ML20097J439
| ML20097J439 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Columbia |
| Issue date: | 01/26/1996 |
| From: | Mckibben J, Meyer W MISSOURI, UNIV. OF, COLUMBIA, MO |
| To: | NRC (Affiliation Not Assigned), NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9602020149 | |
| Download: ML20097J439 (4) | |
Text
.s Research Reactor C:nt:r II Research Park Columbia Missouri 65211 UNIVERSITY OF MISSOURI-COLUMBIA weg314g2 q]
January 26,1996 Director of Nuclear Reactor Regulation ATTN: Document Control Desk MailStation P1-37 U.S. Nuclear Regulatory Commission Washington,DC 20555
REFERENCE:
Docket No. 50-186 University of Missouri Researth Reactor License R-103
SUBJECT:
Report as required by T.S. 6.1.h.(2) regarding reactor operation with the regulating blade in a degraded condition DESCRIPTION On December 27,1995, at 1748 a reactor shutdown was commenced by manual md run-in to investigate a suspected problem with the regulating blade drive mechanism. A senior operator, performing a sample evolution on the reactor brid ;e, suspected a problem when he detected an audible difference in the sound of the regulating b ade dnve mechanism when it operated. The Channel 4 (WRM) chart recorder indicated the regulating blade was maintaining reactor power level within its normal range.
The reactor was shut down because Technical Specification 3.2.a. 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."
After the reactor was shut down, the regulating blade was exercised in the " manual" mode.
The regulating blade appeared to drive in normally, but was noisy and slower than normal when driving out.
An electronics technician was called to investigate the problem with the regulating blade. After disassembly of the regulating blade drive motor and gearbox assembly, the electronics technician found that the dowel pin had failed in the gearbox coupling to the drive motor. This resuhed in a friction fit between the gearbox input shaft and the gearbox coupling $at allowed the shaft to slip in the coupling when the regulating blade was driven out.
The dowel pin was replaced in the gearbox coupling and the yearbox input shaft bearing was replaced. The regulating blade drive was installed and the comphance check (CP-14) was completed to demonstrate its operability. This check included drive speed in both "in" and "out" direction and assured that alanns and rod run-ins associated with the regulating blade were actuating at the expected rod height.
COLUMBIA KANSAS CITY ROLLA ST. LOUIS 9f02020'149 960126
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January 26,1996 Page two De reactor was refueled and returned to operation at 2331 the same day. We estimate that the reactor may have operated five minutes with the regulating blade drive in a degraded condition prior to commencing reactor shutdown.
t ANALYSIS De 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 i
minute by the regulating drive mechanism. The regulating drive mechanism consists of a drive i
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.
In the automatic mode, the regulating blade controls reactor power by comparing the output of j
the Wide Range Monitor (Channel 4) with the level on the power schedule potentiometer set by the reactor 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
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to make minor adjustments to maintam power at the desired level in automatic control.
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The md run-ins associated with the regulating blade (< 10% widxirawn and rod bottomed) were operable during the short time the reactor operated with the regulating blade drive in a l
. degraded condition, because the regulating blade demonstrated it was capable of driving in without i
slipping. Any up power excursion would have resulted in the regulating blade driving in at the i
normal rate to compensate. This was further verified by reviewing the Channel 4 (Wide Ranpc
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Monitor) charts which indicated that the regulating blade was maintaining reactor power withm its j
normal range.
Even though the regulating blade drive in its degraded condition could not meet the strict I
definition of" operable" in T.S.1.12 definition, it was capable of meeting the basis for T.S. 3.2.a.
which is to "... ensure that the normal method of reactivity control is used during reactor operation." The regulating blade could drive in at normal speed, but had a slower positive reactivity insertion rate while driving out. When the regulating blade drive was suspected to have a problem the reactor was shut down to investigate.
t Subsequent investigation by the Electronics Shop indicated that the dowel pin for the gearbox input coupling was missing and had presumably broken shortly before the operator detected an audible difference in the regulating blade drive. The dowel pin and input shaft bearing were replaced, the mechanism tested, and compliance checks relevant to the regulating blade rod run-ins were completed. Review of Electronics Shop maintenance logs indicate no previous failure of this coupling pin which was part of original design.
De regulating blade and its associated rod run-in features are not part of the reactor 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 reactor i
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 regulating blade is automatically shifted to manual control to prevent it from trying to maintain power by shimming. The basis for rod run-ins associated with the regulating blade is to ensure termination of a transient which, in automatic operation, is causing a rapid insertion of the regulating blade.
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Leuer to Director of Nuclear Reactor Regulation January 26,1996 Page thsee While evaluating the safety significance of this event, we acognized that our Limiting Conditions for Operation for regulating blade operability in MURR Technical S xcifications do not include Action requirements (similar to Specification 3.0.2 of power plant Stanc ard Technical Specifications) that would allow implementation of an Action requirement (in this case, a prompt shutdown) within a specified time interval as constituting compliance with the specification.
Technically, the second the regulating blade drive is found to be in a degraded condition we are in non-compliance with Technical Specifications.
The University of Missouri Research Reactor management is developing a safety analysis to suppon 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, Develooment of Technical Soecifications for Reseamh Reactors and ANS-15.18, Administrative Control for Research Reactors where special re x>ns would not be required when a research reactor momentarily operates outside the limiting concitions for operation if prompt remedial action is taken (e.g., a reactor shutdown). This would alleviate the generation of a Licensee Event Repon for conditions which do not pose a safety concem for the mactor or the public.
CORRECTIVE ACTION The control room operator commenced a reactor shutdown to investigate a suspected problem with the agulating blade drive within minutes of the repon from an operator on the reactor bridge that the agulating blade drive sounded different.
A failure of this dowel pin in the gearbox coupling was the first in twenty-seven years of operation. The pin was aplaced by an equivalent dowel pin. No additional long-term cormetive action specific to this failm is planned. The regulatin; blade drive motor and gearbox are already on a six month preventive maintenance schedule which includes an inspection of all set screws and pms.
Sincerely, ENDORSEMENT:
p Reviewed and Approve, b
4.
Walt A. Meyer Jr.
J. Charles McKibben Reactor Manager Associate Director
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Attachment:
Figure 1 g
xc: Mr. Alexander Adams, Jr., USNRC Cl{RISTINEM.ERRANTE Regional Administrator, NRC, Region III Notary Public-Notary Seal STATE OF MISSOURI Dr. John P. McCormick, Interim Vice Provost for Researc 1 and Graduate School Dean, UMC Boonecounty Reactor Advisory Committee My conunission Expires: April M *,
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Reactor Safety Subcommittee
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