the motor and found that the pump shaft had
sheared under the balancing drum lock nut.  This was the same location as in
three failures that occurred in the 1970s. (LER 50-400/93-005) 

D.C. Cook Unit 2

In July 1993, at D.C. Cook Unit 2, a charging pump failed a surveillance test
when it could not deliver the required 454 liters [120 gallons] per minute. 
The rotating element in that pump had been installed in 1987.  The licensee
disassembled the pump and found a 10 centimeter [4 inch], 180 degree
circumferential crack through the number 9 impeller shaft keyway.  Smaller
cracks were found in two other impeller keyway areas. (LER 50-316/93-006)


                                                              IN 94-76
                                                              October 26, 1994
                                                              Page 3 of 5

Braidwood Unit 1 and Sequoyah Unit 2

The NRC staff has received information on two other recent failures of
charging/safety injection pump shafts:  (1) on September 15, 1993, at the
Braidwood Nuclear Station Unit 1 (Braidwood), a charging/safety injection pump
shaft sheared between the 10th and 11th stage impellers, and (2) on
February 7, 1994, at Sequoyah Unit 2, a charging\safety injection pump shaft
failed, resulting in a reactor shutdown as required by plant technical
specifications.  The licensee for Braidwood replaced the pump shaft and
documented the failure on Problem Investigation Report 456-200-93-03600.  The
licensee for Sequoyah reported that the affected pump had not exhibited any
indication of degradation before the shaft failure and that, similar to many
of the other shaft failures, the shaft had failed near the location of the
balancing drum lock nut. (LER 50-328/94-002)

Discussion

Charging/safety injection pumps are important for normal plant operation and
for core cooling during accidents such as a small break loss-of-coolant
accident.  For most of the failure events described above, determination of
the root cause of the failure was inconclusive.  However, the operational
histories of many of the failed shafts showed that they had been operated with
void formation, gas entrainment, or other abnormal conditions within a few
years of the failure.  Operation of the pumps under these conditions may have
caused or contributed to the later failure of the shafts.  Avoiding operation
of charging pumps under abnormal conditions and maintaining vibration levels
within manufacturer recommendations may increase pump reliability.  To
increase the benefit of predictive maintenance programs, Westinghouse
recommends that pump vibrations be monitored at least monthly; preferably,
every two weeks.  This is more frequent than is required by Section XI of the
ASME Boiler and Pressure Vessel Code.  Westinghouse will provide recommended
vibration limits upon request.  

Industry experience in detecting shaft failures in pumps such as the reactor
coolant pump and the recirculation pump is relevant to monitoring programs of
charging/safety injection pumps because the precursors to shaft failure are
similar.  For those pumps, monitoring phase angles as well as monitoring
vibration amplitude is considered to be important in detecting shaft
degradation.  These data are routinely trended by some licensees for detection
of impending shaft failures.  

A description of the analyses and conclusions for some of the above events
follows:

Westinghouse evaluated seven possible root causes for the shaft failure at
Callaway, including material defects, design flaws, errors in fabrication or
processing, assembly or installation defects, off-design or unintended service
conditions, maintenance deficiencies, and improper operation.  Westinghouse
concluded that the shaft failure was most likely the result of a 1986 event in
which the pump had experienced a loss of suction water flow for approximately
seven minutes.  The loss of suction flow increased the vapor-to-liquid ratio
in the pump and caused a dynamic imbalance.  Events of this type could cause   

                                                              IN 94-76
                                                              October 26, 1994
                                                              Page 4 of 5


immediate pump failure or cause cyclic fatigue damage that could lead to
premature shaft failure at a later date. 

A gas entrainment event that occurred on August 20, 1990, was determined to be
the probable cause of the shaft failure at Sequoyah Unit 2.  Problems caused
by gas entrainment are discussed in NRC Information Notice 88-23, "Potential
for Gas Binding of High-Pressure Safety Injection Pumps During a Design Basis
Accident," and its supplements. 

Westinghouse considers an operational phenomenon, such as gas entrainment, may
have led to the shaft failure at Shearon Harris, although the licensee found
no evidence of gas pockets in the charging system.  In May 1991 (two years
before the shaft failure), the licensee reported to the NRC that the charging
system had been in a degraded condition during the previous operating cycle. 
An NRC Special Inspection Team reviewed the event and determined that several
water hammer events could have occurred in the system as a result of
weaknesses in the design of the alternate minimum flow system.  The NRC issued
IN 92-61, "Loss of High Head Safety Injection," and its supplement regarding
that event.  Another concern at Shearon Harris was the fact that the A and B
charging/safety injection pumps are alternated at approximately 2-week
intervals.  Therefore, each pump is started about 25 to 30 times each year. 
Westinghouse believes the high number of starts also could contribute to early
shaft failure.  Problems associated with excessive pump starts include galled
wear rings, increased vibration, and decreased pump performance.  However,
Westinghouse concluded that the available data were insufficient to directly
link the failure of the shaft to the high number of pump starts.  

Although Westinghouse could not conclusively determine the root cause of the
shaft failure at Shearon Harris, Westinghouse made recommendations which could
help prevent or detect impending shaft failures.  Westinghouse suggested that
the licensee conduct a detailed review of the possibility that gas could
become entrained in the charging pump suction piping and the cross connects to
other systems.  Westinghouse also recommended that, when the rotating element
of the pump is replaced, consideration be given to installing the latest shaft
design which has an improved one-piece balance drum lock nut.  Westinghouse
included recommendations for vibration monitoring in Westinghouse Technical
Bulletin TB-79-6.  Westinghouse has not specified a limitation on the number
of pump starts but recommends that pump starts be minimized to maintain pump
reliability.

In addition to the industry actions described above, Westinghouse and the
Westinghouse Owners Group (WOG) are implementing a program to address these
pump shaft failures.  The program includes:  (1) a survey of WOG member
utilities for pump service operating history data, (2) a pump design review,
and (3) a shaft material enhancement evaluation.  The program is intended to
identify any weaknesses in design, maintenance, or operation of the pumps in
order to improve shaft reliability.
.                                                              IN 94-76
                                                              October 26, 1994
                                                              Page 5 of 5


This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.

                                          /S/'D BY BKGRIMES


                                         Brian K. Grimes, Director
                                         Division of Project Support
                                         Office of Nuclear Reactor Regulation


Technical contact:  D. Roberts, RII
                    (919) 362-0601

Attachment:  
List of Recently Issued NRC Information Notices
```

*Page Last Reviewed/Updated Wednesday, February 16, 2011*

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