ML24340A122
| ML24340A122 | |
| Person / Time | |
|---|---|
| Site: | Callaway  (NPF-030) |
| Issue date: | 12/05/2024 |
| From: | Ameren Missouri |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24340A120 | List: |
| References | |
| ULNRC-06907 | |
| Download: ML24340A122 (1) | |
Text
Attachment to Supplemental Information and Response to Request for Confirmation ULNRC-06907 of Information (RCI) for Resolution of Callaway Plant Page 1 of 5 License Renewal Commitments 34 and 35 NOTES:
The U.S. Nuclear Regulatory Commission (NRC) staffs Request for Confirmation of Information (RCI) and NRC audit questions are shown in italics within this attachment to distinguish them from the Ameren Missouri responses.
The references in this attachment are identified on page 3 of the cover letter.
Request for Confirmation of Information (RCI)
Background
In March 2015, the NRC published the final Safety Evaluation Report Related to the License Renewal of Callaway Plant, Unit 1, as NUREG-2172 (ML15068A342). In NUREG-2172, Appendix A, Callaway Plant Unit 1 License Renewal Commitments, Commitment No. 34 provides for three options to fulfill the commitment, and Commitment No. 35 provides for two options to fulfill the commitment. By letter dated December 20, 2023, (Agencywide Documents Access and Management Systems (ADAMS) Package Accession No. ML23354A244), Ameren Missouri (the licensee), submitted to the U.S. Nuclear Regulatory Commission (NRC or staff) a response to Commitment Nos. 34 and 35 of the Callaway, Unit 1 (Callaway) license renewal safety evaluation report. Commitment No. 34 is related to a concern regarding potential failure at the divider plate welds to primary head and tubesheet cladding. Commitment No. 35 is related to a concern regarding potential failure of the primary-to-secondary pressure boundary due to PWSCC cracking of tube-to-tubesheet welds. To address both commitments, the licensee selected Option 2: Analysis. The December 20, 2023 letter included Enclosure 1, Cracking Assessment for Framatome RSG Channel Head Assembly (ML23354A246), which describes the analyses.
Regulatory Basis Section 54.21(a)(3) of Title 10 of the Code of Federal Regulations (10 CFR) requires an applicant to demonstrate that the effects of aging for structures and components will be adequately managed so that the intended function(s) will be maintained consistent with the current licensing basis for the period of extended operation. One of the findings that the NRC staff must make to issue a renewed license (10 CFR 54.29(a)) is that actions have been identified and have been or will be taken with respect to managing the effects of aging during the period of extended operation on the functionality of structures and components that have been identified to require review under 10 CFR 54.21, such that there is reasonable assurance that the activities authorized by the renewed license will continue to be conducted in accordance with the current licensing basis.
Issue
Attachment to Supplemental Information and Response to Request for Confirmation ULNRC-06907 of Information (RCI) for Resolution of Callaway Plant Page 2 of 5 License Renewal Commitments 34 and 35
- a. Section 4.2, Justified Assumptions and Modeling Simplifications, of Enclosure 1
[Reference 3] states that a residual stress of +2 ksi was conservatively assumed to exist through the thickness of the channel head cladding based on the project agreement between Framatome and EPRI. During the audit, the licensee clarified that 2 ksi is identified as a residual stress uncertainty in the EPRI source documents, not residual stress magnitude, and this was misstated in Enclosure 1. Citing a technical reference, the licensee stated that +2 ksi in the cladding was a conservative value based on the small thickness of the cladding relative to the base metal (about 5%), the post-weld heat treatment, pre-service hydrostatic testing, and inservice temperature changes.
- b. Section 4.2 of Enclosure 1 [Reference 3] also states that no residual stress was assigned to the low alloy steel channel head material. Industry studies used a base metal residual stress of +8 ksi due to uncertainty about the location of seam welds. In the audit the licensee stated that the circumferential welds will not affect crack growth because they are too distant from the postulated fatigue crack location. The licensee also noted that there are no longitudinal welds in the tubesheet ring. The licensee provided a stress analysis example with dimensions similar to those of the Callaway SGs to show that residual stress decreases sharply from the weld location and therefore would not affect growth of the postulated fatigue crack.
Request Please confirm the following about the cracking assessment for the Callaway steam generator channel head assemblies.
- a. For cladding residual stress, please confirm the following:
The cracking assessment for the Callaway steam generators concluded residual stress in the channel head cladding is insignificant based on the cladding thickness being a small percentage (about 5 percent) of the base metal thickness, the post-weld heat treatment, the pre-service hydrostatic testing, and inservice temperature changes.
The conclusion was informed by the survey documented in D. P. G. Lidbury, The Significance of Residual Stress in Relation to the Integrity of LWR Pressure Vessels, International Journal of Pressure Vessels and Piping, 17 (1984) 197-328.
- b. For base metal residual stress, please confirm the following:
Circumferential welds in the tubesheet ring are at least 13 inches from any postulated crack in the triple point region.
Finite element analysis for weld geometry approximating the Callaway SG configuration shows that a weld residual stress of approximately 8 ksi attenuates to near zero within 5-6 inches of the weld.
There are no longitudinal welds in the Callaway steam generator tubesheet rings.
Attachment to Supplemental Information and Response to Request for Confirmation ULNRC-06907 of Information (RCI) for Resolution of Callaway Plant Page 3 of 5 License Renewal Commitments 34 and 35 Ameren Missouri Response to RCI
- a. Ameren Missouri confirms the cracking assessment performed for Callaway's replacement steam generators (RSGs) [Reference 3] concluded that the residual stress in the channel head cladding is insignificant based on the cladding thickness being a small percentage (about 5 percent) of the base metal thickness, the post-weld heat treatment, the pre-service hydrostatic testing, and inservice temperature changes. This conclusion was informed by the survey in D.
P. G. Lidbury, The Significance of Residual Stress in Relation to the Integrity of LWR Pressure Vessels, International Journal of Pressure Vessels and Piping, 17 (1984) 197-328.
- b. Ameren Missouri confirms that for the base metal residual stress, the circumferential welds in the tubesheet ring are at least 13 inches from any postulated crack in the triple point region and that a finite element analysis for weld geometry approximating the Callaway RSG configuration shows that a weld residual stress of approximately 8 ksi attenuates to near zero within 5-6 inches of the weld. Lastly, Ameren Missouri confirms that there are no longitudinal welds in the Callaway RSG tubesheet rings.
Audit Question: Aging Management Approach for Tube-to-Tubesheet Welds Please clarify the approach to aging management for the tube-to-tubesheet welds in the Callaway SGs. Page 3 of the submittal cover letter [Reference 1] states that the tube-to-tubesheet welds are not susceptible to primary water stress corrosion cracking and do not require inspection based on the chromium content of at least 22 percent. Section 4.0 of [Reference 4] states that a plant specific aging management program (AMP) is not needed because the chromium content of the tube-to-tubesheet welds is greater than 22 percent.
However, the guidance in LR-ISG-2016-01 states that for SGs with Alloy 690 tubing and Alloy 600 type tubesheet cladding, a plant-specific AMP is unnecessary for a combination of 22 percent chromium in the tube-to-tubesheet welds and compressive stress in the cladding. In the ISG, higher chromium is considered to reduce susceptibility but not eliminate it, and general visual inspection of tube-to-tubesheet welds is a part of the Steam Generators AMP applicable to all pressurized-water reactors.
Ameren Missouri Response to Aging Management Approach to Tube-to-Tubesheet Welds Ameren Missouri confirms that a plant-specific AMP for the tube-to-tubesheet welds in the Callaway RSGs is not needed as stipulated by LR-ISG-2016-01 due to the chromium content of the tube-to-tubesheet welds being greater than 22 percent [Reference 4] and the existence of compressive stress in the cladding, as analyzed in the cracking assessment [Reference 3]. This response serves as a correction to the tube-to-tubesheet weld aging management approach
Attachment to Supplemental Information and Response to Request for Confirmation ULNRC-06907 of Information (RCI) for Resolution of Callaway Plant Page 4 of 5 License Renewal Commitments 34 and 35 information provided in the "Commitment 35 Analytical Evaluation Summary" section of the Reference 1 letter.
It should be noted that Framatome is updating the Reference 4 report to indicate the chromium content and compressive stress requirements. Once complete, Ameren Missouri will be submitting, as a supplement to the NRC, an update to the Framatome Reference 4 report.
Audit Question: Justification of the RTNDT value Section 3.2.5 of Enclosure 1 in the submittal [Reference 3] identifies an RTNDT of 10°F for Callaway based on Reference 9 (Callaway Unit 1 Replacement Steam Generators Section 13: Non-Ductile Failure Risk). Please discuss whether this value is justified by testing.
Ameren Missouri Response to Justification of the RTNDT value Ameren Missouri confirms that the RTNDT value was verified by testing to be 10oF or lower as shown in Callaway's RSG Certified Material Test Reports. Further, the Certified Design Report states the RTNDT value for the Callaway RSGs shall be no higher than 10°F. The Framatome ANP Procurement Specification for the Callaway RSGs also states the RTNDT value shall be no higher than 10°F. Framatome is updating the Reference 3 report to indicate the same. Once complete, Ameren Missouri will be submitting, as a supplement to the NRC, an update to the Framatome Reference 3 report.
Audit Question: Heat Transfer Coefficient Unit Errors The Thermal Analysis discussion in Section 6.1.2 of Enclosure 1 [Reference 3] and Section 7.2 of document 32-9360111-00 include HTC values. In both documents, the HTC units are BTU/hr [British Thermal Unit/hour]-s2-°F [degree Fahrenheit], which appears to be an error because the denominator has two units of time and no unit of surface area. The industry analysis in EPRI report 3002002850, for example, uses HTC units of BTU/hr-ft2-°F and BTU/sec-in2-°F. If the units are incorrect, please provide the correct units and values, and describe any effects on the stress and crack growth analyses.
Ameren Missouri Response to Heat Transfer Coefficient Unit Errors Ameren Missouri confirms that the unit of BTU/hr-s2-oF stated in both Framatome reports is a typographical error. The correct unit is BTU/sec-in2-oF. Ameren Missouri confirms that the HTC values are correct and correctly used in the related finite element analyses. As a result, there is no impact on the stress and crack growth analyses. Framatome is updating the Reference 3 and 32-
Attachment to Supplemental Information and Response to Request for Confirmation ULNRC-06907 of Information (RCI) for Resolution of Callaway Plant Page 5 of 5 License Renewal Commitments 34 and 35 9360111-000 reports to correct the typographical error. Once complete, Ameren Missouri will be submitting, as a supplement to the NRC, an update to the Framatome Reference 3 report.
Audit Question: Bounding Paths for Crack Growth Analysis Section 6.3.1 of Enclosure 1 [Reference 3] explains that the stresses used in the crack growth calculations were extracted from the finite element analyses along several predefined paths.
Please verify that the paths chosen include the limiting (highest) stress path for use in the crack growth analyses.
Ameren Missouri Response to Bounding Paths for Crack Growth Analysis Ameren Missouri confirms that the paths chosen in the Reference 3 report include the limiting (highest) stress path for use in the crack growth analyses. Specifically, in the Reference 3 report, local stresses near the triple point and through the base metal are reviewed for all transient and steady state conditions. The path lines are defined as passing through the locations that experience the most extreme stress conditions (i.e., the maximum in magnitude and/or in variations). Framatome is updating the Reference 3 report to indicate the bounding (limiting) stress path for use. Once complete, Ameren Missouri will be submitting, as a supplement to the NRC, an update to the Framatome Reference 3 report.