ML16091A228

Monticello Nuclear Generating Plant Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations, Supplemental Information to Respond to the Second Request for Additional Information (TAC No. L25058)

ML16091A228
Person / Time
Site:	Monticello
Issue date:	03/29/2016
From:	Gardner P A Northern States Power Co, Xcel Energy
To:	Document Control Desk, Office of Nuclear Material Safety and Safeguards, Office of Nuclear Reactor Regulation
References
L-MT-16-016, TAC L25058
Download: ML16091A228 (10)

Text

fl Xcel Energy Monticello Nuclear Generating Plant 2807 W County Rd 75 March 29, 2016 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Monticello, MN 55362 Director, Division of Spent Fuel Management Office of Nuclear Material Safety and Safeguards Washington, DC 20555-0001 Monticello Nuclear Generating Plant Docket No. 50-263 Renewed Facility Operating License No. DPR-22 Independent Spent Fuel Storage Installation Docket No. 72-58 L-MT-16-016 10 CFR 72.7 Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations.

Supplemental Information to Respond to the Second Request for Additional Information (TAC No. L25058)

References:

1) NSPM letter to NRC Document Control Desk, Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations, L-MT-15-056, dated September 29, 2015 (ADAMS Accession No. ML 15275A023)

2) NRC letter to NSPM, First Request for Additional Information for Review of Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations, dated December 15, 2015 (ADAMS Accession No. ML 15350A064)

3) NSPM letter to NRC Document Control Desk, Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations, Supplemental Information, L-MT-16-003, dated January 29, 2016 (ADAMS Accession No. ML 16035A 187) 4) NRC letter to NSPM, Second Request for Additional Information for Review of Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations, dated March 3, 2016 (ADAMS Accession No. ML 16068A156)

Pursuant to 10 CFR 72.7, "Specific Exemptions", Northern States Power Company, a Minnesota corporation (NSPM), doing business as Xcel Energy, submitted Reference 1 to request an exemption to the requirements of 10 CFR 72.212(b)(3) and 10 CFR 72.212(b)(11) for one NUHOMS Dry Shielded Canister (DSC) designated number 16 due to nonconforming dye penetrant (PT) examinations performed during the loading campaign started in September 2013.

Document Control Desk Page 2 In Reference 2, the NRC Division of Spent Fuel Management (DSFM) Staff transmitted a request for additional information (RAI) associated with their review of the Exemption Request. In Reference 3, NSP.M provide a reply to that RAI. Subsequently, NRC issued a second round of RAls in Reference 4 .. Enclosure 1 provides responses to the RAI questions that were provided in Reference

4. Enclosure 2 provides AREVA Calculation 11042-0205 Revision 3, which supports changes made to address RAl-2-1. The information offered herein does not affect the conclusions associated with exemption criteria or the environmenta I considerations provided in the Reference 1 Exemption Request. Summary of Commitments This letter makes no new commitments and no revisions to existing commitments.

Peter A. Gardner Site Vice President Monticello Nuclear Generating Plant Northern States Power Company-Minnesota Enclosures (2) cc: Administrator, Region Ill, USNRC Rob Kuntz, Project Manager, Monticello Nuclear Generating Plant, USNRC Christian Jacobs, Project Manager, Spent Fuel Management, USNRC Resident Inspector, Monticello Nuclear Generating Plant, USNRC L-MT-16-016 Enclosure 1 MONTICELLO NUCLEAR GENERATING PLANT EXEMPTION REQUEST FOR NONCONFORMING DRY SHIELDED CANISTER DYE PENETRANT EXAMINATIONS, SUPPLEMENTAL INFORMATION This enclosure provides responses from Northern States Power Company, a Minnesota corporation (NSPM), doing business as Xcel Energy, to the Second Request for Additional Information (RAI) provided by the Nuclear Regulatory Commission (NRG) on March 3, 2016. The NRG questions are provided below in italic font and the NSPM response is provided in the normal font. L-MT-16-003.

Enclosure

2. Calculation 11042-0205, Rev. 2 61 BTH /TCP and OTPC Closure Weld Flaw Evaluation RAl-2-1: With respect to Table 6, "Summary of Load Cases and Results," consider the elastic-plastic analysis results to revise the collapse pressure and g-load entries, as appropriate.

This may be done by recognizing that a collapse load should be defined as the loading not to result in the weld strain ductility demand beyond its code allowable extension limit. The staff notes that the listed collapse pressures and side-drop g-loads were calculated with the elastic-perfectly plastic analyses, which would have been replaced, in the present submittal, with the elastic-plastic analysis results. Absent of the collapse loads commensurate with the elastic-plastic analysis, there is no basis for establishing the weld design margins to make safety findings.

Specifically, for the loading conditions analyzed, the results should be listed for demonstrating adequate loading factors of safety in that the calculated weld ductility demands are smaller than the code allowable extension limits. This information is needed to complete the review in accordance with 10 CFR 72.236. NSPM Response:

As noted in the RAI question, elastic-plastic analyses were performed and summarized in Appendix A of the calculation 11042-0205 Rev. 2 to provide plastic strain incurred in the inner top cover plate and outer top cover plate welds to the DSC shell. The maximum strains for accident pressure and drop loads were reported in the Appendix.

Calculation 11042-0205 is revised to demonstrate margin of safety of the aforementioned plastic strains against the weld ductility limits. The weld ductility limit was reduced by a 0.8 factor to account for PAUT limitations and inaccessible area Page 1 of 2 L-MT-16-016 Enclosure 1 around the vent and siphon block. It was also shown that the plastic strain remains stable for loads that are 150% of the original design loads for the NUHOMS 61 BTH DSC. Revision 3 of calculation 11042-0205 is included as Enclosure

2. L-MT-15-056, Enclosure 1 Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations RAl-2-2: Considering the revised collapse analysis, revise the "Executive Summary," and any other sections of the submittal, as appropriate, to recognize that (1) reference made to "stress margins" may need to be revised to reflect the design margins available in the weld and (2) the summary bullets on the lower bound collapse pressures and

• acceleration are to reflect the calculated results using the elastic-plastic analyses.

The Executive Summary should be prepared to be consistent with those of the revised analyses and results documented in Calculation 11042-0205.

This information is needed to complete the review in accordance with 10 CFR 72.236. NSPM Response:

The Attachment to this Enclosure provides a markup of affected pages to reflect the requested changes. RAl-2-3: Considering the revised collapse analysis, revise Enclosure 1, "Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations" for appropriate description of the structural evaluation approach by noting that the collapse load margins rather than the stress margins are used for demonstrating the weld structural performance.

The updated structural analysis approach, including the acceptance criteria, should be consistent with the revised analyses and results documented in Calculation 11042-0205. This information is needed to complete the review in accordance with 10 CFR 72.236. NSPM Response:

The Attachment to this Enclosure provides a markup of affected pages to reflect the requested changes. Page 2 of 2 L-MT-16-016 Enclosure 1 Attachment, Exemption Request Markups for RAl-2-2 and RAl-2-3 Affected Pages of Exemption Request Enclosure 1: 2 of 74 38 of 74 39 of 74 40 of 74 4 pages follow L-MT-15-056 Enclosure 1 Margin of safety is 5. The helium le k and DSC backfill testing results verify confinement barrier integrity.

6. The lack of failure mechanism that adversely affects confinement barrier integrity.

7. Stress margins are available in the welds when assuming conservatively large flaws. These margins are demonstrated by two different methods: (1) structural analysis using an analysis-based Stress Allowance Reduction Factor and theoretically-bounding full-circumferential flaws, and (2) a finite element limit load analysis assuming flaw distributions conservatively derived from PAUT examination.

The PAUT examination and analysis described in Enclosure 4 provides an objective review of volumetrically-identified flaw indications in the accessible DSC 16 ITCP and OTCP welds to conclude that DSG-=!-§ remains in eem-pliance vvith /\SME Section Ill SubsectioR-N-8 stress limits in the presence ef the identified

W!:::

approach, the results showed that the f -Ohtr: t ... <Fn e *

• Further discussion of the PAUT examin tion and analysis is provided in Enclosure 4 . . The peak strains in the welds remain well below the weld material ductility limit when subjected to the accident pressure and drop loads. The peak strains have a margin of safety of 3.69 and 3.60 for accident pressure and drop loads, respectively.

Furthermore, it was shown that the strains in the welds remain stable at 150% of the original design loads for the NU HOMS 61 BTH DSC. Page 2 of 74 L-MT-15-056 Enclosure 1 See additional markup from the first round of RAI (RAl-1) and (2) a finite element limit load analysis assuming flaw distributions conservatively derived from Phased Array Ultrasonic Test (PAUT) examination.

1. Analysis-Based Structural Analysis with Theoretical Flaws: The calculation provided in Enclosure 3 provides an analysis of the structural adequacy of a DSC assuming a theoretical weld flaw existing in both ITCP and OTCP welds. The potential flaw is assumed to reside .in a full circumference around the ITCP and OTCP welds (i.e., throughout the entire weld). For the ITCP weld, the weld depth is assumed to be fully filled, with a critical flaw evaluated for the 360° circumference.

The calculation determined an "acceptable" allowable flaw size, that is, a flaw size that will ensure that ASME weld stress limits are still met. For a 360° circumferential flaw, an allowable flaw depth of 0.1 O" could exist and the weld would still meet ASME weld stress limits5. All the component stresses remain below the stress allowable limits. The flaw is evaluated as either occurring on the surface or subsurface of the weld. For the OTCP weld, the weld depth was reduced to 0.480" to account for the weld depth issues discovered on DSC 16, with an allowable flaw evaluated for the 360° circumference6.

The thermal loads that are secondary loads would tend to reduce with the reduction in stiffness of the components.

Conservatively all secondary stresses were scaled for the 0.480" weld. Like the ITCP weld calculation, the OTCP calculation determined an "acceptable" allowable flaw size, that is, a flaw size that will ensure the weld stress limits are still met. For a 360° circumferential flaw an allowable flaw depth of 0.27" could exist and the weld would still meet ASME weld stress limits. All the component stresses remain below the stress allowable limits. The flaw is evaluated as either occurring on the surface or subsurface of the weld. For conservatism a weld stress reduction factor was applied to both the ITCP and OTCP weld calculations.

Per ISG-15 the stress reduction factor of partial penetration welds with PT examination is 0.80. Since these welds are non-compliant with the PT requirements, 5 Note that the acceptable flaw size calculated herein was based on the design 3/16-inch minimum size of the ITCP weld rather than the 0.25-inch minimum size measured by PAUT. Thus, the margin to allowable stress as calculated for the ITCP is conservative.

6 Note that the acceptable flaw sized calculated herein was based on the 0.48-inch weld size that was subsequently restored to 0.50 inch. Thus, the margin to allowable stress as calculated for the OTCP is conservative.

Page 38of74 L-MT-15-056 Enclosure 1 See additional markup from the first round of RAI. (RAl-1) . the weld reduction factor is reduced beyond 0.80 based on a set of theoretical flaw distributions that might conceivably have gone undetected during DSC closure weld examinations.

Thus, an based stress allowable reduction factor of 0.70 was calculated (see Enclosure

3) to support the value of 0.70 used in the analysis of record. For further conservatism, the weld membrane stress at limiting load for OTCP and ITCP is calculated using square root sum of the squares (SRSS) method, for a 0.480" OTCP weld and 3/16" ITCP weld, while excluding the compressive loads onto the weld. When the modified stress reduction factor and other conservatisms are included as described above are applied for normal conditions, the stress ratio of calculated stress to allowable stress is 0.52 and 0.79 for the ITCP and OTCP welds, respectively.

For accident conditions, the stress ratio is 0.84 and 0.71 for the ITCP and OTCP welds, respectively.

Therefore, adequate design margins exist for the ITCP and OTCP welds when evaluated against conservative stress allowable values. The assumptions of flaw size used in the calculation bound any of the indications found on DSC 16 by Phased Array Ultrasonic Test (PAUT). Therefore, this calculation demonstrates that sufficient margin is included in the welds and a reasonable expectation of satisfactory performance of each DSC for the design service lifetime of the DSC. Potential weld flaws in DSC 16 can be characterized based on the results of the re-inspections performed on the final layer of the OTCP on DSC 16 and the. observations of the actual welding process documented in the SIA report (see section 1.3.2). For the flaws identified in those re-inspections, the maximum indication was approximately 0.035" with a length of 1.6" (see section 1.3.3). The composite effect of all weld flaws reported for DSC 16 results in a conservative flaw characterization defined as a single flaw of 0.050" depth occurring around 45° of the weld circumference, which applies to both ITCP and OTCP welds. In comparison, the allowable calculated flaws bound this assumed flaw by at least twice the depth for the conservatively assumed 360° flaw. Therefore, it is concluded that the flaws that may exist in the ITCP and OTCP welds of DSC 16 are stable for all design basis loading conditions, with significant margins (minimum factor of 2 on flaw depth). This conclusion applies over the entire licensed life of the canister.

This page has been included for continuity.

There are no new markups indicated on this page. Page 39of74 L-MT-15-056 Enclosure 1 will continue to perform its function due to the adequate margins shown for the accident pressure and drop loads Furthermore, the allowable flaw size evaluation indicates that flaws that may exist in the ITCP and OTCP welds are bounded by the allowable flaw size, with significant margins with respect to flaw de and circumferential length (factor of 8). 2. Limit Load Structural Analysis Using PAUT Flaw Distribution:

Structural Analysis Summary: Based on the analysis discussed above, the structural design and redundant barrier functions are adequately maintained, without crediting the noncompliant PT examinations, for all design basis loading conditions postulated to occur over the 20-year lifetime of the affected DSC. 3.3 Conclusion Based on the discussion above, the proposed activity does not adversely affect the criticality safety, shielding/radiological safety, heat removal, confinement integrity or structural support functions of the DSC as described in the UFSAR. In summary, the requested exemption is safe to grant. The integrity of the field closure welds for DSC 16 can be assured with confidence even though the TS required PT examinations were conforming.

The known quality of the welding processes, the use of multi-layer weld technique, the adequate stress margin in the welds to accommodate the flaws identified by PAUT, plus the visual inspection and helium leak testing demonstrate that the field closure weld integrity of DSC 16 is sufficient to ensure that the affected closure welds will contin.ue to perform their design basis functions over the service lifetime of these canisters.

This exemption request further concludes, based on objective evidence, that there is a reasonable assurance of safety for the NRC to approve the requested exemption for the 20-year service lifetime of DSC 16. The reasonable assurance of safety is based on the following:

1. Integrity of the fuel creates a fission product barrier. 2. The quality of the welding process employed provides indication of development of welds. Page 40of74 QF0212, Revision 5 (FP-SC-RSl-04) fl Xcel Energy* SHIPPING DOCUMENT NORTHERN STATES POWER -MN . D/B/A Xcel Energy Page 1 of 1 Monticello Nuclear Plant, 2807 W Hwy. 75, Monticello, MN 55362 Date: 3/29/2016 Shipping Document ('* A Tracking Number: I & -0 /",S Ship To: USN RC 11555 Rockville Pike Rockville, MD 20852-2738 Attention Of: Doc Control Carrier: UPS -Standard Overnight Pro I Tracking No:

RMANo: PO I Contract No: Number of Packages:

1 I Weight: Dangerous Goods/ UN/NA No: Insurance Est. Value Hazardous Materials?

Required?

Reason for Shipment:

Overnight Shipment to USNRC Melody Imholte -Please ensure tracking number is communicated to me -melody.imholte@xenuclear.com Env Submittal to NRC L-MT-16-016

• Da : By s;gnITT t . sh;pp;n d

• u are declaring, to the best of your knowledge, thalthe

(/t{mpped

s ;n compliance wi h Xcel E_nergy Corporate Policies.

Please print and sign your name legibly.

Making Shipment:

• • Date:

I f.p Received By: Date: For will*cafl use only ' I ' I . . l I 'd d t . I t t' l't j d . Ur of this form as a procerura a1 oes no require\ re en ion as a qua I y rerr .

fl Xcel Energy Monticello Nuclear Generating Plant 2807 W County Rd 75 March 29, 2016 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Monticello, MN 55362 Director, Division of Spent Fuel Management Office of Nuclear Material Safety and Safeguards Washington, DC 20555-0001 Monticello Nuclear Generating Plant Docket No. 50-263 Renewed Facility Operating License No. DPR-22 Independent Spent Fuel Storage Installation Docket No. 72-58 L-MT-16-016 10 CFR 72.7 Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations.

Supplemental Information to Respond to the Second Request for Additional Information (TAC No. L25058)

References:

1) NSPM letter to NRC Document Control Desk, Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations, L-MT-15-056, dated September 29, 2015 (ADAMS Accession No. ML 15275A023)

2) NRC letter to NSPM, First Request for Additional Information for Review of Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations, dated December 15, 2015 (ADAMS Accession No. ML 15350A064)

3) NSPM letter to NRC Document Control Desk, Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations, Supplemental Information, L-MT-16-003, dated January 29, 2016 (ADAMS Accession No. ML 16035A 187) 4) NRC letter to NSPM, Second Request for Additional Information for Review of Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations, dated March 3, 2016 (ADAMS Accession No. ML 16068A156)

Pursuant to 10 CFR 72.7, "Specific Exemptions", Northern States Power Company, a Minnesota corporation (NSPM), doing business as Xcel Energy, submitted Reference 1 to request an exemption to the requirements of 10 CFR 72.212(b)(3) and 10 CFR 72.212(b)(11) for one NUHOMS Dry Shielded Canister (DSC) designated number 16 due to nonconforming dye penetrant (PT) examinations performed during the loading campaign started in September 2013.

Document Control Desk Page 2 In Reference 2, the NRC Division of Spent Fuel Management (DSFM) Staff transmitted a request for additional information (RAI) associated with their review of the Exemption Request. In Reference 3, NSP.M provide a reply to that RAI. Subsequently, NRC issued a second round of RAls in Reference 4 .. Enclosure 1 provides responses to the RAI questions that were provided in Reference

4. Enclosure 2 provides AREVA Calculation 11042-0205 Revision 3, which supports changes made to address RAl-2-1. The information offered herein does not affect the conclusions associated with exemption criteria or the environmenta I considerations provided in the Reference 1 Exemption Request. Summary of Commitments This letter makes no new commitments and no revisions to existing commitments.

Peter A. Gardner Site Vice President Monticello Nuclear Generating Plant Northern States Power Company-Minnesota Enclosures (2) cc: Administrator, Region Ill, USNRC Rob Kuntz, Project Manager, Monticello Nuclear Generating Plant, USNRC Christian Jacobs, Project Manager, Spent Fuel Management, USNRC Resident Inspector, Monticello Nuclear Generating Plant, USNRC L-MT-16-016 Enclosure 1 MONTICELLO NUCLEAR GENERATING PLANT EXEMPTION REQUEST FOR NONCONFORMING DRY SHIELDED CANISTER DYE PENETRANT EXAMINATIONS, SUPPLEMENTAL INFORMATION This enclosure provides responses from Northern States Power Company, a Minnesota corporation (NSPM), doing business as Xcel Energy, to the Second Request for Additional Information (RAI) provided by the Nuclear Regulatory Commission (NRG) on March 3, 2016. The NRG questions are provided below in italic font and the NSPM response is provided in the normal font. L-MT-16-003.

Enclosure

2. Calculation 11042-0205, Rev. 2 61 BTH /TCP and OTPC Closure Weld Flaw Evaluation RAl-2-1: With respect to Table 6, "Summary of Load Cases and Results," consider the elastic-plastic analysis results to revise the collapse pressure and g-load entries, as appropriate.

This may be done by recognizing that a collapse load should be defined as the loading not to result in the weld strain ductility demand beyond its code allowable extension limit. The staff notes that the listed collapse pressures and side-drop g-loads were calculated with the elastic-perfectly plastic analyses, which would have been replaced, in the present submittal, with the elastic-plastic analysis results. Absent of the collapse loads commensurate with the elastic-plastic analysis, there is no basis for establishing the weld design margins to make safety findings.

Specifically, for the loading conditions analyzed, the results should be listed for demonstrating adequate loading factors of safety in that the calculated weld ductility demands are smaller than the code allowable extension limits. This information is needed to complete the review in accordance with 10 CFR 72.236. NSPM Response:

As noted in the RAI question, elastic-plastic analyses were performed and summarized in Appendix A of the calculation 11042-0205 Rev. 2 to provide plastic strain incurred in the inner top cover plate and outer top cover plate welds to the DSC shell. The maximum strains for accident pressure and drop loads were reported in the Appendix.

Calculation 11042-0205 is revised to demonstrate margin of safety of the aforementioned plastic strains against the weld ductility limits. The weld ductility limit was reduced by a 0.8 factor to account for PAUT limitations and inaccessible area Page 1 of 2 L-MT-16-016 Enclosure 1 around the vent and siphon block. It was also shown that the plastic strain remains stable for loads that are 150% of the original design loads for the NUHOMS 61 BTH DSC. Revision 3 of calculation 11042-0205 is included as Enclosure

2. L-MT-15-056, Enclosure 1 Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations RAl-2-2: Considering the revised collapse analysis, revise the "Executive Summary," and any other sections of the submittal, as appropriate, to recognize that (1) reference made to "stress margins" may need to be revised to reflect the design margins available in the weld and (2) the summary bullets on the lower bound collapse pressures and

• acceleration are to reflect the calculated results using the elastic-plastic analyses.

The Executive Summary should be prepared to be consistent with those of the revised analyses and results documented in Calculation 11042-0205.

This information is needed to complete the review in accordance with 10 CFR 72.236. NSPM Response:

The Attachment to this Enclosure provides a markup of affected pages to reflect the requested changes. RAl-2-3: Considering the revised collapse analysis, revise Enclosure 1, "Exemption Request for Nonconforming Dry Shielded Canister Dye Penetrant Examinations" for appropriate description of the structural evaluation approach by noting that the collapse load margins rather than the stress margins are used for demonstrating the weld structural performance.

The updated structural analysis approach, including the acceptance criteria, should be consistent with the revised analyses and results documented in Calculation 11042-0205. This information is needed to complete the review in accordance with 10 CFR 72.236. NSPM Response:

The Attachment to this Enclosure provides a markup of affected pages to reflect the requested changes. Page 2 of 2 L-MT-16-016 Enclosure 1 Attachment, Exemption Request Markups for RAl-2-2 and RAl-2-3 Affected Pages of Exemption Request Enclosure 1: 2 of 74 38 of 74 39 of 74 40 of 74 4 pages follow L-MT-15-056 Enclosure 1 Margin of safety is 5. The helium le k and DSC backfill testing results verify confinement barrier integrity.

6. The lack of failure mechanism that adversely affects confinement barrier integrity.

7. Stress margins are available in the welds when assuming conservatively large flaws. These margins are demonstrated by two different methods: (1) structural analysis using an analysis-based Stress Allowance Reduction Factor and theoretically-bounding full-circumferential flaws, and (2) a finite element limit load analysis assuming flaw distributions conservatively derived from PAUT examination.

The PAUT examination and analysis described in Enclosure 4 provides an objective review of volumetrically-identified flaw indications in the accessible DSC 16 ITCP and OTCP welds to conclude that DSG-=!-§ remains in eem-pliance vvith /\SME Section Ill SubsectioR-N-8 stress limits in the presence ef the identified

W!:::

approach, the results showed that the f -Ohtr: t ... <Fn e *

• Further discussion of the PAUT examin tion and analysis is provided in Enclosure 4 . . The peak strains in the welds remain well below the weld material ductility limit when subjected to the accident pressure and drop loads. The peak strains have a margin of safety of 3.69 and 3.60 for accident pressure and drop loads, respectively.

Furthermore, it was shown that the strains in the welds remain stable at 150% of the original design loads for the NU HOMS 61 BTH DSC. Page 2 of 74 L-MT-15-056 Enclosure 1 See additional markup from the first round of RAI (RAl-1) and (2) a finite element limit load analysis assuming flaw distributions conservatively derived from Phased Array Ultrasonic Test (PAUT) examination.

1. Analysis-Based Structural Analysis with Theoretical Flaws: The calculation provided in Enclosure 3 provides an analysis of the structural adequacy of a DSC assuming a theoretical weld flaw existing in both ITCP and OTCP welds. The potential flaw is assumed to reside .in a full circumference around the ITCP and OTCP welds (i.e., throughout the entire weld). For the ITCP weld, the weld depth is assumed to be fully filled, with a critical flaw evaluated for the 360° circumference.

The calculation determined an "acceptable" allowable flaw size, that is, a flaw size that will ensure that ASME weld stress limits are still met. For a 360° circumferential flaw, an allowable flaw depth of 0.1 O" could exist and the weld would still meet ASME weld stress limits5. All the component stresses remain below the stress allowable limits. The flaw is evaluated as either occurring on the surface or subsurface of the weld. For the OTCP weld, the weld depth was reduced to 0.480" to account for the weld depth issues discovered on DSC 16, with an allowable flaw evaluated for the 360° circumference6.

The thermal loads that are secondary loads would tend to reduce with the reduction in stiffness of the components.

Conservatively all secondary stresses were scaled for the 0.480" weld. Like the ITCP weld calculation, the OTCP calculation determined an "acceptable" allowable flaw size, that is, a flaw size that will ensure the weld stress limits are still met. For a 360° circumferential flaw an allowable flaw depth of 0.27" could exist and the weld would still meet ASME weld stress limits. All the component stresses remain below the stress allowable limits. The flaw is evaluated as either occurring on the surface or subsurface of the weld. For conservatism a weld stress reduction factor was applied to both the ITCP and OTCP weld calculations.

Per ISG-15 the stress reduction factor of partial penetration welds with PT examination is 0.80. Since these welds are non-compliant with the PT requirements, 5 Note that the acceptable flaw size calculated herein was based on the design 3/16-inch minimum size of the ITCP weld rather than the 0.25-inch minimum size measured by PAUT. Thus, the margin to allowable stress as calculated for the ITCP is conservative.

6 Note that the acceptable flaw sized calculated herein was based on the 0.48-inch weld size that was subsequently restored to 0.50 inch. Thus, the margin to allowable stress as calculated for the OTCP is conservative.

Page 38of74 L-MT-15-056 Enclosure 1 See additional markup from the first round of RAI. (RAl-1) . the weld reduction factor is reduced beyond 0.80 based on a set of theoretical flaw distributions that might conceivably have gone undetected during DSC closure weld examinations.

Thus, an based stress allowable reduction factor of 0.70 was calculated (see Enclosure

3) to support the value of 0.70 used in the analysis of record. For further conservatism, the weld membrane stress at limiting load for OTCP and ITCP is calculated using square root sum of the squares (SRSS) method, for a 0.480" OTCP weld and 3/16" ITCP weld, while excluding the compressive loads onto the weld. When the modified stress reduction factor and other conservatisms are included as described above are applied for normal conditions, the stress ratio of calculated stress to allowable stress is 0.52 and 0.79 for the ITCP and OTCP welds, respectively.

For accident conditions, the stress ratio is 0.84 and 0.71 for the ITCP and OTCP welds, respectively.

Therefore, adequate design margins exist for the ITCP and OTCP welds when evaluated against conservative stress allowable values. The assumptions of flaw size used in the calculation bound any of the indications found on DSC 16 by Phased Array Ultrasonic Test (PAUT). Therefore, this calculation demonstrates that sufficient margin is included in the welds and a reasonable expectation of satisfactory performance of each DSC for the design service lifetime of the DSC. Potential weld flaws in DSC 16 can be characterized based on the results of the re-inspections performed on the final layer of the OTCP on DSC 16 and the. observations of the actual welding process documented in the SIA report (see section 1.3.2). For the flaws identified in those re-inspections, the maximum indication was approximately 0.035" with a length of 1.6" (see section 1.3.3). The composite effect of all weld flaws reported for DSC 16 results in a conservative flaw characterization defined as a single flaw of 0.050" depth occurring around 45° of the weld circumference, which applies to both ITCP and OTCP welds. In comparison, the allowable calculated flaws bound this assumed flaw by at least twice the depth for the conservatively assumed 360° flaw. Therefore, it is concluded that the flaws that may exist in the ITCP and OTCP welds of DSC 16 are stable for all design basis loading conditions, with significant margins (minimum factor of 2 on flaw depth). This conclusion applies over the entire licensed life of the canister.

This page has been included for continuity.

There are no new markups indicated on this page. Page 39of74 L-MT-15-056 Enclosure 1 will continue to perform its function due to the adequate margins shown for the accident pressure and drop loads Furthermore, the allowable flaw size evaluation indicates that flaws that may exist in the ITCP and OTCP welds are bounded by the allowable flaw size, with significant margins with respect to flaw de and circumferential length (factor of 8). 2. Limit Load Structural Analysis Using PAUT Flaw Distribution:

Structural Analysis Summary: Based on the analysis discussed above, the structural design and redundant barrier functions are adequately maintained, without crediting the noncompliant PT examinations, for all design basis loading conditions postulated to occur over the 20-year lifetime of the affected DSC. 3.3 Conclusion Based on the discussion above, the proposed activity does not adversely affect the criticality safety, shielding/radiological safety, heat removal, confinement integrity or structural support functions of the DSC as described in the UFSAR. In summary, the requested exemption is safe to grant. The integrity of the field closure welds for DSC 16 can be assured with confidence even though the TS required PT examinations were conforming.

The known quality of the welding processes, the use of multi-layer weld technique, the adequate stress margin in the welds to accommodate the flaws identified by PAUT, plus the visual inspection and helium leak testing demonstrate that the field closure weld integrity of DSC 16 is sufficient to ensure that the affected closure welds will contin.ue to perform their design basis functions over the service lifetime of these canisters.

This exemption request further concludes, based on objective evidence, that there is a reasonable assurance of safety for the NRC to approve the requested exemption for the 20-year service lifetime of DSC 16. The reasonable assurance of safety is based on the following:

1. Integrity of the fuel creates a fission product barrier. 2. The quality of the welding process employed provides indication of development of welds. Page 40of74 QF0212, Revision 5 (FP-SC-RSl-04) fl Xcel Energy* SHIPPING DOCUMENT NORTHERN STATES POWER -MN . D/B/A Xcel Energy Page 1 of 1 Monticello Nuclear Plant, 2807 W Hwy. 75, Monticello, MN 55362 Date: 3/29/2016 Shipping Document ('* A Tracking Number: I & -0 /",S Ship To: USN RC 11555 Rockville Pike Rockville, MD 20852-2738 Attention Of: Doc Control Carrier: UPS -Standard Overnight Pro I Tracking No:

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