ML21116A260
| ML21116A260 | |
| Person / Time | |
|---|---|
| Site: | Vogtle  |
| Issue date: | 04/26/2021 |
| From: | NRC/NRR/VPOB |
| To: | |
| Gleaves B | |
| References | |
| Download: ML21116A260 (10) | |
Text
Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16)
Plant Site-Unit: Vogtle Electric Generating Plant (VEGP) - Unit 3 Applies to activities related to the Containment Vessel (CV) ASME N-Interval-Interval Certificate holders execution of a remediation plan before the 10 CFR Dates: 52.103(g) finding, with residual weld material remaining in place for the life of the plant Requested Date for Approval:
ASME Code Components Affected:
T ASME Section III Subsection NE Class MC applies to the Containment Vessel (CV) pressure vessel (CNS-MV-01) shell in the Containment System (CNS).
AF Applicable Code Edition and Addenda:
ASME B&PV Code,Section III, 2001 Edition through the 2002 Addenda NCA-3131 Welding During Construction For Divisions 1 and 2, it is required that all shop and field welding during Code construction be done only by a Certificate Holder holding a certificate appropriate to the scope of welding to be performed. A Certificate Holder may engage individuals by contract for their services DR Applicable Code Requirements:
as welders or welding operators at the site location shown on the Certificate, provided conditions of (a) through (g) below are met. This is an acceptable method of complying with Section IX requirements concerning responsibility for welding.
(a) The work to be done by such welders and welding operators is within the scope of the Certificate.
(b) The use of such welders and welding operators is contained in the Quality Assurance Program of the Certificate Holder. The Quality Assurance Program (NCA-3462, NCA-3562, NCA-3662, or NCA-3762) shall include a requirement for direct supervision and direct technical control of the welders and welding operators by the Certificate Holder during such welding operations, and this program shall be acceptable to the Certificate Holders Authorized Inspection Agency (NCA-5121) performing the inspections.
(c) The welding procedures have been properly qualified by the Certificate Holder, and Code responsibility for such procedures is retained by the Certificate Holder.
(d) The welders and welding operators are qualified by the Certificate Page 1 of 10
Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16)
Holder to perform such procedures.
(e) The Certificate Holder shall have contractual control of the welding operation, including authority to assign or remove welders and welding operators at his discretion.
(f) The Certificate Holder shall be responsible for Code compliance of the completed item or installation including Code Symbol stamping and providing the completed appropriate Data Report Form.
(g) Exceptions to the requirements of this paragraph for furnace brazing operations are specified in NCA-3561(c), NCA-3661(b),
and NCA-3761(b).
Required Qualifications T
NE-4321 Welding Qualifications, Records, and Identifying Stamps -
(a) Each Certificate Holder is responsible for the welding done by his organization, and he shall establish the procedure and conduct AF the tests required by this Article and by Section IX in order to qualify both the welding procedures and the performance of welders and welding operators who apply these procedures.
(b) Procedures, welders, and welding operators used to join permanent or temporary attachments to pressure parts and to make permanent or temporary tack welds used in such welding shall also meet the qualification requirements of this Article.
(c) When making procedure test plates for butt welds, consideration shall be given to the effect of angular, lateral, and end restraint on the weldment. This applies particularly to material and weld metal of 80.0 ksi (552 MPa) tensile strength or higher and heavy sections of both low and high tensile strength material. The DR addition of restraint during welding may result in cracking difficulties that otherwise might not occur.
NE-4411 Identification, Storage, and Handling of Welding Materials Each Certificate Holder is responsible for control of the welding electrodes and other material which is used in the fabrication and installation of components (NE-4120). Suitable identification, storage, and handling of electrodes, flux, and other welding material shall be maintained. Precautions shall be taken to minimize absorption of moisture by electrodes and flux.
NE-4435 Welding of Nonstructural and Temporary Attachments and Their Removal (a) Nonstructural attachments (NE-1132.1) welded to the pressure retaining portion of the containment vessel need not comply with NE-2000 and may be welded with continuous or intermittent fillet or partial penetration welds, provided the requirements of (1)
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Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16) through (4) below are met:
(1) the welding procedure and the welders have been qualified in accordance with NE-4321; (2) the material is identified and is compatible with the material to which it is attached; (3) the welding material is identified and compatible with the materials joined; (4) the welds are postweld heat treated when required by NE-4620.
(b) Removal of temporary attachments shall be accomplished as follows:
T (1) The temporary attachment is completely removed in accordance with the procedures of NE-4211.
(2) As an alternative to (a)(4) above, postweld heat treatment may be deferred until after removal of the attachment.
AF NE-4610 Welding Preheat Requirements In summary, NE-4610 requires materials to be preheated prior to welding if not exempted by Table NE-4622.7(b)-1 and required by the qualified welding procedure specification (WPS). For the specific plate, a preheat of 200º F is required to exempt postweld heat treatment (PWHT).
The Vogtle Unit 3 CV is an ASME Section III, Class MC pressure vessel designed and constructed in accordance with ASME Section III 2001 Edition, 2002 Addenda. Design and construction of the CV is complete as indicated by application of the N-Certificate Holders Certification DR Reason for Request:
Mark.
Subsequent to completion of design and construction of the CV, and contrary to the requirements in NCA-3131, NE-4411, NE-4435(a)(1)
(which invokes NE-4321), NE-4435(a)(4) (which invokes NE-4620) and NE-4610, a contractor that is not an N-Certificate Holder nor properly sub-contracted by the original CV N-Certificate Holder per Section III, welded pieces of Unistrut to the inner diameter (ID) surface of the CV.
Six pieces of 12 gauge (0.105 nominal thickness) Unistrut, each approximately 9 long, were welded to the CV ID in three locations. Two four (4) inch long by 1/4 to 3/8 fillet welds were utilized to attach each piece of Unistrut. Since the contractor performing the welding was not an N-Certificate Holder, nor were they properly subcontracted by the CV N-Certificate Holder, the ASME Section III requirements for welding were not met.
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Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16)
Proposed Alternative:
As an alternative to the requirements in ASME B&PV Section III Code paragraphs NCA-3131, NE-4411, NE-4435(a)(1) (which invokes NE-4321), NE-4435(a)(4) (which invokes NE-4620) and NE-4610, Southern Nuclear Operating Company (SNC) requests authorization to enact a plan of remediation developed by CB&I, the CV N-Certificate Holder. This remediation will be performed by the CV N-Certificate Holder in accordance with their ASME Section III program as follows:
Mechanical methods will be used to cut the Unistrut from the welds The remnant weld material will be ground flush with the CV shell T
surface, with trace amounts of the material remaining in place for the life of the plant Ultrasonic Digital Thickness Measurements of the CV will be taken to confirm thickness prior to, and after grinding the weld flush AF Proposed Alternative and Basis for Use:
Surface exams of the CV will be conducted to identify any discontinuities Supplemental vacuum box testing of remediated areas will be conducted.
Documentation of remediation activities will be on an N-10A Data Report in accordance with Code Case N-802 and this alternative request.
Basis for Use:
DR This proposed alternative to authorize and enact the remediation plan described above provides an acceptable level of quality and safety in accordance with 10 CFR 50.55a. This section outlines the basis of use for the proposed alternative.
NCA-3131 To remediate the Unistrut welding performed by the subcontractor that is not an N-Certificate Holder, the proposed remediation plan that was developed and will be conducted by the CV N-Certificate Holder in accordance with their Section III Quality Assurance Program will remove the subject welded attachments and grind the material flush with the CV shell surface. The remediation plan will also utilize examinations and supplemental testing to verify that no surface indications are present as a result of the welding performed, and no leak path has been introduced to the CV shell.
NE-4411 The weld material used was procured under Stone & Websters ASME Page 4 of 10
Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16)
Section III Quality Assurance Program and maintained under this program until it was issued through a chain of custody to the contractor and checked out by the welders performing the Unistrut welding. The CV N-Certificate Holder has reviewed the certified material test reports (CMTR)s for the weld metal and confirmed that the material is compatible with the CV plate material. Further discussion of the material compatibility is included in a later section.
NE-4435(a)(1) / NE-4321 ASME Section III, paragraph NE-4435(a)(1), states that nonstructural attachments welded to the pressure retaining portion of the CV need not T
comply with NE-2000 and may be welded with continuous or intermittent fillet or partial penetration welds, provided in part that welding procedures and welders have been qualified in accordance with NE-4321. The proposed remediation plan that was developed and will be conducted by the CV N-Certificate Holder in accordance with their Section III Quality Assurance Program will remove the subject welded attachments that AF were welded using procedures and welders that were not qualified in accordance with NE-4321, and grind the material flush with the CV shell surface. The remediation plan will also utilize examinations and supplemental testing to verify that no surface indications or through-wall leakage paths are present as a result of the welding performed.
Additionally, the CV N-Certificate Holder has reviewed the CMTRs for the weld metal used by the unqualified welders and confirmed that the material used is compatible with the CV shell.
NE-4435(a)(4) / NE-4620 Based on the CV base material thickness of 1-3/4 and the fillet weld size DR of 1/4 to 3/8, the material would be exempt from the PWHT by NE-4620 and NE-4622.7 provided that a preheat of 200º F is achieved. The WPS used for the unqualified welding calls for a preheat of 150º F. While this temperature is not the full 200º F required by Code for the PWHT exemption, it does reduce the temperature differential present during the welding process and the potential for adverse effects as opposed to no preheat at all. Based on the size of the Unistrut attachment fillet welds, the potential resulting residual stresses will be highly localized. Any remaining residual stress that may be present after the removal of the welds will be a strain-controlled secondary stress, which will not overstress the shell under pressure loading because of its self-limiting characteristic. Therefore, residual stresses in the affected areas are ineffectual and do not warrant a localized PWHT.
NE-4610 Materials are preheated prior to welding if not exempted by Table NE-4622.7(b)-1 and required by the qualified WPS. For the specific CV plates in question, a preheat of 200º F is required to exempt PWHT. The Page 5 of 10
Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16) achieved preheat of 150º F does not allow for the exemption of PWHT.
As noted, the 150º F preheat temperature is not the full 200º F required by Code for the PWHT exemption, however it does reduce the temperature differential present during the welding process and the potential for adverse effects as opposed to no preheat at all.
Consideration was also given to the limitations of interpass temperature for quenched and tempered material in accordance with NE-4613, in order to avoid detrimental effects on the mechanical properties. The CV N-Certificate Holders WPS limits interpass temperature to 500º F, while the WPS used for the unqualified welds calls for a maximum interpass temperature of 450º F.
The proposed remediation plan includes a surface examination to T
confirm the absence of surface defects that could have been introduced by inadequate preheat or interpass temperature control. The remediation plan also includes supplemental vacuum box testing to verify that no leak path has been introduced to the CV shell.
AF Fracture Toughness in the Heat Affected Zone (HAZ)
The fracture toughness requirement in NE-2300 for the subject weld areas is met.
Per ASME Section III Table NE-2332.1-2, the required Charpy V-notch test (CV) energy values for pressure retaining material other than bolting with specified minimum yield strength over 55 ksi to 75 ksi and thickness over 11/2 to 21/2 inclusive, the average of three specimens is 40 ft-lb, and the lowest one of three is 35 ft-lb. For the specific CV plates, the Code-required average fracture toughness is 40 ft-lb at the lowest service metal temperature of -18.5º F.
An average fracture toughness of 221.25 ft-lb at -58.5º F was achieved DR for the SA-738 Grade B CV shell plates where the Unistrut channels were welded. This result is greater than five times the minimum Code-required value and it was achieved at a significantly lower temperature than required by the code. Production test plate impact tests were performed using the N-Certificate Holders qualified WPS for the heat affected zones of the welds made to confirm that the welding did not adversely impact the facture toughness of the base metal. For the production test plate impact testing, two sections of the 1-3/4 production plate material were welded together. The CB&I production test plates were welded with shielded metal arc welding (SMAW), utilizing a 1/8 E9018M electrode and a qualified maximum heat input of 68.4 kJ/in. The test plates exhibited average toughness values of 237 ft-lb at -29º F, exceeding the Code requirement of 40 ft-lb at -18.5º F. The average fracture toughness value for the E9018M weld filler material is 92.7 ft-lb at -30º F, also exceeding the Code requirement of 40 ft-lb at -18.5º F.
In contrast to these production test plate welds, the Unistrut attachment welds are 4 long, 1/4 to 3/8 fillet welds made on 1-3/4 CV shell plates with a 3/32 E7018 electrode. According to the CMTR provided to SNC, the weld filler material used to attach the Unistrut has an average fracture Page 6 of 10
Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16) toughness value of 133 ft-lb at -20º F, exceeding the Code requirement of 40 ft-lb at -18.5º F. The HAZ in the CV material created by the Unistrut welding is localized on the interior surface of the CV and is not significantly different from other attachment welds on the vessel. It is reasonable to conclude that the heat input used to make the Unistrut attachment welds and the size of the heat affected zone of the Unistrut attachment welds are bounded by the heat input and size of the heat affected zone of the production test plate weld.
Therefore, if the production test plate welding did not adversely impact the fracture toughness of the base metal, it is reasonable to conclude that the Unistrut attachment welds also did not adversely impact the fracture toughness of the CV shell plate base metal.
T Compatibility of Welding Material ASME Section III, paragraph NE-4435(a), states that nonstructural attachments welded to the pressure retaining portion of the CV need not comply with NE-2000 and may be welded with continuous or intermittent AF fillet or partial penetration welds, provided in part that the welding material used is identified and compatible with the materials joined. As previously noted, the CMTR provided to SNC for the weld filler metal used for the unqualified welding was reviewed by the CV N-Certificate Holder. The table below compares the chemical composition of the 3/32 EXCALIBUR 7018 MR' filler metal used to weld the Unistrut pieces to the CV and the CV shell material composition. As shown in the table, the deposited weld metal chemical composition either falls within the range or is below the maximum allowed for SA-738 Grade B. Therefore, the chemical composition of the remnant weld material that will remain on the CV is compatible with the SA-738 Grade B CV shell plate material.
DR Material SA 738 Grade B Excalibur 7018 MR C
(%)
0.20 0.08 Si
(%)
0.15-0.55 0.22 Mn
(%)
0.90-1.50 1.11 P
(%)
0.025 0.01 S
(%)
0.025 0.007 Cu
(%)
0.35 0.03 Ni
(%)
0.60 0.02 Cr
(%)
0.30 0.03 The reported yield strength of the weld filler metal is 67 ksi, which is above the 60 ksi minimum for SA-738 Grade B, and the reported 84 ksi ultimate tensile strength nearly matches the 85 ksi minimum required for SA-738 Grade B.
Unistrut material was purchased commercial grade, therefore, CMTRs Mo
(%)
0.30 0.23 V
(%)
0.07
<0.01 Nb
(%)
0.04 NR are not available for the material that could remain present in the remnant weld material on the CV shell. The CV N-Certificate Holder has reviewed the catalog information for the A1011 SS Gr.33 Unistrut that was used.
The information following is a comparison between the CV shell base metal and the Unistrut.
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Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16)
P-number and Group Number:
Metals with same P-number have similar materials. In this case P-1 means they are both carbon steel.
Metals with the same Group number and same P-Number have similar impact test requirements and weldability.
A1011 Gr. 33 is listed as a P1 Group 1 material in ASME Section IX (QW-422)
Chemistry Carbon (C) and more so Carbon Equivalency (CE) affects the T
hardness and welding process.
The Unistrut and the base metal have similar CE (see below).
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 AF Strength Since the Unistrut material will be remixed and undergoes a level of dilution during the welding process, the amount of Unistrut material remaining in the base metal will be minimal. As such, the base metal is not expected to get weaker with the dilution from the Unistrut material.
P Number Group Number C%
SA-738 B 1
3 0.2 A1011 SS Gr 33 1
1 0.25 DR CE %
Sy (at room temp) 0.48%
(per CBI Spec) 60 ksi 0.47%
33 ksi Welding between P-1 Group-3 and P-1 Group-1 has been performed on other temporary attachments on CV.
Therefore, the mechanical properties of the remnant weld material that will remain on the CV are compatible with the CV shell plate material.
Remediation Plan Basis The proposed remediation is focused on restoration by removal of the Unistrut channels in accordance with NE-4435(b)(1) and acceptability of the final containment vessel boundary. Subsequent examinations and testing will verify the absence of defects that could be present due to a lack of having a qualified welder perform the work and the remnant of residual weld material that will be in place for the life of the plant. The Page 8 of 10
Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16) following examinations and testing will be performed by the CV N-Certificate Holder in accordance with their ASME Section III program:
Ultrasonic digital thickness measurements of the CV to confirm thickness prior to, and after grinding weld flush Surface exams of the CV to identify any discontinuities Supplemental vacuum box testing of remediated areas Supplemental vacuum box testing of remediated areas will be conducted by the CV N-Certificate Holder as part of the remediation plan. This test will verify that no leak path has been created in the disturbed surfaces of the CV.
The Unit 3 CV has successfully passed an SIT per NE-6000. There is no T
explicit Code requirement to reperform pressure test after removal of temporary attachments in NE-4435. The integrity of the minimal area of the CV affected by the Unistrut welding is adequately and conservatively demonstrated by the above discussed engineering, metallurgical and welding evaluations. The surface examinations required by NE-AF 4435(b)(1) along with the proposed vacuum box testing provides reasonable assurance that no through-wall leakage paths resulted from the subject welding and as a result, that the SIT results remain valid. For many pressure boundary components, stresses arising from the SIT are the highest levels that are expected for the life of the components. Each SIT contributes to the cumulative fatigue damage that the CV pressure boundary is required to endure. An additional SIT is not appropriate to address the welding of the Unistrut to the CV as it will introduce an additional fatigue cycle with no beneficial effects.
Conclusion The proposed alternative and plan of remediation recommended by DR Duration of Proposed Alternative:
CB&I, the CV N-Certificate Holder, are supported by the combined evaluations of SNC and CB&I, and provide assurance that the CV structural integrity is maintained such that the safety design function of the CV is not compromised. Therefore, this proposed alternative represents an acceptable level of safety and quality and should be granted pursuant to 10 CFR 50.55a(z)(1).
Life of the plant Page 9 of 10
Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1): Alternative Requirements for ASME Section III Remediation of Unauthorized Containment Vessel Unistrut Welding (VEGP 3-ALT-16)
ASME Boiler & Pressure Vessel Code,Section III, 2001 Edition through 2002 Addenda - Rules for Construction of Nuclear Facility Components
References:
ASME Boiler & Pressure Vessel Code,Section IX, 2019 Edition -
Qualification Standard for Welding, Brazing, and Fusing Procedures; Welders; Brazers; and Welding, Brazing, and Fusing Operators Status: Awaiting NRC authorization T
AF DR Page 10 of 10