ML20100J399
| ML20100J399 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Columbia |
| Issue date: | 02/22/1996 |
| From: | Mckibben J, Meyer W MISSOURI, UNIV. OF, COLUMBIA, MO |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9602290061 | |
| Download: ML20100J399 (3) | |
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Research Reactor Center
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Research Park Columbia. Missouri 65211 i
Telephone (314) 8824211 FAX [314) 882-3443 UNIVERSITY OF MISSOURI-COLUMBIA l
l February 22,1996 Dimetor of Nuclear Reactor Regulation ATTN: DocumentControlDesk Mail Station PI-37 i
U.S. Nuclear Regulatory Commission Washington, DC 20555
REFERENCE:
Docket No. 50-186 University of Missouri Research Reactor License R-103
SUBJECT:
Report as mquimd by T.S. 6.1.h.(2) regarding reactor operation with the regulating blade inoperable for less than 5 minutes DESCRIPTION On January 23,1996, at 0206 the reactor was shutdown by manual scram when the console operator determined that the regulating blade was inoperable. The reactor was shut down because Technical Specification 3.2.a. under Limiting Conditions for Operation (LCO) states, "All control blades, including the regulating blade, shall be operable during reactor operation." The T.S.1.12 definition of operable states, "A system or component is operable when it is capable of performing its intended function in a normal manner." With the regulating blade drive inoperable, the rod mn-ins associated with the regulating blade (<10% withdrawn and rod bottomed), listed under T.S.
3.4.c, were inoperable.
An Electronics Technician investigated the problem with the mgulating blade drive and discovemd a seized bearing on the gearbox input shaft. The bearing was replaced, the regulating blade drive was installed and tested, and the reactor retumed to operation at 0934 the same day.
We estimate from the Wide Range Monitor (WILN1) chart that the mactor may have operated up to five minutes with the regulating blade drive capability deteriorating prior to the manual shutdown.
ANALYSIS The regulating blade system is used to automatically control reactor power at a desired power level (normally 10 MW). The blade is constructed of stainless steel and is driven at 40 inches per minute by the regulating drive mechanism. The regulating drive mechanism consists of a drive servomotor, gearbox assembly, and a ball / lead screw arrangement to translate the rotary motion of the motor and gearbox to the linear motion of the regulating blade.
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t Ixticr to Director of Nuclear Reactor Regulation February 22,1996 Page two i
In the automatic mode, the angulating blade controls reactor power by comparing the output of 1
the Wide Range Monitor (Channel 4) with the level on the power schedule potentiometer set by the tractor operator. Any difference between the wide range monitor indication and the potentiometer setting creates a drive signal to the regulating blade drive mechanism. The blade frequently shims 1
to make minor adjustments to maintain power at the desired level in automatic control.
The rod run-ins associated with the regulating blade (< 10% withdrawn and rod bottomed) i were inoperable during the short time the reactor operated with the regulating blade drive not operable. This is not in compliance with T.S. 3.4.c. The basis for these rod run-ins is to ensure j
termination of a transient which, during automade control,is causing a rapid insertion of the regulating blade.
Investigation of the mgulating blade drive indicated the failure was due to a seized bearing on the gearbox input shaft. This bearing had been replaced with one from spare parts inventory on December 27,1995. Inspection of the bearing revealed that it had seized because the ball retainer or separator had broken. This is an unusual failure mode for roller bearings which normally provide warning of failum (e.g., noise). The surprising element in this case is that the bearing with the shortest time in service had failed, which may indicate that it was defective. The bearing, which had been installed from spare parts inventory, may have lost its original lubricant while in storage. Review of the pmventive maintenance procedure for the regulating rod gearbox revealed it does not provide directions for lubrication or replacement of bearings.
The Reactor Manager, Facilities Operations Manager, Chief Electronics Technician and Chief Machinist discussed the bearing failure and the procedural deficiency regarding bearing replacement or lubrication. The decision was made to rebuild the spare gearbox with all new sealed (self-lubricating) bearings and place it in service. The regulating blade drive preventive maintenance procedure was changed to specify that the gearbox be replaced with a rebuilt gearbox every two years. This will limit the operating time for the gearbox bearings to a fraction of their rated life. Each time a gearbox comes off service, it will be rebuilt with new sealed bearings, have its gears and gear train inspected, and be available, as a spare, if needed.
The regulating blade and its associated rod run-in features are not part of the tractor safety system as defined in Technical Specification 1.18. The total reactivity worth of the regulating blade is 0.0017 Ak and is not considered in any safety analysis to contribute to the mactor shutdown margin of at least 0.02 Ak with any one shim blade fully withdrawn (T.S. 3.1.e). When a reactor scram or rod run-in occurs, the mgulating blade is automatically shifted to manual control to prevent it from trying to maintain power by shimming.
As previously stated in a Licensee Event Report dated January 26,1996, regarding a different failum of the mgulating blade, the management / engineering staff at the University of Missouri Research Reactor are developing a safety analysis to support a request for a Technical Specification revision that would allow a timely reactor shutdown as an action statement for a failure of the regulating blade. This is consistent with ANSI standards ANS-15.1, Development of Technical Specifications for Research Reactors and ANS-15.18, Administrative Control for Research Reactors whem special reports would not be required when a research reactor rpomentarily operates outside the limiting conditions for operation if prompt remedial act;on is taken (e.g., a reactor shutdown). This would alleviate the generation of a Licensee Event Report for conditions which do not pose a safety concern for the reactor or the public.
s IxttI:t to Director of Nuclear Reactor Regulation February 22,19%
Page three CORRECTIVE ACTION The control room operator manually scrammed the mactor when the regulating blade drive was determined to be inoperable. The cause of the mgulating blade drive failure was determined to be a seimd bearing for the gearbox input shaft. The bearing was replaced, the regulating blade drive was tested and retumed to operation.
The engineering / management team that reviewed this failure mcommended that a spare gearbox be rebuilt to meet original specifications with new sealed bearings. This rebuilt gearbox was installed February 12,1996. The gearbox removed from service will be mbuilt with new scaled bearings and placed in spam parts inventory. The preventive maintenance procedure for the mgulating blade drive now specifies that the regulating blade drive gearbox be changed out every two years and rebuilt.
Sincerely, ENDORSEMENT:
Reviewed and Approved 9
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k Walt A. Meyer Jr.
J. Charles McKibben Reactor Manager Associate Dimetor xc:
Mr. Alexander Adams, Jr., USNRC Regional Administrator, NRC, Region III Dr. John P. McCormick, Interim Vice Provost for Research and Graduate School Dean, UMC j
Reactor Advisory Committee Reactor Safety Subcommittee
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