ML15218A033

McGuire, Units 1 and 2 - Revision 136 to MNS-TSB-B 3.1.3, Moderator Temperature Coefficient (Mtc)

ML15218A033
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Site:	McGuire, Mcguire
Issue date:	07/28/2015
From:	Duke Energy Carolinas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
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Download: ML15218A033 (7)

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MCGUIRE NUCLEAR STATIONSUBJECTMNS-TSB-B 3.1.3 RCS MODERATOR TEMPERATURE COEFFICIENT-REV 136Date: 7128/2015 Document Transmittal

1. TR-NUC-MC-000332 Purpose:

IssueReleased By:13225 Haaers Ferry RoadDocument Manaaement MGO2MHuntersville, NC 28078MNSDCRMc@duke-eneray.corn Page 1 of 11: DocumentID.

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PLEASE DISTRIBUE THIS TO TSB MANUAL HOLDERS REFERENCE TRANSM1T~AL TR-NUC-MC000287---FOR DISTRIBUTION ENTITIES MTCB 3.1.3BASESB 3.1 REACTIVITY CONTROL SYSTEMSB 3.1.3 Moderator Temperature Coefficient (MTC)BASESBACKGROUND According to GDC 11 (Ref. 1), the reactor core and its interaction with theReactor Coolant System (RCS) must be designed for inherently stablepower operation, even in the possible event of an accident.

In particular, the net reactivity feedback in the system must compensate for anyunintended reactivity increases.

The MTC relates a change in core reactivity to a change in reactorcoolant temperature (a positive MTC means that reactivity increases withincreasing moderator temperature; conversely, a negative MTC meansthat reactivity decreases with increasing moderator temperature).

Thereactor is designed to operate with a negative MTC over the largestpossible range of fuel cycle operation.

Therefore, a coolant temperature increase will cause a reactivity

decrease, so that the coolant temperature tends to return toward its initial value. Reactivity increases that cause acoolant temperature increase will thus be self limiting, and stable poweroperation will result.MTC values are predicted at selected burnups during the safetyevaluation analysis and are confirmed to be acceptable bymeasurements.

Both initial and reload cores are designed so that theMTC is less than zero when THERMAL POWER is at RTP. The actualvalue of the MTC is dependent on core characteristics, such as fuelloading and reactor coolant soluble boron concentration.

The core designmay require additional fixed distributed poisons to yield an MTC at or nearBOC within the range analyzed in the plant accident analysis.

For somedesigns, the burnable absorbers may burn out faster than the fueldepletes early in the cycle. This may cause the boron concentration toincrease with burnup early in the cycle and the most positive MTC not tooccur at BOC but somewhat later in the cycle. For these core designs,the predicted difference between the BOC MTC, and the most positiveMTC is used to adjust the BOC measured MTC to ensure that the MTCremains less than the limit during the entire cycle. The end of cycle (EOC)MTC is also limited by the requirements of the accident analysis.

Fuelcycles that are designed to achieve high burnups or that have changes toother characteristics are evaluated to ensure that the MTC does notexceed the EOC limit.The limitations on MTC are provided to ensure that the value of thiscoefficient remains within the limiting conditions assumed in the UFSARaccident and transient analyses.

McGuire Units 1 and 2 B3131Rvso o 3B3.1.3-1Revision No. 136 MTCB 3.1.3BASESBACKGROUND (continued)

If the LCO limits are not met, the unit response during transients may notbe as predicted.

The core could violate criteria that prohibit a return tocriticality, or the departure from nucleate boiling ratio criteria of theapproved correlation may be violated, which could lead to a loss of thefuel cladding integrity.

The SRs for measurement of the MTC at the beginning and near the endof the fuel cycle are adequate to confirm that the MTC remains within itslimits, since this coefficient changes slowly, due principally to changes inRCS boron concentration associated with fuel and burnable absorberdepletion.

APPLICABLE The acceptance criteria for the specified MTC are:SAFETY ANALYSESa. The MTC values must remain within the bounds of those used inthe accident analysis (Ref. 2); andb. The MTC must be such that inherently stable power operations result during normal operation and accidents, such as overheating and overcooling events.The UFSAR, Chapter 15 (Ref. 2), contains analyses of accidents thatresult in both overheating and overcooling of the reactor core. MTC isone of the controlling parameters for core reactivity in these accidents.

Both the most positive value and most negative value of the MTC areimportant to safety, and both values must be bounded.

Values used inthe analyses consider worst case conditions to ensure that the accidentresults are bounding (Ref. 2).The consequences of accidents that cause core overheating must beevaluated when the MTC is positive.

Such accidents include the rodwithdrawal transient from any power level (Ref. 3), turbine trip, and loss offorced reactor coolant flow. The consequences of accidents that causecore overcooling must be evaluated when the MTC is negative.

Suchaccidents include sudden feedwater flow increase and steam line break.In order to ensure a bounding accident

analysis, the MTC is assumed tobe its most limiting value for the analysis conditions appropriate to eachaccident.

The bounding value is determined by considering rodded andunrodded conditions, whether the reactor is at full or zero power, andwhether it is the BOC or EOC life. The most conservative combination appropriate to the accident is then used for the analysis (Ref. 2).MTC values are bounded in reload safety evaluations assuming steadystate conditions at BOC and EOC. An EOC measurement orMcGuire Units 1 and 2 B3132Rvso o 3B3.1.3-2Revision No. 136 MTCB83.1.3BASESAPPLICABLE SAFETY ANALYSES (continued) analytical check (Ref. 5) of the EOC MTC is conducted at conditions when the RCS boron concentration reaches approximately 300 ppm. Themeasured or calculated value may be extrapolated to project the EOCvalue, in order to confirm reload design predictions.

MTC satisfies Criterion 2 of 10 CFR 50.36 (Ref. 4). Even though it is notdirectly observed and controlled from the control room, MTC isconsidered an initial condition process variable because of itsdependence on boron concentration.

LCO LCO 3.1.3 requires the MTC to be within specified limits of the COLR toensure that the core operates within the assumptions of the accidentanalysis.

During the reload core safety evaluation, the MTC is analyzedto determine that its values remain within the bounds of the originalaccident analysis during operation.

Assumptions made in safety analyses require that the MTC be lesspositive than a given upper bound and more positive than a given lowerbound. The MTC is most positive at or near BOC; this upper bound mustnot be exceeded.

This maximum upper limit occurs at or near BOC, allrods out (ARO), hot zero power conditions.

For some core designs, theburnable absorbers may burnout faster than the fuel depletes early in thecycle. This may cause the boron concentration to increase with burnupearly in the cycle and the most positive MTC not occur at BOC, butsomewhat later in the cycle. For these core designs, the predicted distance between the BOC MTC, and the most positive MTC is used toadjust the BOC measured MTC to ensure that the MTC remains less thanthe limit during the entire cycle. At EOC the MTC takes on its mostnegative value, when the lower bound becomes important.

This LCOexists to ensure that both the upper and lower bounds are not exceeded.

During operation, the condition of the upper LCO limit at BOC is ensuredthrough measurement.

The lower LCO limit at EOC is ensured eitheranalytically or through measurement.

The Surveillance checks at BOOand EOC on MTC provide confirmation that the MTC is behaving asanticipated so that the acceptance criteria are met.The LCO establishes a maximum positive value that cannot be exceeded.

The BOO positive limit and the EOC negative limit are established in theCOLR to allow specifying limits for each particular cycle. This permits theunit to take advantage of improved fuel management and changes in unitoperating schedule.

McGuire Units 1 and 2 B3133Rvso o 3B 3.1.3-3Revision No. 136 MTCB 3.1.3BASESAPPLICABILITY Technical Specifications place both LCO and SR values on MTC, basedon the safety analysis assumptions described above.In MODE 1, the limits on MTC must be maintained to ensure that anyaccident initiated from THERMAL POWER operation will not violate thedesign assumptions of the accident analysis.

In MODE 2 with the reactorcritical, the upper limit must also be maintained to ensure that startup andsubcritical accidents (such as the uncontrolled control rod assembly orgroup withdrawal) will not violate the assumptions of the accidentanalysis.

The lower MTC limit must be maintained in MODES 2 and 3, inaddition to MODE 1, to ensure that cooldown accidents will not violate theassumptions of the accident analysis.

In MODES 4, 5, and 6, this LCO isnot applicable, since no Design Basis Accidents using the MTC as ananalysis assumption are limiting when initiated from these MODES.ACTIONS A.1If the BOO MTC limit is violated, administrative withdrawal limits forcontrol banks must be established to maintain the MTC within its limits.The MTC becomes more negative with control bank insertion anddecreased boron concentration.

A Completion Time of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> providesenough time for evaluating the MTC measurement and computing therequired bank withdrawal limits.Using physics calculations, the time in cycle life at which the calculated MTC will meet the LCO requirement can be determined.

At this point incore life Condition A no longer exists. The unit is no longer in theRequired Action, so the administrative withdrawal limits are no longer ineffect.B.._If the required administrative withdrawal limits at BOC are not established within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />, the unit must be brought to MODE 2 with keff < 1.0 toprevent operation with an MTC that is more positive than that assumed insafety analyses.

The allowed Completion Time of 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> is reasonable, based onoperating experience, for reaching the required MODE from full powerconditions in an orderly manner and without challenging plant systems.C...1Exceeding the EOC MTC limit means that the safety analysisassumptions for the EOC accidents that use a bounding negative MTCvalue may be invalid.

If the EOC MTC limit is exceeded, the plant mustMcGuire Units 1 and 2 B3134Rvso o 3B 3.1.3-4Revision No. 136 MTCB 3.1.3BASESACTIONS (continued) be brought to a MODE or condition in which the LCO requirements arenot applicable.

To achieve this status, the unit must be brought to at leastMODE 4 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.The allowed Completion Time is reasonable, based on operating experience, for reaching the required MODE from full power conditions inan orderly manner and without challenging plant systems.SURVEILLANCE SR 3.1.3.1REQUIREMENTS This SR requires measurement of the MTC at BOC prior to enteringMODE 1 in order to demonstrate compliance with the positive MTC LCO.Meeting the limit prior to entering MODE 1 ensures that the limit will alsobe met at higher power levels.The BOC MTC value for ARO will be inferred from isothermal temperature coefficient measurements obtained during the physics testsafter refueling.

If appropriate, the ARO value is adjusted to account forany increase in the MTC early in the cycle. The ARO value can then bedirectly compared to the BOC MTC limit of the LCO. If required, measurement results and predicted design values can be used toestablish administrative withdrawal limits for control banks.SR 3.1.3.2In similar fashion, the LCO demands that the MTC be less negative thanthe specified value for EOC full power conditions.

This measurement may be performed at any THERMAL POWER, but its results must beextrapolated to the conditions of RTP and all banks withdrawn in order tomake a proper comparison with the LCO value. Because the RTP MTCvalue will gradually become more negative with further core depletion andboron concentration reduction, a 300 ppm SR value of MTC shouldnecessarily be less negative than the EOC LCO limit. The 300 ppm SRvalue is sufficiently less negative than the EOC LCO limit value to ensurethat the LCO limit will be met when the 300 ppm Surveillance criterion ismet.SR 3.1.3.2 is modified by three Notes that include the following requirements:

a. The SR is not required to be performed until 7 effective full powerdays (EFPD) after reaching the equivalent of an equilibrium RTP allrods out (ARO) boron concentration of 300 ppm for the reasonsdiscussed above. Measurement of the MTC may be suspended for the current operating cycle provided the benchmark criteriaMcGuire Units 1 and 2 B3135Rvso o 3B 3.1.3-5Revision No. 136 MTC8 3.1.3BASESSURVEILLANCE REQUIREMENTS (continued) specified in DPC-NE-1 007-PA and the Revised MTC Prediction specified in the COLR are satisfied.

b. If the 300 ppm Surveillance limit is exceeded, it is possible that theEOC limit on MTC could be reached before the planned EOC.Because the MTC changes slowly with core depletion, theFrequency of 14 EFPD is sufficient to avoid exceeding the EOClimit.c. The Surveillance limit for RTP boron concentration of 60 ppm isconservative.

If the measured MTC at 60 ppm is more positivethan the 60 ppm Surveillance limit, the EOC limit will not beexceeded because of the gradual manner in which MTC changeswith core burnup.REFERENCES

1. 10 CFR 50, Appendix A, GDC 11.2. UFSAR, Chapter 15.3. UFSAR, Section 15.4.4. 10 CFR 50.36, Technical Specifications, (c)(2)(ii).

5. DPC-NE-1 007-PA, "Conditional Exemption of the EOC MTCMeasurement Methodology.

McGuire Units 1 and 2831-6RvsoNo16 B3.1.3-6Revision No. 136