ML20206D003
| ML20206D003 | |
| Person / Time | |
|---|---|
| Site: | University of Missouri-Columbia |
| Issue date: | 03/31/1987 |
| From: | Alger D, Meyer W MISSOURI, UNIV. OF, COLUMBIA, MO |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8704130217 | |
| Download: ML20206D003 (3) | |
Text
,s Research Reactor Facility UNIVERSITY OF MISSOURI g,,,,,cn p,,,
Columbia. Missouri 65211 Telephone (314) 882-4211 Director Office e,i Nuclear Reactor Regulation U. S. Nuclear Regulatory Commission Wehington, D. C.
20555 ATTENTION:
Document Control Desk
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 one of three reactor coolant temperature scrams inoperable.
INTRODUCTION On March 5,1987, while performing a routine semiannual compliance check on the reactor coolant inlet temperature scran unit (980A), the meter relay trip unit did not actuate a reactor scram in response to a simulated high temperature signal.
Operation of the reactor with this meter relay trip unit inoperable deviates from MURR Technical Specification 3.3.a. since one of three reactor coolant temperature scrams required for reactor operation was not operable.
DESCRIPTION March 5,1987 was a regularly scheduled day for maintenance activities.
The reactor was in the shutdown mode, with all control rods fully inserted. A test signal simulating high temperature was fed into the 980A meter relay trip unit as part of the compliance check procedure (CP-88).
The meter indication was deter-mined to be in calibration, but the meter relay contacts, which initiate a scram signal, failed to open.
The compliance check for this meter relay trip unit had previously been performed satisfactorily on October 2,1986.
The failed meter relay trip unit was replaced with a spare unit and the com-pliance check was completed satisfactorily.
The malfunction of the failed unit was investigated by the Electronics Shop.
The capacitance value of an electrolytic capacitor in the meter relay control circuit had decreased to virtually zero and caused the failure.
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y Director March 31,1987 Page 2 ANALYSIS The reactor safety system was still capable of performing its safety function if a high reactor coolant temperature condition had occurred, since both the reactor high outlet coolant temperature scram (9018) and the other reactor inlet high coolant temperature scram (980B) were operable (Hazard Summary Analysis, Addendum 4, Appendix A.3.3.6).
The reactor coolant inlet high temperature instrument channel, 980A, was added to the reactor instrumentation in July 1973.
There was one previous failure of this specific channel on Saptember 22, 1980.
This channel is one of seven instrumentation channels that contain meter relay control units at MURR.
To additional failures of meter relay control units occurred on July 3,1978 (dif-ferential pressure across the reflector) and on April 20,1981 (reactor low pressure).
The meter relay trip units proved extremely reliable for the first twelve years of service (1966 to 1978) before the initial failure occurred on July 3, 1978. The three most recent meter relay unit failures have occurred within the last seven years and are all attributed to reduced capacitance of an electrolytic capacitor used in a common circuit application for each failed trip unit.
The capacitor failure appears to represent the only long term recurring failure mode of these units.
After the first capacitor related failure (October 20,1980), MURR instituted an increased surveillance requirement for the capacitors in each meter relay trip unit application.
The MURR committed to testing the capacitors in each unit at five year intervals. The MURR, in fact, instituted a more rigorous surveillance schedulo by which the capacitors were checked on an annual basis, by replacing the unit and checking the capacitor. The values of capacitance for each unit were recorded for trend analysis in an attempt to predict capacitor failure. This trend analysis was not successful in predicting the most recent capacitor failure.
The trend records for the failed trip unit showed measured values as follows:
Date Value February 22, 1982 447 pf July 27,1983 429 pf March 20, 1984 430 uf March 22, 1985 444 pf March 3, 1986 410 pf The design value of the capacitor is 330 pf 9 6V (with a typical tolerance of + 50%/-10% of the design value).
The common component associated with the last three meter relay trip unit failures has been a capacitor in the unit control circuitry.
These failures may be attributed to the age of these capacitors, since many electrolytic capacitors
.y Director March 31, 1987 Page 3 have a typical design life in operation of 20 years. Most of the trip units have the original capacitors installed in the control circuitry.
CORRECTIVE ACTION The immediate corrective action was to replace the failed meter relay trip unit with a spare trip unit that tested satisfactorily.
Another short term cor-rective action, regarding meter trip units generically, will involve the replace-ment of the capacitors in all spare trip units and the phased replacement of the capacitors in each of the operating' meter relay control units as their respective semiannual compliance tests are scheduled to be completed.
The capacitor failures in the meter relay trip units have not been character-ized by the long term decrease in m?asured capacitances that would lend itself to prediction by trend analysis.
Since this trend analysis has been ineffectual in predicting or preventing the capacite failures, MURR will discontinue the annual surveillance of the capacitance values, MURR will also discontinue evaluatug the capacitors in the meter relay trip units at five year intervals as committec; to in a previous Licensee Event Report (L.E.R. dated October 20,1980).
MURR expects to have these meter relay trip units replaced by more updated equipment in the near future.
In that regard, long term corrective action has already been initiated.
In mid 1985, MURR contracted Stone and Webster E-gineering Corporation to provide an evaluation and proposal for replacing the meter relay trip units, as part of an overall instrumentation and control upgrade.
S'ncerely, O
f Walt A. Meyer, J.
Acting Reactor Manager Endorsement:
Reviewed and Approved g,
Don M. Alger Associate Director mm y _
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U. S. Nuclear Regulatory Commission c/o Document Control Desk y o..y f ;
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Peactor Advisory Committee
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Reactor Safety Subcommittee d'