ML14246A208
| ML14246A208 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Columbia |
| Issue date: | 08/29/2014 |
| From: | Rhonda Butler Univ of Missouri - Columbia |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| LER 14-03 | |
| Download: ML14246A208 (6) | |
Text
UNIVERSITY of MISSOURI RESEARCH REACTOR CENTER August 29, 2014 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Station P 1-37 Washington, DC 20555-0001
REFERENCE:
Docket No. 50-186 University of Missouri-Columbia Research Reactor Amended Facility License R-103
SUBJECT:
Written communication as required by University of Missouri Research Reactor Technical Specification 6.1.h(2) regarding a deviation from Technical Specifications 3.2.a.
The attached document provides the University of Missouri-Columbia Research Reactor (MURR) Licensee Event Report (LER) for an event that occurred on August 14, 2014 that resulted in a deviation from MURR Technical Specifications 3.2.a.
If you have any questions regarding this report, please contact John L. Fruits, the facility Reactor Manager, at (573) 882-5319.
Sincerely, Ralph A. Butler, P.E.
Director RAB:jlb Enclosure xc: Reactor Advisory Committee Reactor Safety Subcommittee Dr. Kenneth Dean, Deputy Provost Mr. Geoffrey Wertz, U.S. NRC 1-7, 9
Mr. Johnny Eads. U.S. NRC v
1513 Research Park Drive Columbia, MO 65211 Phone: 573-882-4211 Fax: 573-882-6360 Web: www.murr.missouri.edu Fighting Cancer with Tomorrow's Technology
Attachment U.S. Nuclear Regulatory Commission August 29, 2014 Licensee Event Report No. 14 August 14, 2014 University of Missouri Research Reactor Introduction On August 14, 2014, with the reactor operating at 10 MW in the automatic control mode, a "Channel 4, 5 or 6 Downscale" annunciation was received. This alarm is initiated when any one of the three Nuclear Instrumentation (NI) Power Range Monitor (PRM) channels decrease below a power level set point of 95%. The duty reactor operator observed the regulating blade height at 7.85 inches and moving in the inward direction at a rapid rate.
All PRM channels indicated a decreasing reactor power level. When the regulating blade reached a height of 5.20 inches (20% withdrawn), the "Regulating Rod 20% Withdrawn" annunciation was received. As the regulating blade continued in the inward direction to a height of 2.6 inches (10% withdrawn), the "Regulating Rod < 10% Withdrawn" annunciation and associated rod run-in occurred. The regulating blade continued moving in the inward direction until it reached the fully inserted position at which time a second rod run-in signal was initiated and the "Regulating Rod Bottomed" annunciation was received. Continued motion of the regulating blade after the initial rod run-in indicated that some type of equipment malfunction had occurred and the reactor was shut down.
Investigation revealed a mechanical failure in the regulating blade drive mechanism reduction gearbox. Failure of the regulating blade to be operable resulted in a deviation from Technical Specification (TS) 3.2.a, which states, "All control blades, including the regulating blade, shall be operable during reactoroperation."
Description of the Regulating Blade and Drive Mechanism The reactivity of the reactor is controlled by five neutron-absorbing control blades. Four of the control blades, referred to as the shim blades, are used for coarse adjustments to the neutron density of the reactor core. The fifth control blade is the regulating blade. The low reactivity worth of this blade allows for very fine adjustments in the neutron density in order to maintain the reactor at the desired power level.
The regulating blade is constructed of stainless steel with an overall length of approximately 30-inches, occupying about 180 of the circular arc around the outer reactor pressure vessel. The blade is driven at 40-inches per minute in both the inward and outward directions by its associated drive mechanism. The regulating blade drive mechanism consists of a servomotor, a reduction gearbox, a lead screw assembly and an overload clutch. The lead screw assembly converts the rotating motion of the servomotor to the linear motion of the regulating blade. The drive mechanism, through a slave gear and chain arrangement, also drives a rod position indication (RPI) encoder transducer and Page 1 of 5
Attachment U.S. Nuclear Regulatory Commission August 29, 2014 a rotary limit switch assembly. The encoder transducer provides an analog signal to the RPI chassis, which converts the analog signal to a digital readout that is displayed on the control room instrument panel and control console. The rotary limit switch assembly actuates two regulating blade position alarm functions (20% and 60% withdrawn) and a rod run-in (< 10% withdrawn). A second rod run-in is initiated by a limit switch, which is independent of the rotary limit switch assembly, when the regulating blade is fully inserted or "bottomed."
The regulating blade may be operated from the control console in either one of two modes: manual or automatic. In the automatic control mode, the regulating blade controls reactor power by comparing the output signal from the NI Wide Range Monitor (WRM) with the setting of the power schedule potentiometer as determined by the reactor operator. If a mismatch does exist, a positive or negative output signal is generated and sent to the servomotor of the regulating blade drive mechanism, which repositions the regulating blade, stepwise, in a direction which minimizes the discrepancy between the power schedule setting and the actual power level. Over the course of the week, while in the automatic control mode, the regulating blade frequently shims to make minor adjustments to maintain power at the desired level.
Detailed Event Description At 03:00 on August 14, 2014, with the reactor operating at 10 MW in the automatic control mode, a "Channel 4, 5 or 6 Downscale" annunciation was received. This alarm is initiated when any one of the three NI PRM channels decrease below a power level set point of 95%. The duty reactor operator observed the regulating blade height at 7.85 inches and moving in the inward direction at a rapid speed. All PRM channels indicated a decreasing reactor power level. As the regulating blade reached the 10% withdrawn position a "Regulating Rod < 10% Withdrawn" annunciation and associated rod run-in occurred. The regulating blade continued moving in the inward direction until it reached the fully inserted position at which time a second rod run-in signal was initiated.
Continued motion of the regulating blade after the initial rod run-in indicated that some type of equipment malfunction had occurred and the reactor was shut down. Failure of the regulating blade to be operable resulted in a deviation from TS 3.2.a, which states, "All control blades, including the regulating blade, shall be operable during reactor operation."
After the reactor was secured, the regulating blade drive mechanism was removed for inspection and troubleshooting. It was determined that the gearbox output shaft had sheared (see attached drawings and photos) thus allowing the lead screw to rotate freely.
Page 2 of 5
Attachment U.S. Nuclear Regulatory Commission August 29, 2014 The weight of the stainless steel regulating blade and associated blade arm and drive tube caused the lead screw assembly to rotate and fully insert.
Safety Analysis Preceding the failure, the reactor had been at continuous full power operation with the regulating blade properly maintaining power level in the automatic control mode for a period of 57 hours6.597222e-4 days <br />0.0158 hours <br />9.424603e-5 weeks <br />2.16885e-5 months <br /> and 22 minutes since the last scheduled startup on August 11,2014.
The regulating blade had been fully operational during that period. A review of the WRM and PRM strip-chart recorders indicated that the power decrease was a direct result of the shaft shear and occurred immediately prior to receiving the "Channel 4, 5 or 6 Downscale" annunciator alarm. It was less than one minute from the point of shaft shear until the "Regulating Rod < 10% Withdrawn" rod run-in occurred and shutdown the reactor.
Failure of the regulating blade to be operable during reactor operation resulted in a deviation from TS 3.2.a. The basis for this specification is to ensure that the normal method of reactivity control is used during reactor operation.
The regulating blade and its associated rod run-in features are not part of the reactor safety system as defined by TS 1.18, which states, "The safety system is that combination of sensing devices, circuits, signal conditioning equipment electronic equipment and electro-mechanical devices that serves to effect a reactor scram, initiate a containment building isolation or activate the primary coolant siphon break system." When a reactor scram or rod run-in occurs, the regulating blade is automatically shifted to manual control to prevent it from operating to maintain power.
In this instance, the rod run-in features associated with the regulating blade were completely functional and operated as designed. When the gearbox output shaft sheared the regulating blade inserted due to the weight of the blade and associated blade arm and drive tube which caused a "Regulating Rod < 10% Withdrawn" rod run-in to occur. This rod run-in terminated operation of the reactor.
Corrective Action:
The reactor was automatically shut down by rod run-in shortly after the regulating blade gearbox output shaft sheared. The regulating blade drive mechanism was inspected and the gearbox output shaft and housing upper plate were replaced. The regulating blade upper drive thrust bearing was also replaced to ensure no abnormal radial loading was Page 3 of 5
Attachment U.S. Nuclear Regulatory Commission August 29, 2014 being applied to the gearbox output shaft. Electronic Shop maintenance procedure EMP-12B, "Regulating Blade," was completed satisfactorily. The applicable sections of compliance procedure CP-14, "Regulating Rod 10% and Rod Bottom Rod Run-In Rod Not in Contact with Magnet Rod Run-In," and the "Regulating Blade Operation And Rod Run-In Function Test" portion of form FM-57, "Long Form Startup Checksheet," were completed satisfactorily to verify proper operation of the regulating blade rotary limit switch assembly.
Additionally on August 25, 2014, during a regularly scheduled maintenance day, the regulating blade drive mechanism was removed and completely refurbished. This refurbishment included replacement of the servomotor, lead screw, ball nut bearing, upper thrust bearing, gearbox output shaft and housing upper plate. Electronic Shop maintenance procedure EMP-1 2B, "Regulating Blade," and the applicable sections of compliance procedure CP-14, "Regulating Rod 10% and Rod Bottom Rod Run-In Rod Not in Contact with Magnet Rod Run-In," were also completed as part of the post maintenance testing.
The cause of the shaft sheer was the repeated stresses applied by routine operation of the regulating blade system. This shaft had been in service since original fabrication of the gearbox and had approximately four years of operational service. Additionally, a review of Electronic Shop machinery history indicated that the upper thrust bearing had not been replaced since November 1981. Bearing wear was probably the most likely cause of inducing radial stress to the gearbox output shaft. To prevent any further failures of this type, the regulating blade drive mechanism biennial preventative maintenance procedure has been revised such that the gearbox output shaft and housing upper plate will be renewed on a biennial basis. An overhaul (every four years) preventive maintenance procedure is being developed to ensure the entire regulating blade drive mechanism is inspected and refurbished on a regularly scheduled basis. Additionally, this event has been entered into the MURR Corrective Action Program as CAP entry No. 14-0047 and any additional improvements or corrective actions will be considered.
If there are any questions regarding this LER, please contact me at (573) 882-5319. I declare under penalty of perjury that the foregoing is true and correct.
ENDORSEMENT:
Sincerely, Reviewed and Approved, John L. Fruits Ralph A. Butler, P.E.
Reactor Manager Director oCHRInS*m i ERRANTE m.
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