ML20151W639

Special Rept:On 980811,swing Shift Operators Noted That CR Would Slowly Drift Inwards When in Manual Control with CR low-raise Switch in Neutral.Cr lower-raise Switch Was Removed,Inspected & Cleaned,Then Reinstalled & Tested

ML20151W639
Person / Time
Site:	University of Michigan
Issue date:	09/11/1998
From:	Jun Lee MICHIGAN, UNIV. OF, ANN ARBOR, MI
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9809160054
Download: ML20151W639 (2)

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M NMGMk./6 MICHIGAN MEMORIAL-PHOENIX PROJECT VGMD gy PHOENIX MEMORIAL LABORATORY FORD NUCLEAR REACTOR ANN ARBOR, MICHIGAN 48109 2100 om et the cueaor September 11,1998 i

l Docket 50-2 I License R-28 United States Nuclear Regulatory Commission Document Control Desk l Washington, D.C. 20555 Re: Notification of Unusual Event:  !

Slow Inward Drift of the Non-Safety Related Control Rod When in Manual Control The purpose of this letter is to infomi you of an unusual event that recent;, occurred at Ford Nuclear Reactor. On 11 August 1998, the swing shift operators noted that the control rod would slowly drift inwards when in manual control with the control rod lower-raise switch in neutral. l The control rod was not supposed to move when the control rod lower-raise switch is in neutral. l The reactor was shutdown as a precautionary measure.

Facility management evaluated the event from a license and regulatory position. The control rod is a non-safety-related rod. The reactor safety system protects against high power and short period I excursions, regardless of cause. The safety analysis states that the control rod reactivity is limited to 0.6 %Bk/k to prevent a prompt criticality. Actual control rod reactivity is about 0.35 %Bk/k, j well below the 0.6 %Bk/k tech spec limit. Management concluded that this unusual event was not I a reportable occurrence; not a tech spec, safety analysis or significant system deficiency; and was not reportable under 10 CFR 21. It was decided that this notification would be made only because the control rod drift resulted in the slow addition of negative reactivity to the reactor. ,

Troubleshooting efforts were begun. The control rod lower-raise switch was removed, inspected and cleaned, then reinstalled and tested. The four solid-state switching relays that send 120 VAC control power to the drive motor were replaced. The drift was still present. It was noted that there were anomalous voltage signals (less than i VAC) at the control rod drive motor wiring terminals.

These anomalous voltage signals disappeared when the drive motor wiring terminals were cleaned and tightened. The control rod drift problem also went away. As a precautionary measure, the drive motor terminations for the three shim-safety rods were also cleaned and re-tightened. Restart ,

authorization was then given by management. I U On 17 August 1998, this slow inward drift of the control rod began to reoccur. Once again, the reactor was shutdown as a precautionary measure. Extensive troubleshooting efforts finally

established that the problem was mechanical, not electrical. Anomalous voltage signals of less than . y, j one volt were still present, but were attributed to the high input impedance of the digital voltmeter being used. The slow inward drift could be duplicated even after the drive motor was physically p

decoupled from the drive assembly.

Disassembly and study of a spare rod drive system showed that the rotating motor shaft had a worm gear which engaged a large drive gear. Staff members agreed that one of the design features of a worm to drive gear coupling was that it should not allow the drive gear to make the worm gear spin. Since the inward control rod drift was making the control rod drive motor spin, a defect in the worm to drive gear coupling was suspected. The gear drive was opened for inspection. It was found that there was significant wear of the worm gear, and that the worm gear was also missing a f

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Phone:(734) 764-6213 Fax: (734) 763-7863 www.umich.edu/~mmpp/

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.i The following half-cycle, during low power operations using manual rod control, the control rod  !

again began to display the slow inward drifting motion with the control rod lower-raise switch in neutral. The reactor was shut down, and the control rod drive assembly was physically removed, disassembled, and the remaining parts of the gear drive inspected. Another gear cavity, located behind the worm gear cavity, was opened and inspected. The gears and bearings in this cavity were in good condition, but the cavity was essentially vacant. Reference to the facility's original l maintenance instructions for the rod drive showed that this cavity was supposed to be packed with  !

Beacon 325 grease. After cleaning and inspecting, the gears and bearing were reinstalled, and the cavity fully packed with new grease (general purpose wheel bearing grease).

There has not been a recurrence of this problem since then.

l Sincerely, q

John lee, Interim Director Michigan Memorial- Phoenix Project 1

xc: Theodore Michaels, USNRC Project Manager Thomas Burdick, USNRC Region III l

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