ML19317F410

RO 270/77-16:on 761227,reactor Operator Exceeded Error Adjusted Power Imbalance Limit.Caused by Software Lockup on Computer Responsible for Calculating Imbalance.Reactor Power Reduced & Computer Program Restored to Operable Status

ML19317F410
Person / Time
Site:	Oconee
Issue date:	01/26/1977
From:	DUKE POWER CO.
To:
Shared Package
ML19317F405	List: ML19317F404 ML19317F410
References
RO-270-77-16, NUDOCS 8001140658
Download: ML19317F410 (2)

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DUKE POWER COMPANY COCONEE UNIT 2 o

Report No.: RO-270/76-16 Report Date: January 26, 1977

' Occurrence'Date_: December 27, 1976 Facility: Oconee Unit 2, Seneca, South Carolina Reactor operation exceeded the error adjusted Identification of Occurrence:

power imbalance limit Conditions Prior to Occurrence: Unit at 100 percent full power Description of Occurrence:

On December 27, 1976'at approximately 0600 hours0.00694 days <br />0.167 hours <br />9.920635e-4 weeks <br />2.283e-4 months <br />, the computer program which The reactor calculates the reactor power imbalance was discovered inoperable.

power imbalance is measured both by the incore and out-of-core detector systems Because of and is normally calculated and displayed by the plant computer.

the better accuracy of the incore system, the incore system is preferred When for the computer imbalance measurement when the plant computer is available.

imbalance program became inoperable, it became necessary to perform a hand calculation of the reactor imbalance from the out-of-core detector indication.

Although the imbalance was within the error adjusted incore system limits at the time the computer program became inoperabic, subsequent hand calculations utilizing the out-of-core detector readings indicated that the imbalance was outside the error-adjusted out-of-core system limits, mainly due to larger error-adjustment factors for the out-of-core system. Action was taken to reduce the imbalance to within the error-adjusted out-of-core system limits; however, as required by Technical Specification 3.5.2.6, a power reduction was initiated approximately two hours after the loss of the incore imbalance indication.

The imbalance was returned within the out-of-core error-adjusted imbalance limits at 0812 hours0.0094 days <br />0.226 hours <br />0.00134 weeks <br />3.08966e-4 months <br /> on December 27, 1976.

Apparent Cause of Occurrence:

This occurrence .resulted from a sof tware lockup on the Unit 2 computer which caused the loss of the computer _ program for calculating reactor power imbalance.

Analysis of Occurrence:

Technical Specification Figure 3.5.2.3B1 provides a limit for reactor power imbalance versus reactor power level to prevent the maximum linear heat rate from exceeding ~ the values used in the LOCA analysis. The existence of statis-tical and _instruaent errors associated with the measurement of reactor power imbalance makes it necessary to operate within an error-adjusted power imbalance envelope'in order to assure that the Technical Specification limit is not actually being exceeded. In this incident, the reactor was operated for two hours outside the error-adjusted out-of-core power imbalance limits, and,

'therefore, it is possible that the limits specified in Technical Spec fica-

. tion Figure 3.5.~2.3B1 were exceeded.

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. Ilowever, the Bases for . Technical Specification _'t.5.2 state that all of the power distribution parameters (quadrant tilt, rod position limits and im-balance) must be at their limits while simultaneously all other engineering and unc'ertainty factors are also at their limits to produce t'ae maximum

. allowable heat rate. In this incident, the maximum- occurrirg quadrant tilt was 0.3% as compared with the operating limit of 3.4% permi'.ted by Technical Specification 3.5.2.4a. Also,' the control rods were' maintained well within

.the normal-maneuvering band. For the.= reasons, it is concluded that the health and safety of the public was not affected by this incident.

Corrective Action:

-Reactor power was reduced in a timely manner to a level where the measured imbalance was within sIecified limits. The computer imbalance calculational program was restored to.an operable status and was verified to be functioning correctly.

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