ML20205H896
| ML20205H896 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Columbia |
| Issue date: | 10/19/1988 |
| From: | Brugger R, Mckibben J MISSOURI, UNIV. OF, COLUMBIA, MO |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8810310208 | |
| Download: ML20205H896 (3) | |
Text
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Roscarch Reactor Facility UNIVERSITY OF MISSOURI October 19, 1988 ceda.O*sUcslil Tef ephone (314) 882-4211 Director of Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Mail Station P1-137 Washington, D. C.
20555
REFERENCE:
Docket 50-186, University of Missouri Research Reactor License R-103
SUBJECT:
Report as required by Technical Specification 6.1.h(2) concerning reactor operation with the regulating blade inoperable.
DESCRIPTION:
At 1145 on September 21, 1988 with the reactor operating at full power in the automatic mode, a channel 4 high power rod run-in occurred.
The duty operator reset the rod run-in and recovered to full power when he determined that the regulating blade would not respond in either the automatic or manual mode.
The reactor was then shutdown by manual rod run-in to repair the regulating blade drive system.
Preceding the failure, the reactor had been in continuous full power operation with the regulating blade properly maintaining power in automatic control for a period of 129 hours0.00149 days <br />0.0358 hours <br />2.132936e-4 weeks <br />4.90845e-5 months <br /> 45 minutes.
The operability prior to the the failure was veri-fied by review of Operations logs and the wide range monitor chart indication.
The regulating blade indication was c5 served changing by the duty operator shortly before the channel 4 high power rod run-in.
The reactor was shutdown to be in accordance with Technical Specification 3.2.a which states "all control blades, including the regulating blade, shall be operable during reactor operation."
ANALYSIS:
The regulating blade controls reactor power at a desired level set by the duty operator (normally 10 MW).
The blade is constructed of stainless steel and drives at 40 inches per minute.
The regulating blade drive mechanism lead screw translates rotary motion of the drive motor and gear box into linear motion of the regulating blade.
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Director of Nuclear Reactor Regulation October 19, 1988 Page 2 In the automatic mode, the regulating blade controls power by balancing the signal received from the wide range monitor (channel 4) with a potentionater set to the desired power level to be maintained. Any dif ference between wide range monitor indication and potentiometer setting creates a drive signal to the regulating blade drive mechanism.
The blade frequently shims to make minor adjustments to maintain power at the desired level in automatic control.
The time interval between shims varies from tenths of a minute to several minutes.
At 1145, September 21, 1988, with the reactor operating in automatic control at 10 KW, a channel 4 high power rod run-in occurred when the reactor power reached approximately 10.9 Kd.
The duty operator terminated the rod run-in af ter finding no obvious cause, and recovered power to 10 MW.
At this time (1200), the duty operator found the regulating blade would not respond in either the automatic or manual mode.
The reactor was then shutdown to investigate and repair the regulating blade problem, it may be concluded that the reactor operated for approximately 15 minutes with the regulating blade inoperable.
The cooling tower fans had been shifted to fast speed shortly before the chan-nel 4 rod run-in.
The subsequent increase in reactor power was caused by the posi-tive reactivity inserted due to the slowly decreasing primary temperature. With the l
regulating blade not responding in automatic to turn the power increase, the trip set point for the channel 4 rod run-in was reached.
The regulating blade is not part of the reactor safety system as defined in Technical Specification 1.18.
When a reactor scram or rod run in occurs the regu.
lating blade is automatically shif ted to manual control to avoid it from attempting to nu1ntain power by shimming.
Electronics Technicians investigating the regulating blade drive problem were able to isolate the problem to the gear box assembly, since the drive motor responded to the manual switch with no resulting movement of the regulating blade.
The gear bvx was disassembled and one internal drive gear was found to have ridden up its shaf t due to a loose set screw, so that it no longer engaged its driven (slave) gear. A spare gear box was installed on the regulating blade mechanism and a test of the regulating rod and its associated rod run-ins was performed.
CORRECTIVE ACTION:
l The reactor was shutdown when the duty operator realized that the regulating blade was not responding in either the manual or the automatic node. The regulating blade drive was repaired and tested.
Director of Nuclear Reactor Regulation October 19, 1988 Page 3 To reduce the possibility of a similar gear box problem recurring, the Elec-tronics Technicians have added a semiannual preventive maintenance regt'irement for the regulating blade drive mechanism that includes a disassembly and visual inspec.
tion of the gears and gear set screws.
Sincerely.
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/ J. C. McKibben Reactor Manager
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Endorsement:
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R. M. Brugger Director xc: Reactor Advisory Comittee
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