ML20046D603

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Application for Amend to License NPF-6,revising LCO for Unit 2 Incore Detection Sys Re TS 3.3.3.2
ML20046D603
Person / Time
Site: Arkansas Nuclear Entergy icon.png
Issue date: 09/24/1993
From: Yelverton J
ENTERGY OPERATIONS, INC.
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML20046D604 List:
References
2CAN099304, 2CAN99304, NUDOCS 9310010052
Download: ML20046D603 (11)
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Text

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IO fd L Tel 501-964-8888 Jerry W. Yelverton Vcc Presdent Operahn ANO September 24,1993 2CAN099304 U. S. Nuclear Regulatory Commission Document Control Desk Mail Station PI-137 Washington, DC 20555

Subject:

Arkansas Nuclear One - Unit 2 Docket No. 50-368 License No. NPF-6 Exigent Technical Specification Change Request Concerning the Arkansas Nuclear One - Unit 2 Incore Detection System Gentlemen:

Attached for your review and approval is a proposed Technical Specification (TS) change revising the Limiting Condition for Operation applying to the Arkansas Nuclear One - Unit 2 (ANO-2)incore detection system. This change modifies the operability requirements specified by TS 3.3.3.2 for the incore detection system by reducing the minimum number of required incore detectors and detector locations from the currently specified 75% to a proposed 50%,

for the remainder of Fuel Cycle 10.

The proposed change has been evaluated in accordance with 10CFR50.91(a)(1) using criteria in 10CFR50.92(c) and it has been determined that this change involves no significant hazards considerations. The bases for these determinations are included in the attached submittal.

c Entergy Operations requests that the effective date for this change be immediately upon NRC issuance of the amendment. We request that this proposed change be considered under exigent circumstances as described in 10CFR50.9)(a)(6) in that failure to act quickly could result in the shutdown of ANO-2.

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U.S.NRC September 24,1993 2CAN099304 Page 2 Very truly yours, f/* 2 d4 E fiWY/c*s Attachments To the best of my knowledge and belief, the statements contained in this submittal are true.

SUBSCRIBED AND SWORN TO before me, a Notary Public in and for Is44m County and the State of Arkarssas, this Mdt. day of la n8 m / 4 ,1993. #

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U. S. NRC September 24,1993 2CAN099304 Page 3 cc: Mr. James L. Milhoan Regional Administrator U. S. Nuclear Regulatory Commission Region IV 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011-8064 NRC Senior Resident Inspector Arkansas Nuclear One - ANO-1 & 2 Number 1, Nuclear Plant Road Russellville, AR 72801 Mr. Roby B. Bevan, Jr.

NIUt Project Manager Region IV/ANO-1 U. S. Nuclear Regulatory Commission NRR Mail Stop 13-H-3 One White Flint North 11555 Rockville Pike Rockville, MD 20852 Mr. Thomas W. Alexion NRR Project Manager, Region IV/ANO-2 U. S. Nuclear Regulatory Commission NRR Mail Stop 13-H-3 One White Flint North 11555 Rockville Pike Rockville, MD 20852 Ms. Greta Dieus Arkansas Department of Health Division of Radiation Control and Emergency Management 4815 West Markham Street Little Rock, AR 72205

4 ATTACHMENT IQ 2CAN099304 PROPOSED TECHNICAL SPECIFICATION AND RESPECTIVE SAFETY ANALYSES IN TIIE MATTER OF AMENDING LICENSE NO. NPF-6 ENTERUY OPERATIONS,INC.

ARKANSAS NUCLEAR ONE, UNIT TWO DOCKET NO. 50-368 1

Attachment to 2CAN099304 Page1 of6 DESCRIPTION OF PROPOSED CII ANGES A footnote has been added to Arkansas Nuclear One - Unit 2 (ANO-2) Technical Specifications (TS) 3.3.3.2.a and TS 3.3.3.2.b which states "For the remainder of fuel cycle 10, the incore detection system may be considered OPERABLE with < 75% and 2 50% of all incore detectors and incore detector locations provided the appropriate penalties (based on a full 1.0% increase in overall uncertainty on the CECOR Eqmeasurement) are applied to the COLSS and CPCs."

HACKGROUND The purpose ofincore detection instrumentation is to provide inputs for measuring the planar radial peaking factors; to perform validation of the Core Protection Calculator (CPC) power distribution; and to provide inputs to the Core Operating Limit Supervisory System (COLSS).

The incore detectors provide a signal representative of core neutron flux to COLSS. COLSS uses the incore detector signals to generate axial shape index, azimuthal power tilt, linear heat rate margin, and departure from nucleate boiling (DNB) margin. COLSS serves to monitor reactor core conditions accurately and provide indication and alarm functions to aid the operator. The incore detectors and COLSS are not required for plant safety since they do not initiate any direct safety-related function during anticipated operational occurrences or postulated accidents. COLSS is independent of the plant protection system. The CPCs operate independently of COLSS using excore neutron detectors to monitor plant safety parameters but are adjusted (calibrated) based upon data obtained from COLSS and the incore detector system. The CPCs provide input to the safety related plant protection system.

The incore detector system at Arkansas Nuclear One - Unit 2 (ANO-2) consists of 44 neutron detector string locations. Two of these locations are permanently occupied by reactor vessel level monitoring instrumentation and a third location is occupied by an alternative vendor incore detection string that was previously being evaluated and is not in use. Each detector string consists of 5 rhodium neutron detectors located at 10, 30, 50, 70, and 90% of core height. Technical Specification (TS) 3.3.3.2.a requires that 75% (or 165) of the 220 possible individual detector positions are operable and TS 3.3.3.2 b requires that 75% (or 33) of the 44 possible detector string locations are operable. An operable string location is defined by a note in T5 3 3 3.2 as having at least 3 operable detectors. TS 3.3.3.2.c requires a suflicient number of operable incore detectors to allow performance of at least 6 tilt estimates with at least one tilt estimate at each of three levels.

As of September 22,1993,174 (or ~79%) of the 220 detector positions and 35 (or -79.5%)

of the 44 string locations are operable (see the attached Figure 1). In addition,6 string locations have 2 inoperable detectors each; therefore, I more inoperable denetor on any of these 6 strings would cause that string to be inoperable. Any 10 detector failures would exceed the TS 3 3.3.2 a limit on the percentage of operable detectors Furthermore, should three selected detectors fail (i.e-, those occurring in strings that already have 2 failed detectors), the TS 3 3 3.2.b limit on the percentage of operable string locations would be exceeded. At this time, ANO-2 has the capability to perform 25 tilt estimates at 5 levels. It is

_ l

Attachment to 2CAN099304 Page 2 of 6 unlikely that the requirements of TS 3.3.3.2.c will be challenged by the incore detector failures during this fuel cycle.

With the portion of COLSS supplied by the incore detection system not operable (i.e., less than 165 detectors or less than 33 detector string locations), the following TS restrictions apply:

1. The linear heat rate (LHR) as determined by any operable CPC shall be reduced to less than 12.1 kWA1 within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> when above 20% rated thermal power (RTM per TS 3.2.1.

Previous studies have shown that a power reduction to approximately 80% to 85% RTP would be required to satisfy TS 3.2.1 when COLSS is not operable for the purposes of monitoring LHR.

2. TS 3.2.2 requires the Planar Radial Peaking Factors (Fxy) used by COLSS and the CPCs to be greater than or equal to the measured F xy etermined d by use of the incore detection system when greater than 20% RTP. TS 3.2 2 is required to be surveilled once per 31 days of accumulated operation in Mode 1 and prior to operation above 70% RTP following a fuel loading. If the incore detection system is not operable and the F xy cannot be measured within the specified surveillance interval, a plant shutdown to Hot Standby is required.
3. TS 3.2.3 requires the azimuthal power tilt to be calculated once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> when above 20% RTP and COLSS is not operable for the purpose of monitoring azimuthal power tilt.

This calculation can be perfonned in accordance with existing plant procedures without the incore detection system.

4. TS 3.2.4 requires the DNBR as determined by any operable CPC shall be increased within 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> to meet the requirements ofTS Figure 3.2-2 with a Control Element Assembly Calculator (CEAC) operable or TS Figure 3.2-3 with no CEAC operable when above 20% RTP. Previous studies have shown that a power reduction to approximately 80% to 85% RTP would be required to satisfy TS 3.2.4 when COLSS is not operable for the purposes of monitoring DNBR.
5. TS 3 2.7 requires the ASI as determined try any operable CPC shall be maintained at

-0.20 s ASI s to.20 when COLSS is not operable for the purposes of monitoring ASI when above 20% RTP. This requirement can be satisfied even without the incore detection system.

Each of the above actions result in additional plant maneuvering with the associated risks of reactor / plant transients and/or additional operator burden.

Entergy Operations is currently performing an evaluation of the detector failure indications, but no clear cause of the failure mechanism has been established as of this date. The detector fabrication, receipt, storage, and installation processes, as well as other industry experience are being examined as a part of this evaluation. Monitored reactor coolant system (RCS)

Attachment to 2CAN099304 Page 3 of 6 chemistry parameters have not indicated any discernible changes from the previous cycle of operation.

ANO-2 typically replaces all 42 detector strings every other refu sling outage. The currently installed detectors are in their second cycle of operation. The detec failure rate during the

% current cycle could not have been predicted because 1) only 6 detectors failed durMg the previous cycle of operation with the existing incore detectors (all 6 detectors have been returned to service during the current cycle and are currently operable), 2) the previous batch ofincores had only 13 detectors failed after 2 cycles ofoperation, and 3) the batch ofincores prior to that had only 10 detectors failed after 2 cycles of operation. Following plant stadup from the mid-cycle outage during May 1993,191 detectors were operable with 14 detectors having failed during the current cycle. At that time, gross failure trend data supported operability of the system until the end of the cycle. Since May 1993, an additional 17 detectors have failed. Failure times and locations ofindividual detectors have indicated no discernible pattern.

Monitoring of failure rates continued and at the end of July, it was determined that operability of the system to the end of the cycle was injeopardy. Therefore, actions were initiated to have a TS change in place by mid-December. The mid-December time frame was based upon the highest failure rate during the cycle of 4 per month and the minimum detector requirement of 165. However, the failures experienced during August and early September have shifled the primary concem to violating the minimum string locations requirement of 33 detector string locations. Most notably,6 incore detector failures have occurred since September 1.

These failures also resulted in the loss of 3 string locations.

Throughout the operating cycle, various indications of core power distribution are monitored.

These indications include symmetrically located incore detector signals, excore neutron detector response, and core exit thermocouple indications. This monitoring has not shown any abnormal trends or sudden changes in the core performance during the current fuel cycle.

DISCUSSION OF CII ANGE The following ANO-2 specific analyses have been performed in support of this change request. First, a new analysis of the overall CECOR power peaking factc; measurement uncertainties has been performed by Entergy Operations. This analysis models the present instrument failures and combines them with various sets of additional postulated failures, so as to model up to 50% of the detectors as failed. The additional element failures are randomly chosen, and each resultant failure pattern is analyzed for its impact an the CECOR measurement uncertainties. The analyzed patterns do not exceed the tilt estimate requirements of the TS or the proposed 50% detector location operability requirement ofTS.

From this analysis, the resultant overall uncenainty on CECOR measured Fxy ncreases i by less than 0.5% compared to the value (6.92%) given in the Entergy CECOR Topical MSS-NA3-P dated August 1,1984. Second, ABB/ Combustion Engineering (ABB/CE) has assessed the impact of up to 50% failed detector locations (including the increased CECOR Fxymeasurement uncertainty) upon the calculations performed by the monitoring system

Attachment to 2CAN099304 Page 4 of 6 (COLSS) and the protection system (CPC). See the attached ABB/CE letter A-93-026 dated September 21,1993, for more details. The impact is accounted for in revised values of certain addressable constants input to these systems For conservatism, a full 1.0% increase in overall uncertainty on the CECOR Fxy measurement was assumed for the generation of these new constants. The following tables show a comparison of the Cycle 10 COLSS and CPC constants and the revised constants.

Table 1 ANO-2 Cycle 10 and Revised COLSS Addressable Constants for a Reduced Number of Operable Incore Detectors Constant Description Value With 275% With <75% and Operable 250% Operable UNCERT Uncertainty Factor for LHR POL 1.09 1.11 EPOL2/4 Uncenainty Factor for DNBR POL 0.11 0.12 Table 2 ANO-2 Cycle 10 and Revised CPC Addressable Constants for a Reduced Number of Operable Incore Detectors Constant Description Value With 275% With <75% and Operable 250% Operable BERR1 Power Uncertainty Factor Used in 1.21 1.22 DNBR Calculation BERR3 Power Uncertainty Factor Used in 1.25 1.26 Local Power Density Calculation The values of the appropriate addressable constants (COLSS EPOL2, EPOL4, and UNCERT; and CPC BERR1 and BERR3) will be changed to the recommended value when operation with <75% and 250% of all incore detectors or detector locations is required. In addition, with the number ofincore detectors or detector locations <75% and 250%, ANO-2 will administratively check the planar rad.%I peaking factor on a once per 15 day interval, which is twice the required frequency of TS 4.2.2.2.b.

ABB/CE has previously analyzed similar situations. Explicit analyses of current and projected incore detector failure patterns were performed for Fort Calhoun Unit 1, Cycle 6; St. Lucie Unit 1, Cycle 4; Calven Cliffs Unit 1, Cycle 8; and Calvert Clifts, Unit 1, Cycle 11. In each case, the licensees requested and were granted TS changes which allowed operation with a reduced complement (below 75%) ofincore detectors A similar situation has also been analyzed for the current cycle of Waterford 3 On September 7,1993, Waterford 3 submitted

Attachment to

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2CAN099304 Page 5 of 6 a proposed TS change allowing operation with a reduced complement ofincore detectors.

This change is currently being reviewed by the NRC.

DETERMINATION OF NO SIGNIFICANT IIAZARDS CONSIDERATION An evaluation of the proposed change has been performed in accordance with 10CFR50.91(a)(1) regarding no significant hazards considerations using the stardards in 10CFR50.92(c). A discussion of these standards as they relate to this amendment request follows:

Criterion 1 - Does Not Involve a Significant Increase in the Probability or Consequences of an Accident Previously Evaluated.

The ANO-2 incore detection system is not required for plant safety since it does not initiate any direct safety-related function during anticipated operational occurrences or postulated acidents. The primary function of the incore detectors is to verify that the core power distribution is consistent with the assumptions used in the safety analysis. Although the requirements for the number and distribution of operable incore detectors is relaxed, sufficient measurements will be required to adequately verify compliance with power distribution Technical Specification (TS) limits. Penalty factors will be applied to the Core Operating Limits Supervisory System (COLSS) and Core Protection Calculators (CPCs) to account for the increased uncertainties of values measured by the incore detectors with the number of operable de:ectors below the current TS requirements. This will ensure that all current TS and fuel design limits are protected and the core power distribution assumptionr n the Safety Analysis Report (SAR) analyses remain valid. Therefore, this condition does not involve a significant increase in the probability or consequences of an accident previously evaluated.

Criterion 2 - Does Not Create the Possibility of a New or DifTerent Kind of Accident from any Previously Evaluated.

This condition does not represent a change in the configuration or operation of the plant. The current TS limits on power distribution as verified by the incore detectors will still be met.

Reducing the minimum number of operable incore detectors does not introduce any new failure modes. Therefore, this condition does not create the possibility of a new or different kind of accident from any accident previously evaluated.

Criterion 3 - Does Not involve a Significant Reduction in the Margin of Safety.

The ANO-2 incore detection system is not required for plant safety since it does not initiate any direct safety-related ftmetion during anticipated operational occurrences or postulated accidents. Suflicient measurements will be required to adequately verify compliance with power distribution TS limits. Use ofincreased measurement uncertainty factors are required commensurate with a reduction .n the minimum number ofincore detector locations. The increased measu ement uncertainty factors assure that power distribution calculations based on the incore detection system will continue to be conservative and that the existing Limiting

Attachment to l 2CAN099304 Page 6 of 6 Conditions for Operation specified for Axial Shape Index, Azimuthal Power Tilt, Radial Peaking Factors, Local Power Density, and Departure from Nucleate Boiling Ratio will not be exceeded. Therefore, this condition does not result in a reduction in the margin of safety.

Therefore, based upon the reasoning presented at,ove and the previous discussion of the amendment request, Entergy Operations has determined that the requested change does not involve a significant hazards consideration.

STATEM ENT OF EXIGENT CIRCUMSTANCES This situation could not have been avoided. The detector failure rate during the current cycle could not have been predicted because 1) only 6 detectors failed during the previous cycle of operation with the existing incore detectors (all 6 detectors have been returned to service during the current cycle and are currently operable), 2) the previous batch ofincores had only 13 detectors failed after 2 cycles of operation, and 3) the batch ofincores prior to that had only 10 detectors failed after 2 cycles of operation. The inmre detector failure mechanism has not been identified at this time, although an evaluation of L.e incore detector failure indications is still in progress. Due to a recent increase in the incore detector failure rate, failure to act quickly to reduce the incore detector requirements for ANO-2 could lead to a plant shutdown. Any 10 detector failures would exceed the TS 3.3.3.2.a limit on the percentage of operable detectors. Furthermore, should three selected detectors fail (i.e., those occurring in strings that already have 2 failed detectors), the TS 3.3.3.2.b limit on the percentage of operable string locations would be exceeded. It is impossible to predict when, or if, add:tional detector failures will occur. The time between failures has varied considerably, but based on the failure history, ANO believes that there is insufficient time to allow for the normal 30 day public comment period. Therefore, given the need to act quickly and the determination that this change does not represent a significant hazard, we request that this change request be considered under exigent circumstances, as described in 10CFR 50.91(a)(6).

Figure 1 Stcttus as of 9/22/93 Shaded Boxes = f ailed incore strings with number of f ailed incores Vhite Boxes with Larger Numbers = # of incore f oitures at that s tring otternative vendor detector (previously evolvated

@ = but never of ficiolly usedt

] = Radcol reoctor vessel level nonitors O = Incore strino A B C D E F G H J K L M N P R 1----- - -----

2----- ---

3------- -

4______ _.

0 0 0 0 0 0 5------

6---- $ y y $

e-- a X i X 9---- y 10 - - -- - 0 1 1 - - -- - -

12 - - - - - $ $ @ l 13 - - - - - - -

14 - - - - - - -

15 - - -- - - - - - -

Total number of Incere Dectector Positions 220 Detector Positions Occupied by Reoctor Vessel Level Monitoring 10 Instrunentation Alternative incore Detector No Lonc,er in Use 5 Incperable Detectors inmediately af ter M;d-Cycle Outoge 14 Inoperable Detectors Since Mid-Cycle Outage 17 Rencin;ng Operable Detector Positions 174 l

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