ML19340A000

Relief Request IR-3-39, Proposed Alternative to ASME Code Weld Preheat Requirements

ML19340A000
Person / Time
Site:	Millstone
Issue date:	12/13/2019
From:	James Danna NRC/NRR/DORL/LPL1
To:	Stoddard D Dominion Energy Nuclear Connecticut
Guzman R
References
EPID L-2019-LLR-0014
Download: ML19340A000 (12)

Text

December 13, 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 - RELIEF REQUEST IR-3-39, PROPOSED ALTERNATIVE TO ASME CODE WELD PREHEAT REQUIREMENTS (EPID L-2019-LLR-0014)

Dear Mr. Stoddard:

By letter dated February 28, 2019 (Agencywide Documents Access and Management System Accession No. ML19064A590), Dominion Energy Nuclear Connecticut, Inc. (the licensee) submitted a request in accordance with paragraph 50.55a(z)(2) of Title 10 of the Code of Federal Regulations (10 CFR) for a proposed alternative to the requirements of Section XI, Rules for lnservice Inspection of Nuclear Power Plant Components, of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) at Millstone Power Station Unit No. 3 (Millstone Unit No. 3). The proposed alternative, IR-3-39, would permit two fillet welds not in compliance with the construction code to remain in service for the remaining life of the fillet welds.

Specifically, pursuant to 10 CFR 50.55a(z)(2), the licensee requested to use the alternative on the basis that complying with the specified requirement would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

The U.S. Nuclear Regulatory Commission (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)(2). Therefore, the NRC staff authorizes the use of the licensees proposed alternative IR-3-39 to permit continued use of fillet welds FW-12 and FW-30 at Millstone Unit No. 3 the remainder of their useful life All other ASME Code,Section XI requirements for which relief was not specifically requested and approved remain applicable, including 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

ML19340A000 *by e-mail OFFICE DORL/LPL1/PM DORL/LPL1/LA DNRL/NPHP/BC*

NAME RGuzman LRonewicz MMitchell DATE 12/12/2019 12/12/2019 10/28/2019 OFFICE DORL/LPL1/BC DORL/LPL1/PM NAME JDanna RGuzman DATE 12/13/2019 12/13/2019 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELIEF REQUEST IR-3-39 PROPOSED ALTERNATIVE TO ASME CODE WELD PREHEAT REQUIREMENTS DOMINION ENERGY NUCLEAR CONNECTICUT, INC.

MILLSTONE POWER STATION UNIT NO. 3 DOCKET NO. 50-423

1.0 INTRODUCTION

By letter dated February 28, 2019 (Agencywide Documents Access and Management System Accession No. ML19064A590), Dominion Energy Nuclear Connecticut, Inc. (the licensee) submitted a request in accordance with paragraph 50.55a(z)(2) of Title 10 of the Code of Federal Regulations (10 CFR) for a proposed alternative to the requirements of Section XI, Rules for lnservice Inspection of Nuclear Power Plant Components, of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code) at Millstone Power Station Unit No. 3 (Millstone 3). The proposed alternative, IR-3-39, would permit two fillet welds not in compliance with the construction code to remain in service for the remaining life of the fillet welds.

Specifically, pursuant to 10 CFR 50.55a(z)(2), the licensee requested to use the alternative on the basis that complying with the specified requirement would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

The U.S. Nuclear Regulatory Commission (NRC or the Commission) 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 preservice examination requirements, set forth in the ASME Code,Section XI.

Paragraph 50.55a(z) of 10 CFR states that alternatives to the requirements of paragraphs (b) through (h) of 10 CFR 50.55a, or portions thereof, may be used when authorized by the Director, Office of Nuclear Reactor Regulation. A proposed alternative must be submitted and authorized prior to implementation. The licensee must demonstrate that (1) the proposed alternative would provide an acceptable level of quality and safety, or (2) compliance with the Enclosure

specified requirements of this section would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

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 the Commission to authorize, the alternatives requested by the licensee.

3.0 TECHNICAL EVALUATION

3.1 Licensees Proposed Alternative Request IR-3-39 3.1.1 ASME Code Component(s) Affected The affected components are 24-inch ASME Code Class 3, B Reactor Plant Component Cooling Water Heat Exchanger (CCP HX) outlet and inlet slip-on flanges, weld numbers FW-12 and FW-30.

3.1.2 Applicable Code Edition and Addenda The welds in question were installed during the third 10-year inservice inspection (ISI) interval at Millstone 3. The third 10-year ISI interval ended on June 22, 2019. The Code of Record for the third 10-year ISI interval at Millstone 3 was ASME Code,Section XI, 2004 Edition (No Addenda).

ASME Code, Section Ill, 1971 Edition with Addenda through Summer 1973 (original Code of Construction)

ASME Code, Section Ill, 2007 Edition through 2008 Addenda (Code of Construction used for Repair/Replacement Activity) 3.1.3 Applicable Code Requirement ASME Code,Section XI, 2004 Edition (No Addenda)

IWA-4221(c) states in part:

As an alternative to (b), the item may meet all or portions of the requirements of different Editions and Addenda of the Construction Code, or Section Ill when the Construction Code was not Section Ill ... Construction Code Cases may also be used.

ASME Code, Section Ill, 2007 Edition through 2008 Addenda ASME Code, Section Ill, ND-4600, contains the following requirements regarding post-weld heat treatment (PWHT) of ASME Ill Class 3 welds:

• ND-4622.1 states that except as otherwise permitted in ND-4622.7, all welds, including repair welds, shall be post-weld heat treated.

• ND-4622.7 states that PWHT is not required for welds exempted in Table ND-4622.7(b)-1.

• Table ND-4622.7(b)-1 states that all welds in P No.1 material over 11/2 inches thick with a nominal thickness of 3/4 inches or less are exempt from PWHT, provided a minimum preheat of 200 degrees Fahrenheit (°F) is applied.

3.1.4 Reason for Request In June 2017, during work order preparation for the Millstone 3 A CCP HX replacement project, the licensee identified that two inside diameter (ID) fillet welds, FW-12 and FW-30, fabricated during the B CCP HX replacement project in spring 2016 were made without a 200 °F preheat as required by the Code of Construction (ASME Code, Section Ill, Table ND-4622.7(b)-1). As illustrated in Figure 1 below, these welds are part of the double fillet-welded slip-on flange connections joining the B CCP HX 24-inch diameter inlet and outlet nozzles to the reactor plant component cooling water piping headers.

These slip-on flanges are welded to 3/8-inch wall thickness SA 106 Grade B piping using an outside diameter (OD) 9/16-inch fillet weld and a 1/4-inch ID fillet weld. In this case, the larger OD fillet welds did not require a 200 °F preheat since the thickness of the flange hub was less than 11/2 inches. However, the smaller ID fillet welds, FW-12 and FW-30, welded to the 1.81-inch thick flange required a 200 °F preheat for PWHT exemption. Multi-pass gas tungsten arc welding (GTAW) was employed to complete these welds. The completed welds satisfied ASME Code-required visual and magnetic particle examinations and inservice leak tests.

These welds have been in place since the spring of 2016 when the B CCP HX was replaced.

3.1.5 Licensees Proposed Alternative and Basis for Use Pursuant to 10 CFR 50.55a(z)(2), the licensee requested an alternative to the requirements of ASME Code,Section XI, IWA-4221(c), to permit fillet welds FW-12 and FW-30, which are not in compliance with the Construction Code, to remain in service.

The licensee stated that its alternative request is supported by the following:

1) The subject welds have passed the ASME Code-required final visual and magnetic particle examinations and post installation in-service leak testing.

No cracking or other recordable indications were observed or are anticipated due to the omitted 200°F weld preheat.

2) Representative weld mock ups and welding procedure qualification testing demonstrate that the subject welds and heat affected zones HAZs [heat affected zones] have acceptable properties with 75°F minimum preheat.

3) The stresses around the ID fillet welds are very low (<25% of the stress allowable) and are primarily compressive. Therefore, the chances of crack initiation or propagation are remote.

4) Code stress analysis demonstrates that if the ID fillet welds are not present (or are postulated to have failed completely) the OD fillet welds alone are fully capable of maintaining structural and pressure boundary integrity.

5) In the highly unlikely event of a failure of both the ID and OD fillet welds, the failure would be evident as a leak that would be detected during routine walkdowns before the failure would compromise the system or component structural integrity.

The licensee also provided, in its February 28, 2019 letter, the results of the mock-up testing and stress analysis referenced above.

3.1.6 Hardship Justification The licensee stated that repair or replacement of the subject welds online or during an outage would eliminate the CCP systems defense in depth should either one of the other two CCP pumps or HXs become inoperable during repair of the B CCP HX. Due to the physical location of the CCP HXs, the tight work area poses an industrial safety risk to plant personnel and risk of damage to plant equipment. Based on mock-up testing and evaluation of the production welds, FW-12 and FW-30 are not prone to cracking and have sufficient strength, ductility, and toughness to perform satisfactorily. The licensee contends that the repair or replacement of FW-12 and FW-30 welds presents a hardship without a compensating increase in the level of quality of safety.

3.1.7 Duration of Proposed Alternative The licensee stated that the duration of the proposed alternative is the remaining useful life of fillet welds FW-12 and FW-30.

3.2 NRC Staffs Evaluation As described above, the welds in question are located on the double fillet-welded, ASME Code Class 3, 24-inch diameter, 150-pound (lb)., SA-105 slip on flanges that join the inlet and outlet nozzles of the B CCP HX to 24-inch diameter, 0.375-inch wall thickness, SA-106 Grade B reactor plant component cooling water piping. The flanges were welded with the GTAW process using SFA 5.18, ER70S-2 weld filler material.

The welds that are the subject of the proposed alternative, identified as FW-12 and FW-30, join the inside of the flange to the end of the pipe (see Figure 1 above). The SA-105 flange material is a carbon steel forging with a minimum tensile strength of 70 thousand pounds per square inch (ksi) and is assigned a P1 Group (Gr) 2 designation in ASME Code,Section IX. The thickness of the flange at the location of FW-12 and FW-30 is 1.81-inch. The piping material is carbon

steel, SA-106 Grade B Seamless, 0.375-inch wall thickness, with a minimum tensile strength of 60 ksi and is assigned a P1 Gr 1 designation in ASME Code,Section IX.

Per Figure 1, the minimum required fillet-weld size is 0.25-inch for FW-12 and FW-30. A fillet-weld size of 0.25-inch corresponds to a nominal weld thickness (throat) of 0.177-inch.

Assuming that the fillet-weld size could be up to the thickness of the pipe material, the fillet-weld thickness would be no greater than 0.266-inch (with a fillet-weld leg equal to the pipe wall thickness).

ASME Code,Section III, Table ND-4622.7(b)-1 provides alternatives to the mandatory PWHT requirements in Table ND-4622.1-1. Table ND-4622.7(b)-1 allows, in part, an exemption from Table ND-4622.1-1 PWHT requirements for all welds in P-No. 1 materials greater than 1.5-inch in thickness where the nominal thickness (thickness of the weld) is 0.75-inch or less, if a 200 °F preheat is applied. Welds such as the flange hub-to-pipe welds shown in Figure 1 in materials 1.5-inch or less in thickness, with a carbon content of 0.30 or less and a nominal thickness of 1.25-inch or less, are exempt from PWHT without the application of preheat.

FW-12 and FW-30 did not receive a 200 °F preheat in accordance with Table ND-4622.7(b)-1.

Preheat slows the weld cooling rate which reduces the potential for the formation of martensite in the heat-affected zone (HAZ), facilitates the diffusion of hydrogen from the weld, and reduces weld shrinkage stresses. Martensite in the HAZ, coupled with sufficient stress and the presence of monatomic hydrogen, can make a weld susceptible to hydrogen-induced cracking (HIC), also known as delayed cracking or underbead cracking. Without a susceptible microstructure, sufficient tensile stress, and a sufficient quantity of monatomic hydrogen, HIC cannot occur.

The degree of weld residual stress in a weld joint is dependent, in part, on the level of restraint that is influenced by the base material and weld metal properties, thickness of the materials being joined, nominal thickness of the weld, and the weld joint geometry. Weld residual stress in carbon steels can be reduced, to some extent, by applying a preheat.

Given the relatively shallow thickness (throat) of welds FW-12 and FW-30, between 0.177-inch minimum and 0.266-inch maximum, the materials involved, and the weld joint geometry, the NRC staff finds that the influence of residual stress on the susceptibility of FW-12 and FW-30 to HIC is low without the presence of a highly susceptible microstructure and the presence of a considerable amount of diffusible hydrogen in the welds. Therefore, the NRC staff focused its review on the HIC susceptibility of FW-12 and FW-30 due to the HAZ microstructure and diffusible hydrogen content. The NRC staff also considered the consequences of potential HIC in welds FW-12 and FW-30 and evaluated the licensees hardship justification.

The NRC staff notes that the licensee stated the completed welds satisfied ASME Code-required visual and magnetic particle examinations and inservice leak tests. These welds have been in place since the spring of 2016 when the B CCP HX was replaced.

During the welding of carbon steels, the region of the HAZ closest to the fusion line in the base material transforms to austenite during heating. Upon cooling, this region can transform into a mixture of several transformation products. The hardest of these transformation products is martensite. This region of the HAZ is known as the coarse grain HAZ (CGHAZ) and exhibits the highest hardness levels in the HAZ. Although carbon provides the most significant contribution to the susceptibility of a carbon steel to HIC due to HAZ hardness, other elements also contribute to hardenability, but to a lesser extent. These elements are manganese (Mn),

chromium (Cr), molybdenum (Mn), vanadium (V), nickel (N), and silicon (Si). Si is not

represented in the formula below. The effects of these elements combined is sometimes referred to as the carbon equivalent (CE). Several empirical CE formulas have been developed to relate the composition of a material to its susceptibility to HIC. The licensee stated that it used the CE formula listed in ASME Code,Section IX, QW-403.26, which is shown below, to calculate the CE of the production flanges. It is one of the most commonly used CE formulas, but there are others that are often used.

+ + +

= + + +

6 6 15 The higher the CE number, the more susceptible a material is to HIC. In general, when a CE number exceeds 0.40, a material is considered susceptible to HIC. The licensee stated that the carbon content of the installed slip on flanges is 0.19 percent with a CE number of 0.42. A material with a CE number of 0.42 would be considered marginally susceptible to HIC and would not be expected to develop HIC unless there was a considerable amount of hydrogen present and a significant amount of restraint/weld residual stress.

The licensee performed mock-up testing using single-pass and multi-pass welds with and without the required 200 °F preheat on 1.9-inch material to represent the thickness of the flange at FW-12 and FW-30. The mock-up testing also included single-pass and multi-pass welds on 1.5-inch material to represent the maximum thickness material that can be welded without preheat in accordance Table ND-4622.7(b)-1. In addition, the mock-up testing included single pass and multi-pass welds on 1-inch material to represent the ASME Code-compliant flange hub OD fillet welds. The mock-up flange contained the same carbon content as the installed flanges at 0.19 percent carbon and a nearly identical CE of 0.43 versus 0.42 for the installed flanges.

Mock-up CGHAZ hardness presented by the licensee in its application is listed in Table 1 below and is given in Vickers Pyramid Number (HV). The licensee also provided HV results for other regions of the mock-up welds. Given that the CGHAZ region has the highest hardness, the CGHAZ would be the most susceptible to HIC. Unless a weld joint is highly restrained or an excessive amount of hydrogen is present, a weld HAZ with an average hardness of 350 HV or less would not be considered particularly susceptible to HIC.

Table 1. Mock-up CG HAZ Micro-Hardness Results Test Mock-up Description GGHAZ Hardness Specimen Low Avg High Number 1A Single-pass 1.9" 75° 450 464 488 1B Multi-pass 1.9" 75° 221 361 444 (Represents FW-12 & FW-30) 2A Single pass 1.9" 200° 399 431 450 2B Multi-pass 1.9" 200° 209 264 355 3A Single pass 1.5" 75° 333 444 523 3B Multi-pass 1.5" 75° 223 305 406 4A Single-pass l" 75° 432 443 451 4B Multi-pass 1" 75° (Represents 205 274 378 Installed Compliant Hub OD Fillet Welds)

As would be expected, the highest hardness in Table 1 is the peak hardness in the single pass weld, test specimen 1A, on 1.9-inch material with no elevated preheat, which results in a high weld cooling rate. Although an average hardness of 464 HV would not indicate a 100 percent martensite structure, it does tend to indicate that some martensite is present. With sufficient hydrogen and residual stress, a weld with these attributes could potentially be susceptible to HIC. Test specimen 3A, which is single-pass and complies with ASME Code requirements, is made on 1.5-inch material with no preheat and shows an average hardness of 444 HV, almost as hard as 1A. Although ASME Code-compliant, this weld could potentially be susceptible to HIC with sufficient tensile stress and diffusible hydrogen in the weld. Test specimen 1B, which represents FW-12 and FW-30, shows an average hardness of 361 HV, which is a considerable decrease from test specimen 1A. As stated by the licensee, this decrease in hardness for the multi-pass weld with no preheat is due to tempering of martensite and grain refinement as a result of multi-pass welding. The NRC staff does not consider weld 1B to be particularly susceptible to HIC due to the tempering of any martensite that was present before subsequent weld passes were applied. This represents the scenario of production welds FW-12 and FW-30. A CGHAZ microstructure containing tempered martensite exhibits high toughness and a low susceptibility to HIC. The NRC staff finds that given the production material chemistry, the application of multi-pass welding, and the results of the licensees mock-up testing, it is unlikely that welds FW-12 and FW-30 contain a CGHAZ microstructure that is highly susceptible to HIC.

As stated by the licensee, the preheat and PWHT requirements in ASME Code,Section III, are intended to be conservative because they address multiple material specifications, product forms, chemistry variations, and multiple weld joint configurations. The NRC staff notes that materials are grouped by P numbers as assigned by ASME Code,Section IX. These P numbers have subgroup designations referred to as Gr numbers. For carbon steel and low alloy steel materials, the higher the P number, the higher the strength. Within each P number, the higher the group number, the higher the strength. Higher strength materials have a higher carbon content and/or other elements that contribute to strength and have a higher hardenability. In general, the higher the P number and Gr number, the higher the susceptibility to HIC. The SA-105 flange material is assigned as a P 1 Gr 2 material, and per SA-105, has a minimum tensile strength of 70 ksi and a maximum of carbon content of 0.35 percent. The flange materials that were installed contained a carbon content of 0.19 percent. The licensee calculated a maximum possible CE for SA-105 material at 0.67 based on the chemistry limits in material specification SA-105 versus the CE of the installed flanges, which was 0.42.

Table ND-4622.7(b)-1 applies the same requirements for P1 Gr 3 materials as it does for P1 Gr 1 and P1 Gr 2 materials. P1 Gr 3 materials generally have higher strength and have a higher hardenability than P1 Gr 2 materials, and therefore, are generally more susceptible to HIC. The carbon content, base material thickness, and nominal thickness (weld throat) are the only factors that differentiate PWHT requirements for P1 materials with a weld configuration similar to FW-12 and FW-30.

The most important component in HIC and most often the easiest to control is diffusible hydrogen introduced into the weld. Diffusible hydrogen in welds is measured in milliliters/100 grams (g) of deposited weld metal (ml/100 g). Table ND-4622.7(b)-1 is not welding process-specific. It is intended to be used for all welding processes. Some welding processes and electrode types produce greater amounts of diffusible hydrogen than others. For example, cellulose shielded metal arc welding electrodes typically produce diffusible hydrogen rates greater than 20 ml/100 g. When Table ND-4622.7(b)-1 was developed, diffusible hydrogen levels in even low hydrogen electrodes such as E7018 were often greater than 8 ml/100 g and even up to 16 ml/100 g. Table ND-4622.7(b)-1 was developed to provide adequate welding requirements for these various levels of diffusible hydrogen. Modern E7018

electrodes generally have no greater than 8 ml/100 g and most often have less than 4 ml/100 g of diffusible hydrogen.

Welds FW-12 and FW-30 were installed using the GTAW process. Barring any external sources of contamination, GTAW produces welds with extremely low diffusible hydrogen levels because no fluxes are involved. The licensee stated that welds made with the GTAW process are capable of a resulting diffusible hydrogen content less than 2 ml/100 g. The NRC staff notes that it is not uncommon for GTAW to result in diffusible hydrogen levels less than 1 ml/100 g. The NRC staff also notes that over 100 ambient temperature temper bead weld repairs using the GTAW process have been applied on carbon steel and low alloy steel components in nuclear plants over the last 20 years. The NRC staff is not aware of HIC occurring in any of the ambient temperature temper bead welds performed using the GTAW process. The NRC staff finds that based on the welding process used, FW-12 and FW-30 would most likely not contain enough diffusible hydrogen to cause HIC.

The licensees stress evaluation documented in Section 7.0 of its application, shows that the overall global stresses in the flange during operation are 1/4 or less than ASME Code allowable stresses, and the stresses in the inside corner of the ID fillet weld are even lower and are predominantly compressive, which is favorable in mitigating cracking. Therefore, the NRC staff finds, based on the licensees stress evaluation, that in the unlikely event that HIC were to occur undetected, it would be unlikely to challenge the pressure boundary, and the OD fillet weld is adequate to carry all anticipated loads.

3.2.1 Hardship Justification The NRC staff finds that repair or replacement of fillet welds FW-12 and FW-30 online or during an outage could degrade the CCP systems defense in depth, should other systems become unavailable during the time of the repair and result in undue hardship, given that the licensee has shown that the welds are capable of continued service without repair or replacement.

Therefore, the NRC staff determined that compliance with the specified ASME Code repair requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

3.3 Summary The NRC staff finds that the proposed alternative will provide reasonable assurance of the structural integrity, and HIC is unlikely to be present in FW-12 and FW-30 because: (1) the residual stress in the ID fillet weld would not be expected to be high; (2) the flange material, CE 0.42, is marginally susceptible to HIC; (3) the GTAW process produces welds with extremely low diffusible hydrogen; (4) the OD fillet weld is capable of carrying all anticipated loads without the ID fillet weld; and (5) service loads on the ID fillet weld are primarily compressive. In addition, complying with ASME Code,Section XI requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.

4.0 CONCLUSION

As set forth above, the NRC staff determined that the proposed alternative provides reasonable assurance of structural integrity of the subject components and that complying with IWA-4221(c) of the ASME Code,Section XI, would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. Accordingly, the NRC staff concludes that the licensee has adequately addressed all regulatory requirements set forth in

10 CFR 50.55a(z)(2). Therefore, the NRC staff authorizes the use of the licensees proposed alternative, IR-3-39, as described in its February 28, 2019 application, to permit continued use of fillet welds FW-12 and FW-30 at Millstone 3 for the remainder of their useful life.

All other requirements of the ASME Code,Section XI, for which relief has not been specifically requested and authorized by NRC staff remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.

Principal Contributor: Robert Davis Date: December 13, 2019