ML20045D728

Nonproprietary WCAP-13650, Safety Evaluation Supporting More Negative Eol Moderator Temp Coefficient Tech Spec for Sequoyah Nuclear Plants.

ML20045D728
Person / Time
Site:	Sequoyah
Issue date:	04/30/1993
From:	Love D, Sheila Ray WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML19303F658	List: ML20045D717 ML20045D721 ML20045D725 ML20045D728
References
WCAP-13650, NUDOCS 9306290355
Download: ML20045D728 (36)

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- WESTINGHOUSE CLASS 3 i

WCAP-13650 1

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l Safety Evaluation Supporting A More l Negative EOL Moderator Temperature Coefficient Technical Specification for the t l Sequoyah Nuclear Plants t

j April 1993

Prepared by: N. A. Pogorzelski J. T. Doman

. l l H. Q. Lam F. B. Baskerville M. M. Baker B. W. Gergos l l

Approved: ,

Approved: /1 .,M l S. Ray, Manager D. S. Lovi, Manager l Core Design E Transient Analysis l l

. Work Performed Under Shop Order TVOS-10239 E

@ 1993 Westinghouse Electric Corporation Energy Systems Business Unit '

P. O. Box 355 Pittsburgh, PA 15230

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ABSTRACT This report proposes a relaxation of the Limiting Condition for Operation and the Surveillance Requirement Moderator Temperature Coefficient Technical Specification values for the end of cycle, rated thermal power condition. Relaxation is sought to improve plant availability and minimize disruptions to normal plant operation, while continuing to satisfy plant safety criteria.

A methodology for establishing the Technical Specification end of cycle Moderator Temperature Coefficient values that are consistent with the plant safety analyses is described herein. Specific application of the methodology to the Sequoyah units provides the Technical Specification Moderator Temperature Coefficient values which are proposed to replace the existing values.

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l TABLE OF CONTENTS Section Title Page

1.0 INTRODUCTION

1-1 1.1 Background 1 -1 1.2 Basis of Current EOL MTC LCO and SR Values 1-1 1

2.0 METHODOLOGY FOR MODIFYING MOST NEGATIVE MTC TECHNICAL 2-1 SPECIFICATION VALUES 2.1 Conversion of Safety Analysis MDC to Technical Specification MTC 2-1 2.2 Conservatism of the ARI to ARO MTC Conversion 2-1 2.3 Alternative MTC Conversion Approach 2-2 2.4 Determining SR MTC from LCO MTC 2-3 2.5 Benefits of the Alternative MTC Conversion Approach 2-3 3.0 MOST NEGATIVE FEASIBLE MTC APPROACH APPLIED TO SEQUOYAH 3-1 3.1 Sequoyah Accident Analysis MDC Assumption 3-1 3.2 Determination of Most Negative Feasible MTC Sensitivities 3-1 3.3 Maximum Allowed Deviations from Nominal Operating Conditions 3-3 3.3.1 Moderator Temperature and Pressure Deviations 3-4 3.3.2 RCCA Insert Deviation 3-4 3.3.3 Axial Flux (Power) Shape Deviation 3-5 3.3.4 Transient Fission Product (Xenon) Concentration Deviation 3-5 3.4 Overall" Delta MTC" Factor for Sequoyah Reloads 3-6 3.5 Proposed Sequoyah Technical Specification EOL MTC LCO Value 3-7 4.0 SAFETY ANALYSIS IMPACT OF MOST NEGATIVE FEASIBLE MTC 4-1 APPROACH  ;

i 5.0 DETERMINATION OF MOST NEGATIVE FEASBILE MTC SURVEILLANCE 5-1 1 VALUE

6.0 CONCLUSION

S 6-1 Appendix A SEQUOYAH PARAMETER SENSITIVITIES A-1 A.1 Determination of Most Negative Feasible A-1 A.2 MTC Sensitivity to Moderator Temperature and Pressure Variation A-1 A.3 MTC Sensitivity to RCCA Insertion A-2 A.4 Sensitivity to Axial Flux (Power) Shape A-2 A.5 Sensitivity to Transient Fission Product (Xenon) Concentration A-3 l A.6 Comparison of Sequoyah Sensitivities to Other Cores A-4 Appendix B SEQUOYAH TECHNICAL SPECIFICATION CHANGE PAGES B-1 i

i REFERENCES R-1 l

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i UST OF TABLES Table Title Page 4-1 FSAR Events that Assume a Constant 0.43 4-2 ,

Ak/k/gm/cc Value of MDC l l

A-1 Comparison of EOL MTC Sensitivities for Sequoyah A-5 with Other Cores l l

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Figure Title Page Figure A-1 Change in MTC with increase in T-Average Above A-6 Nominal T-Average ,

Figure A-2 Delta MTC Versus Axial Flux Difference at EOL, A-7 HFP, ARO '

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1.0 INTRODUCTION

1.1 Background For FSAR accident analyses, the transient response of the plant is dependent on reactivity feedback effects, in particular, the moderator temperature coefficient (MTC) and the Doppler power coefficient. Because of the sensitivity of accident analysis results to the assumed MTC value, it is important that the actual core MTC remain within the bounds of the limiting values assumed in the FSAR accident analyses. While core neutronics analyses will have confirmed that the MTC is within these bounds, the Technical Specifications require that the core MTC also be l

confirmed by measurement as verification of the accuracy of the neutronics predictions. These MTC measurements are performed:

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1. At beginning of cycle, prior to initial operation above 5% rated thermal power, and

2. Within 7 EFPD after reaching an equilibrium boron concentration of 300 ppm.

4 1.2 Basis of Current EOL MTC LCO and SR Values I

In order to ensure a bounding accident analysis, the MTC is assumed to be at its most limiting value for the analysis conditions appropriate to each accident. Currently, the most negative MTC limiting value is based on EOL conditions (specifically with regards to fuel burnup and boron concentration), full power, with rods fully inserted.

Most accident analyses use a constant moderator density coefficient (MDC) designed to bound the MDC at this worst set of initial conditions (as well as at the most limiting set of transient conditions). This value for the MDC forms the licensing basis for the FSAR accident analyses.

Converting the MDC used in the accident analyses to a corresponding MTC is a simple calculation which accounts for the rate of change of moderator density with temperature at the conditions of interest. In this report, the convention followed is to discuss the moderator feedback in terms of MTC rather than MDC. Nevertheless, it is important to note that the accident analyses actually assume a constant MDC value, rather than making any explicit assumption on MTC.

The Technical Specifications place both a Limiting Condition for Operation (LCO) and a Surveillance Requirement (SR) constraint on the MTC, based on the accident analyses assumptions of the MDC described above. The most positive MTC LCO limit applies to Modes 1 and 2, and requires that the MTC be less positive than the specified limit value. The most negative MTC LCO limit applies to Modes 1,2, and 3, and requires that the MTC be less negative than the specified limit value for the all rods withdrawn, end of cycle life, rated thermal power condition.

The Technical Specification SR calls for measurement of the MTC at BOL of each cycle prior to initial operation above 5% rated thermal power in order to demonstrate compliance with the most positive MTC LCO. Similarly, to demonstrate compliance with the most negative MTC LCO, the Technical Specification SR calls for measurement of the MTC prior to EOL (after a 300 ppm FM2490E 1 un(930401)SO 1-1

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.I equilibrium boron concentiation has been attained). However, unlike the BOL situation, the -

300 ppm SR MTC value dffers from the EOL LCO limit value. Because the HFP MTC value will gradually become more negative with further core depletion and boron concentration reduction, a 300 ppm SR value of MTC should necessarily be less negative than the EOL LCO limit. The -

300 ppm SR value is suficiently less negative than the EOL LCO limit value to provide assurance I that the LCO limit will be met when the 300 ppm surveillance criterion is met.

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2.0 METHODOLOGY FOR MODIFYING MOST NEGATIVE MTC TECHNICAL SPECIFICATION VALUES 2.1 Conversion of Safety Analysis MDC to Technical Specification MTC As stated previously, the FSAR accident analyses have assumed a bounding value of the moderator density coefficient (MDC) which ensures a conservative result for the transient analyzed. The process by which this accident analysis most positive MDC is transformed into the most negative MTC LCO value is stated in the Sequoyah Technical Specification BASES Section 3.1.1.3:

"The most negative MTC value equivalent to the most positive Moderator Density Coefficient (MDC), was obtained by incrementally correcting the MDC used in the FSAR analyses to nominal operating conditions. These corrections involved subtracting the incremental change in the MDC associated with a core condition of all rods inserted (most positive MDC) to an all rods withdrawn condition and, a conversion for the rate of change of moderator density with temperature at RATED THERMAL POWER conditions. This value of the MDC was then transformed into the limiting end of cycle life (EOL) MTC value."

In the process of converting the accident analysis MDC into the corresponding MTC for the Technical Specifications, the conversion for the rate of change of moderator density with temperature at rated thermal power conditions involves conventional thermodynamic properties and imposes no undue conservatism on the resulting MTC value. The additional conversion made is to correct the above MDC (MTC) value for the change associated with going from a condition of all rods in (ARI) to one of all rods out (ARO). That is, the accident analysis MDC (MTC) assumes a coefficient determined for a condition of EOL HFP O ppm with all control and shutdown banks fully inserted. This accident analysis MDC (MTC) is corrected back to the ARO condition in order to produce a Technical Specification limit which permits direct comparison against measured values. The effect of the presence of all control and shutdown banks is to make the MTC markedly more negative than an MTC at the ARO condition, hence this conversion has a substantialimpact.

2.2 Conservatism of the ARI to ARO MTC Conversion The use of a substantially negative MTC (positive MDC) value for the transient accident analyses is prudent, in that it produces a more severe result for the transient, which makes the analysis inherently conservative. The drawback to the ARI assumption is that when the conversion to the ARO condition is made, the resulting Technical Specification MTC value is dramatically less negative than the value corresponding to the transient safety calculations, and is even less negative than the expected best estimate values of the EOL MTC for high discharge burnup reload cores. In the worst case, maintaining the EOL MTC Technical Specification limit at its present value could result in requiring that the plant be placed in hot shutdown when, in fact, there exists substantial margin to the moderator coefficient assumed in the accident analyses. Such a situation is unnecessarily restrictive, and results primarily from the ARI to ARO adjustment made between the accident analysis MDC value and the Technical Specification MTC value.

FM2490E-2.un(930401)SO 2-1

In addition to being unnecessarily restrictive, the HFP ARI assumption is inconsistent with the Technical Specification requirements for allowable operation, wherein shutdown banks are not permitted to be inserted during power operation and control banks must be maintained above their insertion limits.

2.3 Alternative MTC Conversion Approach The ARI to ARO basis for converting from the accident analysis MDC value to a Technical Specification LCO MTC value is overly restrictive. The concept proposed here as an alternative to the ARI to ARO conversion is termed the "Most Negative Feasible MTC" approach. This approach maintains the existing accident analysis assumption of a bounding value of moderator coefficient, but offers an alternative method for converting to the Technical Specification LCO MTC value.

The Most Negative Feasible MTC approach seeks to determine the conditions for which a core will exhibit the most negative MTC value that is consistent with operation allowed by the Technical Specifications. As an example, the Most Negative Feasible MTC approach would not require a conversion assumption that all rods be fully inserted at HFP conditions, but would require a conversion assumption that all control banks are inserted the maximum amount that Technical Specifications permit, so as to make the calculated EOL HFP MTC more negative than it would be for an unrodded core.

The Most Negative Feasible MTC approach determines the EOL MTC sensitivity to those design i and operational parameters that directly impact the MTC, and attempts to make this determination in such a manner that the resulting sensitivity for one parameter is independent of the assumed values of the other parameters. As a result, parameters which are mutually exclusive but permissible according to the Technical Specifications (such as an assumption of full power  ;

operation and an assumption of no xenon concentration in the core), and which serve to make l MTC more negative, will have their incrementalimpacts on MTC combined to arrive at a conservative and bounding condition for the most negative feasible MTC. The parameters which

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are variable under normal operation and which affect MTC are:

soluble boron concentration in the coolant a

moderator temperature and pressure RCCA irsertion

+ axial flux (power) shape a transient fission product (xenon) concentration The Most Negative Feasible MTC approach examines each parameter separately, and assesses the impact of variation of that parameter on the EOL MTC. The assessment is performed for multiple core designs that feature combinations of fuel design, discharge bumup, cycle length, and operating temperature which are expected to be similar to future core designs.

FM2490E-2.un(930401)SO 2-2

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When the assessment is complete, the MTC sensitivity associated with each of the above parameters has been identified. One then determines the maximum deviation from " nominal" conditions (ARO, HFP, equilibrium xenon, Tavg on the reference temperature program) that the Technical Specifications permit. and multiplies that deviation by the appropriate MTC sensitivity to arrive at a " delta MTC" factor associated with the parameter.

For example, suppose it is determined that the MTC becomes 1 pcmrF more negative for each 1*F increase in core average operating temperature above nominal (the MTC

• sensitivity" is

-1 pcmFFFF). If the Technical Specifications permit a maximum increase in Tavg of 4 F above the nominal core Tavg, then the moderator temperature " delta MTC" factor is:

(-1 pcmrFFF) x 4 F = -4 pcmFF.

Bounding " delta MTC" factors are determined in this way for each of the above parameters, and these factors are then added to arrive at an overall bounding " delta MTC" factor. This overall

" delta MTC" factor states how much more negative the MTC can become, relative to the nominal EOL HFP ARO MTC value, for normal operation scenarios permitted by the current Technical Specifications. The combination of moderator temperature, rod insertion, xenon, etc., which defines the Most Negative Feasible MTC condition is proposed as a replacement for the ARI to ARO conversion of the current MTC Technical Specification. The conversion for the Most Negative Feasible MTC condition is applied in the same way that the current ARI to ARO conversion is applied, in order to arrive at an EOL ARO HFP MTC Technical Specification limit that remains based on the accident analysis MDC assumption.

2.4 Determining SR MTC from LCO MTC Under the Most Negative Feasible MTC approach, the 300 ppm surveillance value is determined in the manner currently stated in the Sequoyah Technical Specification BASES 3.1.1.3:

... MTC value represents a conservative value (with corrections for burnup and soluble boron) at a core condition of 300 ppm equilibrium boron concentration and is obtained by making these corrections to the limiting EOL MTC value...."

That is, the 300 ppm surveillance value is derived by making a conservative adjustment to the EOL ARO HFP MTC limit value that accounts for the change in the MTC with soluble boron and burnup.

Plant-specific examination of the difference between the 300 ppm HFP MTC and EOL (0 ppm)

HFP MTC suggests that a smaller correction is justified than the 9 pcm/'F which has historically been applied to Westinghouse-designed plants.

2.5 Benefits of the Alternative MTC Conversion Approach The Most Negative Feasible MTC approach is considered to be superior to the ARI to ARO conversion currently specified by the plant Technical Specifications for the following reasons:

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FM2490E-2.un(930401)50 2-3

1. The Most Negative Feasible MTC approach does not require an unduly restrictive 300 ppm surveillance value that could result in repeated MTC surveillance measurements. These repeated measurements are undesirable in that they perturb normal reactor operation.

2. The Most Negative Feasible MTC approach does not alter the FSAR transient accident analysis bases or assumptions, and hence, does not affect the accident analysis conclusions. It retains the concept of a conversion between the accident analysis MDC assumption and the Technical Specification LCO MTC value that assures that the plant cannot experience an MDC which is more severe than that assumed in the accident analyses.

3. The Most Negative Feasible MTC approach is a conservative but reasonable basis to assume for an MTC value for a reload core, and is consistent with operation as defined by other sections of the Technical Specifications. The current methodology is overly conservative and makes assumptions which are inconsistent with the Technical Specification requirements.

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3.0 MOST NEGATIVE FEASIBLE MTC MOST NEGATIVE FEASIBLE MTC APPROACH J l

APPLIED TO SEQUOYAH 3.1 Sequoyah Accident Analysis MDC Assumption l The FSAR accident analyses upon which the Technical Specification EOL HFP LCO MTC limit is based have assumed bounding values of the moderator density coefficient in order to ensure a conservative simulation of plant transient response for Sequoyah. For those transients for which the analysis results are made more severe by assuming maximum moderator feedback, a moderator density coefficient (MDC) of 0.43 Ak/k/gm/cc has been assumed to exist throughout the transient.

When discussing the Technical Specification EOL LCO limit on moderator feedback, it is better to use the moderator temperature coefficient (MTC) than MDC, since temperature is the measurable quantity. For this reason, the Sequoyah accident analysis MDC assumption of 0.43 A.k/k/gm/cc is converted to its equivalent MTC. This conversion depends on the density change to temperature change relationship which prevails for the conditions of interest. For this discussion, the conditions of interest are the core temperature and pressure (hence, density) experienced under normal operation at which the MDC assumes its most extreme (positive) value. These temperature and pressure conditions are the Sequoyah rated thermal power (RTP), full flow vessel average nominal operating conditions of 582.2 F, and 2250 psia, respectively.

At these nominal RTP operating conditions, the accident analysis MDC value of 0.43 Ak/k/gm/cc ,

is equivalent to a HFP MTC of -52.68 pcm/ F. For simplicity, this value of MTC will often be referred to as the " accident analysis MTC", in the dscussion which follows. Howes 3r, it should be remembered that the applicable accident analyses actually assume a constant MDC value of 0.43 Ak/k/gm/cc and make no explicit assumption about MTC.

3.2 Determination of Most Negative Feasible MTC Sensitivities As stated previously, there are a limited number of core operational parameters that directly affect the MTC and are variable under normal core operation. The parameters are as follows:

- soluble boron concentration in the coolant

- moderator temperature and pressure

- RCCAinsertion

- axial flux (power) shape l l

- transient fission product (xenon) concentration The radial flux (power) shape can also vary under normal core operation and will affect the MTC. 1 However, the operational activities that directly affect radial power shape do so through withdrawal )

or insertion of control rods and through xenon concentration; therefore, the impact of radial flux distribution variation on MTC will be an implicit part of the MTC sensitivity to these other parameters.

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FM2490E 3.un(930402p0 3-1

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Soluble boron concentration is certainly variable under normal core operation. However, it is eliminated as a source of sensitivity for this analysis. This is because the EOL HFP ARO MTC Technical Specification limit value is assumed to be essentially a O ppm limit by virtue of the definition of EOL. The most negative MTC value will always occur at a boron concentration of 0 ppm, and therefore, a 0 ppm boron concentration is assumed as the basis of the EOL MTC Technical Specification limit under the Most Negative Feasible MTC approach.

For the remaining parameters, sensitivity analyses were performed by perturbing the parameter of interest in such a way as to induce a change from its nominal EOL value, and then determining the MTC with the parameter held in the perturbed state. A further perturbation was induced and the MTC calculation repeated. This sequence was repeated until sufficient MTC values were generated to reliably determine the trend of MTC change with variation in the value of the independent parameter.

In order to establish trends in MTC that are appropriate and bounding for the Sequoyah reload cores, these sensitivities were determined for three different reload cores. These cores were the Sequoyah Unit 1 Cycles 5 and 6 and Sequoyah Unit 2 Cycle 6 cores. The SR value is based on EOL MTC data from many core designs of Westinghouse plants. In addition, the sensitivities developed for Sequoyah were compared to those developed for several other reload cores to confirm consistency with previous experience and establish confidence that these sensitivities are cycle-independent.

Three other cores were examined for comparison to Sequoyah. Representative reload designs for these cores are summarized below. The MTC sensitivities for these three cores may be found in Table A-1. Note that the sensitivities in Table A-1 were derived to bound a variety of expected

design conditions for the specific core, not just those listed in the descriptions below.

RELOAD A:

This is a reload core for a Westinghouse designed 3 loop plant. It utilizes the Westinghouse 17x17 standard rod diameter fuel design, feeds 52 assemblies in a low leakage loading pattern (L3P) and assumes a region average discharge burnup of 36000 MWD /MTU. This reload also has 400 Wet Annular Burnable Absorbers and a nominal cycle length of 13365 MWD /MTU. The nominal core average temperature for this plant is 592.5 .F. The analysis also considered a transition to

. OFA fuel.

RELOADB:

This is a reload core for a Westinghouse designed 4 loop plant. It utilizes the Westinghouse 17x17 standard rod diameter fuel design, feeds 84 assemblies in a low leakage loading pattern (L3P) and assumes a region average discharge burnup of 36000 MWD /MTU. This reload also has 448 pyrex glass bas and a nominal cycle length of 15800 MWD /MTU. The control rod absorber materialis hafnium. The nominal core average temperature assumedin this analysis is 590.5 F.

FM2490E-3.un(930402) 50 3-2

RELOAD C:

This is a reload core for a Westinghouse designed 4 loop plant using an L3P design for a cycle length of 17000 MWD /MTU and a region average discharge burnup of 38000 MWD /MTU with 17x17 standard fuel. This design feeds 84 assemblies and uses 1728 pyrex glass bas. The control rod absorber materialis silver-indium-cadmium. The core average moderator temperature is 575.3 F.

A comparison of the parameterization of the Moderator Temperature Coefficient for these cores and the Sequoyah cores will be discussed in Appendix A.

Three principal computer codes have been used in the nuclear analysis of the MTC sensitivities.

j PHOENIX-P (two-dimensional), ANC (two-dimensional and three-dimensional) and APOLLO l (one-dimensional). Descriptions and uses for these codes are given below.

PHOENIX P W is a two-dimensional, multi-group transport theory code which utilizes a 42 energy-l group cross-section library derived from ENDF/B/V W data. Itprovidesthecapabilityforcelllattice modeling on an assembly level. In this design, PHOENIX-P is used to provide homogenized, two-group cross-sections for nodal calculations and feedback models. It is also used in a special geometry to generate appropriately weighted constants for the baffle / reflector regions.

ANC(2) is an advanced nodal code capable of two-dimensional and three-dimensional calculations. In this design, ANC is employed as the reference model for all safety analysis calculations, power distributions, peaking factors, critical boron concentrations, control rod worths, reactivity coefficients, etc. In addition,3D ANC is used to validate one- and two-dimensional results and to provide radial (x-y) peaking facton; as a function of axial position. It has the capability of calculating discrete pin powers from the nodalinformation as well.

APOLLO, an advanced version of PANDA (3) is a two-group, one-dimensional diffusion-depletion code. It uses cross-sections generated by a radial averaging of the corresponding 3-D model cross sections and is used as a one-dimensional axial model. Thermal feedback is included in the calculctional models. The axial modelis used for computing axial power distributions, differential rod worths, control rod operating limits (insertion limits, return-to-power limits), etc.

Neutronics calculations and evaluations established the MTC sensitivities for each of the parameters listed above. The description and results of this analysis are provided in Appendix A.

3.3 Maximum Allowed Deviations from Nominal Operating Conditions The concept of maximum allowed deviation from nominal operating conditions is employed to determine the extent to which reactor parameters can vary under normal operation so as to cause the MTC to become more negative. This combination of parameter statepoints then defines the worst allowable initial condition for a transient that employs a most negative MTC (most positive MDC) assumption. It is also necessary to demonstrate that the moderator coefficient assumption used in the accident analysis bounds the parameter changes that occur throughout the transient.

FM2490E 3.un(930402)50 3-3

The adequacy of the constant MDC accident analysis assumption to bound MTC values that occur throughout the transient is examined in Section 4.0.

The bases for the maximum allowed deviation from nominal operating conditions are Technical Specifications that limit the extent of moderator temperature increase, RCCA insertion, and axial power skewing. The deviations permitted by present Sequoyah Technical Specifications are discussed in the following sections.

3.3.1 Moderator Temperature and Pressure Deviations The current nominal design vessel average temperature for Sequoyah is 582.2 F and the current nominal design pressure is 2250 psia. The safety analyses for the Sequoyah plants assume uncertainties of +/- 5.5 F for the vessel average temperature and 40/-42 psi for the pressurizer pressure. For additional conservatism, the uncertainties in RCS temperature and pressure assumed for this sensitivity analysis were increased to [ ]+ F and [_] psi, respectively, l relative to the operating temperatures and pressures appropriate for a given cycle, with only the (a.c) limiting direction of the uncertainty examined. These maximum deviations in temperature and pressure are applied to the MTC sensitivity discussed in Appendix A to obtain a " delta MTC" factor associated with RCS moderator temperature and pressure deviations from nominal. The resulting " delta MTC"is [ ]+ pcm/ F.

l (a,c) 3.3.2 RCCA Insertion Deviation The nominal condition assumption for RCCA placement is complete withdrawal (ARO). This assumption is underscored by the requirement in Technical Specification 3.1.1.3 that the LCO value of EOL MTC is for the "all rods withdrawn" condition. Because some RCCA insertion is allowed during full power operation, and because RCCAinsertion may cause the MTC to be more negative than it would be otherwise, the RCCA insertion deviation is simply the maximum allowable RCCA insertion permitted by the Technical Specifications.

Technical Specifications 3.1.3.5 and 3.1.3.6 place limits on allowable RCCA insertion. Technical Specification 3.1.3.5 precludes Shutdown RCCA insertion in Modes 1 and 2, and Technical Specification 3.1.3.6 limits Control Bank insertion via the Rod insertion Limits (Rlls).

Control rods can be inserted as a function of power level according to the Rlls, and all RCCAs can be inserted at HZP coincident with reactor trip. With greater RCCA insertion, the MTC becomes more negative relative to the ARO MTC, all other parameters being held equal.

However, the Technical Specifications do not allow all other parameters to be held equal. With deeper RCCA insertion, power must be reduced and Tavg will be reduced accordingly. The reduction in Tavg serves to make the MTC more positive, and at EOL 0 ppm conditions, this positive Tavg effect will entirely offset the negative RCCA effect on MTC. For this reason, the maximum RCCA deviation from nominal conditions allowable by the Technical Specifications needs to be assessed at only the HFP condition.

FM2490E-3un(930402)So 3-4 a

At full power, the Rits permit insertion of the lead control bank to 182 steps withdrawn. However, strict application of current RILs in determining the " delta MTC" factor astociated with RCCA insertion may prove to be restrictive if minor changes to the RILs becomes necessary in the future.

For this reason, the HFP RCCA insertion assumed for this analysis is [_]+ steps withdrawn. This l(a,c) additional insertion is expected to bound minor Rll adjustment which may be necessary for optimizing core performance characteristics of future reloads. However, applicability of the ultimate EOL MTC LCO value derived from these sensitivities will be confirmed on a cycle-by-cycle basis as part of the reload design process; therefore, a Rlls adjustment to lead control bank insertion beyond [170]+ steps withdrawn will not necessarily invalidate the revised EOL MTC LCO value. l (a.c)

This insertion also covers possible misalignment of D-bank.

This limiting HFP RCCA insertion of [ _]+ steps withdrawn forms the basis of the determination l (a,c) of MTC sensitivity to HFP RCCA insertion, which is described in Appendix A. The resulting " delta MTC" factor associated with RCCA insertion was conservatively determined to be [ ]+ pcm/ F. l (a.c) 3.3.3 Axial Flux (Power) Shape Deviation As indicated earlier, the MTC is affected by the axial flux shape which exists in the core, primarily as a result of the influence that the axial flux shape has on the rate at which the moderator is heated as it moves up the core. The detailed shape itself is not so important, but rather the

" balance" of the flux shape,in terms of how much moderator heating occurs in the lower half of the core versus the upper half of the core. The influence which axial power shape has on the MTC 1 can, therefore, be captured by quantifying this axial flux " balance", and this balance is best quantified by the core's Axial Flux Difference (AFD).

The discussion of the MTC sensitivity to axial flux (power) shape presented in Appendix A establishes that the more negative the AFD becomes, the more negative the MTC will become.

The axial flux (power) shape deviation is, therefore, determined by how negative the AFD is allowed to become under normal full power operating conditions.

Sequoyah employs a Relaxed Axial Offset Control (RAOC) Technical Specification which sets the ,

allowable AFD limits at +6% and -15% at HFP operation. To assign a " delta MTC" factor  !

attributable to axial flux shape, one need only examine the MTC effect associated with the i

-15% AFD. For future changes,[_]+% AFD is used in the analysis. This [ ]+% AFD deviation j (a.c) is applied to the MTC sensitivity to axial flux (power) shape, which is described in Appendix A, to obtain a " delta MTC" factor associated with this [ _]+% AFD. The resulting " delta MTC" factor I"'I is [ _ ]+ pcm/ F.

3.3.4 Transient Fission Product (Xenon) Concentration Deviation I l

Xenon is the most significant transient fission product in terms of effects on core reactivity and flux i distribution; therefore, its possible impact on MTC is investigated to compute the final " delta MTC" factor to include in the Most Negative Feasible MTC approach. While Technical Specifications l place no limitations on either xenon distribution or overall concentration, the AFD limits discussed above, in effect, place a limitation on the amount of axial xenon skewing that can occur, and the physics of xenon buildup and decay place practicallimits on the concentration.

ru24scs-a uncsao402)so 3-5

Because axial xenon distribution directly impacts axial flux shape, this aspect of xenon effect on the MTC is implicitly included in the axial flux (power) shape deviation discussed above. What l remains to be quantified is the impact of the overall xenon concentration in the core.

Taking the EOL HFP ARO equilibrium xenon concentration to be the nominal xenon condition for the core,it was determined for low leakage core designs of the type characteristic of the Sequoyah )

reloads, that the MTC becomes more negative with a reduced xenon concentration. Accordingly, the most negative MTC results when there is no xenon in the core.

It was established in the discussion on moderator temperature and pressure deviation and on RCCA insertion deviation, that the condition for the most negative MTC requires maximum allowable temperature (minimum allowable density) and, therefore, occurs at full power conditions. While the assumption of achieving full power operation with no xenon in the core is certainly a conservative assumption, the possibility of steady power escalation after an extended shutdown period presents a reasonable scenario for full power operation with a comparatively low xenon concentration in the core. For this reason, the " xenon deviation" to be used in conservatively determining the " delta MTC" factor attributable to transient fission product is a change from HFP ARO equilibrium xenon to no xenon in the core. The resulting " delta MTC" factor is [ 1+ pcm F.

l(ac) 3.4 Overall" Delta MTC" Factor for Sequoyah Reloads The preceding section has concluded that the most adverse operation possible, in terms of achieving the most negative EOL MTC under current Sequoyah Technical Specifications, would feature the following values of key parameters:

- Core Moderator Temperature: [._]+ F above HFP nominal

- Core Moderator Pressure: [__]+ psia below HFP nominal (a.c)

- HFP RCCA insertion: D-bank [_]+ steps withdrawn

- HFP most negative AFD: [ _ ]+%

- HFP xenon concentration: 0 When these maximum allowable deviations from a nominal condition of EOL HFP ARO, with equilibrium xenon, and 0 ppm boron are applied to the individual parameter sensitivities discussed in Appendix A, tne overall" delta MTC" factor is determined. This overall factor for the Sequoyah plants is computed as:

FM2490E-3.un(930402)S0 3-6

(

- Core Moderator Temperature and Pressure

- Factor: [ _ ]+ pcm/ F

- HFP RCCA Insertion Factor: [ __ ]+ pcm/ F

- Axial Flux (Power) Shape Factor: [ _ ]+ pcm/ F

)

- Xenon Concentration Factor: [ _ ]+ pcm/ F Overall " Delta MTC" Factor: [ _ ]+ pcm/ F The interpretation of this overall" delta MTC" factor is as follows. The Technical Specification LCO value of EOL MTC is based on the explicit conditions of unrodded full power operation. This is an appropriate condition for performing an MTC experiment and obtaining results that can be meaningfully compared to design predictions. It is not, however, the condition undar which the MTC can achieve its most negative value under normal operation scenarios permitted by the Technical Specifications. The conservative " delta MTC" formulation has concluded that the actual core MTC can be as much as [_]+ pcm/ F more negative than the EOL MTC LCO value l(a.c) defined by the Technical Specifications.

The individual components of this [ _]+ pcm/ F overall " delta MTC" factor have been l(a.c) determined on a conservative basis and are expected to bound the values predicted for Sequoyah reload cores in the future. While an individual component could conceivably exceed the value cited above, such an occurrence would not invalidate the Most Negative Feasible MTC approach, as long as the total of all the components remains bound by the [ ]+ pcm/ F overall" delta l(a.c)

MTC" factor. Implementation of a revised EOL MTC Technical Specification based on the Most Negative Feasible MTC approach will require that any reload core's overall " delta MTC" factor be demonstrated to be bounded by the licensing basis value, rather than specifically addressing the individual components. Such validation of the Most Negative Feasible MTC ap)proach on a specific basis would be performed as part of the reload core design process (5, The individual components of this " delta MTC" factor have been compared with the values calculated for the other reload cores discussed previously and have been found to be consistent and within the range of previous experience.

3.5 Proposed Sequoyah Technical Specification EOL MTC LCO Value As was pointed out in Section 3.1, Sequoyah FSAR accident analyses have assurned an MDC value which, when converted to an MTC at nominal HFP conditions, is equivalent to an MTC of

-52.68 pcm/ F. At no time may the actual core be allowed to experience an MTC more negative than -52.68 pcm/ F, as this would invalidate an assumption of the accident analyses. The Most Negative Feasible MTC approach guarantees that such a situation will not occur by subtracting from this -52.68 pcm/ F MTC value the [_]+ pcm/ F " delta MTC" factor determined. The resulting value of [ ]+ pcm/ F is the Technical Specification EOL LCO value of MTC under (a.c)

FM2490E.3.un(930402)50 3-7

the Most Negative Feasible MTC aproach. [ )+this I"#

value is [ ] -45.0 pcm/ F, and proposed as the EOL HFP ARO Technical Specification MTC LCO value for Sequoyah reload cores, replacing the current LCO value.

The -45.0 pcm/ F proposed limit provides relief over the limit associated with the current Technical Specification ARO to ARI conversion requirement, yet still represents a conservative formulation.

The scenario of deep RCCA insertion, coupled with high Tavg, low system pressure, and no xenon, represents a compounding of worst case events which can be considered independent, yet are not treated as such in the Most Negative Feasible MTC formulation. Determination that the core MTC is less negative than -45.0 pcm/ F at EOL HFP ARO conditions provides assurance that the assumption of initial condition MTC made in the plant accident analyses remains bounding. Additional assurance that the MTC (MDC) will not become more limiting at any time ,

during a transient is also needed, in order to demonstrate that the accident analysis conclusions remain valid. This additional assurance is the primary subject of Section 4.0.

l l

I FM2490E-3.un(930402)50 3-8 l

l l

4.0 SAFETY ANALYSIS IMPACT OF MOST NEGATIVE FEASIBLE MTC APPROACH .

l The accident analyses conservatively model the various reactivity coeficients to produce a l bounding analysis. As discussed in Section 3.1, the applicable analyses assume a constant MDC of 0.43 Ak/k/gm/cc to bound the predicted moderator reactivity insertion. The events which assume this value for MDC are listed in Table 4-1.

The Most Negative Feasible MTC approach determines the conditions for which a core will exhibit the most negative MTC value that is consistent with operation allowed by the Technical Specifications. Thus, the value for the Most Negative Feasible MTC provides the basis for a conservative initial condition assumption.

Changes in the parameters identified in Section 2.3 could take place during a transient in such a way as to make the MTC more negative than that allowed under normal operation. However, the most adverse conditions seen in these events will not result in a reactivity insertion that would invalidate the conclusions of the FSAR accident analyses. Therefore, the 0.43 Ak/k/gm/cc assumption used as the basis for the Most Negative Feasible MTC Technical Specification will not change.

As discussed in Reference 5, the reactivity coefficients assumed can have a strong influence on accident analysis results. Since the moderator coefficient can be affected by a reload, the conservative nature of the accident analysis assumption must be confirmed on a cycle-specific basis using the methodology discussed in Reference 5. This includes verification that the most I

adverse accident conditions of a constant MDC do not invalidate the conservative nature of the accident analysis assumption. This process ensures the ability to verify that the applicable safety limits are met for each reload design and, consequently, that the Technical Specifications are met.

l l

I i

FM2490E4 mn(9304'M)$0 4-1

i Table 4-1. FSAR Events That Assume A Constant 0.43 Ak/k/gm/cc Value of MDC Section Event 15.2.2 Uncontrolled Rod Cluster Control Assembly Bank Withdrawal at Power 15.2.6 Startup of an inactive Reactor Coolant Loop 15.2.7 Loss of External Electrical Load and/or Turbine Trip l 15.2.10 Excessive Heat Removal Due to Feedwater System Malfunctions ,

15.2.11 Excessive Load increase incident 15.2.14 Spurious Operation of the Safety injection System at Power  ;

15.4.2.2 Major Rupture of a Main Feedwater Pipe 6.2.1.3 Steamline Break inside Containment for Containment Integrity Evaluation Steamline Break Outside Containment for Equipment Qualification Steamline Break Coincident with Rod Cluster Control Assembly Bank Withdrawal at Power These analyses, which are not presented in the FSAR, are used to support the Sequoyah Nuclear Plant license.

)

l I

i FM2490E-4.un(930401)50 4-2 4

DETERMINATION OF MOST NEGATIVE FEASIBLE MTC SURVEILLANCE VALUE l 5.0 1 l

i Section 2.4 indicated the potential conservatism in the difference of 9 pcmfF between the Technical Specification 300 ppm MTC SR value and the EOL HFP ARO MTC LCO value. Typical 17x17 reload designs exhibit a predicted difference between the 300 ppm HFP design MTC and the EOL HFP ARO design MTC which is much less than 9 pcmf F. However, in order to justify the use of a value which is smaller than 9 pcmf F for a given plant, the specific design predictions of the plant must be examined.

Design predictions for several recent Sequoyah reload cores were reviewed in order to determine the difference between the predicted 300 ppm HFP MTC and the predicted EOL HFP MTC.

The magnitude of the difference showed little variation, varying from + to a maximum of (a.c)

+ pcmf F.

The proposed Technical Specification SR value for Sequoyah reload cores is -37.5 pcmf F. This value is 7.50 pcmfF less negative than the EOL LCO MTC value proposed in Section 3.5. The 7.50 pcmf F was chosen to bound the maximum + pcmf F difference observed for recent (a.c)

Sequoyah reload cores, yet afford relief from the 9 pcmf F difference applied by the current Technical Specifications. While the -37.5 pcmf F Technical Specification SR value is expected to be bounding for future Sequoyah fuel management, the validity of this SR value will be confirmed on a cycle-by-cycle basis, as part of the reload core design process described in Reference 5.

FM2490E-5.un(930401)$0 5-1

l

6.0 CONCLUSION

S The present Sequoyah Technical Specification values for the EOL HFP ARO MTC LCO, and for the 300 ppm HFP ARO SR conservatively reflect the FSAR accident analyses MDC assumption.

However, they are considered to be overly restrictive because they assume deviations from nominal plant operation which are not permitted by other Technical Specifications. An alternative adjustment procedure is proposed which is based on a conservative determination of the extent to which a nominal EOL HFP ARO MTC value can be made more negative under the most extreme values of certain operational parameters that are permitted by other Technical Specifications. This Most Negative Feasible MTC approach assumes that these largely independent extreme situations occur simultaneously, and in the worst case, serve to make the EOL HFP MTC [ ]+ pcmf F more negative than it would be at nominal conditions. When this l(a.c) value is subtracted from the MTC equivalent of the accident analyses assumed MDC value, the resulting MTCis[ ]+ pcmfF. The[ ]+ value of-45.0 pcmf Fis proposed l(a,c) as the EOL HFP MTC Technical Specification LCO limit under the Most Negative Feasible MTC approach.

Examination of the difference between the 300 ppm HFP equilibrium boron concentration MTC value and the EOL HFP MTC values concluded that a bounding expected difference between these two MTC values for Sequoyah reload cores is -7.5 pcmf F. This difference is subtracted from the proposed -45.0 pcmf F EOL HFP MTC Technical Specification limit to arrive at a proposed Technical Specification 300 ppm HFP MTC SR value of -37.5 pcmfF.

It is concluded that the Technical Specification EOL MTC LCO and 300 ppm SR values proposed under the Most Negative Feasible MTC approach do not impact conclusions of FSAR accident analyses, because they do not affect the accident analysis assumption of MDC. In addition, the validity of the above-stated LCO and SR MTC values, as well as the plant's ability to comply with them, will be examined for each reload cycle as part of the normal reload design process.

The new EOL MTC LCO and 300 ppm SR MTC values and the revised basis for adjustment overcome the problems inherent with the present version of Technical Specification 3.1.1.3, yet still afford protection. These Technical Specifications continue to require that the surveillance be pedormed, so that any deviations between the operating core and design predictions that might threaten the validity of accident analysis assumptions can be detected, and continued surveillance and appropriate action undertaken.

l i

fM2490E-6.un(930402) 50 6-1

APPENDIX A. PARAMETER SENSITIVITIES A.1 Determination of Most Negative Feasible MTC Sensitivities Investigation of the sensitivity of MTC to core operational parameters that are variable under normal core operation is a fundamental requirement of the Most Negative Feasible MTC approach. Of the parameters discussed in Section 3.2, those that required detailed evaluation are:

- moderator temperature and pressure

- RCCAinsertion

- axial flux (power) shape

- transientiission product (xenon) concentration For each of these parameters, the sensitivity analyses were performed by perturbing the I

parameter in such a way as to induce a change from its nominal EOL value, and then determining an MTC with the parameter held in the perturbed state. A further perturbation was induced and the MTC calculation repeated. This sequence was repeated until sufficient data was obtained to reliably determine the trend of the MTC change with variation in the value of the parameter. The sensitivity analyses were performed for three Sequoyah reload cores. Neutronics calculations and evaluations were pt.no mad for each of these core designs to establish bounding sensitivities to each of the above parameters. The fo! Ming coctions provide a brief description of these calculations, and the " delta MTC" factors outained. The sensitivities were compared to results from other cores to establish consistency with expected behavior.

A.2 MTC Sensitivity to Moderator Temperature and Pressure Variation The decrease in moderator density which accompanies moderator heatup has the effect of reducing neutron moderation. Since water density changes more rapidly with increasing temperature, and because of adodional spectrum hardening effects, the MTC becomes progressively more negative with increasing temperature. The sensitivity of MTC to increasing temperature was determined for each of the reload cores by increasing core reference moderator temperature slightly above the nominal HFP value, while holding the pressure constant at 2250 psia, and then performing an MTC calculation that induced small changes in core K-effective via changes in moderator temperature and density about the reference values. The effects of changes in moderator temperature and density were considered together. After the MTC value was computed, core reference moderator temperature was further increased, and another MTC calculation performed. This process was repeated until the trend of MTC with increasing core reference moderator temperature was clearly established.

Results were recorded for the Sequoyah cores in terms of change in MTC from the nominal HFP MTC as a function of increase in reference moderator temperature above the nominal HFP moderator temperature. The results are shown in Figure A-1.

N2490E-A.un(930402)$0 A-1

To use the MTC sensitivity information of Figure A-1, the maximum allowable temperature and pressure (and, therefore, density) deviations permissible under operation that complies with the Technical Specifications must also be determined. These deviation values, presented in Section 3.3, are combined with the sensitivity data to arrive at a " delta MTC" factor associated with moderator temperature and pressure (and, therefore, density).

For the [ + F temperature deviation cited in Section 3.3.1, Figure A-1 indicates that the (a,c) corresponding " delta MTC" due to temperature increase is conservatively estimated to be

[ ]+ pcm/ F. The " delta MTC" due to the [ ]+

was conservatively estimated not topcm/ exceed F. The[_]+ psi pressure combined " delta MTC" deviation of cited (a.c) in Sectio

[ ]+ ocm/ F was increased to a final value of [ ]+ pcm/ F for conservatism.

A.3 MTC Sensitivity to RCCA Imartion With constant moderator temperature, pressure, and boron concentration, insertion of control rods can make the MTC more negative. As indicated in Section 3.3.2, the most negative MTC situation for this scenario will be at HFP, with RCCAs inserted to the extent allowed by the HFP insertion limits.

To calculate the EOL HFP MTC sensitivity to RCCA insertion, the reload core models had the lead control bank inserted the maximum applicable amount determined from Section 3.3.2 (D-bank at

[ ]+ steps withdrawn), at HFP, with no soluble boron in the core. The MTC value for this l(a.c) condition was then determined by inducing small changes in core K-effective via changes in moderator temperature and density about their reference values. This MTC value was compared to the MTC determined at the same conditions, but with all RCCAs removed from the core.

When the Sequoyah cores were analyzed, it was determined that the maximum change to the EOL 0 ppm HFP ARO MTC which occurred as a result of HFP RCCA insertion to a depth of [ _ ]+

steps withdrawn was [ ]+ pcm/ F. To bound future cycle variations and some minor (*I adjustment to the Rils for optimization of future core designs, the " delta MTC" factor associated with allowable HFP RCCA insertion was increased to [ ]+ pcm/ F. l(a,c)

A.4 Sensitivity to Axial Flux (Power) Shapo MTC is not directly affected by axial flux distribution itself; it is affected by the impact that the axial flux distribution has on the rate at which the moderator is heated as it moves up the core. The MTC is also affected by the "importance weighting" that the axial flux shape imparts to different regions of the core. -

In general, the accumulated burnup in the bottom half of the core exceeds that in the top half of the core. Other things being equal, higher burnup results in a more negative MTC as a result of isotopic impacts on flux spectrum. A more negative axial flux (power) shape allocates a greater importance to the lower regions of the core where bumups are greater, thereby accentuating this effect.

FM2490E-A.un(930402)so A-2

^

lt may also be shown that, in general, as the power distribution becomes more bottom skewed, the average moderator temperature increases for a constant core temperature rise. Therefore, both the importance weighting effect and the moderator axial heating rate effect indicate that a more negative Axial Flux Difference results in a more negative MTC.

This effect was investigated for the Sequoyah cores at EOL HFP 0 ppm conditions. A specific axial flux shape was induced and then, holding this flux shape approximately constant, the MTC was determined by observing the small changes in core K-effective which resulted from variation in moderator temperature and density about their reference values. A different axial flux shape was induced, and the MTC calculation repeated. This process was repeated until the behavior of MTC with variation in axial flux shape (as quantified by Axial Flux Difference) was clearly identified.

Curves of " delta MTC" as a function of Axial Flux Difference (AFD) for the reload cores are shown ,

in Figure A-2. Because more negative AFD values result from RCCA insertion, this axial flux shape MTC sensitivity implicitly captures part of the RCCA MTC sensitivity not included in the

" delta MTC" factor of the previous section.

Section 3.3.3 concludes that a negative value of HFP AFD deviation that is expected to bound future Sequoyah reload cores is [ _]+%. Using a value which conservatively bounds the data of (a.c)

Figure A-2, the " delta MTC" factor corresponding to a [ _]+% AFD deviation is [ _)+ pcm/ F.

A.5 Sensitivity to Transient Fission Product (Xenon) Concentration Xenon is the most significant transient fission product in terms of effects on core reactivity and flux distribution. Therefore, its possible impacts on the MTC were investigated to compute the final

" delta MTC" factor to include in the Most Negative Feasible MTC approach. While the Technical Specifications place no limitations on either xenon distribution or overall concentration, the AFD limits discussed in Section 3.3.3, in effect, place a limitation on the amount of axial xenon skewing that can occur, and the physics of xenon buildup and decay place practical limits on the concentration.

Because the axial xenon distribution directly impacts the axial flux shape, this aspect of xenon effect on MTC is implicitly included in the Axial Flux Shap i" delta MTC" factor discussed in Appendix A.4. What remains to be determined is the sent ;vity to the overall xenon concentration in the core. Calculations to determine this sensitivity wet a performed.

For the Sequoyah cores, it was found it.at the most negative MTC resulted when all xenon was removed from the core. The largest " delta" from the refemnce (equilibrium xenon) MTC that occurred when all xenon was removed was less than [ _]+ pcm/ F. This value was rounded to (a.c)

[ _]+ pcm/ F for the final " delta MTC" factor attributable to xenon. No further uncertainty is added, simply because the scenario of operating at full power with no xenon in the core is itself sufficiently conservative as to be bounding.

FM2490E-A.un(930402)$0 A-3

A.6 Comparison of Sequoyah Sensitivities to Other Cores in Section 3.2, three other cores were described whose MTC sensitivities were to be compared with the results for Sequoyah so that the reasonableness of the sensitivities could be confirmed on the basis of previous experience. Table A-1 summarizes this comparison. As can be seen in this Table, although there is some variation in the sensitivities for the indvidual effects, the overall sum of the contributions for Sequoyah is consistent with previous experience. The source of the variations in individual effects can be understood in differences in the ranges of parameter variations for the dfferent cores. For example, Sequoyah has a lead bank worth that is greater than Reloads B and C, resulting in a larger contribution from the RCCA parameter.

rumoe-Awoao402po A-4

Table A-1. Comparison of EOL MTC Sensitivities for Sequoyah with Other Cores Parameter _G910 Seauovah Reload A Reload B Reload C

- - + a, c I

t FM2490E-A.un(930402)so A-5

+ a,c 4

Figure A-1 Change in MTC With increase in T Average Above Nominal T- Average s

FM2490E-Aun(930402)$0 A-6

+

_ a,c 4

Figure A-2 Delta MTC Versus Axial Flux Difference AT EOL, HFP, ARO ru24 sos Auncsso4o2)so A-7

APPENDIX B. SEQUOYAH TECHNICAL SPECIFICATION CHANGE PAGES The BASES in the Technical Specification Section 3/4.1.1.3, Moderator Temperature Coefficient, is changed from the current BASES:

3/4.1.1.3 MODERATOR TEMPERATURE COEFFICIENT (MTC)

The limitations on MTC are provided to ensure that the value of this coeficient remains within the limiting conditions assumed for this parameter in the FSAR accident and transient analyses.

The MTC values of this specification are applicable to a specific set of plant conditions; accordingly, verification of MTC values at conditions other than those explicitly stated will require extracolation to those conditions in order to permit an accurate comDarison.

The most negative MTC value equivalent to the most positive moderator density coeficient (MDC), was obtained by incrementally correcting the MDC used in the FSAR analyses to nominal operating conditions. These corrections involved subtracting the incremental change in the MDC associated with a core condition of all rods inserted (most positive MDC) to an all rods withdrawn condition and, a conversion for the rate of change of moderator density with temperature at RATED THERMAL POWER conditions. This value of the MDC was then transformed into the limiting end of cycle life (EOL) MTC value. The 300 ppm surveillance limit mtc value represents a conservation value (with corrections for burnup and soluble boron) at a core condition of 300 ppm equilibrium boron concentration and is obtained by making these corrections to the limiting EOL MTC value.

The surveillance requirements for measurement of the MTC at the beginning and near the end of each fuel cycle are adequate to confirm that the MTC remains within its limits since this coeficient changes slowly due principally to the reduction in RCS boron concentration associated with fuel burnup.

t FM2490E-B.un(930401)$0 B-1

WESTINGHOUSE PROPRIETARY CLASS 2 To the new BASES:

3/4.1.1.3 MODERATOR TEMPERATURE COEFFICIENT The limitations on moderator temperature coefficient (MTC) are provided to ensure that the value of this coefficient remains within the limiting condition assumed in the FSAR accident and transient analyses.

The MTC values of this specification are applicable to a specific set of plant conditions; accordingly, verification of MTC values at conditions other than those explicitly stated will require extrapolation to those condtions in order to permit an accurate comparison.

The most negative MTC, value equivalent to the most positive moderator density coefficient (MDC), was obtained by incrementally correcting the MDC used in the FSAR analyses to nominal operating conditions.

These corrections involved: (1) a conversion of the MDC used in the FSAR Safety analyses to its equivalent MTC, based on the rate of change of moderator density with temperature at RATED THERMAL POWER conditions, and (2) subtracting from this value the largest differences in MTC observed between EOL, all rods withdrawn, RATED THERMAL POWER condtions, and those most adverse conditions of moderator temperature and pressure, rod insertion, axial power skewing, and xenon concentration that can occur in normal operation and lead to a significantly more negative EOL MTC at RATED THERMAL POWER. These corrections transformed the MDC value used in the FSAR safety analyses into the limiting End of Cycle Life (EOL) MTC value.

The 300 ppm surveillance limit MTC value represents a conservative MTC value at a core condition of 300 ppm equilibrium boron concentration, and is obtained by making corrections for burnup and soluble boron to the limiting EOL MTC value.

The Surveillance Requirements for measurement of the MTC at the beginning and near the end of the fuel cycle are adequate to confirm that the MTC remains within its limits since this coefficient changes slowly due principally to the reduction in RCS boron concentration associated with fuel burnup.

Item 4 is added to the ADMINISTRATIVE CONTROLS in the Technical Specification Section 6.9.1.14.a as:

4. WCAP-13631-P-A, " Safety evaluation supporting a more negative EOL moderator temperature coefficient technical specification for the Sequoyah Nuclear Plants", March 1993 (W Propriethry).

(Methodology for Specification 3.1.1.3 - Moderator Temperature Coefficient.)

FM2490E B(930407);5o B2

The LCO EOL MTC and the SR MTC in the COLR are changed as follows:

2.0 OPERATING LIMITS The cycle-specific parameter limits for the specifications listed in Section 1.0 are presented in the following subsections. These limits have been developed using the NRC-approved methodologies specified in TS 6.9.1.14. ,

2.1 Moderator Temoerature Coefficient (Specification 3.4.1.1.3) ,

I

[3/4.1.1.3] ,

2.1.1 The moderator temperature coefficient (MTC) limits are:

The BOUARO/HZP-MTC shall be less positive than 0 Ak/k/ F (BOL limit). j With the measured BOUARO/HZP-MTC more positive than -1.18x10-5 Ak/k/ F (as-measured MTC limit), establish control rod withdrawal limits to ensure the MTC remains less positive than 0 Ak/k/ F for all times in core life.

The EOL/ARO/HZP-MTC shall be less negative than -4.5x10 4 Ak/k/ F.

2.1.2 The 300 ppm surveilla 'ce limit is:

The measured 300 ppm /ARO/RTP-MTC should be less negative than or equal to -3.75x104 Ak/k/ F. ,

where: BOL stands for Beginning of Cycle Life ARO stands for ALL Rods Out HZP stands for Hot Zero THERMAL POWER EOL stands for End of Cycle Life RTP stands for RATED THERMAL POWER l

l FM2490E-B un(930401) 50 B-3

REFERENCES

1. Nguyen, T. O. et. al. Qualification of the PHOEN/X-P/ANC Nuclear Design System for Pressurized Water Reactor Cores, WCAP-11596-P-A (Westinghouse Proprietary),

June 1988.

2. Liu, Y. S., et al. ANC: A Westinghouse Advanced Nodal Computer Code, WCAP-10965-P-A (Westinghouse Proprietary), December 1985.

3. Barry, R. F., et al. The PANDA Code WCAP-7048 (Westinghouse Proprietary), April 1967.

4. Ford, W. E., et. al.CSRL-V: Processed ENDF/B-V 227-Neutron-Group and Point-wise Cross Section Libraries for Criticality Safety, Reactor and Shielding, NUREGICR-2306, ORNilCSD/TM-160 (1982).

u. Davidson, S. L., Kramer, W. R., Westinghouse Reload Safety Evaluation Methodology, WCAP-9272-P-A (Westinghouse Proprietary), July 1985.

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