ML20043A340

LER 90-013-00:on 900413,determined That Axial Shape Index Channels Out of Spec & Inoperable.Caused by Inadequate Understanding of Design Basis for Excore/Incore Comparison. Design Basis for Excore/Incore improved.W/900514 Ltr

ML20043A340
Person / Time
Site:	Calvert Cliffs
Issue date:	05/14/1990
From:	Denton R, Milbradt M BALTIMORE GAS & ELECTRIC CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-013, LER-90-13, NUDOCS 9005210221
Download: ML20043A340 (6)

Text

A B Ai.TIMORE OAS AND ELECTRIC l l

CHARLES CENTER e P.O. BOX 1475

• BALTIMORE, MARYLAND 21203 1475 R. E. DENTON us,.Au n CALVERI CUf f S fdUCLI AR POWER PLAs41 DEPARtWENT May 14, 1990 I

U. S. Nucisar Regulatory Commission Docket No. 50 317 Document Control Desk License No. DPR 53 Washington, D. C. 20555 i

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Dear Sirs:

The attached LER 90 13, Revision 0, is being sent to you as required under 10 CFR 50.73.

Should you have any questi,ons regarding this' report, we would be pleased to discuss them with you.

Very truly yours, heR,E.Denton MDM/1r cc: Thomas T. Martin Director, Office of Management Information and Program Control Messrs: G. C. Creel C. H. Cruse J. R. Lemons L. B. Russell R. P. Heibel ,

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F ACILITv NAME 14 DOCat? NUhESEM (2) FACE i3i Cnivert Cliffs, Unit 1 o l6 l 0 l0 l 0 l3 l 1l7 1 loFl015 l Axial Shape Index Channels Out of Specification Caused by inadequate Understanding of Design Bases Results in Entering T.S. L.C.O. 3.0.3 '

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on April 13, 1990, at approximately 2000, Calvert Cliffs Unit 1 entered Technical Specification (TS) Limiting Condition For Operation (LCO) 3.0.3. LCO 3.0.3 was

entered af ter three of the four Reactor Protective System (RPS) Axial Shape Index l

(ASI) channels were determined to be inoperable. At the time the Unit was in MODE 1 with Reactor Power at about 23%.

The ASI channels were declared inoperabic after comparing the Excore Nuclear Instrumentation ASI to the Incore Nuclear Instrumentation ASI. Due to the changing neutron flux distribution in the core the difference between the Excore ASI and Incore ASI exceedec' the allowable tolerance.

The root cause of this event has been determined to be an inadequate understanding of the design basis for the Excore/Incore ASI comparison. This lack of understanding led to the establishment of overly restrictive tolerances.

Correct. m actions included increasing the allowable difference between Excore ASI and Incore ASI. Additionally the design basis for the Excore/Incore ASI comparison will be improved, CC f e,m 366 (6891

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l I. DESCRIPTION OF EVENT '

on April 13,- 1990 at approximately 2000, Calvert Cliffs Unit i entered l Technical Specification (TS) Limiting Condition For Operation (LCO) 3.0.3.  !

i LCO 3.0.3 was entered after three of the four Reactor Protective System (RPS)

Axial Shape Index (ASI) channels were determined to be inoperable. At the j time the Unit was in MODE 1 with Reactor Power at about 23%. l i

As required by TS Surveillance 4.3.1.1.1., a licensed utility operator )

performed a CHANNEL CilECK between the four ASI channels. The four channels l l receive input from the Power Range Safety Excore Nuclear Instrumentation (NIs)'  ;

and provide indication on Control Room Panel 1C04 Included in the surveillance is a comparison between the ASI measured by the NIs and the ASI measured by the Incore Nuclear Instrumentation (INCA).

The differences between the values for ASI, shown on channels A, B, and D and ]

the INCA ASI value, were discovered to be outside the allowed surveillance '

tolerances. The allowed tolerances and required actions if the tolerances are exceeded, are listed on the Control Room Operator's (CRO) Log Sheet, i Immediately after discovering the problem, the Shift Supervisor on duty 1 1

declared the three channels inoperable, in accordance with the guidelines  ?

l listed on the Log Sheet, and . reviewed the ACTION requirements for TS 3.3.1.1, With three out of the four ASI channels inoperable, the ACTION requirement, in  !

TS 3.3.1.1 did not apply, thus the Shift Supervisor entered LCO 3.0.3.

The Shift Supervisor also contacted the Nuclear Engineering Unit (NEU) for  ;

additional guidance. NEU confirmed. the INCA system was functioning correctly l and suggested the problem appeared to be in the Excore NIs. The . Shift j Supervisor then issued a Priority 1 Maintenance Request to have the Instrument I Maintenance shop calibrate the three ASI channels.

At 2250, LCO 3.0.3 was exited after Chennel A was corrected. The Excore ASI indication on Channels B and D came back into specification without correction j from the technician.

I II. CAUSE OF EVENT As a . result of an event in July of 1988 involving the Excore Nuclear Instrumentation and the RPS Delta T Channels (LER 88 007,-Docket No. 50-317), i more detailed criteria were added to the CRO's Log Sheet for the comparison of the four ASI. channels to the INCA system. New tolerances were added for rodded operations and the limits for unrodded operation were redefined.

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flat ## asag apsee e sepumust asse esameerer MC Femi 354 W (IM Specifically, a limiting tolerance which defined inoperability for a channel, was- added. This tolerance limit was based on a review of past operating '

history.

The root cause of this event was an inadequate understanding of the design i

basis for the Evcore NIs/ INCA ASI comparison. Specifically, there was an ,

inadequate understanding of when, during power escalations, it was important '

to start doing Excore NIs/ INCA ASI comparisons. Due to this lack of understanding, overly restrictive tolerances were incorrectly established af ter the July,19'88 event.

The event being reported in this LER occurred during a recent startup of Unit

1. This was the first startup since 1988 in which the'new ASI limits were in place when the CHANNEL CHECK was performed.

After an outage the NI ASI channels are calibrated to the values of INCA when ,

the plant is at a power level between 85 90%. Contributing to this event were the requirements to perform the NI/ INCA ASI comparison (CHANNEL CHECK) at a lower power level and during trensients, prior to calibrating the NI ASI Channels.

III. ANALYSIS OF EVENT l

i Throughout these events the reactor was in a safe . condition. Although the l Excore NI ASI and the INCA ASI differed by more than the allowed amount, the actual core ASI (ASI measured by INCA) was considerably below the allowed ASI limit of the Limited Safety System Setting in TS 2.2.1. The TS limits for the-Excore ASI versus Linear Heat Rate (LHR) Power, and Excore ASI versus Departure From Nucleate Boiling Ratio (DNBR) Power were never approached. The limits for the IllR and DNBR don' t apply until 90% power and per TS Table l 4,3 1, Reactor Protective Instrumentation Surveillance Requirements, THERHAL POWER is restricted to less than or equal to 90% power until the Excore NIs are calibrated with the Incore Detectors.

l Additionally, there are no accident scenarios in Chapter 14 of the Calvert Cliffs Final Safety Analysis Review (PSAR) for which ASI is the primary RPS trip. ASI only serves as a backup trip.

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0l0 0 l4 OF 0 l5 textan . w m er as.4w on IV. CORRECTIVE ACTIONS Immediate Upon discovering ASI Channels A, B, and D were outside their allowable '

tolerances, the Shift Supervisor declared all three channels inoperable, in accordance with guidance contained in the CRO's Log Sheet. After declaring the channels inoperable, LCO 3.0.3 was entered.

Additionally, the Instrument Maintenance Shop was contacted and Channel A was calibrated to within the allowed specifications. As the flux in the core redistributed the Excore NI ASI Indication on Channels B and D came back into specification without corrective action by the technician, i

ShortaTerm To allow for a greater flexibility at lower power levels, the allowed toler.ance between the Excore NI ASI and the Incore Detection System . ASI was ,

increas.ed by .10 ASI units for power levels below 40%. To compensato for this .

relaxation of the tolerance, the margin between the RPS ASI trip setpoint and the TS ASI limit was also increased by .10 ASI units. These changes were recommended to the Plant Manager by the Plant Operations and Safety Review Committee (POSRC) and approved on April 14, 1990.

On April 18, 1990, POSRC recommended ' approval of a change to the allowed tolerance for powcr levels equal to or above 40%. The tolerance and the margin between the RPS ASI trip setpoint and the TS limit were both increased by .020 ASI units.

Lonc-Term To determine the appropriate allowable tolerances between the Excore NI ASI channels and INCA ASI the design basis will be modified to address the purpose of the ASI comparison, Included in this evaluation will be a determination of the minimum power level at which the comparison will be performed. The results of this evaluation will incorporated into the plant Operating Instructions and CR0 Log Sheet.

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V. ADDITIONAL INFORMATION There have been eight' previous events at Calvert Cliffs involving the Axial Shape Index: '

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1. Docket No. 50 317, LER 75-22 i

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