ML20082L128

Nonproprietary Safety Evaluation Supporting More Negative Eol Moderator Temp Coefficient TS for South Texas Project Units 1 & 2

ML20082L128
Person / Time
Site:	South Texas
Issue date:	06/30/1991
From:	Love D, Silva F WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML20082L118	List: ML20082L125 ML20082L128 ST-HL-AE-3831, Forwards Nonproprietary WCAP-13005 & Proprietary WCAP-12942, Safety Evaluation Supporting More Negative Eol Moderator Temp Coeffcient TS for South Texas Project Units 1 & 2. Proprietary WCAP-12942 Withheld (Ref 10CFR2.790)
References
WCAP-13005, NUDOCS 9109030247
Download: ML20082L128 (39)

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WCAP-13005

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I SAFETY EVALUATION SUPPORTING A MORE NEGATIVE EOL MODERATOR TEMPERATURE

E C EFFICIENT TECHNICAL SPECIFICATION

^ g FOR THE S0lTTH TEXAS PROJECT UNITS 1 AND 2 June, 1991

• l R. C. Cobb iW R. A. Wiley G. K. Roberts y

APPROVED:

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_.,_ /.PPROVED:

G. h, kockus4 b~

F. J, Llva, Manager GD. S. Love, Manager 4

i Core D sign Transient Analysis I lI lI I

C 1991 by Westinghouse Electic Corporation I

l WESTINGHOUSE ELECTRIC CORPORATION Comercial Nuclear Fuel Division I

P. O. Box 355 Pittsburgh, Pennsylvania 15230 l

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ABSTIMCT

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This repott propos es a relaxatic ' of t he 1.imiting C tion for Operation and

,4 Survoillance Requirements values of Moderator Temperature Coefficient for the end of cycle, rated thermal power condition.

Relaxation is sought in order to improve plant availability and minimize disruptions to normal plant operation I

while continuing to satisfy plant safety criteria. A methodology for establishing Technical Specification end of cycle Moderator Temperature Coefficient values that are consistent with the plant safety analyses is described herein.

Specific application of t'ne methodology to South Texas Project Units 1 and 2 provideo Technical Specification Moderator Temperature Coefficient values which are proposed to replace the existing values. This replacement is accomplished without affecting the basis of the safety analysir.,

I which remains bounding.

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TABLE OF CGNTENTS I

Seetion ILtle Egge

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1.0 INTRODUCTION

1-1

1.1 Background

1-1 J

1.2 Basis of Current EOL MTC 12 LCO and SR Values 2.0 METHODOLOGY FOR MODIFYING MOST I

NECATIVE MTC TECHNICAL SPECIFICATION VALUES 21 2.1 Conversion of Safety Analysis 21 MDC to Technical Specification MTC 1

2.2 Conservatism of the ARI to ARO 2-2 MTC Conversion 2.3 Alternative MTC Conversion 2-2 Approach l

2.4 Determining SR MTC from LCO MTC 24 2.5 Benefits of the Alternative 2-5 MTC Conversion Approach 3.0 MOST NECATIVE FEASIBLE MTC APPROACH APPLIED TO SOUTH TEXAS PROJECT UNITS 1 AND 2 31 3.1 South Texas Project Units 1 and 2 31 Accident Analysis MDC Assumption 1

3.2 Determination of Most Negative 32 Feasible MTC Sensitivities 3.3 Maximum Allowed Deviations from 35 I

Nominal Operating Conditions 3.4 Overall "AMTC" Factor for the South 3-10 Texas Project Units 1 and 2 Reloads l

3.5 Proposed South Texas Project Units 1 3-12 l

and 2 Technical Specification EOL MTC LCO Value 4.0 SAFETY ANALYSIS IMPACT OF MOST NECATIVE 4-1 FEASIBLE MTC APPROACH 5.0 DETERMINATION OF MOST NECATIVE FEAc'BLE 51 MTC SURVEILIANCE VALUE

6.0 CONCLUSION

S 6-1 REFERENCES R-1 APPENDIX A DETERMINATION OF MOST NECATIVE FEASIBLE MTC SENSITIVITIES A-1 A.1 MTC Sensitivity to Moderator A2 Temperature and Pressure Variation A.2 MTC Sensitivity to RCCA Insertion A-4 A.3 Sensitivity to Axial Flux A-4 (Power) Shape A.4 Sensitivity to Transient Fission A-5 Product (Xenon) Concentration wea13005.wp 11

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LIST OF TABLES

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I Table Title Egge f

4.1 UFSAR Chapter 15 Non LOCA Events That Asswne 4-2 l

I a Constant 0.43 Ap/gm/cc MDC Value 6.1 South Texas Project Units 1 and 2 Moderator 63

[

I Temperature Coefficient Limiting Conditions for Operation and Surveillance Requirement I

LIST OF ILLUSTRATIONS I

Figure Title Eage A.1 Change in MTC with Increase in Moderator A-7 I

Temperature above Nominal Moderator Temperature A.2 AMTC versus Axial Flux Difference A-8 at EOL, HFP, ARO I

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1.0 INTRODUCTION

This report presents the details of a proposed relaxation of the Technical Specification Limiting Condition for Operation (LCO) and Surveillance Requirements (SR) values of Moderator Temperature Coefficient for the end of cycle, rated thermal power (RTP) condition for the South Texas units.

This section of the report provides some background on the need for Moderator Temperature Coefficient (MTC) limits within the Technical Specifications, and the basis of the current values of those limits.

Section 2 describes the methodology for modifying the end of cycle life (EOL), RTP MTC LCO and SR values, and Section 3 presents the results of.he application of this methodology for the determination of a revised EOL RTP MTC LCO value for the South Texas units.

Section 4 discusses the impact of this revised approach on the safety analyses.

The derivation of a revised EOL RTP MTC SR value for the I

South Texas units is given in Section 5, and the conclusions of the report are summarized in Section 6.

Appendix A provides the details of the determination of the proposed revised EOL RTP MTC LCO value presented in Section 3.

The revisions proposed in this report are accomplished by modifying the assumptions used in the conversion of the limiting moderator dennity coefficient (MDC) assumed in the safety analyses to a corresponding MTC.

The basis of the safety analyses is not affected, and the safety analyses therefore remain bounding.

I

1.1 Background

For the Updated Final Safety Analysis Report (UFSAR) accioent analyses, the transient response of the plant is dependent on reactivity feedback ef fects, in particular, the MTC and the Doppler power coefficient.

Because of the sensitivity of accident analyses results to the MTC value assumed, it is important that the actunt core MTC remain within the bounds of the limiting values assumed in the UFSAR accident analyses. While core neutronic analyses will have predicted that the MTC is within these bounds, the Technical Specifications require that the core MTC also be confirmed by measurement, as verification of the accuracy of the neutronic predictions.

wca13005.=y 1-1

I These MTC measurements are performed:

1) at beginning of-cycle life (BOL), prior to initial operation above 5%

rated thermal power, and I

2) within ? Effective Full-Power Days (EFPD) after reaching an equilibrium boron concentration of 300 ppm.

I 1.2 Basis of Current EOL MTC LCO and SR Values In order to ensure bounding accident analyses, 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 the control rods fully inserted.

I Most accident analyses use a constant Moderator Density Coefficient (MDC) which is designed to bound the MDC based upon the most limiting set of initial conditions as well as transient conditions.

This MDC value forms the licensing basis for the UFSAR accident analysis.

I

~,onverting the MDC used in the accident analyses to an equivalent 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.

I Navertheless, it is important to note that the accident analyses actually assume a constant MDC value rather than making any explicit assumption on MTC.

Technical Specifications place both Limiting Condition for Operation (LCO) and Surveillance Requirement (SR) values on the'MTC which are based on the accident analysis MDC assumptions 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 lin.it value. The most negative MTC LCO limit applies to Modes 1, 2, and 3, and requires that the HTC be less negative than the specified limit value I

for the all rods withdrawn, EOL RTP condition.

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iil The Technical Specification SR calls for measurement of the MTC at BOL of each w

j cycle prior to initial operation above 5% RTP, in order to demonstrate compliance with the most positive MTC LCo.

Similarly, to demonstrate compliance t

with the most negative MTC LCO, the Technical Specification SR calls for measurement of the MTC prior to EOL (near 300 ppm equilibrium boron concentration).

The HFP MTC value will gradually become more negative with 1ig further core depletion and boron concentration reduction.

The 300 ppm SR value

!E of the MTC should therefore be less negative than the EOL LCO limit.

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2.0 MET 110D0!AGY FOR MODIFYING MOST NEGATIVE MTC TECHNICAL SPECIFICATION VALUES 2.1 Conversion of Ssfety Analysis MDC to Technical Specification MTC As stated previously, the UFSAR 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 the accident analysis most I

positive MDC is transformed into the most negative MTC LCO value is stated in the 5,uth Texas Project Technical Specification BASES, Section 3/4.1.1.3:

"The most negative MTC value, equivalent to the most positive moderator density coefficient (MDC), was obtained by incrementally correcting the MDC i

used in the FSAR accident analyses to nominal operating conditions. These

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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

W moderator density with temperature at RATED THERMAL POWER conditions.

This value of the MDC was then transformed into the limiting MTC value" In the process of converting the accident analysis MDC into a 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 chante associated with going from a condition of ARI to one

'I of ARO.

The accident ana1ysis MDC (MTC) assumes a coefficient determined for a condition of EOL, HFP at 0 ppm with all cor. trol and shutdown banks fully inserted.

The f

accident analysis MDC (MTC) is corrected back to the ARO condition in order to produce a Technical Specification limit which permits direct comparison against I

measured values.

The effect of the presence of all control and shutdown banks is to make the MTC markedly more negative than a MTC value at the ARO condition.

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2.2 Conservatism of the ARI to ARO MTC Conversion

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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.

The limitation of the ARI assumption is that it 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.

In addition, the HFP ARI assumption is inconsistent with technical specification requirements for allowable operation.

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 hasis for converting f rom the accident analysis MDC value to a corresponding Technical Specification LCO MTC value is overly restrictive.

The proposed concept is termed the "Most Negative Feasible MTC" approach.

This approach maintains the existing accident analysis assumption of a bounding value of moderator density coefficient.

At the same time, it of fers 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 plant 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 the plant technical specifications permit, so as to make the calculated EOL MTC more negative than it would be for an unrodded core.

The Most Negative Feasible MTC approach determines EOL MTC sensitivity to those design and operational parameters that directly impact MTC, and attempts to make this determination in such a manner that the resulting sensitivity for one wea13005.wp 2-2

I parameter is independent of the assumed values of the other parameters.

As a result, parameters which are mutually exclusive but permissible according to the plant 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 the MTC more negative, will have their incremental impacts combined to I

arrive at a conservative and bounding condition for the most negative feasible MTC.

The parameters which are variable under normal operation and which directly affect MTC are:

soluble boron concentration in the coolant moderator temperature and pressure RCCA insertion axial flux (power) shape I

transient fission product (xenon) concentration Note that this list does not include the relative core power level since the LCO applies specifically to the rated thermal power condition.

The Most Negative Feasible MTC approach examines each parameter separately, and assesses the impact of variation in that parameter on the EOL MTC.

This assessment is performed for multiple core designs that feature combinations of fuel design, discharge burnup, cycle length, and operating temperature expected to envelope future core designs of the plant of interest.

I 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, average temperature on the reference temperature program) that the Technical Specifications permit, and multiplies that devirtion by the appropriate MTC sensitivity to arrive at a "MTC" factor associated with the parameter.

I For example, suppose it is determined that the MTC becomes 1 pcm/ F more negative for each l'F increase in core average operating temperature above I-nominal (the MTC "sennitivity" is -1 pcm/ F/"F).

If the technical specifications permit a maximum increase in the average temperature of 4"F above the nominal core average temperature, then the moderator temperature " MTC "

factor is:

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( 1 pcm/*F/ F) x 4 F - 4 pcm/'F.

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

This overall "t2MTC" f actor states how much more negative the MTC I

can become, relative to the nominal EOL HFP ARO MTC value, for normal operation scenarios permitted by the current technical specifications.

The conditions of moderator temperature, rod insertion, xenon, etc., which defined the Most Negative Feasible MTC condition become the conversion proposed as a replacement for the AR1 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.

I 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 BASES for the South Texas Project MTC Technical Specifications:

I "The 300 ppm surveillance MTC value represents a conservative value (with corrections for burnup and soluble boron) at a core condition of 300 ppm I

equilibrium boron concentration and is obtained by making these corrections to the limiting EOL MTC value."

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

Plant-specific examination of the difference between the 300 ppm HFP MTC and the 0 ppm EOL HFP MTC suggests that the current 9 pcm/*F which has historically been applied to k'estinghouse-designed plants remains applicable for future reloads of South Texas Project Units 1 and 2.

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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 specified by current plant Technical Specifications for the following reasons:

1.

The Most Negative Feasible MTC approach does not require an unduly positive 300 ppm surveillance value that would result in repeated MTC surveillance measurements.

These repeated measurements are undesirable in that they entail perturbations to normal reactor operation.

2.

The Most Negative Feasible MTC approach does not alter the UFSAR 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 MLC assumption and the Technical Specification LCO MTC value that assures that the plant cannot experience a MDC which is more severe than that asswned in the accident analyses.

3.

The Most Negative Feasible MTC condition is a conservative but reasonable basis to assume for a 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 technical specification requirements.

Additionally, the Most Negative Feasible MTC approach retains the " built in safeguard" of a requirement for a 300 ppm surveillance measurement to be performed in order to verify that the reactor is operating in a regime that is bounded by the accident analysis input assumptions, wce13005 wp 2-5

3.0 MOST NEGATIVE FEASIBLE MTC APPROACH APPLIED TO SOUTH TEXAS PRO,iECT UNITS 1 AND 2 I

Using the Most Negative Feasible MTC approach described in Section 2, a revised EOL RTP MTC LCO has been developed for the South Texas Project units.

The assumptions involved, and the results obtained are presented in this section.

3.1 South Texas Project Units 1 and 2 Accident Analysis MDC Assumption The UFSAR accident analyses as.umed bounding values of moderator density coefficient in order to ensure a conservative simulation of plant transient response for the South Texas Project units.

For those transients in which analysis tesults are made more severe by assuming maximum moderator feedback, a moderator density coefficient (MDC) of 0.43 Ap/gm/cc* has been assumed to exist I

throughout the transient.

When discussing the Technical Specification EOL LCO limit on moderator feedback, it is more common to refer to the moderator temperature coefficient (MTC) rather than the MDC.

For this reason, the South Texas Project Units accident analysis MDC assumption of 0.43 Ap/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 I

conditions of interest are the core temperature and pressure (hence, density) experienced under the current and reduced T m conditions for normal operation at which the MDC assumes its most extreme (positive) value. These temperature and pressure conditions for the South Texas Project units are a range of full flow RTP vessel average nominal operating temperature ** conditions between 582.3'F (core average temperature of 585.6 F) and 593.0 F (core average temperature of 596.5*F) and a RTP pressure of 2250 psia, respectively.

I Ap - AK/K I

Vessel average temperature is the average of the hot and cold leb temperatures, as indicated in the control room.

Core average temperature is the average temperature of the coolant within the active fuel region which excludes the bypass flow.

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I Assuming these nominal RTP operating conditions with a vessel average temperature of 582.3'F, the accident analysis MDC value of 0.43 ap/gm/cc is equivalent to -53.8 pcm/ F.

Assuming the nominal RTP operating conditions with a vessel average temperature of 593 F, the accident analysis MDC value of 0.43 Ap/gn/cc is equivalent to 57.6 pcm/'F.

For simplicity, these values of MTC will often be referred to as the " accident analysis MTC", in the discussion which follows. Howeser, it should be remembered that the applicable accident analyses actually assume a constant MDC value of 0.43 Ap/gm/cc and make no explicit assumption about the 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 MTC and are variable under normal core operation. The list I

of parameters is as follows:

soluble boron concentration in the coolant moderator temperature and pressure RCCA insertion axial flux (power) shape transient fission product (xenon) concentration The radial flux (power) shape can also vary under normal core operation and will I

affect MTC.

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 the MTC will be an implicit part of the MTC sensitivity to these other parameters.

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 ascumed to be essentially a O ppm limit by virtue of the definition of EOL.

The most negative f

MTC value will always occur at a boron concentration of 0 ppm, and therefore, a O ppm boron concentration is assumed as the basis of the EOL MTC Technical Specification limit under the Most Negative Feasible MTC approach.

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s 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 perf%rming a MTC determination with the parameter held in the perturbed state.

A further perturbation was induced and the MTC calculation was performed again. This sequence was repeated until a sufficient number of I

MTC values were generated to reliably determine the trend of MTC change with variation in the value of the independent parameter.

I In order to establish trends in MTC that are appropriate and bounding for the South Texas reload cores, these i.ensitivities were determined for six dif f erent reloads identified as A through F.

Reloads D and F are South Texas cores, while the othets are similar to the South Texas designs.

These cores exhibit design features that are expected to bound those of future South Texas reload cores I

(such as increased discharge burnup, higher enrichment, longer cycles, and advanced fuel product features).

A brief description of the six reload core designs follows:

RELOAD A:

This is a reload core for a Westinghouse designed 4 loop plant. It utilizes the Westinghouse 17x17 standard diameter fuel rod design, feeds 84 assemblies in a low leakage loading pattern (L3p) and I

assumes a region average discharge burnup of 36000 MVD/MTU.

This I

reload also has 448 pyrex glass burnable absorbers (bas) and a nominal cycle length of 13800 MWD /MTU.

The nominal core average temperature assumed in this analysis is 590. 5 F.

I RELOAD B:

This core extends the above design to higher burnup and new product features.

It is a L3P design feeding 76 assemblies with axial blankets, approximately 4500 part-length integral fuel burnable absorbers (IFBA), high enrichments, and a region average discharge burnup of 45000 MVD/MTU.

The cycle length is 16500 MVD/MTU and all other core operating parameters are assumed to be the same as Reload A.

RELOAD C:

This reload is a conceptual equilibrium 24 month (21830 MWD /MTU) cycle design with advanced fuel product features including the wceDOOS.wp 3-3 l

features in RElDAD B plus Zircaloy grids and Intermediate Flow Mixer (1FM) grids.

A thimble plug removal analysis is also assumed which increases the core bypass flow and raises the nominal core average moderator temperature to 591. 3*F.

REMAD D:

This reload was an original core design for South Texas Project s

Unit 1 Cycle 2.

It is an annual cycle L3P design using the I

Westinghouse 14-foot length 17x17 ft.el assembly with standard diameter fuel rods.

The discharge burnup for this design is necessarily low since this is the first reload core for South Texas Project Unit 1.

This design has 416 pyrex glass bas.

The nominal vessel average moderator temperature is 593.0 F (core average temperature of 596.5 F) at full power.

RELOAD E:

This reload core is a Westinghouse 4-loop plant with 17x17 standard diameter fuel rods.

A L3P design is used for a cycle I

length of 17000 MVD/MTU and a region average discharge burnup of 38000 MVD/MTU.

This design also feeds 84 assemblies and uses 1728 pyrex glass bas. The control rod absorber material is silver-indium-cadrnium (Ag-In-Cd).

The nominal core average moderator temperature is 575.3 F.

RELOAD F:

This reload is the actual core design for South Texas Project Unit I

1 Cycle 4.

A L3P design is used for a cycle length of 16620 MVD/MTU and an expected region average discharge burnup of I

approximately 38000 MVD/MTU. Vestinghouse 14-foot length 17xl?

fuel assemblies with standard diameter fuel rods are used for this design.

This design has 1376 pyrex glass bas and Ag-In-Cd control l

rods, A range of vessel average temperatures between 582.3'F (core average temperature of 585.6*F) and 593"F (core average temperature of 596.5*F) at full power was assumed.

Reloads A, B, D, E, and F were used in the c.iiculation of the sensitivities to the four parameters described previously.

To provide additional information regarding the effect of extremely long cycles and high discharge burnups on the m a n ce s.-p 3-4 i

s MTC sensitivity to moderator temperature and pressure and xenon concentration, Reload C was also used in the determination of sensitivities.

L The core neutronic models of these six reload cores were derived using standard Westinghouse procedures and computer methods.

For reloads A, B,C, D and E, the ARK code, which has evolved from the LEOPARD (D and CINDER (U codes, was used to perform the fast and thermal spectrum calculations and is the basis for all B

cross sections, depletion rates, and reactivity feedback models.

For reload F, the PHOENIX-P code (M was used as the basis for these parameters.

ANC(H and PA1ADON( 0, nodal analysis theory codes used in two and three dimensions, were used for core neutronic calculations to determine MTC sensitivity for the six reload cores. APOLLO, an advanced version of PANDA (D, was used as an axial neutronic model of the reload cores to determine the MTC sensitivity to variations in axial flux shape.

The neutronic calculations and evaluations performed for the six reload core I

designs established 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 l

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 PDC) assumption.

It is also necessary to demonstrate that the parameter changes that occur throughout the transient do not result in a MTC value which is unbounded by the moderator coefficient essumption used in the accident analysis. The adequacy of the constant MDC accident analysis assumption to bound MTC values that occur throughout the transient is examined in Section 4.

The bases for the maximum allowed deviation from nominal operating conditions are technical specifications that limit the extent of the moderator temperature increase, RCCA insertion, and axial power skewing.

The deviations permitted by I

wee D005,wp 3-5 l

L the present South Texas Project Unita 1 and 2 Technical Specifications, and

(

possible future perturbations to those technical specifications values, are discussed in the following sections.

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i wea13003.wp 3-6

4 Moderator Temperature and Pressure Deviations

~

Technical Specification 3.2.5 establishes LCO and SR values of DNB related parameters such as the reactor coolant system (RCS) average temperature and 4

pressurizer pressure.

For both of the Snuth Texas Project unito Technical I

Specification 3.2.5 states a maximum allowable indicated RCS average temperature of 598.0 F and a minimum allowable indicated pressurizer pressure of 2201 psig.

Because the current nominal design vessel average temperature for the South 4

Texas Project units is 593.0 F.

the 598.0 F technical specification limit represents a 5.0'F maximum allowable average temperature increase over nominal conditions. The current nominal design pressure for the South Texas Project units is 2250 psia (2235 psig); therefore, the 2201 psig technical specification limit represents a 34 psi maximum reduction from nominal system pressure.

The g

safety analyses for the South Texas Project units assume uncertainties of 5

+/- 5'F for the vessel average temperature and +/- 46 psi for the pressurizer pressure.

To be conservative, these deviations in RCS temperature and pressure were 4

increased to [

)+"F and [

}+ psi, respectively, for this sensitivity l(a,c) 1 analysis, These conservative deviations from the nominal RCS temperature and pressure also apply to the Tam reduction case.

The maximum temperature and pressure deviations are applied to the MTC sensitivity to temperature and

!g pressure, which is described in Appendix A, to obtain a "AMTC" factor associated E

with RCS moderator temperature and pressure deviations from nominal. The resulting "AMTC" is [

]+ pcm/ F.

l(a,c)

RCCA Insertion Deviation I

The nominal condition assumption for Rod Cluster Control Assembly (RCCA) placement is complete withdrawal of the rods to the full out position (FOP).

Normally, by EOL, the rods are fully out of the fuel.

This assumption is underscored by the requirement in Technical Specification 3.1.1.3 that the LCO I

value of EOL MTC be for the "all rods withdrawn" condition.

Becav a some RCCA insertion is allowed during full power operation, and because RCCA insertion will generally cause the MTC to be more negative than it would be otherwise, the I

wea13005 wp 3-7

s RCCA insertion deviation is simply the maximum allowable RCCA insertion permitted by the technical specifications.

s c

Technical Specifications 3/4.1.3.5 and 3/4.1.3.6 place limits on allowable RCCA l

Insertion.

Technical Specification 3/4.1.3.5 precludes Shutdown RCCA insertion in Modes 1 and 2, and Technical Specification 3/4.1.3.6 limits Control Bank insertion via the Rod Insertion Limits (RILs).

At full power, the Rlls for the South Texas Project units permit insertion of the lead control bank to 174 steps withdrawn.

Ilowe ve r, strict applicatman of these current RILs in determining the "AMTC" factor associated with RCCA insertion may prove to be restrictive if minor changes to the RILs become necessary in the future.

For to:s reason, the liFP RCCA insertion assumed for this analysis is [

}' steps l( a, c )

withdrawn. This additiona! insertion is expected to bound minor RIL adjustment which may be necessary ror optimizing core performance characteristics of future South Texas reloads.

This limiting ilFP RCCA insertion of [

}' forms the basis for the determination of MTC sensitivity to llFP RCCA l(a,c) insertion for the South Texas Project units, which in described in Appendix A.

The resulting "AMTC" factor associated with RCCA insertion was determined to be

[

l' pern/ F.

This is much smaller than the substantial impact on the MTC l(a,c)

I which would be expected for the insertion of all 57 RCCAs since D bank includes only five RCCAs. This "AMTC" factor is applicable over the range of vessel average temperatures as described in Section 3.1.

Asial Flux (Power) Shatse Deviatten As indicated earlier, 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 I

detailed shape itself is not so important, but rather the " balance" of the flux shape, in terms of how much moderator heati..g occur 4 in the lower half of the core versus the upper half of the core The influence which axial power shape l

l

>.e 1

has on MTC can, therefore, be captured by quantifying this axial flux " balance".

This balance is best quantified by the core's Axial Flux Difference (AFD).

I 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 I

the MTC will become.

The axial flux (power) shape deviat

.n is, therefore, determined by how negative the AFD is allowed to become under normal, full power operating conditions.

The South Texas units employ a CAOC Technical Specification which sets the allowable full power AFD limits at +3% and -12%.

To assign a "MTC" factor attributable to axial flux shape, one need only examine the MTC effect associated with the -12% " deviation" from a most negative expected target AFD value of approximately [

]+ %.

However, to account for possible future changes I

in the most negative HFP AfD limit, an AFD value of [

)* % is selected as the (a,c) basis of the axial flux (power) shape deviation. This [

]* % AFD deviation is applied to the MTC sensitivity to axial flux (power) shape, which is described in Appendix A, to obtain a "MTC" factor associated with an AFD deviation from a perfectly balanced axial flux shape.

The resulting "MTC" factor is [

}'

l(a.c)

-pcm/ F.

Again, this "MTC" factor is applicable over the range of vessel l

average temperatures as described in Section 3.1.

Transient Fission Produer (Xanon) Concentration Deviation Xenon is the most significant transient fission product in terms of effects on core reactivity and flux distribution.

Therefore, its possible impact on MTC is investigated to compute the final "MTC" factor to include in the Most Negative Feasible MTC approach. While the plant technical specifications 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 practical limits on the concentration.

I Because axial xenon distribution directly impacts the axial flux shape, this aspect of xenon effect on MTC is implicitly included in the axial flux (power)

I g

3.e

shape deviation discussed above.

k' hat remains to be quantified is the impact of l

the overall xenon concentration in the core.

L I

Taking the EOL hFP ARO equilibrium xenon concentration to be the nominal xenon L

condition for the core, ic was determined for low leakage cora designs of the type characteristic of South Texas reloads that the MTC becornes more negative with a reduced xenon concentration. Accordingly, the most negative MTC results I

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 the maximum allowable temperature (minimum allowable density) and, therefore, occurs at full power conditions. k'hll e 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 tn extended shutdown period presents a reasonable scenario for full power operation I

with a comparatively low xenon concentration in the core, For this reason, the

" xenon deviation" to be used in conservatively determining the "AMTC" factor attributable to transient fission product is a change from HFP ARO equilibrium xenon to no xenon in the core. The resulting "AMTC" factor for the range of vessel average temperatures as specified for the South Texas Project units in Section 3.1 is (

}+ pcm/ F.

l(a,c) l 3.4 Overall "AMTC" Factor for South Texas Project Units 1 and 2 Reloads The preceding section has concluded that the most adverse operation possible, in terms of achieving the most negative EOL MTC under current South Texas Project Units 1 and 2 Technical Specification, would feature the following values of key parameters:

l

- Core Moderator Temperature:

[

]*

F above HFP nominal P

- Core Moderator Pressure:

[

]+ psia

- HFP RCCA inserticn:

[

l

]+ steps (a,c) withdrawn

- HFP mosc negative AO:

[

l' %

- HFP xenon concentration:

0%

g I

.c. u o o t.p 3-10

m.

When these maxirnum allowable deviations from a nominal condition of EOL liFP ARO, with equilibrium xenon, and 0 ppm boron are applied to the individual parameter sensitivities discussed in Appendix A, the overall "AMTC" factor is determined.

This overall factor for the South Texas units is computed as follows:

Core Moderator Temperature and Pressure Factor:

[

j' pctn/ F HFP RCCA Insertion Factor:

[

]* pcm/'F Axial Flux (Power) Shape Factor:

[

]' pcm/*F (a,c)

Xenon Concentration Factor:

l' ocm/*F Overall "AMTC" Factor:

[

]+ pcm/ F The interpretation of this overall "AMTC" 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 f or perforrning a MTC experiaient and obtaining results that can be meaningfully compared to design predictions.

It is not, however, the condition under which the MTC can achieve its most negative value under normal operation scenarios permitt-d by the plant technical specifications.

The conservative "AMTC" formulation has concluded that the actual core MTC can be as much as [

l' pcm/ F more negative l(a,c) than the predicted HFP, ARD, EOL MTC.

The individual comporents of this [

]' pcm/'F overall "AMTC" factor have been l(a c) determined on a conservative basis and are expected to bound the values predicted for South Texas 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 af all the components remains bound by the [

l' pcm/ F overall "AMTC l(a.c) 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 "AMTC" factor be dernonstrated to be bounded by the licensing basis value, rather than specifIcally addressing the individual components.

Such validation of the Most Negative Feasible MTC approach on a cycle-specif te basis would be performed as part of the reload core design process described in Reference 6.

wt a D00 5 wp 3-11

I 3.5 Proposed South Texas Project Units 1 and 2 Technical Specification EOL MTC LCO Value I

As was pointed out in Section 3.1, the safety analyses which support the South Texas Project units UFSAR have assumed a MDC value which, when converted to a MTC at nominal HFP conditions, is equivalent to a MTC of between -53.8 pcm/ F and -57.6 pcm/ F for the range of full power vessel average temperatures between I

582.3 F and 593.0 F, respectively.

At no time may the actual core be allowed to experience a MTC more negative than these limits, as this would invalidate an assumption of the safety analyses.

The Most Negative Feasible MTC approach guarantees that such a situation will not occur by subtracting from this equivalent MTC value the [

]

• pcm/ F " AMTC" factor determined for both South l(a,c)

Texas Project units.

The resulting range of equivalent MTC values is then calculated to be [

]* pcm/ F to [

]* pcm/ F for the range of full power l(a,c) vessel average temperatures between 582. 3"F and 593.0 F, respectively, and would be the technical specification EOL LCO value of MTC under the Most Negative I

Feasible MTC approach. As an additioral measure of conservatism, this range is modified to -44.0 pcm/*F to -48.0 pcm/ F for the range of full power vessel average temperatures between 582.3 F and 593.0 F, respectively, and proposed as the EOL HFP ARO Technical Specification MTC LCO value for both South Texas reload cores.

This range replaces the current LCO value of -40 pcm/*F.

The EOL MTC LCO is presented as a function of vessel average temperature in Table 6,1.

I The range of limits proposed 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 average temperature, low system pressure, and no xenon, represents a compounding of worst-case events which can be considered independent, yet the Most Negative Feasible MTC approach assumes that they occur simultaneously.

Determination that the core MTC is less negative than this range of limits at EOL HFP ARO conditions provides assurance that the assumption on initial condition MTC made in the plant accident analyses remains bounding.

I 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.

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4.0 EAFETY ANALYSIS IMPACT OF MOST NEGATIVE FEASIBLE MTC APPROACH Most of the South Texas Project (STP) non LOCA accident analyses conservatively model the various reactivity coefficients to produce bounding results.

As discussed in Section 3.1, the applicable analyses assume a constant MDC of 0,43 ap/gm/cc to bound the predicted moderator reactivity insertion.

The STP non 14CA events which assume this EOL MDC value 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 proviSs the basis for a contervative 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 I

will not result in a reactivity insertion that would invalidate the conclusions of the UFSAR accident analyses. Therefore, the 0.43 ap/gm/cc assumption used as the basis for the Most Negative Feasible MTC Technical Specification will not change.

I As aiscussed in Reference 6, the reactivity coefficients assumed can have a I

strong influence on the accident analysis results.

Since the moderator coefficient can be affected by a reload, the conservative nature of this accident analysis assumption must be confirmed on a cycle-specific basis using 3

the methodology discussed in Reference 6.

This includes verification that the most adverse accident conditions discussed above do not invalidate the conservative nature of the accident analysis assumption. This process ensures j

the ability to verify that the applicable safety limits are met for each reload design and, consequently, that the STP Tech'nical Specifications are met.

I iI I

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~~

I TABLE 4.1

^

UFSAR Chapter 15 Non-1ACA Evento That Assume A Constant 0.43 Ap/gm/cc MDC Value Section Event I.

15.1,1 Feedwater System Malfunctions Causing a Reduction in Feedwater Temperature I

15.1.2 feedwater Syste.s Aalfunccions Causing an Increase in feedwater Flow 15.1.3 Excessive Ir. crease in Secondary Steam Flow 15.2.2 Loss of External Electrical Load 15.2.3 Turbine Trip 15.2,8 Feedwater System Pipe Break 15.4,2 Uncontrolled Rod Cluster Control Assembly Bank Withdrawal at Power 15.4.4 Startup of an Inactive Reactor Coolant Pwsp at an incorrect Temperature 15.5.2 Chemical and Volume Control System Malfunction That Increases Reactor Coolant Inventory Steamline Break at Power

• I This plant-specific analysis, which is not presented in the UFSAR, is used to support the STP plant license.

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5.0 DETERMINATION OF MOST NECATIVE FEASIBLE MTC SURVEILLANCE VALUE Section 2.4 discussed the adequacy of the 9 pcm/ F b tween the Technical Specification 300 ppm MTC SR value and the EOL !TP ARO MTC LCO value for both South Texas t nits.

Typical 17xl? 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 pcm/ F.

I Design predictions for recent South Texas 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 [

]+ pcm/*F to a maximum of [

} + pcm/ F.

l(a,c)

In reviewing the differences between predicted 300 ppm and EOL MTC values for the reload cores of 4-loop plants which are similar to the South Texas units, an important trend was discerned.

It was observed that the higher core average I

enrichments associated with increasing the discharge burnup tended to decrease the magnitude of the MTC difference. As future South Texas reload cores are expected to increase enrichment and fuel discharge burnup levels, it is anticipated that the difference between the HFP 300 ppm MTC and the HFP E01. MTC will become less in the future. This [

]+ pcm/ F should, therefore, bound the l(a,c) 300 ppm MTC to EOL MTC differences for future South Texas reload cores.

I The proposed technical specification SR value for both South Texas Project reload cores ranges from -35 pcm/ F to -39 pea / F for a range of vessel average I

temperatures between 582.PF and 593.0"F, respect!vely. This value is 9.0 pcm/*F less negative than the EOL LCO MTC value proposed in Section 3.5. The 9.0 pcm/ F adequately bounds the expected "AMTC" maximum [

]+ pcm/ F l(a,c) difference observed for recent South Texas reload cores. While this range of 300 ppm Technical Specification SR value is expected to be bounding for future South Texas fuel management scenarios, the validity of this SR value will be confirmed on a cycle-by-cycle basis as part of the reload core desig,n process described in Reference 6.

The EOL MTC SR is presented as a function of vessel average temperature in Table 6.1.

I I

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6.0 CONCLttS10NS The present South Texas Project Units 1 and 2 Technical specification values are

-40 pcm/ F for the EOL HFP ARO MTC LCO, and 31 pcm/ F for the 300 ppm HFP ARO SR.

These are overly restrictive and could potentially require repeated I

deviations from nominal plant operation.

If the 300 ppm HFP ARO SR is not met, then the MTC must be measured every 14 EFPD for the remainder of the cycle.

An alternative methodology 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 conditions permitted by technical specifications.

The Most Negative Feasible MTC approach assumes that these largely independent extreme situations occur simultaneously.

In the worst case, they serve to make the EOL HFP MTC [

]* pcm/"F more negative than it would be at nominal l(a,c)

I conditions.

k' hen this value is subtracted f rom the MTC equivalent of the accident analysis asstuned MDC value, the resulting range of MTC values is calculated to be [

]+ pcm/ F to [

]* pcm/ F for the range of vessel l(a,c) average temperatures between 582.3 F ano 593 F, respectively. The more conservative range of -44 pcm/ F to -48 pcm/'F is proposed as the EOL HFP MTC Technical Specification LCO limit under the Most Negative Feasible MTC approach.

I Examination of the difference between the 300 ppm HFP equilibrium boron concentration MTC value and the EOL HFP MTC values concluded that a bounding I

expected difference between these two MTC values for both South Texas reload cores is -9.0 pcm/ F.

This dif ference is subtracted f rom the proposed range of EOL HFP MTC Technical Specification limit to arrive at a proposed Technical Specification 300 ppm HFP MTC SR range of -35 pcm/cF to -39 pcm/ F for the range of vessel average temperatures between 582.3 F and 593.0 F, respectively, The EOL MTC LCO and SR are presented as a function of vessel average temperature in I

Table 6.1.

It is concluded that the Technical Specification EOL MTC LCO and 300 ppm SR I

values proposed under the Most Negative Feasible MTC approach are bounded by the UFSAR accident analyses.

Each rr'oad cycle will be evaluated to ensure that the core design remains within these bounds.

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~"~

_.. - _. _. _.. _ _. _ _ _. _ _ _ _ _ _ _ _... _ _ _ _ _ _ _ _ _ _ _ _ _ _. _ _. _ _.. ~. _ _.. _ _. _ -

I i

The new EOL MTC LCO and 300 ppro SR MTC values and tho revised basis for f

adjustment overcome the liraitations inherent with the present version of l

Technical Specification 3/4.1.1.3, without affo: ting the basis of the accident analyses.

These technical spect(1 cations continue to require that surveillance t

be perforrned, so that any deviations between the operating core and design I

predictions that reight threaten the validity of accident analysis assunptions j

can be detected, and continued surveillance and appropriate action undertaken.

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oca1300$.wp 6-2

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Table 6.1

• l I

South Texas Project Units 1 and 2 Moderator Temperature Coeffleient Limiting Condition for Operation and Surveillance Requirement HTP Vessel Average Limiting Condition Surveillance l

Temperature For Operation Requirement

('T)

(dK/K/'F)

(dK/K/*F) 582.3

4. 4 x 10

3. 5 x 10

583.0 4.4 x 10

3.5 x 10**

t

' g 584.0

4. 5 x 10

3.6 x 10

3 585.0

-4. 5 x 10**

3. 6 x 10

586.0 4.6 x 10

3.7 x 10

$87.0

4. 6 x 10

3.7 x 10

588.0

4. 6 x 10

3.7 x 10**

589.0

-4. 7 x 10

- 3. 8 x 10

590.0 4.7 x 10

3,8 x 10

I 591.0 4.7 x 10"

3. 8 x 10

$92.0 4.8 x 10

3.9 x 10**

[

593.0

-4. 8 x 10

3. 9 x 10

.I i

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wea13005.wp 6*3

I REPER ENCES 1.

Barry, R. F., " LEOPARD A Spect rtun Dependent Non-Spatial Deplet ion Code for the lhM 7094," VCAP 3269 26 Septettber 1963.

2.

England, T.

R., " CINDER - A One Point Depletion and Tission Product Program," VAPD TM 334, August 1962.

3.

Liu, Y.

S., et al., "ANC: A Vestinghouse Advanced Nodal Cottputer Code,"

VCAP 10965 P A, Septetober 1986, 4.

Camden, T. M., Kersting, P. J., Carlson, V. R., "PA1ADON Vestinghouse Nodal Computer Code," VCAP 9485 P A, December 1978.

Barry, R. F., "The PANDA Code, VCAP 7048, April 1967.

6.

Davidson, S.

L.,

Kramer, W.

R.,

ed., * 'estinghouse Reload Safety I

Evaluation Methodology," WCAP 9272 P A, July 1985.

7.

Nguyen, T.Q. et. al.,

• Qualification of the PHOENIX P/ANC Nuclear Design I

System for Pressurized Water Reactor Cores," VCAP 11596 P-A, June 1988.

(Vestlughouse Proprietary).

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g R.1

1 I

I

.,mm.

i i

t DETERMINATION OF MOST NEGATIVE FEASIBLE E

Mrc sEnsITiv1 TIE

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=ca n005.wp A-1 r

Investigation of the sensitivity of MTC to core operational parameters that are variable under normal core operation is a fundamental requirettent of the Most Negative reasible MTC approach. Of the parameters discussed in Section 3.2, those that. required detailed evaluation are:

moderator teroperature and pressure

+ F.CCA insertion

- axial flux (power) si' ape transient fission product (xenon) concentration For each of these parameters, the sensitivity analyses were performed by perturbing the parameter in such a way as to induce a change f rom it s noittnal EOL value, and then performing a MTC determination with the parameter held in the perturbed state.

A further perturbation was induced and the M1C calculation repeated.

This sequence was repeated until sufficient data was obtained to reliably determine the trend of MTC change with variation in the value of the parameter.

The sensitivity analyses were performed ior a recent typical South Texas reload, as well as five additional reload cores.

These cores exhibit design features that may be incorporated into future South Texas reloads, such as increased discharge burnup, high enrichments, longer cycles and advanced product features.

Neutronic calculations and evaluations were periorized for each of these core designs to establish bounding sensitivities to each of the above paratteters.

The fbilowing sections provide a brief description of these calculations, and the "AMTC" factors obtained.

A.1 MTC Sensitivity to Moderator Temperature and Pressure Variation The decrease in moderator density which accottpanies moderator heatup has the effect of reducing neutron moderation.

With a low soluble boron concentration in the moderator, this results in a negative moderator temperature coefficient.

Since water density changes more rapidly with increasing temperature, and because of additionni spectrum hardening effectn, the MTC becomes progressively more negative with increasing temperature.

The sensitivity of MTC to increasing tertpa rature was determined for each of the reload cores by increasing core reference moderator teroperature slightly above the nominal HFP value, while mances wp A2

I holding pressure constant at 2250 pala, and then performing a MTC calculation

~

that induced small changes in core F 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 tetsperature was further increased, and I

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 six reload cores in terms of change in MTC from the nominal HFp MTC as a function of increa,e in reference moderator temperature above the nominal llFP moderator temperature. The results are shown in Figure A.1.

As expected, Reloads D and F exhibit the strongest sensitivity of MTC to increases in moderator temperature, due to the higher nominal liFP reference I

temperature.

A curve which conservatively bounds the MTC sensitivity results of the six reloads is also shown in Figure A.1.

This bounding curve is used in determining the "MTC" factor.

To use the bounding MTC sensitivity information of Figure A.1, the maximum allowable temperature and pressure (and, therefore, density) deviations permissible under operation that complies with Technical Specification must also be determined.

These deviation values, presented in Section 3.3, are combined with the sensitivity data to arrive at a "MTC" factor associated with moderator I

temperature and pressure (and, therefore, density). For the [

)* 'F l(a, c )

temperature deviation cited in Section 3.3, Figure A.1 indicates that the corresponding "MTC" due to temperature increase is [

J ' pcm/'F.

The "MTC" due to the [

)+ psi pressure deviation cited in Section 3.3 was I

conservatively determined to not exceed [

)+ pcm/'F.

The combined "MTC" (a,c) factor is, therefore, [

]' pcm/'F.

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.3

s A.2 MTC Sensitivity to RCCA Insertion F

L Vith constart tooderator teroperatur e, pressure, and boron concentration, insertion of control rods makes MTC more negative. As indicated in Section 3, the most negative MTC situation will always exist at HTP, with RCCA's inserted to the extent allowed by the llFP insertion 11tnits.

To calculate the EOL llFP MTC sensitivity to RCCA insertion, the reload core roodels had the lead control bank inserted the maxitoum applicable atmount determined ftom Section 3.3 ([ )* steps withdrawn), at HFP, with no soluble l( a, c )

~

boron in the core.

The MTC value for this condition was then determined by inducing small changes in core f. 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 f rorn the core It was deterinined that the maxiinum change to the EOL 0 ppm ilFP ARO MTC which would occurred for the South Texas Project Units as a result of HFP RCCA insertion to a depth of ( )* steps withdrawn was [

]* pern/ r.

Civen that l(a,c) suf ficient conservatists was inherent to this analysis, a bounding "tJiTC* value of [

]* pcm/'F was assumed.

l(a, c )

A.3 l

Sensitivity to Axial Flux (Power) Shape MTC is not so much directly affected by axial flux distribution itself, but is affected via the impact which the axial flux distribution has on the rate at which the tnoderator is heated as it moves up the core, and via the importance weighing which the axial flux chape 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 teore negative MTC as a result of isotopic linpacts on flux spectrum.

A more negative axial flux (power) shape allocates a greater "importance weighing" to the lower regions of the core where burnups are greater, thereby accentuating this effect, i

l m.n oo t -r A4 l

I It may also be shown that, in general, as the power distribution becomes reore bottom skewed, the average moderator tercperature increases for a constant core tereperature rise.

Therefore, both the importance weighting effect and the stoderator axial heating rate ef fect indicate that a more negative Axial Flux Difference results in a more negative MTC.

I This effect was investigated for four of the reload cores at "0L HFP O ppm conditions with no xenon in the core (xenon was removed so at to not cotoplicate flux skewing strategy).

A specific axial flux shape was induced and then, holding this flux shape approximately constant, the MTC vas determined by observing the small changes in core K effective which resulted from variation in moderator teroperature 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 I

quantified by Axial Flux Difference) was clearly identified.

Curves of "MTC" as a function of Axial Flux Difference (AFD) for the four reload cores are shown in Figure A.2.

Note that a zero AFD is taken as the reference point, therefore, "MTC" is fixed at zero for an AFD of zero.

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 "MTC" factor of the previous section.

Section 3.3 concludes that a negative value of HFP AFD that is expected to bound future South Texas reload cores is [

l' t, Using a value which bounds the l(a,c) most conservative trend of Figure A.4, the "MTC* factor corresponding to

[

]' 4 AFD is [

]* pcm/"F.

l(a,c)

I A.4 Sensitivity to Transient Fission Product (Xenon) Concentration I

Xenon is the most significant transient fission product in terms of effects on core reactivity and flux distribution, therefore, its possible impacts on MTC I

were investigated to compute the final "MTC" factor to include in the Most Negative Feasible MTC approach. While Technical Specifications place no limitations on either xenon distribution or overall concentration, the AFD liinits discussed in Section 3.3, in ef fect, place a limitation on the amount of I

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I axial xenon skewing that can occur, and the physics of xenon buildup and decay place practical limits on the concentration.

I Because axial xenon distribution directly impacts axial flux shape, this aspect of xenon effect on MTC is implicitly included in the Axial Flux Shape *tMTC" I

factor discussed in Section A.3.

k'ha t remains to be detertained is the sensitivity to overall xenon concentration in the core.

Calculations to determine this sensitivity were performed.

For all six reload cores, it was found that the most negative MTC resulted when all xenon was removed from the core.

The largest "A" from the reference I

(equilibrium xenon) MTC that occurred when all xenon was removed was

[

)* pcm/ F.

This value becornes the final "tMTC* f actor attributable to l(a,c) xenon.

No further uncertainty is added, simply because the scenario of I

operating at full power with no xenon in the core is itself sufficiently conservative as to be bounding.

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s Figure A.2 tJ4TC Versus Axial Flux Dif ference At EOL, IIFP, ARO

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