ML25184A397

Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A

ML25184A397
Person / Time
Site:	Palo Verde   (NPF-041, NPF-051, NPF-074)
Issue date:	07/03/2025
From:	Spina J Arizona Public Service Co
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
WCAP 18240-P-A, 102-08966-JLS/MDD
Download: ML25184A397 (1)

Text

10 CFR 50.90 A member of the STARS Alliance LLC Callaway

• Diablo Canyon

• Palo Verde

• Wolf Creek JENNIFER L. SPINA Vice President Nuclear Regulatory and Oversight Palo Verde Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7605 Tel 623.393.4621 102-08966-JLS/MDD July 3, 2025 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

Subject:

Palo Verde Nuclear Generating Station Units 1, 2, and 3 Docket Nos. STN 50-528, 50-529, and 50-530 Renewed Operating License Nos. NPF-41, NPF-51, NPF-74 Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A By letter dated January 17, 2025 [U. S Nuclear Regulatory Commission (NRC) Agencywide Documents and Access Management System (ADAMS) Accession No. ML25017A380 (non-proprietary) and ML25017A381 (proprietary)], Arizona Public Service Company (APS) submitted a License Amendment Request (LAR) for Palo Verde Nuclear Generating Station (PVNGS) Units 1, 2, and 3. The proposed license amendment would revise Section 5.6.5.b, Core Operating Limits Report (COLR), of the PVNGS Technical Specifications, by adding the NRC-approved Westinghouse Topical Report WCAP-18240-P-A, Westinghouse Thermal Design Procedure (WTDP), to the list of referenced analytical methods for the determination of reactor core operating limits. Consistent with WCAP-18240-P-A, the proposed license amendment also would authorize the option for discretionary use of a single NRC-approved thermal-hydraulic subchannel code in the Core Protection Calculator System and Core Operating Limit Supervisory System setpoints analyses, in lieu of the two NRC-approved thermal-hydraulic codes that are currently used.

On February 13, 2025, the NRC staff accepted the LAR for review (ADAMS Accession No. ML25045A085). Upon review of the LAR, the NRC Nuclear Systems Performance Branch determined that additional information was required to complete the review of the application. This information was provided to APS as a Request for Additional Information (RAI) under ADAMS Accession No. ML25140A162, dated May 20, 2025. On June 4, 2025, a public meeting was held between the NRC staff and APS representatives to discuss the RAI.

A summary of this meeting was documented under ADAMS Accession No. ML25162A115.

The Enclosure to this letter provides the APS response to the NRC RAI. No new commitments are being made to the NRC by this letter.

102-08966-JLS/MDD U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A Page 2 Should you need further information regarding this response, please contact Michael D.

DiLorenzo, Acting Director of Nuclear Regulatory Affairs at (623) 393-3495.

I declare under penalty of perjury that the foregoing is true and correct to the best of my knowledge.

Executed on July 3, 2025 (Date)

Sincerely, JLS/MDD/cmr

Enclosure:

Palo Verde Nuclear Generating Station (PVNGS) Units 1, 2, and 3 - Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A cc:

J. D. Monninger NRC Region IV Regional Administrator W. T. Orders NRC NRR Project Manager for PVNGS E. R. Lantz NRC Senior Resident Inspector for PVNGS Spina, Jennifer (Z08962)

Digitally signed by Spina, Jennifer (Z08962)

Date: 2025.07.03 13:47:21 -07'00'

Enclosure Palo Verde Nuclear Generating Station (PVNGS) Units 1, 2, and 3 Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A

PVNGS Units 1, 2, and 3 Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A 1

Introduction By letter dated January 17, 2025, Arizona Public Service Company (APS) submitted a License Amendment Request (LAR) for Palo Verde Nuclear Generating Station (PVNGS)

Units 1, 2, and 3, [U.S. Nuclear Regulatory Commission (NRC) Agencywide Documents Access and Management System (ADAMS) Accession Nos. ML25017A380 (non-proprietary) and ML25017A381 (proprietary), (Reference 1)]. The proposed license amendment would revise Section 5.6.5.b, Core Operating Limits Report (COLR), of the Palo Verde Technical Specifications (TS), by adding the NRC-approved Westinghouse Topical Report (TR) WCAP-18240-P-A, Westinghouse Thermal Design Procedure (WTDP), (Reference 2), to the list of referenced analytical methods for the determination of reactor core operating limits. Consistent with WCAP-18240-P-A, the proposed license amendment would also authorize the option for discretionary use of a single NRC-approved thermal-hydraulic subchannel code in the Core Protection Calculator System (CPCS) and Core Operating Limit Supervisory System (COLSS) setpoints analyses, in lieu of the two NRC-approved thermal-hydraulic codes that are currently used.

The NRC accepted the LAR for review on February 13, 2025, [ADAMS Accession No. ML25045A085, (Reference 3)]. Upon further review of the LAR, the NRC Nuclear Systems Performance Branch staff determined that additional information was required to complete the review of the application. A Request for Additional Information (RAI) containing two questions was issued to APS on May 20, 2025, [ADAMS Accession No. ML25140A162, (Reference 4)].

The regulatory basis for the RAI is General Design Criterion 10, Reactor Design, of Appendix A to 10 CFR Part 50, which states the following:

The reactor core and associated coolant, control, and protection systems shall be designed with appropriate margin to assure that specified acceptable fuel design limits are not exceeded during any condition of normal operation, including the effects of anticipated operational occurrences.

This enclosure provides the APS responses to the NRC RAI questions. For each question, the NRC RAI question is provided first, followed by the APS response.

NRC RAI Questions and APS Responses NRC Question 1 The Enclosure Attachment 6, Section 3.6, [of the LAR], states:

Adoption of the WTDP methodology does not require changes to the RFO [radial power fall off] penalty License Condition. A radial power fall off curve penalty shall continue to be applied to accommodate the effects of TCD [thermal conductivity degradation] on FATES3B predictions for Westinghouse CE16NGF fuel.

a. Explain why the adoption of WTDP methodology does not require changes to the RFO penalty license condition.

PVNGS Units 1, 2, and 3 Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A 2

b. Provide a discussion of the method of applying the radial power fall off curve penalty to accommodate the effects of TCD on FATES3B code predictions.

c. How much penalty should be applied to accommodate the effects of TCD so that the fuel performance analysis of the Westinghouse CE16NGF provides conservative results?

APS Response to NRC Question 1, Subpart a Adoption of WTDP methodology does not require changes to the RFO penalty license condition because it involves different, burnup-dependent design criteria specified in Section 4.2, Fuel System Design, of NUREG-0800, (Reference 5), that are analyzed with disparate computer codes. WTDP does not involve use of the FATES3B computer code, and as such, use or non-use of WTDP does not impact application of the License Condition radial fall off penalty.

Differences between WTDP methodology and the RFO penalty license condition are as follows:

WTDP is a thermal-hydraulic analysis methodology that, as noted in the Palo Verde LAR, (Reference 1), would utilize the VIPRE-W or VIPRE-01 subchannel computer codes to evaluate the potential for departure from nucleate boiling (DNB) and consequential fuel rod failure due to overheating of fuel rod cladding. The VIPRE-W and VIPRE-01 codes model conditions exterior to and at the cladding surface including, but not limited to, Reactor Coolant System (RCS) pressure and temperature, reactor coolant subchannel flow rate, and heat flux at the cladding surface. The NRC-approved critical heat flux correlations for the fuel types used at Palo Verde predict lower DNB ratio (DNBR) values at higher heat fluxes, so the limiting hot fuel rod for reactor core design is typically located in a higher power fresh fuel assembly at or near the beginning of its operating life. Along with CPCS and COLSS setpoints analyses for cycle-specific core designs, adoption of WTDP would ensure compliance with respect to the DNBR safety limit (SL) specified in Palo Verde TS 2.1.1, Reactor Core SLs; the power distribution limit specified in TS 3.2.4, Departure from Nucleate Boiling Ratio (DNBR); the low DNBR reactor trip function specified in TS Table 3.3.1-1, Reactor Protective System Instrumentation; and the RCS operating limits specified in TS 3.4.1, RCS Pressure, Temperature, and Flow Departure from Nucleate Boiling (DNB) Limits.

The RFO penalty license condition pertains to a fuel performance methodology that utilizes the Westinghouse (formerly Combustion Engineering) FATES3B computer code to evaluate the potential for fuel rod failure due to overheating (melting) of fuel pellets, cladding strain, and cladding creep.

The RFO penalty license condition was introduced in January 2018 via Palo Verde license amendment 205, (Reference 6), for implementation of Westinghouse CE16NGF fuel, and modified in March 2020 via license amendment 212, (Reference 7), during implementation of Framatome CE6HTP fuel, such that the license condition would also apply to future Westinghouse fuel designs that may be analyzed with the

PVNGS Units 1, 2, and 3 Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A 3

FATES3B code. The FATES3B code is an NRC-approved methodology for Palo Verde in that it is described in Combustion Engineering TR CEN-372-P-A, Fuel Rod Maximum Allowable Gas Pressure, (Reference 8), which is referenced in TS 5.6.5.b, Core Operating Limits Report (COLR). The FATES3B code models burnup-dependent conditions interior to fuel rods including, but not limited to, fuel pellet radial growth, fuel rod internal gas pressure, cladding strain, cladding creep, and linear heat rates that may result in incipient centerline melt. Unlike WTDP, the limiting FATES3B fuel rods for reactor core design analyses that model Westinghouse fuel are typically located in lower power, previously irradiated fuel assemblies between the middle and end of their operating lives. The FATES3B code does not, however, explicitly model the burnup-dependent effects of TCD, so the license condition addresses this non-conservatism by imposing a penalty on Westinghouse fuel analyses to accommodate the effects of TCD. Along with CPCS and COLSS setpoints analyses for cycle-specific core designs, FATES3B analyses and the license condition ensure compliance with respect to the burnup-dependent peak fuel centerline temperature limits specified in Palo Verde TS 2.1.1, Reactor Core SLs; the power distribution limit specified in TS 3.2.1, Linear Heat Rate (LHR);

and the high local power density reactor trip function specified in TS Table 3.3.1-1, Reactor Protective System Instrumentation.

In summary, adoption of WTDP methodology does not require a change to the RFO penalty license condition, because WTDP involves subchannel thermal-hydraulic analyses with the VIPRE-W or VIPRE-01 computer codes, not fuel performance analyses with the FATES3B code. The TCD-related RFO penalty specified in the license condition shall continue to be applied as before, when Palo Verde performs core reload fuel performance analyses for Westinghouse fuel with the FATES3B code.

APS Response to NRC Question 1, Subpart b The method of applying the radial power fall off curve penalty to accommodate the effects of TCD on FATES3B code predictions is described in detail in Section 3.5.4, Fuel Performance, of the proprietary NRC Safety Evaluation (SE) associated with Palo Verde license amendment 205, (Reference 6), for implementation of Westinghouse CE16NGF fuel.

In summary, fuel performance analyses were performed with FATES3B using bounding (conservative) power histories, axial power shapes, and burnup-dependent peaking factor reductions, with the intention of demonstrating acceptable fuel performance throughout the design lifetime of Westinghouse fuel rods. These analyses produced burnup-dependent RFO curves for Westinghouse fuel rods both with and without burnable absorbers. These RFO curves were then adjusted (corrected) as specified in the license condition to accommodate the effects of TCD. On an ongoing basis, Palo Verde reactor core design analyses compare cycle-specific predicted powers to the TCD-corrected bounding RFO curves, in order to verify that the results of the bounding analyses remain applicable and continue to bound the cycle of interest.

APS Response to NRC Question 1, Subpart c The burnup-dependent penalties that are applied to accommodate the effects of TCD are proprietary, and are described in detail in Section 3.5.4, Fuel Performance, of the

PVNGS Units 1, 2, and 3 Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A 4

proprietary NRC SE associated with Palo Verde license amendment 205, (Reference 6), for implementation of Westinghouse CE16NGF fuel. That SE also notes that code-to-code comparisons were made between FATES3B and PAD5, a newer NRC-approved fuel performance code that explicitly accounts for TCD. Hence, Palo Verde cycle-specific core designs are required to use a reduced value for a specific proprietary input parameter to ensure consistency with the input used in the PAD5 comparisons.

NRC Question 2 Enclosure Attachment 6, Section 5.3 provides the fuel failure calculation for the loss of flow (LOF) event using the statistical method described in NRC-approved TR CENPD-183-A. Table 1 in this TR lists the acceptable conservative assumptions for the analysis.

For conservative DNBR results, confirm that the demonstration analysis is based on the conservative assumptions listed in Table 1 of CENPD-183-A for the loss of flow (LOF) event, and most adverse initial conditions and reactivity coefficients, as well as the maximum system response delay. Provide justification if any of the initial conditions used are not biased and any of the Table 1 assumptions were not used in the analysis.

APS Response to NRC Question 2 Prior to 2003, Palo Verde fuel failure analyses for LOF events utilized the conservative assumptions listed in Table 1 of Combustion Engineering TR CENPD-183-A, Loss of Flow, C-E Methods for Loss of Flow Analysis, (Reference 9), which is referenced in TS 5.6.5.b, Core Operating Limits Report (COLR). For example, an initial core power level of 102% of rated thermal power (RTP) was used, as specified in Item 1 of Table 1 of CENPD-183-A.

Starting in 2003, however, the licensing basis methodology for Palo Verde LOF fuel failure analyses was modified to utilize assumptions that were generally consistent with, but holistically more conservative than those listed in Table 1 of CENPD-183-A. The modified methodology supported license amendments related to steam generator replacements and uprated power operations for all three Palo Verde units, as documented in NRC SEs for Unit 2 license amendment 149, (Reference 10), and for Units 1, 2, and 3 license amendment 157, (Reference 11). Both SEs also are referenced in TS 5.6.5.b, effectively clarifying how CENPD-183-A is utilized in Palo Verde licensing basis LOF event analyses.

The current licensing basis fuel failure analyses for LOF events are described in Appendix 15E, Limiting Infrequent Event, of the Palo Verde Updated Final Safety Analysis Report (UFSAR). The bounding analyses described therein explain how the modified methodology has been applied for Westinghouse CE16NGF fuel [license amendment 205, (Reference 6)]

and for Framatome CE16HTP fuel [license amendment 212, (Reference 7)]. The Palo Verde methodology models a hypothetical composite event that effectively bounds fuel failure calculations for all postulated infrequent events, including any moderate frequency anticipated operational occurrence (AOO) in combination with a coincident single active failure. The composite event analyses assume that some unspecified initiating event degrades all thermal margin [also called Required Overpower Margin] that is normally preserved by the Core Operating Limit Supervisory System (COLSS), such that conditions in the reactor core are brought to a DNBR specified acceptable fuel design limit (SAFDL). This assumption is conservative with respect to Table 1 of CENPD-183-A, in that in order to use

PVNGS Units 1, 2, and 3 Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A 5

up all the thermal margin, the initial fuel rod cladding heat flux is set to 117% of RTP for both Westinghouse CE16NGF fuel and Framatome CE16HTP fuel, rather than 102% of RTP as specified in CENPD-183-A.

The most limiting single active failure for DNBR degradation is then assumed to occur, that is, a coincident loss of power (LOP) that results in the coastdown of all four reactor coolant pumps (RCPs). The reactor trips almost immediately on a low DNBR trip generated by the CPCS, and DNBR degrades during the ensuing transient because of reduced coolant flow through the core, even though control element assemblies (CEAs) are simultaneously falling into the core, thereby mitigating the DNBR transient by reducing the fuel rod cladding heat flux. This limiting infrequent event with respect to DNBR degradation is therefore described as a postulated LOF from SAFDL conditions.

The demonstration analyses described in Attachment 6 of the WTDP LAR, (Reference 1),

were performed in a manner similar to the bounding analyses presented in UFSAR Appendix 15E, with the following exceptions:

WTDP methodology was used to generate an example DNBR SAFDL of 1.20 for the initial reactor core conditions for both Westinghouse CE16NGF fuel and Framatome CE16HTP fuel, whereas the UFSAR analyses used the current licensing basis thermal-hydraulic analysis methodology to generate DNBR SAFDLs of 1.25 and 1.27, respectively, for these fuel types. As previously described, the initial fuel rod cladding heat flux was set to 117% of RTP for both the UFSAR analyses and the LAR demonstration analyses, but the LAR demonstration analyses adjusted the numerical values of certain input parameters (for example, subchannel coolant flow rate), such that initial conditions for the transient analysis corresponded with the WTDP example DNBR SAFDL of 1.20.

The VIPRE-W thermal-hydraulic analysis code was used in the LAR demonstration analyses, instead of the CETOP-D code described in UFSAR Appendix 15E. VIPRE-W is more refined than CETOP-D in that it models more reactor coolant subchannels than CETOP-D.

For the purpose of demonstrating the statistical convolution technique for calculating fuel failure, the calculational framework used in the LAR demonstration analyses was identical to the convolution methodology described in both WCAP-18240-P-A, (Reference 2), and CENPD-183-A, (Reference 9). The WTDP LAR demonstration analyses, however, utilized cycle-specific reactor core design values to calculate fuel failure, rather than the bounding design values used in the UFSAR analyses. This allowed for a straightforward comparison that demonstrated how Palo Verde cycle-specific fuel failure analyses would predict fewer fuel failures with adoption of the WTDP methodology.

Consistent with the original intent of Table 1 of CENPD-183-A, the following assumptions were utilized in the WTDP LAR demonstration analyses:

No credit was taken for control room operator action (for example, responding to pre-trip alarms) prior to the event. Operators were also assumed to not take control of the plant until thirty minutes after event initiation.

PVNGS Units 1, 2, and 3 Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A 6

No credit was taken for keeping the RCPs electrically connected to the main turbine-generator following event initiation. The event was initialized with the reactor core at DNBR SAFDL conditions, and the coincident LOP was modeled to immediately remove electrical power from the RCPs, consistent with the licensing basis analyses in UFSAR Appendix 15E. Flow coastdown then degraded DNBR below the initial SAFDL conditions, resulting in a reactor trip.

Modeling of the time-dependent negative reactivity insertion due to CEA scram accounted for the response time of the Reactor Protective System (RPS). A CEA solenoid holding coil time delay following opening of the reactor trip breakers was also included to account for how the magnetic flux of the control element drive mechanism holding coils must decay before CEA motion can begin.

No credit was taken for moderator void reactivity feedback. Input parameter values for establishing initial conditions were selected to minimize voiding in the core.

Input parameter values for moderator temperature reactivity feedback and Doppler reactivity feedback were selected to conservatively model the time-dependent reactivity response during the transient.

Conservative instrument uncertainties and system response times were utilized to produce the most adverse DNBR degradation transient response.

References

1. Letter number 102-08888 from J. L. Spina (APS) to U.S. Nuclear Regulatory Commission (NRC), Palo Verde Nuclear Generating Station Units 1, 2, and 3; Docket Nos. STN 50-528, 50-529, and 50-530; Renewed Operating License Nos. NPF-41, NPF-51, and NPF-74; License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP-18240-P-A, dated January 17, 2025. [NRC ADAMS Accession Nos. ML25017A380 (non-proprietary) and ML25017A381 (proprietary).]

2. Westinghouse topical report WCAP-18240-P-A and WCAP-18240-NP-A, Westinghouse Thermal Design Procedure (WTDP), Revision 0, dated April 2020.

[NRC ADAMS Accession Nos. ML20104C041 (non-proprietary) and ML20120A343 (proprietary).]

3. Email from W. Orders (NRC) to M. DiLorenzo (APS),

Subject:

Acceptance Review, dated February 13, 2025. [NRC ADAMS Accession No. ML25045A085.]

4. Email from W. Orders (NRC) to M. DiLorenzo (APS),

Subject:

Request for Additional Information Re: License Amendment Request to Revise Technical Specifications (TS),

Section 5.6.5.b to Add Westinghouse Topical Report (TR) WCAP-18240-P-A, dated May 20, 2025. [NRC ADAMS Accession No. ML25140A162.]

PVNGS Units 1, 2, and 3 Response to Request for Additional Information Regarding License Amendment Request to Adopt Westinghouse Thermal Design Procedure Topical Report WCAP 18240-P-A 7

5. NUREG-0800 (formerly issued as NUREG-75/087), Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants; LWR [Light Water Reactor] Edition, Section 4.2, Fuel System Design, Revision 2, dated July 1981.

[NRC ADAMS Accession No. ML052340660.]

6. Letter from S. P. Lingam (NRC) to R. S. Bement (APS), Palo Verde Nuclear Generating Station, Units 1, 2, and 3 - Issuance of Amendments to Revise Technical Specifications to Support the Implementation of Next Generation Fuel (CAC Nos.

MF8076, MF8077, and MF8078; EPID L-2016-LLA-0005), dated January 23, 2018.

[NRC ADAMS Accession Nos. ML17319A107 (nonproprietary) and ML17319A103 (proprietary).]

7. Letter from S. P. Lingam (NRC) to M. L. Lacal (APS), Palo Verde Nuclear Generating Station, Units 1, 2, and 3 - Issuance of Amendment Nos. 212, 212, and 212 to Revise Technical Specifications to Support the Implementation of Framatome High Thermal Performance Fuel (EPID L-2018-LLA-0194), dated March 4, 2020. [NRC ADAMS Accession Nos. ML20031C947 (non-proprietary) and ML20031C968 (proprietary).]

8. Combustion Engineering topical report CEN-372-P-A and CEN-372-NP-A, Fuel Rod Maximum Allowable Gas Pressure, dated May 1990. [NRC ADAMS Accession Nos.

ML20043H533 (nonproprietary); proprietary version transmitted from Combustion Engineering to NRC via letter dated June 11, 1990 (ML20043H532).]

9. Combustion Engineering topical report CENPD-183-A, Loss of Flow, C-E Methods for Loss of Flow Analysis, dated June 1984. [NRC ADAMS Accession No. ML20097K029.]

10. Letter from B. M. Pham (NRC) to G. R. Overbeck (APS), Palo Verde Nuclear Generating Station, Unit 2 (PVNGS-2) - Issuance of Amendment on Replacement of Steam Generators and Uprated Power Operations (TAC No. MB3696), dated September 29, 2003. [NRC ADAMS Accession No. ML032720538.]

11. Letter from M. B. Fields (NRC) to J. M. Levine (APS), Palo Verde Nuclear Generating Station, Units 1, 2, and 3 - Issuance of Amendments RE: Replacement of Steam Generators and Uprated Power Operations and Associated Administrative Changes (TAC Nos. MC3777, MC3778, and MC3779), dated November 16, 2005. [NRC ADAMS Accession Nos. ML053130275 and ML053220229.]