ML23068A364

Supplement to Proposed Alternate Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection (Relief Request RR-ENG-4-06)

ML23068A364
Person / Time
Site:	South Texas
Issue date:	03/09/2023
From:	Georgeson C South Texas
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
NOC-AE-23003947, EPID L-2023-LLR-0004, STI: 35442883, RR-ENG-4-06
Download: ML23068A364 (1)

Text

March 9, 2023 NOC-AE-23003947 10 CFR 50.55a STI: 35442883 Attention: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 South Texas Project Units 1 and 2 Docket Nos. STN 50-498 and STN 50-499 Supplement to Proposed Alternate Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection (Relief Request RR-ENG-4-06) (EPID: L-2023-LLR-0004)

References:

1. Letter; C. Georgeson (STP) to Document Control Desk (NRC); Proposed Alternate Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection (Relief Request RR-ENG-4-06); February 1, 2023; (NOC-AE-23003940) (ML23032A484).

2. Letter; D. Galvin (NRC) to T. Powell (STP); South Texas Project, Units 1 and 2 - Supplemental Information Needed for Acceptance of Requested Licensing Action Re: Proposed Alternative to the Requirements of the ASME Code (EPID L-2023-LLR-0004); March 6, 2023; (AE-NOC-23003363) (ML23061A175).

By Reference 1, STP Nuclear Operating Company (STPNOC) submitted a relief request to propose alternative frequency to the containment unbonded post-tensioning system inservice inspection. Based on discussion with NRC staff and feedback provided in Reference 2, STPNOC is providing this revised relief request. Please replace the original in its entirety with the attached. The revised relief request removes the request for the containment concrete inspection IWL-2410(a) interval extension.

There are no new commitments in this letter.

If there are any questions regarding this letter, please contact Zachary Dibbern at (361) 972-4336 or me at (361) 972-7806.

Digitally signed by C. H.

C. H. Georgeson Georgeson Date: 2023.03.09 09:03:51 -06'00' Christopher Georgeson General Manager, Engineering

Enclosure:

Revised Relief Request RR-ENG-4-06, Alternative Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection cc:

Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 1600 E. Lamar Boulevard Arlington, TX 76011-4511

NOC-AE-23003947 Enclosure Enclosure Revised Relief Request RR-ENG-4-06, Alternative Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection

NOC-AE-23003947 Enclosure Page 1 of 15 Revised Relief Request RR-ENG-4-06, Alternative Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection

1. ASME Code Components Affected

Description:

Inservice Examination of Concrete Containment Unbonded Post-Tensioning System Component: Concrete Containments, Units 1 and 2 Code Class: CC Examination Categories and Code Item Numbers:

Category L-B, Unbonded Post-Tensioning System Code Item Numbers: L2.10, L2.20, L2.30, L2.40, and L2.50

2. Applicable Code Edition American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (BPV) Code,Section XI, Subsection IWL, 2013 edition. The 2013 edition is applicable for the fourth 10-year Containment Inservice Inspection Interval (CISI) that began Sept. 9, 2019.

(Reference 9.2)

3. Applicable Code Requirements

3.1 IWL-2420 UNBONDED POST-TENSIONING SYSTEMS (a) Unbonded post-tensioning systems shall be examined in accordance with IWL-2520 at 1, 3, and 5 years following the completion of the containment Structural Integrity Test and every 5 years thereafter.

3.2 IWL-2421 Sites With Multiple Plants (a) For sites with multiple plants, the requirements of IWL-2420 may be modified if the containments utilize the same prestressing system and are essentially identical in design, if post-tensioning operations for each subsequent containment constructed at the site were completed not more than 2 years apart, and if the containments are similarly exposed to or protected from the outside environment.

(b) When the conditions of (a) are met, the inspection dates and examination requirements may be as follows.

(1) For the containment with the first Structural Integrity Test, all examinations required by IWL-2520 shall be performed at 1, 3, and 10 years, and every 10 years thereafter.

In addition, the examinations required by IWL-2524 and IWL-2525 shall be performed at 5 and 15 years and every 10 years thereafter.

NOC-AE-23003947 Enclosure Page 2 of 15 (2) For each subsequent containment constructed at the site, all examinations required by IWL-2520 shall be performed at 1, 5, and 15 years, and every 10 years thereafter.

In addition, the examinations required by IWL-2524 and IWL-2525 shall be performed at 3 and 10 years and every 10 years thereafter.

STP completed the 30th year surveillance in 2018. The next scheduled surveillance is the 35th year surveillance. IWL-2421(b)(1) is applicable to Unit 1. IWL-2421(b)(2) is applicable to Unit

2. The next surveillance is due July 29, 2023 in Unit 1 and September 30, 2023 in Unit 2.

These nominal dates may be extended up to 1 year per IWL-2420(c), which makes the true due dates July 29, 2024 in Unit 1 and September 30, 2024 in Unit 2.

The applicable requirements of IWL-2520 are paraphrased and summarized below.

IWL-2520 Examination of Unbonded Post-Tensioning Systems

• IWL-2521 requires random selection of tendons for surveillance.

• IWL-2522 requires tendon force measurements in selected tendons.

• IWL-2523 requires detensioning of one tendon per group, followed by removal and destructive examination of a single wire prior to retensioning. The removed wire is not replaced.

• IWL-2524 requires examination of anchorages of selected tendons. This requires removal of the end cap.

• IWL-2525 requires laboratory analysis of samples of the corrosion protection medium (grease) taken from each end of each examined tendon.

• IWL-2526 requires replacement of corrosion protection medium and measurement of difference in quantity between removal and replacement to identify potential voids in the duct.

Use of IWL-2421(b) means that the requirements of IWL-2522 (tendon force measurement) and IWL-2523 (detension, wire removal and examination) are applied in alternating units at 5-year intervals (i.e. 10-year intervals between force measurements in a single unit) and all other surveillance requirements are performed in both units at each 5-year surveillance. In subsequent discussion, the unit in which all surveillance requirements are applied is described as full surveillance and the unit in which all except IWL-2522 and IWL-2523 are applied is described as partial surveillance.

4. Reason for Request

The currently required surveillance presents potential risk of injury to plant personnel and exposes the plant to potential physical damage without sufficient benefit in safety or quality to offset the costs and risks. Potential risks and hazards associated with surveillance are discussed further in Section 6.7. Past surveillance results in years 1, 3, 5, 10, 15, 20, 25, and 30 confirm that the STP post-tensioning system is in excellent condition in both units. The system is performing as designed and trending toward acceptable level of prestress force through 100 years, well beyond the Period of Extended Operation. Therefore, it is appropriate to implement an alternative surveillance schedule that reduces the number of tendon surveillances while maintaining an acceptable level of quality and safety, as allowed by 10 CFR 50.55a(z)(1).

NOC-AE-23003947 Enclosure Page 3 of 15

5. Proposed Alternative Surveillance 5.1 The following is proposed as an alternative to IWL-2421(b):

(1) For Unit 1, all examinations required by IWL-2520 shall be performed at 1, 3, and 10 years, and every 10 years thereafter. In addition, the examinations required by IWL-2524 and IWL-2525 shall be performed at 5, 15, 25, and 40 years and every 10 years thereafter.

(2) For Unit 2, all examinations required by IWL-2520 shall be performed at 1, 5, 15, 25, and 40 years, and every 10 years thereafter. In addition, the examinations required by IWL-2524 and IWL-2525 shall be performed at 3 and 10 years and every 10 years thereafter.

The current surveillance schedule requires surveillance every 5 years, with the two units staggered such that full surveillance is performed in one unit and partial surveillance (exempting IWL-2522 and 2523) is performed in the other unit. The Proposed Alternative will have the effect of eliminating the IWL-2520 35th year surveillances in both units, and thereafter performing full surveillance in both units every 10 years, beginning at the 40th year. The eliminated 35th year surveillances would be a partial surveillance in Unit 1 and a full surveillance in Unit 2.

6. Basis for Conclusion that Proposed Alternative has Acceptable Level of Quality and Safety 6.1 STP Design STP has 2 PWR units, each having a prestressed concrete containment. The 2 containments are essentially identical and located 600 feet apart. Each containment has the following design parameters:

• Inside radius = 75 feet

• Cylinder walls are 4 feet thick

• Hemispherical dome thickness is 3 feet

• Basemat thickness is 18 feet

• Inside surface (cylinder, dome and basemat) is lined with 3/8-inch carbon steel plate

• Inside height (top of basemat to top of dome) = 241 feet

• There are 3 buttresses, 120 degrees apart

• Each tendon is offset 120 degrees from the adjacent one below, such that every 3rd hoop tendon is anchored in the same buttress.

• Hoop tendons cover the full circumference, starting and ending in the same buttress

• There are 133 hoop tendons (108 in the cylinder wall and 25 in the dome)

• There are 96 Inverted-U tendons that are anchored in a tendon gallery underneath the basemat.

• Each tendon has 186 wires, each 1/4-inch diameter

• Steel wires are ASTM A421-77, type BA

• Wires have Guaranteed Ultimate Tensile Strength (GUTS) = 240 ksi

• Minimum installation prestress force was specified as 70% of GUTS, or 168 ksi

NOC-AE-23003947 Enclosure Page 4 of 15 The applicable design requirements include the following:

• Rated pressure capacity, P = 56.5 psig

• One-time Structural Integrity Test performed at 1.15 x P = 65 psig

• Minimum prestress force = 20% greater than required to offset internal pressure

• Design provides 1% additional steel wires to compensate for potential wire breakage 6.2 Regulatory History Initially, Regulatory Guide (RG) 1.35 Proposed Revision 3, was applicable to STP. The surveillances conducted in years 1, 3, 5, and 10 implemented the requirements of RG 1.35.

Per the requirements of the Regulatory Guide, the surveillance schedule was based on years following the Structural Integrity Test. The Unit 1 Structural Integrity Test was completed in March 1987 and the Unit 2 Structural Integrity Test was completed in September 1988. In 1996, the NRC mandated a transition to ASME BPV Code,Section XI, Subsection IWL. (See 61 FR 41303, August 8, 1996.)

The transition mandate included a requirement to perform an expedited surveillance, which would be used thereafter (instead of Structural Integrity Test dates) to establish surveillance dates. STP performed the expedited surveillance coincident with the scheduled 10th year surveillances in 1998. The completion dates of the expedited surveillances, which thereafter define the due dates for subsequent surveillances, are July 29, 1998 in Unit 1 and September 30, 1998 in Unit 2.

Information Notice (IN) 99-10, Degradation of Prestressing Tendon Systems in Prestressed Concrete Containments discussed and highlighted several problems seen throughout the industry. Multiple plants observed liftoff forces that were significantly below predicted values.

Also, the variety of methods used by plants to calculate force trends did not meet NRC expectations. The IN clarified the NRCs position regarding the proper method to be used to calculate tendon force trends.

RG 1.35 was withdrawn in 2015 (80 FR 52067, August 27, 2015).

STP performed the 15th and 20th year surveillances using the 1992 edition of subsection IWL. During review of the License Renewal Application, STP committed to change to the 2004 edition. (See License Renewal Commitment 22, UFSAR Table 19A.4-1.) The Commitment was implemented, and the 2004 edition was used during the 25th and 30th year surveillances.

6.3 Prior STPNOC Relief Request STPNOC submitted a Relief Request in 2001, seeking an extension (doubling) of surveillance intervals specified in IWL-2421(b). The submittal (Reference 9.3) was revised and resubmitted in 2002 (Reference 9.4). The submittals made use of surveillance data through the 10th-year surveillances completed in 1998. STP made a presentation to the NRC in April 2003 to discuss the revised submittal (Reference 9.14).

During and following the presentation, the NRC provided the following feedback to STP.

• STP had only 10 years of surveillance data, but 10 years of data was not sufficient to establish a trend of excellent performance. The NRC pointed out that Calvert Cliffs

NOC-AE-23003947 Enclosure Page 5 of 15 (mentioned prominently in IN 99-10) discovered major problems during their 20th year surveillance after having acceptable surveillance results through 10 years.

• The NRC had recently seen a trend of poor performance throughout the industry, as summarized in IN 99-10.

Based on this feedback, STP withdrew the Relief Request (Reference 9.5).

6.4 Significant Developments Since 2003 This Relief Request is substantially similar to the one STP submitted in 2001 and withdrew in 2003. Therefore, it is appropriate to review and consider new information that wasnt available in 2003:

• STP has surveillance data from surveillances conducted at years 15, 20, 25, and 30. The surveillance results confirm the ongoing excellent performance of the post-tensioning system in both units. These results should address the NRCs previous feedback that 10 years of data was insufficient.

• Industry performance has improved in this area and the NRC has approved similar relief requests at multiple other plants, e.g. Three Mile Island (2019), Vogtle (2019), Millstone (2020), Byron and Braidwood (2021), and Palo Verde (2022).

• STP submitted a license renewal application (LRA) in 2010 (Reference 9.6). During the extensive review, the NRC specifically reviewed the post-tensioning system, which is described in Section 4.5 of the LRA, Revision C. The LRA submittal was based on 20th year surveillance data. STP received seven Requests for Additional Information (RAI) focused specifically on the post-tensioning system. As documented in Sections 3.0.3.1.8, 3.0.3.2.23, and 4.5 of the Safety Evaluation Report (SER) that accompanied approval of the license extension, the NRC concurred with STPs conclusion that the prestress force trends were projected to be acceptable for the duration of the Period of Extended Operation. Additionally, STP has data that shows a favorable trend that goes beyond 60 years.

6.5 Surveillance Results (Years 1 - 30)

The License Renewal Application, Section 4.5, includes surveillance results through the 20th year. Liftoff testing was performed in Unit 1 during the 20th year surveillance. The most recent liftoff data used in Unit 2 was from the 15th year. Since then, the 25th year surveillance included liftoff testing in Unit 2 and the 30th year surveillance included liftoff testing in Unit 1.

The following pages show the trend line figures included in the LRA next to the corresponding updated figure using the most recent data. For each group of tendons, a comparison of the LRA figure to the updated figure shows very little change in trend. For the inverted-U tendons, the trend line forecast value at year 60 declined by about 1% compared to the trends shown in the LRA figures, while the 60-year forecast for hoop tendons increased by about the same amount. Overall, the figures show stable trends. In all cases, the trend line is substantially above the Minimum Required Value (MRV) for the entire Period of Extended Operation (through 60 years) and beyond.

NOC-AE-23003947 Enclosure Page 6 of 15 Through 30 years, there have been 164 liftoff values measured during surveillances at STP.

The IWL acceptance criteria (greater than or equal to 95% of predicted value) was met 162 of 164 times. The other 2 were both 94% of predicted. They are classified as failures, even though 94% of predicted is above the MRV. The most recent failure occurred in 1992 during the Unit 1 fifth year surveillance.

Grease volume discrepancies have been a recurring phenomenon at STP, but not recently.

Several of these have been reported to the NRC.

• Reference 9.7 (Unit 1, year 3)

• Reference 9.8 (Unit 1, year 5)

• Reference 9.9 (Unit 1, year 10)

• Reference 9.10 (Unit 2, year 3)

• Reference 9.11 (Unit 2, year 5)

• Reference 9.12 (Unit 2, year 10)

• Reference 9.13 (Unit 2, year 15)

In each case, evaluations supported the conclusion that the volume discrepancies had minimal consequence and the post-tensioning system remained operable. The acceptance criterion became less stringent following the transition from RG 1.35 to IWL. The limit specified in IWL-3221.4 is 10% of net duct volume vs 5% in RG 1.35. Only one of the 7 occurrences happened since switching to IWL, and it was 19 years ago. Surveillances at years 20, 25, and 30 met the acceptance limit in IWL-3221.4.

Ineffective wires have been identified and tracked throughout the life of the plant. The majority of these were identified during initial construction. Most of the others were deliberately damaged during surveillance, as specified by IWL-2523.2. The design includes a 1% breakage allowance. Since each unit has 229 tendons, with 186 wires per tendon, the total number of wires is 229 x 186 = 42,594. The 1% breakage allowance implies 426 wires are assumed to be ineffective in each unit. The actual number of ineffective wires identified through 30 years of surveillance are as follows:

• Unit 1 - 23 wires, of which 14 were identified during initial construction and 8 were removed during surveillances.

• Unit 2 - 38 wires, of which 23 were identified during initial construction and 8 were removed during surveillances.

The total number of ineffective wires identified at STP is 61; 37 of which were identified during construction and 16 of which were removed during surveillance. Only 8 other wires have been found ineffective during the surveillances. The number of ineffective wires is less than one-tenth the 1% allowance.

There have been no noteworthy adverse findings related to other surveillance elements (concrete exterior inspection, wire strength and elongation, corrosion protection chemistry) during any of the surveillances conducted during years 1-30. In summary, the only surveillance items during years 1-30 that did not meet the acceptance limits were 2 liftoff tests that were 1% low, most recently in 1992; non-consequential grease volume discrepancies identified during 7 surveillances (most recently in 2004); and 8 wires out of 85,000 that were found ineffective.

NOC-AE-23003947 Enclosure Page 7 of 15 Liftoff Force Regression Analysis for Unit 1 Hoop Tendons Figure 6.5.1 is Fig. 4.5-2 in LRA, Revision C. It used data through year 20. Figure 6.5.2 is an updated version that includes 30th year surveillance data. The figures are not directly comparable because the top figure uses a semi-log plot. In Figure 6.5.1, the forecast value at year 60 is 1275 kips. In Figure 6.5.2, the forecast at year 60 has increased slightly to 1283 k, which remains above the minimum required value (1169 k). The trend is essentially stable.

Figure 6.5.1 Figure 6.5.2

NOC-AE-23003947 Enclosure Page 8 of 15 Liftoff Force Regression Analysis for Unit 2 Hoop Tendons Figure 6.5.3 is Fig. 4.5-4 in LRA, Revision C. It used data through year 15. Figure 6.5.4 is an updated version that includes 25th year surveillance data. The figures are not directly comparable because the top figure uses a semi-log plot. In Figure 6.5.3, the forecast value at year 60 is 1273 kips. In Figure 6.5.4, the forecast at year 60 has increased slightly to 1283 k (MRV = 1169 k). The trend is essentially stable.

Figure 6.5.3 Figure 6.5.4

NOC-AE-23003947 Enclosure Page 9 of 15 Liftoff Force Regression Analysis for Unit 1 Inverted-U Tendons Figure 6.5.5 is Fig. 4.5-2 in LRA, Revision C. It used data through year 20. Figure 6.5.6 is an updated version that includes 30th year surveillance data. The figures are not directly comparable because the top figure uses a semi-log plot. In Figure 6.5.5, the forecast value at year 60 is 1349 kips. In Figure 6.5.6, the forecast at year 60 has declined roughly 1% to 1334 k, but still far above the minimum required value (1150 k). The trend is essentially stable.

Figure 6.5.5 Figure 6.5.6

NOC-AE-23003947 Enclosure Page 10 of 15 Liftoff Force Regression Analysis for Unit 2 Inverted-U Tendons Figure 6.5.7 is Fig. 4.5-4 in LRA, Revision C. It used data through year 15. Figure 6.5.8 is an updated version that includes 25th year surveillance data. The figures are not directly comparable because the top figure uses a semi-log plot. In Figure 6.5.7, the forecast value at year 60 is 1366 kips. In Figure 6.5.8, the forecast at year 60 has declined very slightly to 1362 k, but it is still far above the minimum required value (1150 k). The trend is essentially stable.

Figure 6.5.7 Figure 6.5.8

NOC-AE-23003947 Enclosure Page 11 of 15 6.6 Applicability of IWL-2521(a)

RG 1.35, used during the first 10 years of surveillances, established a schedule for multi-unit plants that allowed liftoff force measurements in alternating units if the Structural Integrity Tests were separated by no more than 2 years. STP complied with that since the SIT tests are separated by 18 months (March 1987 in Unit 1 and September 1988 in Unit 2).

When the NRC mandated a transition to IWL, STP noted that IWL-2521(a) was slightly different from RG 1.35 in its definition of the 2-year limit. Whereas RG 1.35 said the Structural Integrity Tests must be separated by no more than 2 years, IWL-2521(a) applies the 2-year limit to completion of post-tensioning operations. The term is not defined within IWL. At STP, physical work (tensioning of last tendon) was completed 26 months apart (January 1986 in Unit 1 and March 1988 in Unit 2), which exceeds the 2-year limit. However, the Installation Final Reports were signed 23 months apart (July 1986 in Unit 1 and June 1988 in Unit 2), which is within the 2-year limit. In the nuclear industry, it is customary to regard work as incomplete until it is signed. Therefore, STP concluded in 2003 that STPs two units met the intent of IWL-2421(a). STP has followed that schedule in subsequent surveillances (15th, 20th, 25th, and 30th years). The first paragraph of the License Renewal SER, page 3-35 documents NRC scrutiny of the multi-unit surveillance schedule and the transition from RG 1.35 to IWL. The NRC did not express disagreement with STPs use of IWL-2521(a).

6.7 Risks Associated with Surveillances Performance of these surveillances, particularly liftoff testing, requires use of a crane to lift a work platform to access the tendon ends. Given the locations of the containment buttresses, this cannot be accomplished without lifting Heavy Loads, as defined by Generic Letter 81-07 and NUREG 0612, over buildings housing safety-related equipment. The specific buildings effected are the Fuel Handling Building, the Electrical Auxiliary Building, and the Mechanical Auxiliary Building. Potential load drops and crane collapses onto the roofs of these buildings have been evaluated and shown to be acceptable in the sense that the safety-related functions would be maintained. Nevertheless, surveillance of the post-tensioning system introduces some risk of damage to safety-related buildings from load drops. To comply with the guidance in Generic Letter 81-07 and NUREG 0612, lifts of heavy loads over safety-related structures should be minimized.

In addition, surveillance of the post-tensioning system introduces some risk of injury to personnel performing the surveillance and to personnel working below them. The personnel risks include:

• High locations (risk is to personnel performing work and also personnel under them).

• Each tendon is tensioned to roughly 1.5 million pounds, which represents a dangerous amount of stored elastic energy. Liftoff testing and detensioning introduces slight risk of sudden release of energy due to tendon failure or equipment mishandling or malfunction.

• Liftoff testing is performed using a hydraulic ram with hydraulic fluid contained in hoses under high pressure.

• The Corrosion Protection Medium (grease) is heated to facilitate placement in tendon ducts. Mishandling could result in burns.

NOC-AE-23003947 Enclosure Page 12 of 15 6.8 Acceptability of Proposed Alternative Surveillance Schedule As shown in section 6.5, the performance has been excellent, with tendon force trends remaining above the Minimum Required Value (MRV) for the 40-year life for which the plant was designed, but also through the entire Period of Extended Operation and also beyond that through the 100th year of plant life. In addition, the MRV was established very conservatively, meaning that the trend lines shown in section 6.5 understate the actual available margin.

The conservatism of the design includes the following elements:

• The maximum calculated accident pressure is 41.2 psig, but the containment was designed for 56.5 psig rated pressure. This represents a margin of 37%.

• The post-tensioning system was designed to provide a minimum of 20% larger prestress force than required to offset internal pressure. This represents an additional 20% margin. The internal pressure that would balance the prestress force would be 56.5 x 1.2 = 67.8 psig, which exceeds the maximum accident pressure by 64.6%.

• The design took no credit for concrete strength in tension, assuming it to be zero.

While concrete is weak in tension, its strength is greater than zero.

• The post-tensioning system was designed to provide the strength necessary to resist internal pressure without any credit taken for the liner plate or the reinforcing bars.

The liner plate by itself could resist approximately 15 psi, as shown below.

Hoop stress = PR/T = (15 psi)(75 ft x 12 in/ft)/(0.375 in) = 36,000 psi = 36 ksi (yield stress)

• The ultimate pressure capacity of the containment was estimated to be 141 psig.

The large design margin means that minor degradation would not threaten the containment's ability to perform its design function. The containment surveillance is intended to identify degradation early before it becomes major degradation. If a containment had very small design margin, it would be important to perform the surveillance relatively frequently to ensure degradation is identified before margin is lost. Given the large design margin, it is unlikely all margin will be eroded prior to the next surveillance interval.

The most recent surveillance of the post-tensioning system was performed at STP in 2018, which was the 30th year surveillance. The Proposed Alternative Surveillance, if approved, would skip the 35th year surveillance elements associated with IWL-2520 and perform full surveillances in both units during the 40th and 50th years. The surveillance elements are considered individually below.

Tendon Forces As stated in IWL-2421(b), liftoff testing occurs at 10-year intervals in each individual unit, with the two units staggered on 5-year alternating schedules. The 30th year surveillance included liftoff testing in Unit 1, but not in Unit 2. Liftoff testing is scheduled to occur again in Unit 1 in years 40 and 50. Therefore, the proposed alternative would make no change to the Unit 1

NOC-AE-23003947 Enclosure Page 13 of 15 surveillance schedule for liftoff testing. Liftoff testing is scheduled to occur in Unit 2 in years 35, 45 and 55. The proposed alternative would change that to years 40 and 50, aligned with Unit 1 liftoff testing. That would introduce a one-time surveillance interval of 15 years between liftoff tests in Unit 2 (between years 25 and 40), with 10-year intervals thereafter.

The one-time 15-year interval is acceptable because the trend lines established by regression of liftoff data through the 25th year remain significantly above the Minimum Required Value between years 25 and 40 and beyond. Liftoff testing performed in Unit 1 at year 30 showed no appreciable change in trend in that unit between the 20th year and 30th year data (see Section 6.5). Since the units are essentially identical and subject to the same conditions, it is reasonable to assume Unit 2 likewise did not degrade significantly between the 25th year and 30th year.

Therefore, the proposed alternative surveillance schedule provides an acceptable level of quality and safety with respect to liftoff forces.

Concrete Exterior Examination The examinations required by IWL-2410 and IWL-2510 will continue unchanged, at 5-year intervals.

Wires Wire damage or breakage since construction has been very low. Nearly all wires determined to be ineffective after construction were damaged either deliberately or inadvertently during surveillances. No degradation mechanism other than the surveillance itself has been observed during the life of the plant. Total wire breakage is less than one-tenth the 1%

allowance (426 wires per unit) included in the design. The proposed alternative surveillance, which reduces the number of surveillances, will have the beneficial effect of reducing the opportunities to damage wires. Therefore, the proposed alternative surveillance provides an acceptable level of quality and safety, with respect to tendon wires.

Corrosion Protection Medium The purpose of the corrosion protection medium is to protect the tendons. Tendon liftoff force testing and wire removal and examination confirm that the tendons are behaving as designed. Chemistry samples of the corrosion protection medium have met acceptance limits during all surveillances. Volume discrepancies of the medium have been evaluated as being non-consequential and have not occurred during the three most recent surveillances. The proposed alternative surveillance skips the 35th-year surveillance and examines both units at 10-year intervals thereafter. This proposed alternative schedule provides an acceptable level of quality and safety, with respect to the corrosion protection medium.

Anchorage Components The proposed alternative surveillance extends the interval for visual anchorage inspections from 5 years to 10 years, but the 5-year interval for general visual exterior inspection IWL-2511 will remain unchanged at 5 years. Those 5-year inspections will include the anchorage area outside the end caps; the entire concrete surface area defined by IWL 2524.1 is visible with the end caps in place. Other than ineffective wires, anchorage inspections throughout the life of the plant have not identified any noteworthy issues. Therefore, the proposed alternative surveillance provides an acceptable level of quality and safety.

NOC-AE-23003947 Enclosure Page 14 of 15

7. Duration of Proposed Alternative

STPNOC requests approval to use the proposed alternative surveillance schedule during the fourth and fifth 10-year Containment Inservice Inspection Intervals. (The fourth interval ends September 8, 2029 and the fifth interval will begin September 9, 2029 and end September 8, 2039.)

8. Precedents

8.1 Vogtle Electric Generating Plant, Units 1 and 2 - Inservice Inspection Alternative VEGP-ISI-ALT-19-01 for Containment Tendon Inservice Inspection Extension (EPID L-2019-LLR-0017);

July 11, 2019; ML19182A077.

8.2 Three Mile Island Nuclear Station, Unit 1 - Relief from The Requirements Of The ASME Code Re: Examination And Testing For Containment Unbonded Post-Tensioning System (EPID-L-2018-LLR-0132); September 19, 2019; ML19226A023.

8.3 Millstone Power Station, Unit 2 - Proposed Alternative RR-05-05 to the Requirements of the ASME Code Re: Containment Unbonded Post-Tensioning System Inservice Inspection Requirements (EPID L-2019-LLR-0120); October 20, 2020; ML20287A471.

8.4 Braidwood Station, Units 1 And 2, And Byron Station, Units 1 and 2 - Proposed Alternatives to The Requirements of The American Society of Mechanical Engineers Boiler & Pressure Vessel Code (EPIDS L-2020-LLR-0099 and L-2020-LLR-0100); August 3, 2021; ML21134A006.

8.5 Palo Verde Nuclear Generating Station, Units 1, 2, And 3 - Relief Request 67 For an Alternate Frequency to Containment Unbonded Post-Tensioning System Inservice Inspection (EPID L-2021-LLR-0050); May 12, 2022; ML22124A241.

9. References 9.1 American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (BPV)

Code,Section XI, Subsection IWL, 2013 edition.

9.2 Inservice Inspection Program Plan and Inservice Testing Program Snubber Inservice Test Plan for the Fourth Ten-Year Interval; May 20, 2021; ML21144A109; NOC-AE-21003806.

9.3 Request for Approval of an Alternative Approach for Containment Tendon Surveillances (RR-ENG-37); September 26, 2001; ML012750097; NOC-AE-00000981.

9.4 Revised Request for Approval of an Alternative Approach for Containment Concrete and Tendon Surveillances (RR-ENG-37); August 20, 2002; ML022380251; NOC-AE-02001364.

9.5 Withdrawal of Relief Request, "Request for Approval of an Alternative Approach for Containment Tendon Surveillances (RR-ENG-37)"; August 26, 2003; ML032450235; NOC-AE-03001544.

9.6 License Renewal Application; October 25, 2010; ML103010257; NOC-AE-10002607 9.7 Special Report Regarding an Evaluation of The Unit 1 Third Year Containment Tendon Surveillance; May 5, 1990; ML20042F117; ST-HL-AE-003451.

NOC-AE-23003947 Enclosure Page 15 of 15 9.8 Special Report Regarding an Evaluation of The Unit 1 Fifth Year Containment Tendon Surveillance; April 22, 1992; ML20095G411; ST-HL-AE-004064.

9.9 Special Report Regarding a Evaluation of the Unit 1 Tenth Year Containment Tendon Surveillance; August 12, 1998; ML20237B536; NOC-AE-00000250.

9.10 Special Report Regarding an Evaluation of The Unit 2 Third Year Containment Tendon Surveillance; March 30, 1992; ML20091C260; ST-HL-AE-004037.

9.11 Special Report Regarding an Evaluation of The Unit 2 Fifth Year Containment Tendon Surveillance; September 29, 1993; ML20057D861; ST-HL-AE-004587.

9.12 Special Report Regarding an Evaluation of Unit 1 and Unit 2 Containment Tendon Conditions; September 24, 1998; ML20153G254; NOC-AE-00000304.

9.13 Summary Report Regarding Results of the Unit 2, Year 15 Containment Tendon Surveillance; June 16, 2004; ML041730259; NOC-AE-04001735.

9.14 04/15/2003 Summary of Meeting between NRC and South Texas Project Nuclear Operating Company RE: Risk-Based Proposal for Alternative Selection Criteria for Containment Tendon Testing.; April 25, 2023; ML031150748