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 L NRC/NRR/DIPM/IROB
To:
Telson, R - NRR/DIPM/IROB - 415-2256
References
TAC MC6269 IN-05-008
Download: ML050730093 (5)

UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555April 5, 2005NRC INFORMATION NOTICE 2005-08:MONITORING VIBRATION TO DETECTCIRCUMFERENTIAL CRACKING OF REACTORCOOLANT PUMP AND REACTORRECIRCULATION PUMP SHAFTS

ADDRESSEES

All holders of operating licenses for nuclear power reactors, except those who havepermanently ceased operations and have certified that fuel has been permanently removedfrom the reactor vessel.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alertaddressees to the importance of timely detection of circumferential cracking of reactor coolantpump (RCP) and reactor recirculation pump (RRP) shafts to minimize the likelihood ofconsequential shaft failures. It is expected that recipients will review the information for applicability to their facilities andconsider actions, as appropriate, to avoid similar problems. However, the suggestions in this INare 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 rootcause was determined to be fatigue initiated by thermal stresses that, combined withmechanical stresses, caused cracks to propagate. GE recommended countermeasuresincluding shaft vibration monitoring, inspection of shafts with greater than 80,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> ofservice, 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 shaftinspections. The licensee had operated the B RRP for several refueling cycles with vibrationlevels approaching vendor limits. During this time, the licensee also identified failed anddegraded RRP seals and concluded that the most likely causes of the failed and degraded RRPseals were a possible bow in the pump shaft and low reliability of the seal purge system. The licensee's decision to restart following the fall 2004 refueling outage without correcting thiscondition led to heightened public interest and prompted a close NRC review. The staffevaluated site-specific technical details, related domestic and international operatingexperience, and the generic safety aspects of vibration-related shaft and seal failure. Circumferential cracking of RCP and RRP shafts had previously been reported at severalfacilities including Sequoyah, Palo Verde, St. Lucie, and Grand Gulf. In addition, reactorcoolant pump shafts at Crystal River separated completely during operation on two occasions(see IN 86-19 and IN 89-15).The staff evaluated the licensee's determination that the Hope Creek unit could be safelyreturned to power with the existing pump shaft and the interim compensatory measuresimplemented to provide reasonable assurance that a shaft failure could be detected in itsincipient stage and operators would take prompt action to prevent the occurrence of a potentialshaft and seal failure. The licensee committed to (1) replace the B pump shaft at the nextoutage of sufficient duration and to (2) establish a comprehensive program of enhancedcontinuous vibration monitoring to ensure timely detection of circumferential crack propagationwith proceduralized contingency actions for plant operators to act promptly at specifiedadministrative vibration limits to reduce pump speed or shut the pump down completely. Thesame monitoring regime was implemented for the A RRP.The Hope Creek licensee implemented a program to continuously monitor the synchronousspeed (1X) vibration amplitude, two times synchronous speed (2X) vibration amplitude, 1Xphase angle, and 2X phase angle. These parameters provide a more sensitive leadingindicator of circumferential crack initiation and propagation giving the operators enough time torespond. Alarm limits were established using the ASME OM standard, "Reactor Coolant andRecirculation Pump Condition Monitoring." GE SIL 459 indicates that all Byron Jackson (now Flowserve) RRP shafts inspected haveshown some degree of thermally induced cracking. The cracking occurs near the pumpthermal barrier where the cold seal purge system water mixes with the hot reactor coolantwater. 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 mechanicalloads, the axial cracks can change direction and propagate circumferentially. The time it takesto transition from slow-growing axial cracks to more rapidly growing circumferential cracksdepends on the magnitude of the mechanical loads on the pump shaft. It could take years. Onthe 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 anddegradation or failure of the pump seal package resulting in leakage of reactor coolant throughclearances around the upper portion of the pump shaft. However, at Crystal River - where theonly two instances of shaft failure occurred at domestic nuclear power plants - there was noevidence of seal degradation. A loss-of-coolant accident can occur if leakage through the sealsof a RRP or RCP exceeds the capacity of the normal makeup systems. Thus circumferentialshaft cracking that leads to shaft or seal failure is a safety concern. As noted above, vibration-monitoring systems are available to detect circumferential cracking ofpump 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 shaftfailure. Although overall pump vibration limits are necessary for assessing gaps and clearancesin the pump, they are not the most appropriate indicator of shaft cracking. Monitoring the 1Xand 2X steady-state vectors (1X and 2X amplitudes and phase angles) provides a betterindication of changes in shaft integrity resulting from circumferential crack propagation. Licensees should be alert to the possibility of circumferential RCP or RRP shaft cracking andshould evaluate the information in this IN and determine what actions, if any, are prudent toprovide early detection of circumferential shaft cracking and prevent failure of RRP or RCPshafts and shaft seals.GENERIC IMPLICATIONSA significant number (about half) of the BWR RRP pump shafts currently in service are olderand have more hours of operation than those at Hope Creek and many have not beeninspected as recommended in GE SIL 459-S2.About a half-dozen BWR RRPs were identified as having higher vibration levels than HopeCreek. Such issues would not necessarily be reported to the NRC. The staff contacted threeBWR licensees whose plants had been reported to have higher vibration levels than HopeCreek. 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 pumpvibration, the vibration acceptance criteria used, and why the current vibration levels areacceptable. These licensees indicated that they have either replaced their pump shafts (or willin the near future) or are taking steps to monitor RRP vibration and have establishedacceptance criteria to detect anomalous behavior.Operating experience suggests that pressurized water reactor (PWR) RCPs are not immune tovibration-related shaft and seal failure concerns similar to BWR RRP concerns. PWR RCP sealfailure can be more safety significant than BWR RRP seal failure because (1) PWR reactorcoolant systems operate at higher pressures, increasing the differential pressure across thepump seals and (2) PWR RCPs, unlike BWR RRPs, typically can not be isolated from thereactor coolant system following a seal failure. In addition, while a number of BWR RRP shaftshave cracked, several PWR RCP shafts have completely severed.

CONTACT

This information notice requires no specific action or written response. Please direct anyquestions about this matter to the technical contact(s) listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager./RA/Patrick L. Hiland, ChiefReactor Operations BranchDivision of Inspection Program ManagementOffice of Nuclear Reactor RegulationTechnical Contacts:William Poertner, NRRRoss Telson, NRR(301) 415-5787(301) 415-2256E-mail wkp@nrc.govE-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.

IN 2005-08