ML19308B116

Provides Addl Info Re Unit 1 Cycle 4 Core Tilt Condition. Table Encl Depicting History of Core Tilt Indications Observed as Function of Core Burnup & Reactor Power

ML19308B116
Person / Time
Site:	Oconee
Issue date:	12/14/1977
From:	Parker W DUKE POWER CO.
To:	Case E Office of Nuclear Reactor Regulation
References
NUDOCS 7912130733
Download: ML19308B116 (6)

Text

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,2 7e) *'FCmh - *;, *"""**" 1 NRC OISTRIBUTION Pen PART 50 DOCKET MATERI AL CATE CP CCCUMENT FROM: 12/14/77 TO.' Duke Power Company Charlotte, North Carolina cAra macaivao Mr. Edson G. Case William O. Parker, Jr. 12/19/77 mPuTPORM NUMSER CP CCPiES RECEIVED PnCP ds- -ra ONcTons:so ONE M iciNAt. MLAss Piso CccPv ENCLOSu ME CSCNPTICM Consists of additional info. concerning investigations into the cause af the Cycle 4 tilt..w/att table which ptm _ is a history of the core tilt indications observed as a function of core burnup and reactor power...

PLANT NAME: Oconee Unit No. 1

. RJL 12/20/77 (4-P)+(1-P) ( &dG.C FOR ACTION /INFORMATION SAI'ETY

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4QQ.n, SgW.J A RE: Oconee Unit 1  :,y M Docket No. 50-269 4, N

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Dear Sir:

In letters dated October 4 and November 9, 1977, a core tilt condition was described which had been observed during the startup of the Oconee ,

Cycle 4 core. Technical Specification amendments were issued by the Commission on November 23, 1977 to permit the full power operation of the core in an unrodded mode with a conservative limit on core power tilt. The purpose of this letter is to provide additional information concerning the investigations into the cause of the Cycle 4 tilt. The attached Table provides a history of the core tilt indications observed as a function of core burnup and reactor power.

A more indepth analysis of the Oconee 1, Cycle 3 core has been performed utilizing unprocessed incore detector signals. All failed detector locations were eliminated from this calculation to reduce any assymetry caused by substitution routines. This analysis indicated that a tilt of approximately 1.5% occurred from 150 EFPD throughout the duration of the cycle. This tilt was well within the Technical Specification limits and did not constitute any safety or operational problems during Cycle 3; hence, an indepth analysis was considered unnecessary at that time. Sub-sequent calculations have revealed that a high probability exists that the higher Cycle 4 tilt has resulted from the Cycle 3 tilt being magni-fled by the core shuffle scheme. The tilt in Cycle 3 would cause the fuel assemblies in the northwest quadrant of the core to receive a higher burnup than those in the southeast quadrant. During the refueling, fuel assemblies in the northwest and southeast quadrants were moved across the core into a region of higher importance (i.e., towards the center of the core). This shuffle pattern placed more reactive fuel assemblies (due to the burnup effect) in the northwest quadrant than in the southeast quadrant, thereby contributing to the quadrant tilt measured at the beginning of Cycle 4.

The following discussion is provided to explain the manner in which the burnup distribution is normally measured by the incore detector system.

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q Mr. Edron G. Cica, Acting Director Page Two December 14, 1977 The segment and assembly burnups are calculated by the On-Line Computer using the following procedure:

(a) The raw signals generated by the rodium detectors in the flux environment are corrected to a value for an undepleted detector of nominal sensitivity.

(b) The corrected signals are converted to segment powers (1/7 of the axial height of any assembly) using a series of pre-calculated factors which convert the signals to power under a variety of core conditions. The calculated powers are then normalized to the core heat balance.

(c) The burnup for a segment is calculated by multiplying the calculated power by the time increment since the last calculations and adding this in a running sum.

B(1) = B(1-1)+P*At where:

B(1) = Burnup at time 1, MWD /MTU B(1-1) = Burnup at time 1-1, MWD /MTU P = Power generated in the segment, MW At = Delta time between 1-1 and 1, days (normally about 10 minutes)

Since the core tilt during Cycle 3 was exceedingly small, it was not considered in the design of Cycle 4. The burnup distributions for all cycles were computed on a quarter-core basis assuming symmetry about the core major axes. These computations were performed using B&W's version of the PDQ 07 Code in two energy groups with a pin by pin repre-sentation of the flux solution. The fuel depletion and isotopic changes are calculated using a system of equations patterned after those employed in the CINDER Program described in WAPD-TM-334 (August, 1962).

An operating limit more conservative than the Technical Specification limit is established on the allowable quadrant power tilt in order to account for the uncertainty in the tilt measurement. The accuracy of the incore detector system for tilt measurements may be described by

't defining the following terms:

Uncertainty. This terms refers to the potential for error in the measured value due to instrument error, signal noise, and calculational uncertainty.

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i Mr. Edcen G. C 23, Acting Dirc.ctor Pigs Thrsa December 14, 1977 Observabilitv. This term refers to the potential for err)r in the measured value due to the limitations of measuring core power with an instrument system having a limited number of sensors distributed geome-trically throughout the core.

The uncertainty is equal to 0.39 (percent quadrant power tilt). The observability is equal to 0.106 times the quadrant power tilt in percent.

The total measurement uncertainty is thus equal to the sum of these terms.

These terms are applied to the quadrant power tilt Technical Specification envelope to obtain an operating envelope within the Technical Specification envelope.

In order to alleviate the effects of quadrant tilt during the operation of Oconee 1, Cycle 4 and future cycles the core is being operated in a conservative manner in an unrodded mode. Additionally, the fuel shuffle procedure is being revised for future reloads to minimize the effects of a residual tilt from the previous cycle.

In order to gain a better understanding of this tilt phenomenon, additional investigations are in progress as follows:

(a) The computer study will be completed in which Oconee 1, Cycle 3 is depleted with a tilt. This will be used as input for Cycle 4, and the calculated ,.ero power ejected rod worths and power tilts will be compared in detail with measured data. l I

(b) Investigations are currently in progress into the Cycle 3 plant 1 operations that could potentially affect the Cycle 3 quadrant )

power tilt. Some steps in this investigation are:

(1) examine detailed incore detector data to determine axial level dependence of the tilt, tilt surface contour, and consistency of tilt behavior.

, (2) Investigate plant operation during Cycle 3 to determine any l anomalous behavior, other than tilt, which may have introduced a tilt in the core.

(3) Investigate Cycle 3 thermal hydraulic plant parameters in a 1 similar manner. l (4) Examine data from ether B&W plants to determine if similar behavier has occurred elsewhere.

(5) Perform computer studies of "vele 3 to quantify.the effect of various parameters on core tilt.

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Mr. Edson G. Casa, Acting Director

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Page Four December 14, 1977 i

Further status reports will be submitted to the Commission as information becomes available.

Very truly yours, i

%DL.m O. f William O. Parker, Jr.

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• INCORE DETECTOR DATA FOR OCONEE 1 CYCLE 4 TILT i

Tilt (f.)

EFPD % FP g g Z YZ, ZW 0.0 15 5.66 1.26 -6.94 0.01 0.3 40 3.66 0.73 -3.14 0.76 1.0 40 3.81 0.84 -5.36 0.71 1.6 31 3.33 1.39 -4.99 0.28 3.5 40 2.88 0.69 -4.42 0.84 5.8 75 2.88 0.82 -3.86 0.16 6.4 75 2.87 0.84 -3.82 0.12 17.9 74 2.65 0.61 -3.12 0.14 28.4 99 2.30 0.46 -2.52 -0.23 l

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