ML25294A152

Cycle 25 Core Operating Limits Report

ML25294A152
Person / Time
Site:	South Texas
Issue date:	10/21/2025
From:	Georgeson C South Texas
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
NOC-AE-25004132, STI: 35800624
Download: ML25294A152 (1)

Text

October 21, 2025 NOC-AE-25004132 10 CFR 50.36 STI: 35800624 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Unit 2 Docket No. STN 50-499 Unit 2 Cycle 25 Core Operating Limits Report In accordance with Technical Specification 6.9.1.6.d, STP Nuclear Operating Company submits the attached Core Operating Limits Report (COLR) for Unit 2 Cycle 25. The report covers the core design changes made during the 2RE24 refueling outage.

There are no commitments in this letter.

If there are any questions regarding this report, please contact Derrick Johnson at (361) 972-7088 or me at (361) 972-7806.

Christopher H. Georgeson Gen Mgr Strategic Proj &

Eng Design drj

Attachment:

South Texas Project Unit 2 Cycle 25 Core Operating Limits Report, Revision 0 cc:

Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 1600 E. Lamar Boulevard Arlington, TX 76011-4511 C. H. Georgeson Digitally signed by C. H.

Georgeson Date: 2025.10.21 10:15:41 -05'00'

NOC-AE-25004132 Attachment Attachment South Texas Project Unit 2 Cycle 25 Core Operating Limits Report, Revision 0

Core Operating Limits Report Page 1 of 17 SOUTH TEXAS PROJECT Unit 2 Cycle 25 CORE OPERATING LIMITS REPORT U2C25-0 Revision 0

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 2 of 17 1.0 CORE OPERATING LIMITS REPORT This Core Operating Limits Report for STPEGS Unit 2 Cycle 25 has been prepared in accordance with the requirements of Technical Specification 6.9.1.6. The core operating limits have been developed using the NRC-approved methodologies specified in Technical Specification 6.9.1.6.

The Technical Specifications affected by this report are:

1) 2.1 SAFETY LIMITS

2) 2.2 LIMITING SAFETY SYSTEM SETTINGS

3) 3/4.1.1.1 SHUTDOWN MARGIN

4) 3/4.1.1.3 MODERATOR TEMPERATURE COEFFICIENT LIMITS

5) 3/4.1.3.5 SHUTDOWN ROD INSERTION LIMITS

6) 3/4.1.3.6 CONTROL ROD INSERTION LIMITS

7) 3/4.2.1 AFD LIMITS

8) 3/4.2.2 HEAT FLUX HOT CHANNEL FACTOR

9) 3/4.2.3 NUCLEAR ENTHALPY RISE HOT CHANNEL FACTOR

10) 3/4.2.5 DNB PARAMETERS 2.0 OPERATING LIMITS The cycle-specific parameter limits for the specifications listed in Section 1.0 are presented below.

2.1 SAFETY LIMITS (Specification 2.1):

2.1.1 The combination of THERMAL POWER, pressurizer pressure, and the highest operating loop coolant temperature (Tavg) shall not exceed the limits shown in Figure 1.

2.2 LIMITING SAFETY SYSTEM SETTINGS (Specification 2.2):

2.2.1 The Loop design flow for Reactor Coolant Flow-Low is 98,000 gpm.

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 3 of 17 2.2.2 The Over-temperature T and Over-power T setpoint parameter values are listed below:

Over-temperature T Setpoint Parameter Values 1

measured reactor vessel T lead/lag time constant, 1 = 8 sec 2

measured reactor vessel T lead/lag time constant, 2 = 3 sec 3

measured reactor vessel T lag time constant, 3 = 2 sec 4

measured reactor vessel average temperature lead/lag time constant, 4 = 28 sec 5

measured reactor vessel average temperature lead/lag time constant, 5 = 4 sec 6

measured reactor vessel average temperature lag time constant, 6 = 2 sec K1 Overtemperature T reactor trip setpoint, K1 = 1.14 K2 Overtemperature T reactor trip setpoint Tavg coefficient, K2 = 0.028/°F K3 Overtemperature T reactor trip setpoint pressure coefficient, K3 = 0.00143/psi T

Nominal full power Tavg, T 592.0 °F P

Nominal RCS pressure, P = 2235 psig f1(I) is a function of the indicated difference between top and bottom detectors of the power-range neutron ion chambers; with gains to be selected based on measured instrument response during plant startup tests such that:

(1)

For qt - qb between -70% and +8%, f1(I) = 0, where qt and qb are percent RATED THERMAL POWER in the top and bottom halves of the core respectively, and qt + qb is total THERMAL POWER in percent of RATED THERMAL POWER; (2)

For each percent that qt - qb is more negative than -70%, the T Trip Setpoint shall be automatically reduced by 0.0% of its value at RATED THERMAL POWER; and (3)

For each percent that the magnitude of qt - qb is more positive than +8%, the T Trip Setpoint shall be automatically reduced by 2.65% of its value at RATED THERMAL POWER (Reference 3.6 and Section 4.4.1.2 of Reference 3.7).

Over-power T Setpoint Parameter Values 1

measured reactor vessel T lead/lag time constant, 1 = 8 sec 2

measured reactor vessel T lead/lag time constant, 2 = 3 sec 3

measured reactor vessel T lag time constant, 3 = 2 sec 6

measured reactor vessel average temperature lag time constant, 6 = 2 sec 7

Time constant utilized in the rate-lag compensator for Tavg, 7 = 10 sec K4 Overpower T reactor trip setpoint, K4 = 1.08 K5 Overpower T reactor trip setpoint Tavg rate/lag coefficient, K5 = 0.02/°F for increasing average temperature, and K5 = 0 for decreasing average temperature K6 Overpower T reactor trip setpoint Tavg heatup coefficient K6 = 0.002/°F for T > T, and K6 = 0 for T T T

Indicated full power Tavg, T 592.0 °F f2(I) = 0 for all (I)

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 4 of 17 2.3 SHUTDOWN MARGIN (Specification 3.1.1.1):

The SHUTDOWN MARGIN shall be:

2.3.1 Greater than 1.3% for MODES 1 and 2*

• See Special Test Exception 3.10.1 2.3.2 Greater than the limits in Figure 2 for MODES 3 and 4.

2.3.3 Greater than the limits in Figure 3 for MODE 5.

2.4 MODERATOR TEMPERATURE COEFFICIENT (Specification 3.1.1.3):

2.4.1 The BOL, ARO, MTC shall be less positive than the limits shown in Figure 4.

2.4.2 The EOL, ARO, HFP, MTC shall be less negative than -62.6 pcm/F.

2.4.3 The 300 ppm, ARO, HFP, MTC shall be less negative than -53.6 pcm/F (300 ppm Surveillance Limit).

Where:

BOL stands for Beginning-of-Cycle Life, EOL stands for End-of-Cycle Life, ARO stands for All Rods Out, HFP stands for Hot Full Power (100% RATED THERMAL POWER),

HFP vessel average temperature is 592 F.

2.4.4 The Revised Predicted near-EOL 300 ppm MTC shall be calculated using the algorithm from the document referenced by Technical Specification 6.9.1.6.b.10:

Revised Predicted MTC = Predicted MTC + AFD Correction - 3 pcm/F If the Revised Predicted MTC is less negative than the COLR Section 2.4.3 limit and all of the benchmark data contained in the surveillance procedure are met, then an MTC measurement in accordance with S.R. 4.1.1.3b is not required.

2.5 ROD INSERTION LIMITS (Specification 3.1.3.5 and 3.1.3.6):

2.5.1 All banks shall have the same Full Out Position (FOP) of either 259, 254, or 256 steps withdrawn.

2.5.2 The Control Banks shall be limited in physical insertion as specified in Figure 5.

2.5.3 Individual Shutdown bank rods are fully withdrawn when the Bank Demand Indication is at the FOP and the Rod Group Height Limiting Condition for Operation is satisfied (T.S. 3.1.3.1).

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 5 of 17 2.6 AXIAL FLUX DIFFERENCE (Specification 3.2.1):

2.6.1 AFD limits as required by Technical Specification 3.2.1 are determined by Constant Axial Offset Control (CAOC) Operations with an AFD target band of +5, -10%.

2.6.2 The AFD shall be maintained within the ACCEPTABLE OPERATION portion of Figure 6, as required by Technical Specifications.

2.7 HEAT FLUX HOT CHANNEL FACTOR (Specification 3.2.2):

2.7.1 FQ RTP = 2.55.

2.7.2 K(Z) is provided in Figure 7.

2.7.3 The Fxy limits for RATED THERMAL POWER (Fxy RTP) within specific core planes shall be:

2.7.3.1 Less than or equal to 2.102 for all cycle burnups for all core planes containing Bank "D" control rods, and 2.7.3.2 Less than or equal to the appropriate core height-dependent value from Table 1 for all unrodded core planes.

2.7.3.3 PFxy = 0.2.

These Fxy limits were used to confirm that the heat flux hot channel factor FQ(Z) will be limited by Technical Specification 3.2.2 assuming the most-limiting axial power distributions expected to result for the insertion and removal of Control Banks C and D during operation, including the accompanying variations in the axial xenon and power distributions, as described in WCAP-8385. Therefore, these Fxy limits provide assurance that the initial conditions assumed in the LOCA analysis are met, along with the ECCS acceptance criteria of 10 CFR 50.46.

2.7.4 The Fxy limits from Section 2.7.3 above are not applicable in the following core plane regions:

2.7.4.1 Upper and lower core plane regions as presented in Table 2, and 2.7.4.2 Grid plane regions as presented in Table 2, and 2.7.4.3 Core plane regions within +/- 2% of core height (+/- 3.36 inches) about the bank demand position of the bank D control rods.

2.7.5 Core Power Distribution Measurement Uncertainty for the Heat Flux Hot Channel Factor 2.7.5.1 If the Power Distribution Monitoring System (PDMS) is operable, as defined in the Technical Requirements Manual Section 3.3.3.12, the core power distribution measurement uncertainty (UFQ) to be applied to the FQ(Z) and Fxy(Z) using the PDMS shall be calculated by:

UFQ = (1.0 + (UQ/100))*UE Where:

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 6 of 17 UQ = Uncertainty for power peaking factor as defined in Equation 5-19 from the document referenced by Technical Specification 6.9.1.6.b.11.

UE =

Engineering uncertainty factor of 1.03.

This uncertainty is calculated and applied automatically by the Power Distribution Monitoring System (PDMS).

2.7.5.2 If the moveable detector system is used, the core power distribution measurement uncertainty (UFQ) to be applied to the FQ(Z) and Fxy(Z) shall be calculated by:

UFQ = UQU*UE Where:

UQU = Base FQ measurement uncertainty of 1.05.

UE =

Engineering uncertainty factor of 1.03.

2.8 ENTHALPY RISE HOT CHANNEL FACTOR (Specification 3.2.3):

2.8.1 FH RTP = 1.62 2.8.2 PFH = 0.3 2.8.3 Core Power Distribution Measurement Uncertainty for the Enthalpy Rise Hot Channel Factor 2.8.3.1 If the Power Distribution Monitoring System (PDMS) is operable, as defined in the Technical Requirements Manual Section 3.3.3.12, the core power distribution measurement uncertainty (UFH) to be applied to the FH N

using the PDMS shall be the greater of:

UFH = 1.04 OR UFH = 1.0 + (UH/100)

Where:

UH = Uncertainty for power peaking factor as defined in Equation 5-19 from the document referenced in Technical Specification 6.9.1.6.b.11.

This uncertainty is calculated and applied automatically by the Power Distribution Monitoring System.

2.8.3.2 If the moveable detector system is used, the core power distribution measurement uncertainty (UFH) shall be:

UFH = 1.04

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 7 of 17 2.9 DNB PARAMETERS (Specification 3.2.5):

2.9.1 The following DNB-related parameters shall be maintained within the following limits (nominal values from Reference 3.1, as annotated below): 1 2.9.1.1 Reactor Coolant System Tavg 595 °F 2, 2.9.1.2 Pressurizer Pressure > 2200 psig 3, 2.9.1.3 Minimum Measured Reactor Coolant System Flow > 403,000 gpm4.

3.0 REFERENCES

3.1 Letter from Brown, P. A. (Westinghouse) to K. Aucoin (STPNOC), "South Texas Project Electric Generating Station Unit 2 Cycle 25 Final Reload Evaluation" NF-TG-25-030 (ST-UB-NOC-25000020) dated July 23, 2025.

3.2 NUREG-1346, Technical Specifications, South Texas Project Unit Nos. 1 and 2.

3.3 STPNOC Calculation ZC-07035, Rev. 3, Loop Uncertainty Calculation for RCS Tavg Instrumentation, Section 10.1. (STI: 35343289) 3.4 STPNOC Calculation ZC-07032, Rev. 7, Loop Uncertainty Calculation for Narrow Range Pressurizer Pressure Monitoring Instrumentation, Section 2.3, Page 9. (STI: 35191115) 3.5 Letter from J. S. Wyble (Westinghouse) to T. J. Jordan (STPNOC), STP Nuclear Operating Company Units 1 & 2 Power Uprate PCWG Parameters, ST-WN-NOC-00-000072 dated December 15, 2000. (STI: 31218644) 3.6 Letter from J. M. Ralston (Westinghouse) to D. F. Hoppes (STPNOC), South Texas Project Electric Generating Station Units 1 and 2 Documentation of the f1(I) Function in OTT Setpoint Calculation, NF-TG-11-93 (ST-UB-NOC-11003215) dated November 10, 2011.

(STI: 33079478) 3.7 Document RSE-U2, Rev. 13, Unit 2 Cycle 25 Reload Safety Evaluation and Core Operating Limits Report. (CR Action 24-1128-53).

1 A discussion of the processes to be used to take these readings is provided in the basis for Technical Specification 3.2.5.

2 Includes a 1.9 F measurement uncertainty per Reference 3.3, Page 37.

3 Limit not applicable during either a Thermal Power ramp in excess of 5% of RTP per minute or a Thermal Power step in excess of 10% RTP. Per Technical Specification 3.2.5 Bases, this includes a 10.7 psi measurement uncertainty as read on QDPS display, which when added to the safety analysis limit of 2189 psig gives the COLR limit listed above. The 10.7 psi uncertainty is bounded by the 9.6 psi averaged measurement calculated in Reference 3.4.

4 Includes the most limiting flow measurement uncertainty of 2.8% from Reference 3.5.

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 8 of 17

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 9 of 17

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 10 of 17

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 11 of 17

-3.0

-2.0

-1.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 0

10 20 30 40 50 60 70 80 90 100 Moderator Temperature Coefficient (pcm/°F)

Rated Thermal Power (%)

Figure 4 MTC versus Power Level Unacceptable Acceptable (0, 5.0)

(70, 5.0)

(100, 0.0)

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 12 of 17 1

• Control Bank A is already withdrawn to Full Out Position. Fully Withdrawn region shall be the condition where shutdown and control banks are at a position of 259, 254, or 256 steps withdrawn.

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 13 of 17

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 14 of 17

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 15 of 17 Table 1 (Part 1 of 2)

Unrodded Fxy for Each Core Height for Cycle Burnups Less Than 9,000 MWD/MTU (Subject to the exclusion zones of COLR Section 2.7.4)

Core Height (Ft.)

Axial Point Unrodded Fxy Core Height (Ft.)

Axial Point Unrodded Fxy 14.0 1

7.258 6.8 37 1.973 13.8 2

5.668 6.6 38 2.041 13.6 3

4.063 6.4 39 2.024 13.4 4

2.699 6.2 40 1.963 13.2 5

2.500 6.0 41 1.942 13.0 6

2.205 5.8 42 1.961 12.8 7

2.109 5.6 43 1.972 12.6 8

2.101 5.4 44 1.973 12.4 9

2.055 5.2 45 2.018 12.2 10 1.999 5.0 46 2.083 12.0 11 1.967 4.8 47 2.090 11.8 12 1.988 4.6 48 2.045 11.6 13 2.039 4.4 49 1.979 11.4 14 2.021 4.2 50 1.986 11.2 15 1.957 4.0 51 1.990 11.0 16 1.915 3.8 52 1.974 10.8 17 1.910 3.6 53 1.980 10.6 18 1.900 3.4 54 2.016 10.4 19 1.890 3.2 55 2.040 10.2 20 1.913 3.0 56 1.994 10.0 21 1.966 2.8 57 1.947 9.8 22 1.987 2.6 58 1.952 9.6 23 1.939 2.4 59 1.959 9.4 24 1.902 2.2 60 1.968 9.2 25 1.920 2.0 61 2.006 9.0 26 1.926 1.8 62 2.080 8.8 27 1.925 1.6 63 2.148 8.6 28 1.937 1.4 64 2.165 8.4 29 2.004 1.2 65 2.162 8.2 30 2.076 1.0 66 2.219 8.0 31 2.018 0.8 67 2.605 7.8 32 1.952 0.6 68 3.535 7.6 33 1.938 0.4 69 5.089 7.4 34 1.941 0.2 70 7.234 7.2 35 1.938 0.0 71 10.837 7.0 36 1.924

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 16 of 17 Table 1 (Part 2 of 2)

Unrodded Fxy for Each Core Height for Cycle Burnups Greater Than or Equal to 9,000 MWD/MTU (Subject to the exclusion zones of COLR Section 2.7.4)

Core Height (Ft.)

Axial Point Unrodded Fxy Core Height (Ft.)

Axial Point Unrodded Fxy 14.0 1

6.326 6.8 37 2.143 13.8 2

5.070 6.6 38 2.186 13.6 3

3.809 6.4 39 2.166 13.4 4

2.734 6.2 40 2.123 13.2 5

2.601 6.0 41 2.101 13.0 6

2.335 5.8 42 2.099 12.8 7

2.161 5.6 43 2.086 12.6 8

2.096 5.4 44 2.070 12.4 9

2.025 5.2 45 2.090 12.2 10 2.004 5.0 46 2.130 12.0 11 2.002 4.8 47 2.128 11.8 12 2.033 4.6 48 2.067 11.6 13 2.077 4.4 49 2.016 11.4 14 2.068 4.2 50 2.013 11.2 15 2.031 4.0 51 2.001 11.0 16 1.985 3.8 52 1.986 10.8 17 2.007 3.6 53 1.989 10.6 18 2.020 3.4 54 2.028 10.4 19 2.022 3.2 55 2.055 10.2 20 2.055 3.0 56 1.985 10.0 21 2.118 2.8 57 1.912 9.8 22 2.146 2.6 58 1.883 9.6 23 2.098 2.4 59 1.860 9.4 24 2.058 2.2 60 1.837 9.2 25 2.072 2.0 61 1.836 9.0 26 2.081 1.8 62 1.866 8.8 27 2.084 1.6 63 1.885 8.6 28 2.094 1.4 64 1.856 8.4 29 2.137 1.2 65 1.860 8.2 30 2.181 1.0 66 1.952 8.0 31 2.136 0.8 67 2.283 7.8 32 2.093 0.6 68 2.937 7.6 33 2.082 0.4 69 3.926 7.4 34 2.100 0.2 70 5.289 7.2 35 2.105 0.0 71 7.726 7.0 36 2.103

Unit 2 Cycle 25 Core Operating Limits Report Rev. 0 U2C25-0 Page 17 of 17 Table 2 Core and Grid Plane Fxy Exclusion Zones Axial Point Core Core Axial Point Core Core Height (in.)

Height

(%)

Top/Bottom Grid Height (in.)

Height

(%)

Top/Bottom Grid 1

168.0 100.0 Excluded 37 81.6 48.6 2

165.6 98.6 Excluded 38 79.2 47.1 Excluded 3

163.2 97.1 Excluded 39 76.8 45.7 Excluded 4

160.8 95.7 Excluded 40 74.4 44.3 5

158.4 94.3 Excluded Excluded 41 72.0 42.9 6

156.0 92.9 Excluded Excluded 42 69.6 41.4 7

153.6 91.4 Excluded 43 67.2 40.0 8

151.2 90.0 Excluded 44 64.8 38.6 9

148.8 88.6 45 62.4 37.1 10 146.4 87.1 46 60.0 35.7 Excluded 11 144.0 85.7 47 57.6 34.3 Excluded 12 141.6 84.3 48 55.2 32.9 Excluded 13 139.2 82.9 Excluded 49 52.8 31.4 14 136.8 81.4 Excluded 50 50.4 30.0 15 134.4 80.0 Excluded 51 48.0 28.6 16 132.0 78.6 52 45.6 27.1 17 129.6 77.1 53 43.2 25.7 18 127.2 75.7 54 40.8 24.3 Excluded 19 124.8 74.3 55 38.4 22.9 Excluded 20 122.4 72.9 56 36.0 21.4 Excluded 21 120.0 71.4 Excluded 57 33.6 20.0 22 117.6 70.0 Excluded 58 31.2 18.6 23 115.2 68.6 Excluded 59 28.8 17.1 24 112.8 67.1 60 26.4 15.7 25 110.4 65.7 61 24.0 14.3 26 108.0 64.3 62 21.6 12.9 Excluded 27 105.6 62.9 63 19.2 11.4 Excluded 28 103.2 61.4 64 16.8 10.0 Excluded Excluded 29 100.8 60.0 Excluded 65 14.4 8.6 Excluded 30 98.4 58.6 Excluded 66 12.0 7.1 Excluded 31 96.0 57.1 Excluded 67 9.6 5.7 Excluded 32 93.6 55.7 68 7.2 4.3 Excluded 33 91.2 54.3 69 4.8 2.9 Excluded Excluded 34 88.8 52.9 70 2.4 1.4 Excluded Excluded 35 86.4 51.4 71 0.0 0.0 Excluded Excluded 36 84.0 50.0