Information Notice 2005-08, Monitoring Vibration to Detect Circumferential Cracking of Reactor Coolant Pump and Reactor Recirculation Pump Shafts

Monitoring Vibration to Detect Circumferential Cracking of Reactor Coolant Pump and Reactor Recirculation Pump Shafts

ML050730093
Person / Time
Site:	Hope Creek
Issue date:	04/05/2005
From:	Hiland P NRC/NRR/DIPM/IROB
To:
Telson, R - NRR/DIPM/IROB - 415-2256
References
TAC MC6269 IN-05-008
Download: ML050730093 (5)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 April 5, 2005 NRC INFORMATION NOTICE 2005-08: MONITORING VIBRATION TO DETECT

CIRCUMFERENTIAL CRACKING OF REACTOR

COOLANT PUMP AND REACTOR

RECIRCULATION PUMP SHAFTS

ADDRESSEES

All holders of operating licenses for nuclear power reactors, except those who have

permanently ceased operations and have certified that fuel has been permanently removed

from the reactor vessel.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert

addressees to the importance of timely detection of circumferential cracking of reactor coolant

pump (RCP) and reactor recirculation pump (RRP) shafts to minimize the likelihood of

consequential shaft failures.

It is expected that recipients will review the information for applicability to their facilities and

consider actions, as appropriate, to avoid similar problems. However, the suggestions in this IN

are not NRC requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

General Electric (GE) Nuclear Services Information Letter (SIL) 459-S2, issued October 21,

1991, informed GE boiling water reactor (BWR) owners of shaft cracking in RRPs. The root

cause was determined to be fatigue initiated by thermal stresses that, combined with

mechanical stresses, caused cracks to propagate. GE recommended countermeasures

including shaft vibration monitoring, inspection of shafts with greater than 80,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of

service, and measures to reduce mechanical and thermal stresses.

At Hope Creek, RRPs had accumulated over 130,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of service without pump shaft

inspections. The licensee had operated the B RRP for several refueling cycles with vibration

levels approaching vendor limits. During this time, the licensee also identified failed and

degraded RRP seals and concluded that the most likely causes of the failed and degraded RRP

seals were a possible bow in the pump shaft and low reliability of the seal purge system.

The licensees decision to restart following the fall 2004 refueling outage without correcting this

condition led to heightened public interest and prompted a close NRC review. The staff

evaluated site-specific technical details, related domestic and international operating

experience, and the generic safety aspects of vibration-related shaft and seal failure.

Circumferential cracking of RCP and RRP shafts had previously been reported at several

facilities including Sequoyah, Palo Verde, St. Lucie, and Grand Gulf. In addition, reactor

coolant pump shafts at Crystal River separated completely during operation on two occasions

(see IN 86-19 and IN 89-15).

The staff evaluated the licensees determination that the Hope Creek unit could be safely

returned to power with the existing pump shaft and the interim compensatory measures

implemented to provide reasonable assurance that a shaft failure could be detected in its

incipient stage and operators would take prompt action to prevent the occurrence of a potential

shaft and seal failure. The licensee committed to (1) replace the B pump shaft at the next

outage of sufficient duration and to (2) establish a comprehensive program of enhanced

continuous vibration monitoring to ensure timely detection of circumferential crack propagation

with proceduralized contingency actions for plant operators to act promptly at specified

administrative vibration limits to reduce pump speed or shut the pump down completely. The

same monitoring regime was implemented for the A RRP.

The Hope Creek licensee implemented a program to continuously monitor the synchronous

speed (1X) vibration amplitude, two times synchronous speed (2X) vibration amplitude, 1X

phase angle, and 2X phase angle. These parameters provide a more sensitive leading

indicator of circumferential crack initiation and propagation giving the operators enough time to

respond. Alarm limits were established using the ASME OM standard, "Reactor Coolant and

Recirculation Pump Condition Monitoring."

GE SIL 459 indicates that all Byron Jackson (now Flowserve) RRP shafts inspected have

shown some degree of thermally induced cracking. The cracking occurs near the pump

thermal barrier where the cold seal purge system water mixes with the hot reactor coolant

water. The cracks initiate as axial cracks in the pump shaft. Axial cracks are generally benign, grow slowly, and do not affect the operation of the pump. However, given sufficient mechanical

loads, the axial cracks can change direction and propagate circumferentially. The time it takes

to transition from slow-growing axial cracks to more rapidly growing circumferential cracks

depends on the magnitude of the mechanical loads on the pump shaft. It could take years. On

the other hand, circumferential shaft cracking can propagate rapidly and, if not detected early, may result in complete severance of the shaft.

Circumferential shaft cracking or shaft separation could result in pump damage and

degradation or failure of the pump seal package resulting in leakage of reactor coolant through

clearances around the upper portion of the pump shaft. However, at Crystal River - where the

only two instances of shaft failure occurred at domestic nuclear power plants - there was no

evidence of seal degradation. A loss-of-coolant accident can occur if leakage through the seals

of a RRP or RCP exceeds the capacity of the normal makeup systems. Thus circumferential

shaft cracking that leads to shaft or seal failure is a safety concern. As noted above, vibration-monitoring systems are available to detect circumferential cracking of

pump shafts. As circumferential cracks propagate, the stiffness of the pump shaft changes.

These changes are detectable through changes in the pump vibration signature prior to shaft

failure. Although overall pump vibration limits are necessary for assessing gaps and clearances

in the pump, they are not the most appropriate indicator of shaft cracking. Monitoring the 1X

and 2X steady-state vectors (1X and 2X amplitudes and phase angles) provides a better

indication of changes in shaft integrity resulting from circumferential crack propagation.

Licensees should be alert to the possibility of circumferential RCP or RRP shaft cracking and

should evaluate the information in this IN and determine what actions, if any, are prudent to

provide early detection of circumferential shaft cracking and prevent failure of RRP or RCP

shafts and shaft seals.

GENERIC IMPLICATIONS

A significant number (about half) of the BWR RRP pump shafts currently in service are older

and have more hours of operation than those at Hope Creek and many have not been

inspected as recommended in GE SIL 459-S2.

About a half-dozen BWR RRPs were identified as having higher vibration levels than Hope

Creek. Such issues would not necessarily be reported to the NRC. The staff contacted three

BWR licensees whose plants had been reported to have higher vibration levels than Hope

Creek. The three plants included Susquehanna Units 1 and 2, Peach Bottom Units 2 and 3, and Browns Ferry Units 2 and 3. The staff discussed with each licensee how it monitors pump

vibration, the vibration acceptance criteria used, and why the current vibration levels are

acceptable. These licensees indicated that they have either replaced their pump shafts (or will

in the near future) or are taking steps to monitor RRP vibration and have established

acceptance criteria to detect anomalous behavior.

Operating experience suggests that pressurized water reactor (PWR) RCPs are not immune to

vibration-related shaft and seal failure concerns similar to BWR RRP concerns. PWR RCP seal

failure can be more safety significant than BWR RRP seal failure because (1) PWR reactor

coolant systems operate at higher pressures, increasing the differential pressure across the

pump seals and (2) PWR RCPs, unlike BWR RRPs, typically can not be isolated from the

reactor coolant system following a seal failure. In addition, while a number of BWR RRP shafts

have cracked, several PWR RCP shafts have completely severed.

CONTACT

This information notice requires no specific action or written response. Please direct any

questions about this matter to the technical contact(s) listed below or the appropriate Office of

Nuclear Reactor Regulation (NRR) project manager.

/RA/

Patrick L. Hiland, Chief

Reactor Operations Branch

Division of Inspection Program Management

Office of Nuclear Reactor Regulation

Technical Contacts: William Poertner, NRR Ross Telson, NRR

(301) 415-5787 (301) 415-2256 E-mail wkp@nrc.gov E-mail rdt@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

ML050730093 OFFICE OES:IROB:DIPM TECH EDITOR EMEB:DE SC:CI&T:EMEB C:EMEB

NAME RTelson PKleene WPoertner DTerao Eimbro

DATE 03/17/2005 03/22/2005 03/17/2005 03/18/2005 03/18/2005 OFFICE D:DE LPD1:DLPM A:SC:OES:IROB:DIPM C:IROB:DIPM

NAME MMayfield DSCollins (E- m ail) EJBenner PLHiland

DATE 03/20/2005 04/05/2005 04/05/2005 04/05/2005