ML19340A001
| ML19340A001 | |
| Person / Time | |
|---|---|
| Site: | Millstone  (NPF-049) |
| Issue date: | 12/18/2019 |
| From: | James Danna NRC/NRR/DORL/LPL1 |
| To: | Stoddard D Dominion Energy Nuclear Connecticut |
| Guzman R | |
| References | |
| EPID L-2019-LLR-0033 | |
| Download: ML19340A001 (9) | |
Text
December 18, 2019 Mr. Daniel G. Stoddard Senior Vice President and Chief Nuclear Officer Innsbrook Technical Center 5000 Dominion Blvd.
Glen Allen, VA 23060-6711
SUBJECT:
MILLSTONE POWER STATION, UNIT NO. 3 - PROPOSED ALTERNATIVE REQUEST IR-4-01, USE OF ENCODED PHASED ARRAY ULTRASONIC EXAMINATION TECHNIQUES IN LIEU OF RADIOGRAPHY (EPID L-2019-LLR-0033)
Dear Mr. Stoddard:
By letter dated March 29, 2019 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML19106A050), Dominion Energy Nuclear Connecticut, Inc. (the licensee), submitted Proposed Alternative IR-4-01 for the Fourth 10-year Inservice Inspection (ISI) interval at Millstone Power Station, Unit 3 (MPS3). The licensees request describes alternative requirements to the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, Paragraph IWA-4221 and Subsubarticle IWA-4520. Proposed Alternative IR-4-01 uses phased array ultrasonic testing (PAUT) in lieu of the required radiographic testing (RT) in the examination of welds in ferritic and austenitic piping as part of repair and replacement activities at MPS3.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1), the licensee requested to use the proposed alternative on the basis that the alternative would provide an acceptable level of quality and safety.
The NRC staff has reviewed the subject request and concludes, as set forth in the enclosed safety evaluation, that the licensee has adequately addressed all regulatory requirements set forth in 10 CFR 50.55a(z)(1). Therefore, the NRC staff authorizes the use of Proposed Alternative IR-4-01 for the remainder of the Fourth 10-year ISI interval for MPS3.
All other ASME Code,Section XI, requirements for which relief was not specifically requested and approved remain applicable, including the third-party review by the Authorized Nuclear Inservice Inspector.
D. Stoddard If you have any questions, please contact the Millstone Project Manager, Richard Guzman, at 301-415-1030 or by e-mail to Richard.Guzman@nrc.gov.
Sincerely,
/RA/
James G. Danna, Chief Plant Licensing Branch I Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-423
Enclosure:
Safety Evaluation cc: Listserv
Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION PROPOSED ALTERNATIVE REQUEST IR-4-01 USE OF ENCODED PHASED ARRAY ULTRASONIC EXAMINATION TECHNIQUES IN LIEU OF RADIOGRAPHY DOMINION ENERGY NUCLEAR CONNECTICUT, INC.
MILLSTONE POWER STATION, UNIT NO. 3 DOCKET NO. 50-423 EPID: L-2019-LLR-0033
1.0 INTRODUCTION
By letter dated March 29, 2019 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML19106A050), Dominion Energy Nuclear Connecticut, Inc. (the licensee), submitted Proposed Alternative IR-4-01 for the Fourth 10-year Inservice Inspection (ISI) interval at Millstone Power Station, Unit 3 (MPS3). The licensees request describes alternative requirements to the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, Rules for Inservice Inspection of Nuclear Power Plant Components, Paragraph IWA-4221 and Subsubarticle IWA-4520. Proposed Alternative IR-4-01 uses phased array ultrasonic testing (PAUT) in lieu of the required radiographic testing (RT) in the examination of welds in ferritic and austenitic piping as part of repair and replacement activities at MPS3.
Specifically, pursuant to Title 10 of the Code of Federal Regulations (10 CFR) 50.55a(z)(1), the licensee requested to use the proposed alternative on the basis that the alternative would provide an acceptable level of quality and safety.
The U.S. Nuclear Regulatory Commission (NRC) staff evaluation of the proposed alternative request is contained herein.
2.0 REGULATORY EVALUATION
Adherence to Section XI of the ASME Code is mandated by 10 CFR 50.55a(g)(4), which states, in part, that ASME Code Class 1, 2, and 3 components will meet the requirements, except the design and access provisions and the pre-service inspection (PSI) requirements, set forth in the ASME Code,Section XI.
Paragraph (z) of 10 CFR 50.55a states, in part, that alternatives to the requirements of 10 CFR 50.55a(b)-(h) may be used, when authorized by the U.S. Nuclear Regulatory Commission (NRC or Commission) if: (1) the proposed alternative would provide an acceptable level of quality and safety; or (2) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Pursuant to 10 CFR 50.55a(z)(1), the licensee is proposing an alternative to Paragraph IWA-4221 and Subsubarticle IWA-4520 of the 2013 Edition of the ASME Code,Section XI.
Based on the above, and subject to the following technical evaluation, the NRC staff finds that regulatory authority exists for the licensee to request, and for the Commission to authorize, the alternative requested by the licensee.
3.0 TECHNICAL EVALUATION
3.1 Proposed Alternative IR-4-01 ASME Code Components Affected All ASME Code,Section XI ferritic and austenitic piping welds requiring radiography during repair/replacement activities at MPS3.
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Applicable Code Edition and Addenda===
The proposed alternative applies to inspections performed during the Fourth 10-Year ISI interval which commenced on February 23, 2019 and will end on February 22, 2029. The Code of record for the Fourth 10-year ISI interval is the ASME Code,Section XI, 2013 Edition.
Applicable Code Requirements Paragraph IWA-4221 of the 2013 Edition of the ASME Code,Section XI requires the owner to meet the applicable Construction Code requirements when performing repair and replacement activities.
Subsubarticle IWA-4520 of the 2013 Edition requires that welding or brazing areas and welded joints made for fabrication or installation of items be examined in accordance with the Construction Code identified in the Repair/Replacement Plan with certain specified exceptions.
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Reason for Request===
The licensee stated that replacement of piping is periodically performed in support of the Flow Accelerated Corrosion (FAC) program as well as other repair and replacement activities. The use of encoded PAUT in lieu of RT to perform the required examinations of the repaired or replaced welds would eliminate the safety risk associated with performing RT, which includes planned exposure and the potential for accidental personnel exposure to plant workers.
Encoded PAUT also minimizes the impact on other outage activities normally involved with performing RT.
In addition, the licensee stated that encoded PAUT is equivalent or superior to the ASME Code-required RT examination for ferritic and austenitic piping repair or replacement welds for detecting and sizing critical (planar) flaws, such as cracks and lack of fusion. PAUT provides sizing capabilities for both depth and length dimensions of the flaw. RT does not provide depth sizing capabilities.
Proposed Alternative The licensee proposed to perform encoded PAUT in lieu of RT. Important aspects of the proposed alternative include:
The written ultrasonic examination procedure will be qualified by performance demonstration.
Ultrasonic examination personnel shall be qualified to detect and size flaws with the qualified procedure using blind performance demonstration testing.
The examination volume shall include 100 percent of the weld volume and the weld-to-base metal interface.
All detected axial and circumferential flaws shall be evaluated as planar flaws and compared to the preservice acceptance standards for volumetric examination in accordance with ASME Code,Section XI, Articles IWB-3000, IWC-3000, or IWD-3000, as applicable.
The examinations will be spatially encoded, and the data will be recorded.
The licensee will store electronic data files for the encoded PAUT as archival-quality records permitting off-line analysis of images built from the data.
The proposed alternative is based on and very similar to the provisions of ASME Code Case N-831-1, Ultrasonic Examination in Lieu of Radiography for Welds in Ferritic or Austenitic Pipe Section XI, Division 1.
Basis for Use In its submittal, the licensee stated that the basis for this proposed alternative is that encoded PAUT is equivalent or superior to RT for detecting and sizing critical (planar) flaws. It also stated that the basis for the proposed alternative was developed from the ASME Code, code cases, relevant industry experience, articles, and the results of RT and encoded PAUT examinations. It further stated that the examination procedure and personnel performing examinations are qualified using representative piping conditions and flaws that demonstrate the ability to detect and size flaws that are both acceptable and unacceptable to the defined acceptance standards.
Duration of Proposed Alternative The licensee is requesting approval of this proposed alternative for the duration of the Fourth 10-year ISI interval.
3.2
NRC Staff Evaluation
The NRC staff has evaluated Proposed Alternative IR-4-01 pursuant to 10 CFR 50.55a(z)(1) to determine if the proposed alternative provides an acceptable level of quality and safety. UT, like RT, is a volumetric inspection technique that is commonly used to inspect welds in nuclear power plants and in other industries. Ultrasonic examinations are not the same as radiographic examinations as they use different physical mechanisms to detect and characterize discontinuities. These differences in physical mechanisms result in several key differences in sensitivity and discrimination capability. The staff divided its review of the capabilities and limitations of the application of PAUT in lieu of RT for: (1) ferritic steel welds and (2) austenitic steel welds.
Ferritic Steel Welds The NRC staff has been assessing the effectiveness of the use of ultrasound in lieu of radiography for ferritic steel welds since 2009, including literature reviews, detailed evaluations of previous relief requests and proposed alternatives, and confirmatory experimental work to validate the findings. An assessment of the use of UT in lieu of RT by the NRC is described in the 2015 document NUREG/CR-7204, Applying Ultrasonic Testing In Lieu of Radiography for Volumetric Examination of Carbon Steel Piping (ADAMS Accession No. ML15253A674). This report included evaluation of the use of UT in lieu of RT for ferritic steel welded pipes and plates with thicknesses ranging from 0.844 inches to 2.2 inches.
In NUREG/CR-7204, the NRC staff stated that:
Considering overall detections/non-detections for the piping specimens, as well as the Navy plates, it appears that [phased array ultrasonic inspection] PA-UT, based on the techniques applied in this study, provides an equally effective examination for identifying the presence of fabrication flaws in carbon steel welds. The PA-UT parameters applied were shown to be more effective for planar flaws, but slightly less effective for small volumetric flaws, than RT.
Based on this research, the NRC staff finds that there is sufficient technical basis to support the use of UT in lieu of RT for ferritic steel welds. While the spatial resolving power of UT is lower than that of RT, the UT methods can provide more contrast (signal-to-noise ratio in UT) than RT. UT has a higher sensitivity to planar flaws and similar sensitivity to volumetric flaws and can detect cracks and lack of fusion defects more effectively than simple RT. The higher spatial resolving power of RT allows RT to effectively discriminate between different types of planar and volumetric flaws. RT provides a clear image of many flaws, allowing the examiner to distinguish between slag, porosity, undercut, and cracks by looking at the image. UT generally presents all indications as similar-looking regions, and multiple inspection angles are required to distinguish planar flaws from volumetric flaws, and different types of volumetric flaws provide nearly identical indications to UT techniques. In ferritic materials, advanced PAUT methods can detect, size and differentiate between planar flaws such as cracks and lack of fusion defects and volumetric flaws such as slag and porosity.
Austenitic Steel Welds The Electric Power Research Institute (EPRI) Technical Report (TP) No. 3002010297, Technical Basis for Substituting Ultrasonic Testing for Radiographic Testing for New, Repaired, and Replacement Welds for ASME Section XI, Division 1, Stainless Steel Piping, (June 2017) summarizes EPRIs performance-based approach based on the ASME Code,Section XI, Appendix VIII to demonstrate the effectiveness of the encoded PAUT for detection and sizing fabrication flaws in the austenitic stainless-steel piping welds.
When compared to the information for ferritic steel materials, the primary difference is that the ability to discriminate between planar and volumetric flaws has not demonstrated for the more challenging austenitic materials. Austenitic welds have larger grain sizes than ferritic welds, and the austenitic weld grains are anisotropic, meaning that sound goes faster in some crystalline directions than others. These large anisotropic grains can redirect the ultrasonic beam and provide reflections, creating increased noise. While detection and sizing of flaws is possible in an austenitic weld, it is significantly more challenging to discriminate between a volumetric flaw and a planar flaw. For this reason, this proposed alternative does not attempt to discriminate between flaw types. All flaws detected using angle-beam ultrasonic testing will be treated as planar flaws and will be evaluated against the preservice acceptance standards of ASME Section XI, IWB-3400, IWC-3400, or IWD-3400 for ASME Code Class 1, 2, or 3 welds, respectively. Since it is not necessary to differentiate between planar and volumetric flaws, the primary weakness of UT in lieu of RT in austenitic welds is mitigated.
Given the above, the NRC staff considered whether the proposed alternative applies UT in a way that provides reasonable assurance of finding structurally-significant flaws.
Important aspects of this proposed alternative include:
Ultrasonic examination procedures shall be qualified by using either a blind or a non-blind performance demonstration using a minimum of 30 flaws covering a range of sizes, positions, orientations, and types of fabrication flaws. The demonstration set shall include specimens to represent the minimum and maximum diameter and thickness covered by the procedure.
The flaw through-wall heights for the performance demonstration testing shall be based on the applicable acceptance standards for volumetric examination in accordance with IWB-3400, IWC-3400 or IWD-3400. At least 30 percent of the flaws shall be classified as acceptable planar flaws, with the smallest flaws being at least 50 percent of the maximum allowable size based on the applicable [a/l] aspect ratio for the flaw.
The examination volume shall include 100 percent of the weld volume and the weld-to-base metal interface.
The electronic data files for the PAUT examinations will be stored as archival-quality records. In addition, hard copy prints of the data will be included as part of the PAUT examination records to allow viewing without the use of hardware or software.
Ultrasonic examination personnel shall demonstrate their capability to detect and size flaws by performance demonstration using the qualified procedure. The demonstration specimen set shall contain at least 10 flaws covering a range of sizes, positions, orientations, and types of fabrication flaws.
Based on the inspection and qualification requirements described in the licensees request for alternative and the evaluation results reported in NUREG/CR-7204 and EPRI Technical Report No. 3002010297, the NRC staff concludes that there is reasonable assurance that the encoded PAUT, applied and qualified as proposed by the licensee, will provide an adequate level of quality and safety because 1) in ferritic steel welds, encoded PAUT provides capability for detection and sizing fabrication flaws, and 2) in austenitic steel welds, all flaws similarly detected by encoded PAUT will be treated as planer flaws and will subsequently be evaluated against appropriate preservice acceptance standards. Therefore, the staff finds the licensees request for alternative acceptable.
4.0 CONCLUSION
As set forth above, the NRC staff concludes that the licensee's proposed alternative to use PAUT in lieu of RT provides reasonable assurance of structural integrity and leak tightness of ferritic and austenitic piping welds requiring radiography during repair/replacement activities.
Thus, ultrasonic examination using the procedure described in Proposed Alternative IR-4-01 will provide an acceptable level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all the regulatory requirements set forth in 10 CFR 50.55a(z)(1). Therefore, the NRC staff authorizes the use of Proposed Alternative IR-4-01 for the remainder of the Fourth 10-year ISI interval for MPS3.
The NRC staff notes that the approval of Proposed Alternative IR-4-01 does not imply or infer the NRC approval of ASME Code Case N-831-1 for generic use.
All other requirements of the ASME Code for which relief has not been specifically requested remain applicable, including third-party review by the Authorized Nuclear lnservice Inspector.
Principal Contributor: B. Fu Date: December 18, 2019
- by e-mail OFFICE DORL/LPL1/PM DORL/LPL1/LA DNRL/NPHP/BC*
NAME RGuzman LRonewicz MMitchell DATE 12/12/19 12/12/19 12/02/19 OFFICE DORL/LPL1/BC DORL/LPL1/PM NAME JDanna RGuzman DATE 12/18/19 12/18/19