ML25128A186
| ML25128A186 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 05/08/2025 |
| From: | Constellation Energy Generation |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML25128A184 | List: |
| References | |
| RS-25-093 | |
| Download: ML25128A186 (1) | |
Text
May 8, 2025 Enclosure A Page 1 of 22 Enclosure A Dresden Nuclear Power Station, Units 2 and 3, Subsequent License Renewal Application Responses to NRC Request for Additional Information Set 2 and Request for Confirmation of Information Set 3
==
Introduction:==
This enclosure provides responses to the NRC Request for Additional Information Set 2 and Request for Confirmation of Information Set 3, organized according to the following table of contents:
Table of Contents Responses to NRC Request for Additional Information Set 2:
RAI 4.3.3-3................................................................................................................................ 2 RAI 4.3.6-1................................................................................................................................ 5 RAI 4.7.5-1................................................................................................................................ 8 RAI B.2.1.33-1..........................................................................................................................10 RAI B.2.1.33-2..........................................................................................................................16 RAI 3.5.2.2.2.1-1.......................................................................................................................19 Responses to NRC Request for Confirmation of Information Set 3:
RCI B.2.1.33-1..........................................................................................................................21 RCI 3.5.2.2.2.1-1.......................................................................................................................22
May 8, 2025 Enclosure A Page 2 of 22 Responses to NRC Request for Additional Information Set 2 as follows:
RAI 4.3.3-3 Regulatory Basis Pursuant to 10 CFR 54.21(c), the SLRA must include an evaluation of time-limited aging analyses (TLAAs). The applicant must demonstrate that (i) the analyses remain valid for the subsequent period of extended operation, (ii) the analyses have been projected to the end of the subsequent period of extended operation, or (iii) the effects of aging on the intended function(s) will be adequately managed for the subsequent period of extended operation.
=
Background===
The following report indicates that in the detailed EAF analysis for the limiting locations, the Fen calculations use the average temperature approach considering the threshold temperature for a material type (
Reference:
Structural Integrity Associates (SIA) 2200483.305P, Environmentally Assisted Fatigue Calculations for Sentinel Locations at Dresden, Revision 1).
Issue SLRA Section 4.3.3 does not clearly discuss the following: (1) whether the average temperature approach is used only for simple, linear transients; and (2) if not, why the conservatism of the applicants approach is comparable to or greater than that of the modified rate approach described in NUREG/CR-6909, Rev. 1, Section 4.4 (i.e., plant-specific demonstration of the adequacy of the applicants approach).
Request Clarify the following: (1) whether the average temperature approach is used only for simple, linear transients; and (2) if not, why the conservatism of the applicants approach is comparable to or greater than that of the modified rate approach described in NUREG/CR-6909, Rev. 1, Section 4.4 (i.e., plant-specific demonstration of the adequacy of the applicants approach).
Constellation Response:
The average temperature approach was used in the detailed EAF analysis for simple transients and for complex transients, as explained below.
NUREG/CR-6909 Revision 1, Section 4.1.4 states:
For simple, linear transients, an average temperature that considers the threshold temperature of 150°C may be used to calculate Fen for a specific stress cycle or load set pair. Complex thermal transients that have multiple increasing and decreasing temperature excursions should be evaluated using the maximum temperature for the specific stress cycle or load set pair unless information is available to justify the use of an average temperature.
May 8, 2025 Enclosure A Page 3 of 22 Based on this guidance, it is concluded that all but a few of the transients assumed in the detailed EAF analysis are simple linear transients which do not have multiple increasing and decreasing temperature excursions. For these simple transients, the detailed EAF analysis uses the average temperature of these transients to determine the Fen value for specific stress cycles or load pairs.
NUREG/CR-6909 Revision 1, Section 4.1.4 does allow the use of average temperatures for transients that are not simple linear transients if information is available that justifies the use of this approach. Industry information is available that justifies the use of the average temperature approach for transients that are not simple in a paper entitled Use of Average Temperature in Fen Calculations, Proceedings of the ASME 2020 Pressure Vessel & Piping Conference, PVP2020-21009. This paper compares Fen values based on the modified rate approach with those using the average temperature approach. The paper documents case studies where the modified rate approach and average temperature approach are used on various BWR and PWR reactor pressure vessel components of varying materials. The paper concluded that in every case study, the total CUFen value was greater (more conservative) with a Fen value based on the average temperature approach rather than a Fen value based on the modified rate approach.
To provide additional confirmation that the use of the average temperature approach is conservative for complex thermal transients, as opposed to the modified rate approach, an actual transient associated with the A feedwater nozzle was evaluated (item number 2 in SLRA Table 4.3.1-3). The transient occurred at Dresden Unit 2 in November 2019, during a reactor startup from a refueling outage. This transient is considered a complex thermal transient since the feedwater nozzle experienced at least two decreasing temperature excursions and at least three increasing temperature excursions.
At Dresden Units 2 and 3, the SI:FatigueProTM software monitors the feedwater nozzles (items 1 through 4 in SLRA Table 4.3.1-3) using stress based fatigue monitoring methodology. The SI:FatigueProTM stress based fatigue monitoring methodology calculates Fen values based on the modified rate approach.
For comparison, the Fen value was also calculated using the average temperature value in accordance with the guidance in NUREG/CR-6909 Revision 1 Appendix A, using equation A.2 on page A-1. A value of 1.77 for the O* value was input. This value corresponds to dissolved oxygen levels during the time of the transient. The minimum strain rate is used in all cases, which is conservative because it results in larger Fen values.
The temperature profile of the November 2019 transient is provided in Figure 1 and the Fen results based on the two methodologies are presented in Table 1 below.
May 8, 2025 Enclosure A Page 4 of 22 Table 1 - Results Date Calculated FatigueProTM Fen Value Based on Modified Rate Approach Tmax Tmin Tave Calculated Fen Value Based on Average Temperature Approach November 11th through 17th 2019 2.1 551°F 302°F(1) 426°F 3.5 Note 1:
For the minimum temperature, the threshold temperature for carbon steel was used instead of the actual temperature, as recommended in NUREG/CR-6909 Revision 1.
The Fen value calculated using the average temperature approach is significantly greater than the Fen value calculated by SI:FatigueProTM using the modified rate approach. This plant specific demonstration provides additional confirmation that the average temperature approach is conservative when calculating the Fen value and environmentally adjusted cumulative usage factor (CUFen) for complex thermal transients at Dresden.
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Figure 1: 11/11/2019 to 11/17/2019 Temperature Data
May 8, 2025 Enclosure A Page 5 of 22 RAI 4.3.6-1 Regulatory Basis Pursuant to 10 CFR 54.21(c), the SLRA must include an evaluation of time-limited aging analyses (TLAAs). The applicant must demonstrate that (i) the analyses remain valid for the subsequent period of extended operation, (ii) the analyses have been projected to the end of the subsequent period of extended operation, or (iii) the effects of aging on the intended function(s) will be adequately managed for the subsequent period of extended operation.
=
Background===
SLRA Section 4.3.6.2 addresses the thermal fatigue TLAA for the DNPS Unit 2 jet pump riser repair/mitigation clamps (also called jet pump riser brace (JPRB) clamps). SLRA Section 4.3.6.2 provides the following explanation regarding the JPRB clamps. During the fall 2001 refueling outage for DNPS Unit 2, a crack was detected in the JPRB for jet pumps 9/10. The SLRA also explains that a mechanical clamping system designed to structurally replace these welds was installed on both jet pump risers in 2003.
Issue SLRA Section 4.3.6.2 does not clearly discuss which specific welds of the jet pump assembly are structurally replaced by the JPRB clamps. The staff also noted that typically two jet pumps share one riser pipe. However, the SLRA explains that the mechanical clamp system was installed on both jet pump risers of the jet pumps 9/10, indicating that the jet pumps 9/10 have two riser pipes rather one riser pipe.
In addition, SLRA Section 4.3.6.2 explains that the JPRB clamps were also installed on the other JPRBs of Unit 2 to preclude high cycle fatigue cracking concerns. However, the SLRA section does not clearly discuss the following items related to the JPRB clamps for vibration mitigation: (1) whether these vibration mitigation clamps are also subject to the fatigue TLAA in a similar manner as the repair clamp(s) are subject to the fatigue TLAA; (2) frequency and method of the periodic inspections for the repair and vibration mitigation clamps; and (3) whether the results of these inspections confirm the absence of cracking in the clamps due to fatigue or other degradation mechanisms (e.g., stress corrosion cracking).
Request
- 1.
Discuss the following items: (1) which specific welds of the jet pump assembly (jet pumps 9/10) are structurally replaced by the JPRB repair clamps; (2) the degradation mechanism that led to the installation of the repair clamps; and (3) how many risers are associated with one pair of jet pumps.
- 2.
Clarify whether the vibration mitigation clamps as well as the repair clamps are subject to the fatigue TLAA for jet pumps. In addition, describe the number of the repair clamps and the number of non-repair vibration mitigation clamps installed inside the Unit 2 reactor vessel, respectively.
May 8, 2025 Enclosure A Page 6 of 22
- 3.
Describe the following items: (1) frequency and method of the periodic inspections for the repair and vibration mitigation clamps; and (2) whether the results of these inspections confirm the absence of cracking in the clamps due to fatigue or other degradation mechanisms (e.g., stress corrosion cracking).
- 4.
Revise SLRA Sections 4.3.6.2 and A.4.3.8 as needed based on the discussion above (e.g., (1) how many repair and vibration mitigation clamps are installed respectively and (2) whether the vibration mitigation clamps as well as the repair clamps are subject to the fatigue TLAA).
Constellation Response to Request 1:
During the fall 2001 refueling outage for DNPS Unit 2, a crack was detected in the JPRB for jet pumps 9 and 10. This is a known industry aging concern described in BWRVIP-41, Revision 4.
Section 5.3 identifies the cause of the cracking.
For Dresden Unit 2, there are 20 jet pumps in total. Two Jet Pumps (pair) share a single riser pipe. Each riser pipe has a Jet Pump Riser Brace (JPRB), for a total of 10. BWRVIP-41 Revision 4, BWR Jet Pump Assembly Inspection and Flaw Evaluation Guidelines, Figure 1-1 provides a sketch of this arrangement.
The Dresden Unit 2 JPRB configuration contains a double leaf configuration with an upper and lower leaf on each side of the riser pipe. There are a total of 20 JPRB upper leaves and 20 JPRB lower leaves. Refer to Table 2.1 and Figure 2-3 of BWRVIP-41 Revision 4, for a depiction of the double leaf configuration. During the fall 2001 refueling outage for Dresden Unit 2, a crack was detected on the leaf brace to block weld (RB-4b) on the JPRB to Jet Pumps 9 and
- 10.
In 2003 the mechanical repair clamp was installed to structurally replace the RB-4b weld attaching the upper riser brace leaf to the small block which is in turn welded to the vessel reactor wall on the Jet Pump 9 side of the JPRB. This weld is shown on figure 2-3 of BWRVIP-41 Revision 4.
Constellation Response to Request 2:
As shown on figure 2-3 of BWRVIP-41 Revision 4, each of the 10 JPRBs has an upper and lower leaf pair on each side of the riser pipe. Therefore, there are a total of 20 upper and lower leaf pairs that straddle the 10 jet pump riser pipes. The mechanical repair clamp installed in 2003 was installed on only one of the upper and lower leaf pairs associated with the jet pump 9 and 10 JPRB, (installed on the Jet Pump 9 side of the JPRB). This is illustrated as the upper and lower leaf pair associated with weld RB-4b on figure 2-3 of BWRVIP-41 Revision 4. The remaining 19 upper and lower leaf pairs of the other JPRBs were not found to have cracks and did not require mechanical repair clamps. However, in the same 2003 outage, Vibration Mitigation Clamps (VMCs) were installed on the remaining 19 upper and lower leaf pairs to mitigate high vibration fatigue concerns.
May 8, 2025 Enclosure A Page 7 of 22 Since these components may be susceptible to fatigue, fatigue evaluations were performed on the one mechanical repair clamp that was installed on one jet pump 9 and 10 JPRB upper and lower leaf pair and on the 19 VMCs installed on the remaining 19 upper and lower leaf pairs.
Constellation Response to Request 3:
The one mechanical repair clamp and each of the 19 VMCs were inspected in 2005, one cycle after installation, with no relevant indications identified. The one mechanical repair clamp and seven (7) of the 19 VMCs were again inspected in 2007 with no relevant indications identified.
In 2013 visual inspections were performed on five (5) of the 19 VMCs and the one repair clamp with no relevant indications identified. In 2019 visual inspections were performed on five (5) of the 19 VMCs and the one repair clamp with no relevant indications identified. In 2023 visual inspections were performed on five (5) of the 19 mitigation clamps and the one repair clamp, with no relevant indications identified. In summary all 19 VMCs have been inspected at least one time since installation and 16 VMCs have been inspected at least two times; with no relevant indications identified. In addition, the mechanical repair clamp has been inspected five times since installation with no relevant indications identified.
For the mechanical repair clamp and the VMCs, Dresden imposed a medium priority classification, and the inspection frequency up to 2014 was 25% every 6 years. This matched the recommended inspection frequency in BWRVIP 41 for the RB-4 welds. In 2014 the BWR Owners Group revised the inspection frequency recommendation for the RB-4 welds in BWRVIP-41 Revision 4 to 25% every 12 years. Therefore, inspection frequencies for the mechanical repair clamp and the VMCs is currently 25% every 12 years.
The inspections utilize VT-1 inspection requirements which are intended to detect discontinuities and imperfections on the surface of components, including such conditions as cracks, wear, corrosion, or erosion.
Therefore, based on previous inspections and the material properties of the mechanical repair clamps and VMCs, future inspections are expected to confirm the absence of cracking due to fatigue or other degradation mechanisms such as stress corrosion cracking.
Constellation Response to Request 4:
Review of SLRA Section 4.3.6.1 and Appendix A.4.6.3.1 found that the descriptions of the location of the crack, found in 2001 and the installed repair clamp were imprecise. As a result, SLRA Sections 4.3.6.1 and Appendix A.4.3.6.1 are revised as shown in Enclosure B.
May 8, 2025 Enclosure A Page 8 of 22 RAI 4.7.5-1 Regulatory Basis Pursuant to 10 CFR 54.21(c), the SLRA must include an evaluation of time-limited aging analyses (TLAAs). The applicant must demonstrate that (i) the analyses remain valid for the subsequent period of extended operation, (ii) the analyses have been projected to the end of the subsequent period of extended operation, or (iii) the effects of aging on the intended function(s) will be adequately managed for the subsequent period of extended operation.
=
Background===
SLRA Section 4.7.5 addresses the fatigue TLAA for Dresden Unit 2 core spray replacement piping. The applicant explained that, in November 2009, all four lower sections of the core spray system were replaced and thus removed all known piping flaws of the core spray piping inside the Unit 2 reactor pressure vessel.
The SLRA also indicates that, since the fatigue analysis for the replacement piping assumed the 40-year service life since the replacement (November 2009) until November 2049, the analysis was reevaluated to extend the evaluation period by additional 5 years (total 45 years) beyond the end of subsequent period of extended operation for Unit 2 (i.e., December 2049). The applicant dispositioned the fatigue TLAA in accordance with 10 CFR 54.21(c)(1)(i).
Issue SLRA Section 4.7.5 does not clearly describe the transients and transient cycles used in the existing 40-year fatigue analysis in comparison with the projected cycles for the service of the replacement core spray piping throughout the end of the subsequent period of extended operation to confirm that the transient cycles evaluated in the existing 40-year fatigue analysis are bounding for the projected cycles for the service of the replacement core spray piping.
Request Describe the transients and transient cycles used in the existing 40-year fatigue analysis in comparison with the projected cycles for the service of the replacement core spray piping throughout the end of the subsequent period of extended operation to confirm that the transient cycles evaluated in the existing 40-year fatigue analysis are bounding for the projected cycles for the service of the replacement core spray piping.
Constellation Response:
Fatigue analyses were performed on various components of the Unit 2 Core Spray System lower sectional replacement (LSR) to demonstrate the structural integrity and functional adequacy of the repair. Specifically, low cycle fatigue analyses were performed at the limiting locations of the pipe flange, the lower anchor, and bolts. The fatigue analyses were performed based on enveloping 500 transient occurrences over the 40-year design life. The specific transients that were enveloped were specified in the Dresden Unit 2 and 3 reactor vessel thermal cycle diagram and are documented in Table 1 below. While transients with a large
May 8, 2025 Enclosure A Page 9 of 22 temperature rate of change, specifically startup and shutdown, would incur more fatigue damage on LSR components, other transients with smaller temperature rate changes were also selected and are included in the 500 transient occurrences.
Table 1 below provides a comparison of the number of specified transient occurrences that are the basis for the assumed 500 occurrences versus the projected number of occurrences from the date of installation of the LSR in November 2009 until the end of the SPEO.
The first and second column of Table 1 documents the design transients and transient occurrences specified for 40-years in the thermal cycle diagram, which are the basis for the 500 enveloping occurrences. Note that although the total number of specified design transient occurrences shown in the second column is 463, the LSR fatigue analyses rounded up to 500.
Since the LSR was installed during a Unit 2 refueling outage which ended December 1 of 2009, all transients that occurred prior to December 1 of 2009 are excluded from the comparison. The fifth column documents the projected number of occurrences from December 2009 to the end of the SPEO. This column shows that the number of projected transient occurrences from December 2 of 2009 until the end of the SPEO are less than or equal to the specified occurrences for each listed transient. Therefore, the fatigue analyses are bounding for the projected transient occurrences from the date of installation until the end of SPEO.
Table 1 - Dresden Unit 2 Transient Events and Projections Transient Number of Specified Design Occurrences for 40 years 80-Year Projections from SLRA Table 4.3.1-1 Number of Occurrences through December 1, 2009 Projected Number of Occurrences from December 2, 2009, Until end of SPEO Startup/Shutdown 120 283 214 69 Loss of Feedwater Flow 80 10 8
2 Scram 200 227 169 58 SRV Blowdown 1
4 3
1 Improper Start of a Recirc Loop 10 2
1 1
Sudden Start of Recirc Loop 10 1
0 1
Turbine Trip 40 74 50 24 Overpressure to 1250 psig 1
1 0
1 Overpressure to 1375 psig 1
1 0
1 Total 463 603 445 158
May 8, 2025 Enclosure A Page 10 of 22 RAI B.2.1.33-1 Regulatory Basis Title 10 of the Code of Federal Regulations Section 54.21(a)(3) requires the applicant to demonstrate that the effects of aging for structures and components will be adequately managed so that the intended function will be maintained consistent with the current licensing basis (CLB) for the period of extended operation. As described in the SRP-SLR, an applicant may demonstrate compliance with 10 CFR 54.21(a)(3) by referencing the GALL-SLR Report when evaluation of the matter in the GALL-SLR Report applies to the plant.
=
Background===
Units 2 and 3 chimney is a tapered, cantilevered reinforced concrete structure with a height of 310 feet. SLRA Section 2.4.12 designates this chimney as a safety-related structure that provides a discharge path for treated gaseous waste.
During the NRC onsite audit from December 10-12, 2024, the staff observed significant surface efflorescence (white and black) and some minor spalling on the exterior of the Units 2 and 3 chimney from the ground up to about 25 feet elevation below the vent stack. The staff reviewed the work orders (WOs) from the past 10 years, including WO 01512813, WO 01601817, WO 01698314, WO 01793913, WO 01888740, WO-04867735, WO 04995283, WO 05112098, WO 05218686, WO 0532099, and WO 05429441, which indicate that chimney degradations in this area develop slowly over time and are likely to continue during the Subsequent Period of Extended Operation (SPEO). However, these WOs focus on recent degradation trends and do not consider long-term degradation patterns. Despite increasing the inspection frequency to annually for aviation safety purposes, no significant corrective actions have been taken to address the concrete conditions.
SLRA Section B.2.1.33, as modified by SLRA Supplement 2 (ML25072A153), enhances the acceptance criteria program element in the Structures Monitoring program by using quantitative second tier criteria from Chapter 5 in ACI 349.3R-02 for structural concrete. Due to the chimneys unique configuration and past operating experience, second tier criteria maybe insufficient to adequately manage effects of aging for the Units 2 and 3 chimney.
The staff also reviewed UFSAR Sections 3.3.1.1.2, 3.3.2.2.3, 3.7.2.4, and 3.8.4.3 along with UFSAR Figures 3.3-1, 3.3-2, 3.7-11, 3.7-12, 3.7-13, 3.8-44 to 3.8-46 and found that critical sections of the Units 2 and 3 chimney are located at 60 feet above grade for tornado loading and 208 feet above grade for safe shutdown earthquake (SSE) loading. However, UFSAR does not include calculation results for the chimneys lower sections, where observable degradations are occurring.
Issue SLRA does not make clear whether the observed degradations affect the Units 2 and 3 chimneys ability to perform its intended function. With the potential further degradations of the chimney, the Structures Monitoring program lacks specific degradation limits or acceptance criteria for the chimney that would trigger corrective actions, that require testing or engineering evaluations. Without these clear thresholds, it is uncertain how the Units 2 and 3 chimney will maintain its intended function during the SPEO.
May 8, 2025 Enclosure A Page 11 of 22 Request
- 1.
Update the SLRA to incorporate recent operating experience and degradation trends related to the Units 2 and 3 chimney.
- 2.
Evaluate whether the observed degradations affect the Units 2 and 3 chimneys ability to perform its intended function.
- 3.
Based on the chimneys unique configuration (height, critical loading sections, environmental exposure), aligning with the rate of degradation in the lower sections of the chimney and in accordance with Chapter 5 of ACI 349.3R-02, establish clear degradation limits or acceptance criteria (e.g., crack width, spalling depth and dimension, extent of efflorescence) with a technical basis for the Units 2 and 3 chimney, which will trigger corrective actions and ensure the chimneys structural integrity and intended function during the SPEO. Propose program enhancements if required.
Constellation Response to Request 1:
Inspections confirming the structural adequacy of the Units 2 and 3 chimney have been performed since 1975. An inspection report was prepared by an outside engineering consultant for the 1995 inspection. Additional inspection reports were reviewed for inspections performed in 1996, 1998, 1999, 2000, and 2002. Inspections records were also reviewed every year since 2012. From these reports, it can be qualitatively inferred that no significant degradation existed.
The conditions around the circumference of the chimney are similar with no section being significantly worse than another.
Narrow cracking at the bottom of the chimney was noted in the 1995 report. The inspection report in 1995 identified no significant degradation affecting structural adequacy. The 1995 inspection report postulated that the cracking was due to temperature effects and the cracks are described as follows:
The walkdown revealed that there were several cracks in the concrete shell. These cracks were the typical temperature cracks which pose no structural adequacy concern.
The cracks were typically hairline in size. No rusting discoloration was noted. Some cracks were visibly wet, while others showed a dried discoloration from previous leakages. The cracks were typically vertical. The cracks appeared randomly located.
Subsequent inspection reports identify only minor changes in concrete condition. Inspection reports from 2014 through 2024 include color photographs, which allow for comparisons to previous years and do not identify discernible differences in the degradation. The 2024 report compares the 2014 and 2024 photos. A comparison of the photos in the reports from 2014 to 2024 reveal only minor differences in the concrete conditions.
May 8, 2025 Enclosure A Page 12 of 22 Following the NRC walkdown in December 2024 during the onsite audit, a structural assessment was made for license renewal purposes to address the conditions at the bottom of the chimney. The following observations were made regarding the conditions at the base of the chimney:
Primarily vertical cracks were observed around the base of the chimney. The cracks are often long (greater than 10 feet), relatively narrow, with somewhat consistent spacing between the cracks.
Rust staining due to reinforcing steel corrosion was not identified.
Horizontal cracks, also narrow, are much less common, shorter, and more randomly spaced.
Mineral deposits, a cloudy white in color, with a relatively smooth surface, were observed in some cracks but not all. The mineral deposits were relatively hard and strongly adhered to the concrete.
Map cracking was sometimes observed in a narrow band of a few inches along some of the vertical and horizontal cracks, but not all. The map cracking was randomly dispersed and covers only a small portion of the overall area.
The cracking and mineral deposits are much more common at the bottom of the chimney (about bottom 25) than at higher sections. Little cracking or mineral deposits were visible at higher elevations.
The comparisons of the photos in the reports from 2014 to 2024, which reveal only very small differences in the concrete conditions, are consistent with a comparison of the descriptions of the chimney conditions in the 1995 report and the December 2024 walkdown. Considering the previous degradation description in 1995, and the photo comparison between the 2014 and 2024 inspections, the extent of the degradation has increased very slowly, and the degradation does not appear to be worsening at a noticeably higher rate in later years when compared to earlier years. To date, the chimney degradation trend in this area has developed slowly over time and is likely to continue at a similar rate during the Subsequent Period of Extended Operation (SPEO).
Up to this point, with the chimney being more than 50 years old, there have been no significant material losses, indications of changes in material properties, or cracks that would affect chimney cross sections due to crack size, locations, or orientation. Therefore, there has been no degradation that would affect material cross sections considered in the chimney analysis.
May 8, 2025 Enclosure A Page 13 of 22 Constellation Response to Request 2:
The base of the chimney is 25-7-1/2 in diameter with a wall thickness of 2-9. It should be noted that the base of the chimney is not the critical section in the analysis referred to in the UFSAR so all safety factors will be higher for the chimney base than what is shown for the upper portions of the chimney.
Following the NRC walkdown in December 2024, a structural assessment was made for license renewal purposes to address the conditions at the bottom of the chimney. It was concluded that the structural integrity of the reinforced concrete chimney was not affected by the current aging effects, due to the relatively minor nature of the cracking considering the thickness of the base, the crack orientation, and crack locations, as well as the lack of any indication of reinforcing steel corrosion.
The cracks observed are primarily vertical cracks around the base of the chimney.
Vertical cracking is typical for a chimney and poses no structural adequacy concern since the direction and location of the vertical cracking does not affect the cross section of the chimney considered in the analysis.
No circumferential cracks were identified that would indicate an overstress condition due to bending loads at the base of the chimney.
The magnitude and extent of minor spalling has a negligible effect on the cross section considered in the analysis since the affected areas are shallow (less than the depth of the reinforcing steel cover) and localized.
Since the chimney base is above grade and not continuously exposed to water, any efflorescence due to potential leaching is not expected to be significant. The efflorescence has not been associated with any concrete degradation.
Rust staining due to reinforcing steel corrosion was not identified so there is no indication of any loss of cross section of the reinforcing steel.
Based on these observations, it can be concluded that the observed degradations do not affect the Units 2 and 3 chimneys ability to perform its intended function.
May 8, 2025 Enclosure A Page 14 of 22 Constellation Response to Request 3:
The inspections will be continued under the current program. As discussed above, it is concluded that the aging effects, which are only very slowly progressing, are being adequately managed and the chimney intended functions will be maintained during the SPEO.
Considering the concrete used for the original construction, the relatively mild environment that the base of the chimney is exposed to, and the structural margins, significant concrete degradation is not expected at the base of the chimney. The cracks at the bottom of the chimney are related to temperature effects such as expansion and contraction, which are self-limiting and do not affect the structural performance of the chimney. The cracks may widen at the surface due to some freeze-thaw degradation but the effect on the overall cross section is negligible. Since the chimney base is above grade and not continuously exposed to water, any efflorescence due to potential leaching is not expected to be significant. The efflorescence has not been associated with any concrete degradation and is likely due to salts in the groundwater wicking up into the base of the chimney and the extent is very small when compared to the overall chimney. Structural performance would be impacted when there are significant material losses that affect the geometric properties of the chimney used in the analysis, indications of changes in material properties or cracks that would affect chimney cross sections due to crack size, locations, or orientation. To date after more than 50 years, there has been no degradation that would affect material cross sections considered in the chimney analysis.
SLRA Section B.2.1.33, as modified by SLRA Supplement 2 (ML25072A153), enhances the acceptance criteria program element in the Structures Monitoring program by using quantitative second tier criteria from Chapter 5 in ACI 349.3R-02 for structural concrete. SLRA Section B.2.1.33 also enhances the monitoring and trending program element in the Structures Monitoring program to perform inspections to establish quantitative baseline inspection data prior to the subsequent period of extended operation for structures in which prior inspection results are inadequate to establish a quantitative baseline to allow for effective monitoring and trending. Quantitative inspection records have not been developed during the inspections of the base of the chimney.
Instead of establishing new, unique degradation limits for the chimney, that would result in the initiation of further detailed evaluations and development of potential corrective actions, a new enhancement will be identified to require a detailed engineering evaluation of the chimney base in accordance with section 5.3 of ACI 349.3R-02, including consideration and identification of any material testing, and future inspection changes, as well as other corrective actions such as repairs, to ensure that the chimneys structural integrity and intended functions during the SPEO will be maintained, while considering the degradation trends that will be quantitatively established as a result of the implementation of the other enhancements mentioned above.
The establishment of hypothetical degradation limits or acceptance criteria (e.g., crack width, spalling depth and dimension, extent of efflorescence or other aging effect or combination of aging effects) with a technical basis for the Units 2 and 3 chimney, is problematic due to the number of different potential combinations of different aging effects and degree of degradation for the different aging effects that might be considered. The new enhancement will trigger the corrective actions to ensure the chimneys structural integrity and intended function during the SPEO. The new enhancement will be identified to require the detailed engineering evaluation of the chimney base, including consideration and identification of any material testing, and future inspection changes, as well as any other corrective actions such as repairs, to ensure that the
May 8, 2025 Enclosure A Page 15 of 22 chimneys structural integrity and intended functions during the SPEO will be maintained, considering the degradation trends. Propose program enhancements if required.
This approach considers the concerns identified for the chimney in the issue statement above, and that the chimney is a tall cantilever structure with no redundant load paths, and that no quantitative trending has been performed to date. The detailed evaluation will be initiated following the establishment of the baseline inspection results and completed no later than two years after entering the SPEO in order to allow for the planning, implementation, and data processing for any testing data required as a result of the initial detailed evaluation results.
Enhancement #19 of the Structures Monitoring aging management program will be added to provide a more detailed explanation of the plant specific actions.
SLRA Appendix A, Section A.2.1.33, and Appendix B, Section B.2.1.33, are revised as shown in Enclosure B. SLRA Appendix A, Section A.5, Commitment 33 is revised as shown in Enclosure C.
May 8, 2025 Enclosure A Page 16 of 22 RAI B.2.1.33-2 Regulatory Basis Title 10 of the Code of Federal Regulations Section 54.21(a)(3) requires the applicant to demonstrate that the effects of aging for structures and components will be adequately managed so that the intended function will be maintained consistent with the current licensing basis (CLB) for the period of extended operation. As described in the SRP-SLR, an applicant may demonstrate compliance with 10 CFR 54.21(a)(3) by referencing the GALL-SLR Report when evaluation of the matter in the GALL-SLR Report applies to the plant.
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Background===
SLRA Table 3.5.2-2, as modified by SLRA Supplement 2 (ML25072A153), added a Table 2 AMR item for the galvanized steel cable tray and its supports in the Units 2 and 3 Crib House exposed to water-flowing environment, which will be managed under the Structures Monitoring program.
During the NRC onsite audit from December 10-12, 2024, the staff observed significant and excessive corrosion of the south side lower cable tray and its supports running between 2A and 2B circulating water pumps in the Units 2 and 3 Crib House. This deterioration was due to periodic water intrusion from electrical penetrations (ductbanks) located above the tray.
Interviews with the applicants staff revealed that this is a reoccurring issue. In 1996, a wider galvanized steel cable tray was installed underneath the original tray due to excessive deterioration of the original sheet metal cable tray from the same water in-leakage problem.
Over the years, the applicant had conducted engineering evaluations, testing and various mitigative actions to address the cable tray degradation and water in-leakage in the southeast portion of the Crib House.
SLRA Sections 3.5.2.2.2.1, item 4, and 3.5.2.2.2.3, item 3 indicate that inaccessible concrete elements of Group 6 structures (e.g., Crib House) are subject to an aggressive environment.
Test results from groundwater and raw water samples taken in 2023 confirm chloride levels exceeding 500 ppm, which defines an aggressive environment. This chloride exposure is detrimental to the south side cable tray and its supports due to recurring water intrusion from the electrical penetrations.
Issue The SLRA does not clearly demonstrate whether the observed degradation affects the ability of the south side lower cable tray and its supports in the Units 2 and 3 Crib House to perform their intended function. Given the significant and recurring degradation, the Structures Monitoring program lacks specific degradation limits or acceptance criteria for the cable tray and its supports exposed to water-flowing environment that would trigger corrective actions. Without clear thresholds, it is uncertain how these components will maintain their intended function during the SPEO.
Request
- 1.
Update the SLRA to incorporate operating experience related to the south side lower cable tray and its supports in the Units 2 and 3 Crib House.
May 8, 2025 Enclosure A Page 17 of 22
- 2.
Evaluate whether the observed degradation affects the ability of the south side lower cable tray and its supports in the Units 2 and 3 Crib House to perform their intended function. Clarify whether these components will be replaced prior to entering the SPEO.
- 3.
Based on past operating experience, establish clear degradation limits or acceptance criteria with a technical basis for the south side lower cable tray and its supports in the Units 2 and 3 Crib House. These criteria should trigger corrective actions for prompt repair or replacement to ensure their structural integrity and intended function during the SPEO. Propose program enhancements if required.
Constellation Response to Request 1:
During periods of heavy rain, groundwater in-leakage from a nearby cable duct bank drains into the Unit 2 and 3 Crib House via unsealed cable penetrations. The penetrations are designed to allow water to drain along the wall to a floor drain trench, however some of the water drains along the cables coming from the penetrations and into the closed bottom pan of the cable tray assembly. Over the years, water intrusion into the cable tray assembly resulted in areas of degradation due to corrosion. To protect the cable tray assemblies from further degradation, the station investigated sealing the cable penetrations, however this was determined to be unadvisable. The cable duct bank and penetrations are designed to drain through the penetrations to prevent the cables from being permanently submerged. As a compensatory measure, the station installed a new wider reinforcement tray below the existing tray in 1997.
Since then, ground water in-leakage and other sources of water have contributed to the degradation of some components of the new cable tray assembly. In 2016, a fire piping elbow was reported in the Corrective Action Program (CAP) to be leaking above the cable tray and repaired in 2017. An inadvertent activation of the deluge system, reported in CAP in 2016, flooded the cable tray assembly and resulted in standing water in the cable tray assembly.
The cable tray is inspected on a five-year frequency during the Crib House Structures Monitoring inspections. Inspection reports from 2016 and 2021 document evidence of corrosion of some cable tray pans and supports. Photos found in these inspection reports compared against the current condition of the cable tray pans and supports show a slow progressing degradation.
Constellation Response to Request 2:
Design information indicates that the cable tray supports perform a critical structural function in the assembly. Although the design allows a maximum tray span of 13 feet between supports to carry all design loads, the supports are field located no more than 6 feet from each other resulting in significant margin in the support configuration.
The cables within the cable tray are continuously supported by the cable tray pan. While certain cable tray pans are experiencing degradation, no evidence indicating that the wall loss has extended through wall has been identified. As such, the cables remain adequately supported and the intended function of the cable tray pans to support the cables is maintained.
To address the current condition of the cable tray assemblies, DNPS will enhance the structures monitoring program to require the replacement of degraded components of the cable tray
May 8, 2025 Enclosure A Page 18 of 22 assemblies to return the assembly to full structural design capacity. This replacement will be performed prior to entering the subsequent period of extended operation. To address ongoing degradation issues associated with this cable tray, subsequent replacements will be performed at a 20-year interval.
SLRA Table 3.5.2-2, Component Supports Summary of Aging Management Evaluation, Appendix A, Section A.2.1.33, and Appendix B, Section B.2.1.33 are revised as shown in Enclosure B. SLRA Appendix A, Section A.5, Commitment 18 is revised as shown in Enclosure C.
Constellation Response to Request 3:
Since these cable trays will be replaced on a time-based frequency, they will no longer be subject to aging management review and, therefore, DNPS will not rely on inspections to ensure aging is adequately managed and, as such, inspection acceptance criteria are not required.
May 8, 2025 Enclosure A Page 19 of 22 RAI 3.5.2.2.2.1-1 Regulatory Basis Title 10 of the Code of Federal Regulations Section 54.21(a)(3) requires the applicant to demonstrate that the effects of aging for structures and components will be adequately managed so that the intended function will be maintained consistent with the current licensing basis (CLB) for the period of extended operation. As described in the SRP-SLR, an applicant may demonstrate compliance with 10 CFR 54.21(a)(3) by referencing the GALL-SLR Report when evaluation of the matter in the GALL-SLR Report applies to the plant.
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Background===
SRP-SLR states that cracking and distortion due to increased stress levels from settlement could occur in below-grade inaccessible concrete areas of structures for all Groups, and reduction in foundation strength, and cracking due to differential settlement and erosion of porous concrete subfoundations could occur in below-grade inaccessible concrete areas of Groups 1-3, 5-9 structures.
SLRA Table 3.5.1 claims that Table 1 AMR item 3.5.1-044, as modified by SLRA Supplement 2 (ML25072A153), aligns with NUREG-2191. Specifically, it indicates that the Structures Monitoring program will manage cracking and distortion of reinforced concrete in all inaccessible areas exposed to a groundwater/soil environment for yard and switchyards structures.
SLRA Section 3.5.2.2.2.1, Item 3, as modified by SLRA Supplement 2 (ML25072A153),
indicates that this aging effect and mechanism is insignificant for the Group 1, 2, 3, 6, and 9 concrete building structures founded on rock or natural compacted soil, and the DNPS Mark 1 containment and its associated support is established on the Reactor Building foundation slab, which rests on solid rock as specified in UFSAR Section 2.5.4. Consequently, cracking and distortion from increased stress levels due to settlement are not applicable to Group 4 internal containment structures.
Additionally, DNPS does not have concrete tanks or concrete missile barriers categorized as Group 7 structures, which would require aging management. Group 5 structure is evaluated as a Group 2 structure and concrete foundations for group 8 structures within the scope of subsequent license renewal are evaluated for aging management in SLRA Table 3.5.2-16 for yard structures.
Furthermore, SLRA Table 3.5.1 indicates that Table 1 AMR item 3.5.1-046, as modified by SLRA Supplement 2 (ML25072A153), is not used. The aging effects of reduction of foundation strength and cracking due to differential settlement and erosion of porous concrete subfoundation are managed under Table 1 AMR item 3.5.1-044.
UFSAR Section 2.5.4 states, Examination of cores from borings at the site and excavation for the construction of Units 1 and 2 show that all footings for major structures have a foundation of sound rock which eliminates the potential problems of soil consolidation and differential settlement. However, this statement does not explicitly identify which Units 1 and 2 concrete structures are supported on rock, nor does it clarify whether the Unit 3 in-scope concrete structures are similarly supported on rock.
May 8, 2025 Enclosure A Page 20 of 22 Issue The SLRA does not explicitly identify which in-scope concrete structures are supported on rock, nor does it provide an adequate technical basis for identifying which concrete structures are rock-supported.
Request
- 1.
Evaluate and provide a technical basis for whether all in-scope concrete structures (excluding yard and switchyard structures) are supported on rock, and identify those that are not.
- 2.
For concrete structures that not supported on rock or cannot be identified to be supported on rock, provide the corresponding Table 2 items associated with Table 1 AMR item 3.5.1-044.
Constellation Response to Request 1:
Excluding the Switchyard Structures and Yard Structures, all concrete structures within the scope of the subsequent license renewal are supported on rock. This conclusion is substantiated through a review of station design documentation, which includes drawings, specifications, the Updated Final Safety Analysis Report (UFSAR), various calculations, and modification documentation.
As stated in the UFSAR section 2.5.4, sound rock prevents potential problems associated with soil consolidation and differential settlement. Station design drawings and documentation indicate that the foundations of these structures were constructed on rock, ensuring stability and integrity.
The technical basis for the evaluation to determine whether all in-scope concrete structures (excluding yard and switchyard structures) is based upon a review of station design documentation, which includes drawings, specifications, the Updated Final Safety Analysis Report (UFSAR), various calculations, and modification documentation. This evaluation determined that all in-scope concrete structures (excluding yard and switchyard structures) are supported on rock.
Constellation Response to Request 2:
Excluding the Table 2 items associated with the Table 1 AMR item 3.5.1-044 for the Switchyard Structures and Yard Structures, it has been determined that there are no additional Table 2 items associated with Table 1 AMR item 3.5.1-044 necessary for the other structures within the scope of subsequent license renewal, as they are supported on rock.
May 8, 2025 Enclosure A Page 21 of 22 Responses to NRC Request for Confirmation of Information Set 3 as follows:
RCI B.2.1.33-1 Operating experience in SLRA Section B.2.1.33 describes effectiveness of the AMP in managing aging effects of Units 2 and 3 Reactor Building roof. The Reactor Building roof was replaced in 2019.
During the NRC onsite audit from 12/10/2024 to 12/12/2024, the staff observed significant damage and vegetation growth at some areas of Units 2 and 3 Turbine Building roof, which could eventually lead to roof leakage. The staff reviewed WO 1508043 and finds that the Turbine Building roof will be replaced once the contract is awarded.
Confirm that building roofs with significant degradation that resulted in a potential loss of intended function for components and protected by the roofs will eventually be replaced.
When necessary, temporary measures are implemented to ensure that roof leakage does not impair the operability of underlying equipment.
Confirm that the Turbine Building roof will be replaced prior to entering the SPEO.
Constellation Response:
At DNPS, when significant degradation of a building roof that could result in a potential loss of intended function for components protected by the roof is identified, the roof is replaced in accordance with the corrective action program. Temporary measures are implemented to ensure that roof leakage does not impair the operability of underlying equipment, when necessary.
Replacement of the Units 2 and 3 Turbine Building roof is currently planned to be performed prior to the subsequent period of extended operation.
May 8, 2025 Enclosure A Page 22 of 22 RCI 3.5.2.2.2.1-1 SLRA Section 3.5.2.2.2.1, Item 4, as modified by SLRA Supplement 2 (ML25072A153), states, Operating experience at DNPS, which inspects for concrete deterioration due to any aging effect and mechanism, has identified increases in porosity and permeability and loss of strength due to the effects of carbonation. Concrete degradation that would indicate a reduction in its structural integrity has not been identified. The inconsequential increase in porosity and permeability, and loss of strength due to leaching of calcium hydroxide and carbonation does not impact the intended functions of concrete structures.
SLRA Section 3.5.2.2.2.3, Item 3 states, Some minor leaching of calcium hydroxide has been observed on DNPS Group 6 concrete structures; however, it did not result in a loss of intended function.
SRP-SLR Report states that a plant-specific program is not required for the reinforced concrete exposed to flowing water if evaluation determined that the observed leaching of calcium hydroxide and carbonation in accessible areas has no impact on the intended function of the concrete structure.
In addition, SLRA Section 3.5.2.2.2.1, Item 1, as modified by SLRA Supplement 2 (ML25072A153), states, The inconsequential loss of material (spalling, scaling) and cracking due to freeze-thaw does not impact the intended functions of concrete structures.
SLRA Section 3.5.2.2.2.3, Item 1, as modified by SLRA Supplement 2 (ML25072A153), states, The inconsequential loss of material (spalling, scaling) and cracking due to freeze-thaw does not impact the intended functions of concrete structures.
SRP-SLR Report states that a plant-specific program is not required if documented evidence confirms that the existing concrete had air content of 3 percent to 8 percent and subsequent inspection did not exhibit degradation related to freeze-thaw.
Confirm that evaluations exist and demonstrate that the observed leaching of calcium hydroxide and carbonation has no impact of the intended functions of the concrete structure, which was described in SLRA Sections 3.5.2.2.2.1, Item 4 and 3.5.2.2.2.3, Item 3.
Confirm that evaluations exist and demonstrate that the observed loss of material (spalling, scaling) and cacking due to freeze-thaw has no impact of the intended functions of the concrete structure, which was described in SLRA Sections 3.5.2.2.2.1, Item 1 and 3.5.2.2.2.3, Item 1.
Constellation Response:
CEG confirms that the information provided above is correct.