ML113360526
| ML113360526 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 12/09/2011 |
| From: | Nancy Salgado Plant Licensing Branch 1 |
| To: | George Gellrich Calvert Cliffs |
| Pickett D, NRR/DORL/LPL1-1, 415-1364 | |
| References | |
| TAC ME5423 | |
| Download: ML113360526 (14) | |
Text
{{#Wiki_filter:UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555*0001 December 9, 2011 Mr. George H. Gellrich, Vice President Calvert Cliffs Nuclear Plant, LLC Calvert Cliffs Nuclear Power Plant 1650 Calvert Cliffs Parkway Lusby, MD 20657-4702
SUBJECT:
RELIEF REQUEST FOR MODIFICATIONS TO PRESSURIZER HEATER SLEEVE AND LOWER LEVEL NOZZLE PENETRATIONS (RR-PZR-01) CALVERT CLIFFS NUCLEAR POWER PLANT, UNIT NO.1 (TAC NO. ME5423)
Dear Mr. Gellrich:
By letter dated January 31, 2011, Calvert Cliffs Nuclear Power Plant, LLC, the licensee, requested Nuclear Regulatory Commission (NRC) approval to perform modifications to pressurizer heater sleeve and lower level nozzle penetrations at Calvert Cliffs Nuclear Power Plant Unit No. 1 (Agencywide Document Access Management Systems (ADAMS) Accession No. ML110340059). The licensee proposed to perform modifications (repairs) as an alternative to the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code). The proposed alternative is described in Relief Request RR-PZR 01 and is applicable to the fourth 10-year in-service inspection (lSI) interval which commenced on October 10, 2009, and will end on June 30,2019. By letter dated May 4, 2011, the licensee responded to the NRC's request for additional information (RAI) questions and submitted weld anomaly analyses and J-groove weld flaw evaluations (ADAMS Accession No. ML11126A186). By letter dated September 19, 2011, the licensee provided the result of the corrosion analysis of the pressurizer shell (ADAMS Accession No. ML11264A021). On the basis of information submitted, the NRC staff concludes that the proposed alternative in RR-PZR-01 will provide an acceptable level of quality and safety for the repair of the pressurizer heater sleeve and lower level nozzle penetrations. Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), the staff authorizes the proposed alternative described in RR-PZR-01 for the fourth 10-year lSI interval at the Calvert Cliffs Nuclear Power Plant, Unit No.1. The NRC staff notes that the repairs performed in accordance with the provisions of this relief request may remain in place for the design life of the modifications. All other ASME Code, Section XI requirements for which relief was not specifically requested and approved in this relief request remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector.
G. Gellrich - 2 Please contact Douglas Pickett at 301-415-1364 or Douglas.Pickett@nrc.gov if you have any questions. Sincerely, i-:/i..?7J.,/..'C? ;::r...-rr-/ ;;-' --1f?" ~ ~ J~ C l w Nancy L Salgado, Chief Plant Licensing Branch 1-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket No. 50-317
Enclosure:
Safety Evaluation cc w/encl: Distribution via Listserv
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555*0001 SAFETY EVALUATION BY THE OFFICE NUCLEAR REACTOR REGULATION RELIEF REQUEST RR-PZR-01 MODIFICATIONS TO PRESSURIZER HEATER SLEEVE AND LOWER LEVEL NOZZLE PENETRATIONS CALVERT CLIFFS NUCLEAR POWER PLANT. UNIT NO.1 CALVERT CLIFFS NUCLEAR POWER PLANT, LLC DOCKET NO. 50-317
1.0 INTRODUCTION
By letter dated January 31, 2011, Calvert Cliffs Nuclear Power Plant, LLC, the licensee, requested Nuclear Regulatory Commission (NRC) approval to perform modifications to pressurizer heater sleeve and lower level nozzle penetrations at Calvert Cliffs Nuclear Power Plant, Unit No. 1 (Agencywide Document Access Management Systems (ADAMS) Accession No. ML110340059). The licensee proposed to perform modifications (repairs) as an alternative to the requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code). The proposed alternative is described in Relief Request RR-PZR 01 and is applicable to the fourth 10-yearinservice inspection (lSI) interval which commenced on October 10, 2009, and will end on June 30,2019. By letter dated May 4, 2011, the licensee responded to the NRC's request for additional information (RAI) questions and submitted weld anomaly analyses and J-groove weld flaw evaluations (ADAMS Accession No. ML11126A186). By letter dated September 19, 2011, the licensee provided the result of the corrosion analysis of the pressurizer shell (ADAMS Accession No. ML11264A021). The pressurizer heater sleeve and lower level (instrument) nozzles are constructed of Alloy 600 material. The J-groove weld joining the nozzles to the pressurizer bottom head uses nickel based Alloy 82/182 filler material, which is susceptible to primary water stress-corrosion cracking (PWSCC) in the pressurized-water reactor (PWR) environment.
2.0 REGULATORY EVALUATION
Pursuant to Title 10 of the Code of Federal Regulations (10 CFR) Section 50.55a(g)(4), ASME Code Class 1,2, and 3 components (including supports) must meet the requirements, except Enclosure
- 2 the design and access provisions and the preservice examination requirements, set forth in the ASME Code, Section XI, "Rules for Inservice Inspection (lSI) of Nuclear Power Plant Components," to the extent practical within the limitations of design, geometry, and materials of construction of the components. The regulations require that inservice examination of components and system pressure tests conducted during the first 10-year interval and subsequent intervals comply with the requirements in the latest edition and addenda of Section XI of the ASME Code, incorporated by reference in 10 CFR 50.55a(b), 12 months prior to the start of the 120-month interval, subject to the limitations and modifications listed therein. Pursuant to 10 CFR 50.55a(a)(3) alternatives to ASME Code requirements may be authorized by the NRC if the licensee demonstrates that: (i) the proposed alternatives provide an acceptable level of quality and safety, or (ii) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety. The Code of Record for the fourth 10-year lSI interval is the 2004 Edition, no addenda, of the ASME Code, Section XI. To perform the modification, the licensee proposed to use ASME Code Case N-638-1, Similar and Dissimilar Metal Welding Using Ambient Temperature Machine GTAW Temper Bead Technique, Section XI, Division 1. The NRC has approved Code Case N-638-4 as documented in NRC Regulatory Guide (RG) 1.147, Revision 16. By letter dated May 4,2011, the licensee clarified that RG 1.147, Revision 16 was published in October 2010 and that was after the Design Specification for the nozzle repair in Relief Request RR-PZR-01 had been issued. Therefore, the proposed relief request did not reference Code Case N-638-4. The NRC staff did not find significant differences between N-638-1 and N-638-4 in terms of the applicable requirements to the proposed modifications. The staff finds that it is acceptable for the licensee to use Code Case N-638-1 for the proposed modifications.
3.0 TECHNICAL EVALUATION
3.1 PROPOSED RELIEF REQUEST RR-PZR-01
3.2 ASME Code Components Affected
Components: Pressurizer heater sleeve and lower level nozzle penetrations Code Class: Class 1 Examination Category: B-P Code Item Number: B15.10 Heater Sleeves: 1.156 Inch Nominal Outside Diameter Instrument Nozzles: 1.315 Inch Nominal Outside Diameter Material: Stainless Steel Type 316/316L 3.3
Applicable Code Edition and Addenda
ASME Code, Section XI, 2004 Edition, no Addenda and Section III, 1965 Edition, including Addenda through winter 1967
- 3 3.4
Applicable Code Requirement
The licensee listed the following applicable requirements from the 2004 Edition, no Addenda of ASME Code, Sections III and XI for the modification of degraded pressurizer heat sleeve and lower level nozzles. Section XI, IWA-4221(a), (b), and (c) require repair/replacement activities to meet the Owner's Requirements and the Construction Code. Section XI, IWA-4400, -4411 and -4411(a) require welding, brazing, metal removal, and installation be performed in accordance with the Owner's Requirements and the Construction Code. Section XI, IWA-4610(a) requires thermocouples and recording instruments be used to monitor the process temperatures. Section XI, IWA-4422.1, -4422.1(b), and -4611.1(a) require defects be removed, reduced to an acceptable size, or analyzed. Section XI, IWB-2420(b) and -2420(c) requires successive examinations of flaws that are allowed to remain in service. Section XI, IWB-3142.4 allows disposition of flaws by evaluation. Section XI, IWA-3300(a), -3300(b) and -3420 require detected flaws be sized and characterized. Section III, NB-5245 requires incremental and final surface examination of partial penetration welds. Section III, NB-5331 and -5331 (b) require imperfections be investigated and provide criteria for disposition of the indications. Code Case N-638-1 provides requirements for automatic or machine Gas Tungsten Arc Welding (GTAW) of Class 1 components without the use of preheat or postweld heat treatment. Code Case N-638-1, paragraph 3.0(d) requires the maximum interpass temperature for field applications be 350 degrees F regardless of the interpass temperature during qualification. Code Case N-638-1, "Reply" paragraph, states that the nondestructive examination requirements of the Construction Code need not be met, provided the requirements of paragraphs 1.0 through 5.0, and all other requirements of IWA-4000 are met. Code Case N-638-1, paragraphs 4.0(b) and 4.0(e), require certain areas to be examined and the ultrasonic examination acceptance criteria to be followed. 3.5 Proposed Alternatives and Bases for Use As an alternative to the above ASME Code requirements, the licensee proposed to repair the pressurizer heater sleeve and lower level nozzle penetrations based on the ambient
-4 temperature inside diameter temper bead (lDTB) welding method to restore the pressure boundary of the pressurizer penetrations. The IDTB welding method based on Code Case N-638-1 is performed with a remotely operated welding tool using the machine Gas Tungsten Arc Welding (GTAW) process and the ambient temperature bead method with 50 degrees F minimum preheat temperature without post-weld heat treatment. 3.6 Duration of Proposed Alternative The provisions of this alternative are applicable to the fourth 10-year lSI interval which commenced on October 10, 2009, and will end on June 30, 2019. The modifications performed in accordance with the provisions of this alternative shall remain in place for the design life of the modifications. 4.0 STAFF EVALUATION The NRC staff notes that should the licensee follow articles IWA-4221, -4400 and -4411 of the ASME Code Section XI, the licensee would need to either repair the degraded sleeve/lower level nozzle to its original condition or replace it. Instead, the licensee asked relief from these requirements and proposed an alternative repair method as described in Relief Request RR-PZR-01. Because the licensee's proposed repair method will not remove or examine the potential flaws in the J-groove weld. the licensee asked relief from articles IWA-3300, IWA-4422.1, IWA-4611.1, IWB-2420, IWB-3142, and IWB-3420 of the ASME Code, Section XI. The licensee asked relief from the ASME Code, Section III, NB-5245 and NB-5331 and Section XI. IWA-4610(a) as part of the installation process. The licensee also asked relieffrom certain requirements of Code Case N-638-1. Pursuant to CFR 50.55a(a)(3)(i), the NRC staff reviewed the proposed alternative repair method to verify that it provides an acceptable level of quality and safety in terms of the design, installation, flaw evaluations, examinations and corrosion analysis. The majority of licensee's response to the staff's RAI questions is documented in its May 4, 2011, submittal. 4.1 Desjgn The new heater sleeve and lower level nozzles will be made of stainless steel 316 base material with less than 0.03% maximum carbon content, and will use ER309 weld material with 0.03% maximum carbon content, both of which are less susceptible to PWSCC than the materials that are used in the original nozzles. The stainless steel material for the new sleeve nozzles is dual certified (316/316L) as it meets the low carbon content requirement of the "L" grade to minimize sensitization and the mechanical properties of the "non-L" grade of higher strength. The diagrams for machining, welding and examination for the nozzle repair are shown in Relief Request RR-PZR-01. By letter dated May 4, 2011, in response to RAI Question 8, the licensee provided a repaired heater sleeve configuration, identifying the new attachment weld as part of the new pressure boundary. The axial distance between the bottom end of the original heater sleeve and the top of the new weld is approximately 1 inch. This is the gap that the licensee analyzed for general corrosion as discussed later in this safety evaluation. The thickness of the new sleeve is approximately 0.225 inches.
- 5 The NRC staff noted that PWR operating experience (not necessary at Calvert Cliffs) has shown that cracking has occurred in stainless steel components resulting from contact with halogens (such as chlorides and fluorides) or being sensitized. In RAI Question 2, the staff asked the licensee to discuss whether RG 1.44, "Control of the Use of Sensitized Stainless Steel," will be followed in welding of the new sleeve, measures to limit the contact with halogens, and operating experience using the stainless steel heater sleeves in pressurizers. In response to RAI Question 2, the licensee stated that since the stainless steel base material and weld filler are limited to 0.03% maximum carbon content, base material sensitization is not a concern and RG 1.44 is not a specified requirement. Further, since ambient temperature temper bead welding is used for installation, the heat input will be low, providing further assurance that base material sensitization will not occur. The licensee's cleaning procedures will restrict contact and contamination of halogens and other contaminants on replacement items. All the Westinghouse pressurizer heater sleeves (heater wells) are stainless steel (either 304 or 316). The licensee is aware of only a single adverse operating experience where a stainless steel heater sleeve degraded due to stress-corrosion cracking as discussed in NRC Information Notice 2006-27 (ADAMS Accession No. ML062490396). The licensee stated that starting in 2006, 25 PWR plants visually inspected about 2000 stainless steel heater sleeves with no signs of leakage. The NRC staff finds that the material selected and installation procedures will minimize cracking in the replacement nozzles. In response to RAI Question 19, the licensee explained that the new sleeve and weld will satisfy the design requirements of the ASME Code, Section III. The licensee stated that the repair configuration complies with ASME Code, Section III, Figure NB-4244(d)-2(d) and is qualified for the applicable ASME Code, Section III, NB-3200 requirements. To meet the ASME Code, Section III requirements, the licensee used 3-dimensional finite element models that include a segment of the pressurizer bottom head and its cladding, remnant of the original nozzle and its weld, and replacement nozzle and its connection to the head. Thermal and pressure transients are applied to the models. Thermal analyses are performed to calculate the thermal distributions at key locations in the models for each transient event. The licensee also performed elastic structural analyses to demonstrate that the new sleeve and heater will not be ejected from the nozzle penetration. The NRC staff finds that the licensee has qualified the proposed repair design in accordance with the ASME Code, Section III, and has demonstrated by analyses that the replacement nozzles will not be ejected from the bottom head of the pressurizer. 4.2 Installation The basic installation procedures (steps) for the proposed repair are summarized as follows:
- 1. Remove the lower portion of the degraded nozzle by machining.
- 2. Examine the machined area that is to be welded using liquid penetrant testing (PT).
- 3. Weld the new sleeve nozzle to the pressurizer bottom head per Code Case N-638-1.
- 4. Machine the new attachment weld and adjacent area to provide a surface for examination.
- 5. Examine the new attachment weld and adjacent area using PT and ultrasonic testing (UT).
I n response to RAI Question 15, the licensee stated that the entire installation process from start to finish was demonstrated using a full scale mock-up as it would be implemented in the field.
- 6 The process included all machining, welding, and non-destructive examinations using all equipment and tooling that will be used to implement the repair. The engineering drawings and modification procedures were used as the basis to control the work to ensure acceptable results were demonstrated and achieved. The repair process was demonstrated in all of the various sleeve locations and was validated on ten coupons. In response to RAI Question 4, the licensee clarified that for installation step 2, the PT examination will be performed on the area to be welded and will include all new machined surfaces in the bore. Also, after the lower part of the original sleeve is removed, the licensee* will visually inspect the bore to verify no damage has occurred. These examination areas will ensure suitability for welding and that flaw propagation into the new weld will not occur. In response to RAI Question 11, the licensee explained that for installation step 3, before welding, the new nozzle is inserted into the bottom head bore and held in place using a positioning tool instead of roll expansion inside the bore. The radial clearance and crevice between the outer surface of the new nozzle and the bore is not exposed to reactor coolant so crevice corrosion is not a concern. The counter bore is initially machined to facilitate machining the bore that interfaces with the replacement nozzle. The (interface) length of the new weld in the bore is approximately 13/16 inches. The new nozzle is positioned with respect to the upper weld prep bevel and is independently verified by the quality control inspector. In response to RAI Question 6, the licensee stated that for installation step 4, the final machined length will extend from approximately 1 inch below the new attachment weld, across the weld face to approximately 0/4 inch above the new weld. Approximately 1132 inch thickness will be removed from the low alloy steel bore, approximately 1/8 inch thickness will be removed from the new attachment weld, and approximately 1/32 inch thickness will be removed from the lower (new) heater sleeve during final machining after welding. There will be some cold work induced due to the machining process but the level of cold work will be far below 20%. Standard Review Plan Section 5.2.3.11.4 of NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition, limits the yield strength of cold worked austenitic stainless steel to 90 ksi (kilo-pounds per square inch) ( approximately 20% cold work) based upon concerns about susceptibility to stress-corrosion cracking of higher strength materials. The machining is performed in accordance with an established procedure. Typical surface finishes are expected to be 125 roughness height reading or better based on mockup results. In response to RAI Question 7, the licensee stated that for installation step 5, the heat affected zone depth is approximately 3/32 inches so it will extend beyond the weld face apprOXimately 3/32 inches on both the top and bottom edges of the new weld. The residual tensile stresses are, as expected, most significant in the new weld with stresses ranging as high as 50 ksi to 65 ksi. However, both the axial and hoop residual stresses decay to approximately 10 ksi or less approximately % inches above the top of the new weld in the low alloy steel bore and approximately % inch below the bottom of the weldin the new heater sleeve and the low alloy steel bore. After welding is completed, the licensee proposed to perform PT across the weld face and extend 1/2" above the weld on the bore and 1/2" below the weld on the sleeves inside diameter. Based on the diagrams in the January 31 and May 4, 2011, submittal, the licensee will also perform UT covering the same area as PT.
- 7 As discussed above, the ASME Code, Section XI, IWA-4610(a) requires that thermocouples and recording instruments be used to monitor process temperatures. However, it is impractical to measure welding interpass temperature directly because of the nozzle penetration configuration. The licensee proposed to determine the maximum interpass temperature by heat-flow calculations and a test coupon per Code Case N-638-1. The NRC staff finds that the use of heat-flow calculations and a test coupon to determine the maximum interpass temperature is acceptable because this indirect interpass temperature method is permitted per Code Case N-638-4 and is addressed in the conditions imposed on N-638-4 in RG 1.145, Revision 16. The NRC staff finds that the proposed installation procedures are based on a full scale mock up to ensure quality control and suitability of the proposed repair and, therefore, are acceptable. The staff notes that the licensee will follow Code Case N-638-1 which provides welding requirements to minimize fabrication defects in the new weld. 4.3 Flaw Evaluations As a result of the repair, the licensee will not remove or perform future examination of the potential flaws in the J-groove weld and triple point location. Therefore, the licensee asked relief from IWA-3300, IWA-4422.1, IWA-4611.1, I WB-2420, IWB-3142, and IWB-3420. In lieu of these ASME Code requirements, the licensee performed flaw evaluations to demonstrate the structural integrity of the pressurizer with flaws remaining in service in the J-groove weld and new weld (triple point anomaly) without performing future examinations. Triple Point Anomaly The triple point is a location in the repaired nozzle where the low alloy steel pressurizer bottom head, the new stainless steel sleeve/nozzle, and the new stainless steel attachment weld intersect. As a result of welding, the new sleeve to the bottom head, an anomaly may be formed at the triple point location based on the licensee's mock-up testing. The anomaly is an irregularly shaped, very small void that does not exceed 0.05 inches in length. In the flaw evaluation, the licensee assumed the anomaly to exist around the entire bore circumference at the triple point elevation. The licensee performed a fracture mechanics analysis to justify, in accordance with the ASME Code, Section XI, for operating with the postulated triple point anomaly. The anomaly is modeled as a 0.05 inch, semicircular "crack like" defect, extending 360 degrees around the circumference at the triple point location. The licensee assumed two possible propagation paths for the subject anomaly flaw. In the horizontal direction, for path 1, the licensee postulated a circumferential and an axial flaw originated from the outside diameter (OD) of the nozzle and propagate across the heater sleeve/lower level nozzle wall thickness to the inside diameter (ID) of the nozzle. In the vertical direction, for path 2, the flaw propagates up the outside surface of the new weld between the new weld and pressurizer lower head. A continuous surface flaw is postulated to lie along this cylindrical interface between the weld and head. This flaw, driven by radial stresses, may propagate along either the new stainless steel attachment weld or the low alloy steel head. The licensee used the evaluation procedures and acceptance criteria of IWB-3612 and Appendix C of ASME Code, Section XI to demonstrate that the D.OS-inch weld anomaly in a repaired sleevellower level nozzle is acceptable for a 40-year deSign life. The detection of the O.OS-inch triple point weld anomaly is discussed in the Examinations section below.
- 8 J-Groove Weld Cracking ASME Code, Section XI, IWA-3300(a) and (b) requires that flaws are to be characterized and evaluated to determine acceptability in accordance with IWB-3500 and/or IWB-3600 as applicable. As a result of the repair, the upper portion of the original sleevellower level nozzle with the attached J-groove weld will remain in service. The J-groove weld is made of Alloy 82/182 weld which is susceptible to PWSCC.lf the J-groove weld is cracked, the crack may propagate into the pressurizer bottom head and challenge its structural integrity. It is difficult to examine the original J-groove weld with UT due to the weld and penetration geometry. In lieu of future examinations of the J-groove weld, the licensee assumed the worst-case tlaw in the J-groove weld in its flaw evaluation. The licensee postulated that a radial crack in the J-groove weld would propagate via PWSCC, through the J-groove weld and butter, to the interface with the low alloy steel bottom head. Crack growth through the original J-groove weld would tend to relieve the residual stresses in the weld as the crack grew to its final size until its growth is arrested at the low alloy steel interface. The licensee assumed that a small flaw could initiate in the low alloy steel bottom head and grow by fatigue. The licensee postulated that a small flaw in the bottom head would combine with a large PWSCC flaw in the J-groove weld to form a radial comer flaw. The licensee performed the flaw evaluation in accordance with the ASME Code, Section XI, IWB-3600 and demonstrated that a repaired heater sleeve is acceptable for a 35-year design life, while a repaired lower level instrument nozzle would be acceptable for 32 years of fatigue crack growth into the low alloy steel head. The NRC staff notes that 32 years would allow the repaired nozzle to remain in service until the end of the period of license renewal. The NRC staff finds that the licensee has demonstrated by analysis that the remnant heater sleevellower level nozzles will not affect the structural integrity of the pressurizer bottom head, and the triple point anomaly will not grow significantly to affect the structural integrity of the new attachment weld. 4.4 Examinations As discussed in the installation procedure above, the licensee will inspect before and after installing the new heater sleeve and lower level nozzles. The licensee provided the following additional information regarding the examinations. The licensee stated that Code Case N-638-1 paragraph 4.0(b) requires the final weld surface and the preheated band around the area defined in paragraph 1.0(d) to be examined using surface and ultrasonic methods. Paragraph 1.0(d) of the code case defines this area as at least 1-1/2times the component thickness or 5 inches, whichever is less. The licensee asked relief from inspecting this area. As an alternative, the licensee proposed to examine the new weld surface and the 1/2 inch minimum distance above and below the new weld. The licensee stated that the final examination of the new weld and immediate surrounding area within the bore will be sufficient to verify that defects have not been induced in the low alloy steel bottom head material due to the welding process and will assure the integrity of the sleeve/nozzle and the new weld. The NRC staff finds that the licensee proposed examination coverage is acceptable in lieu of the area defined by Paragraph 1.0(d) in N-638-1 because the proposed examination
- 9 area is sufficiently large to include the heat affected zones which are susceptible to potential fabrication defects. The licensee stated that Code Case N-638-1, Paragraph 4.0(e) requires that the ultrasonic examination acceptance criteria be in accordance with the ASME Code, Section XI, IWB-3000. However, IWB-3000 does not have any acceptance criteria that directly apply to the subject partial penetration weld configuration. As documented in RG 1.147, Revision 15, the NRC has approved Code Case N-638-1 with the condition that UT volumetric examinations be performed with personnel and procedures qualified for the new weld volume and qualified by demonstration using representative samples which contain construction type flaws. The licensee requested relief from examination and acceptance criteria requirements as specified in N-638-1 paragraphs 4.0(b) and 4.0(e). In lieu of the requirements in paragraph 4.0(e) of N-638-1, the licensee proposed to use the acceptance criteria of the 2004 Edition of the ASME Code, Section III, NB-5331, to disposition all flaws identified within the new weld volume, except for the triple point anomaly as discussed above. In addition, ASME Code, Section III, NB-5245 requires incremental and final surface examination of partial penetration welds. Due to the welding layer disposition sequence (i.e., each layer is deposited parallel to the penetration centerline), the licensee could not perform incremental (progressive) PI. Instead, the licensee proposed to perform a UT and a final surface PT examination. The NRC staff finds that the proposed UT and a final PT will provide an acceptable level of examination quality that will detect potential fabrication defects. In response to RAI Question 12, the licensee stated that the UT examination volume includes the weld and adjacent base metal areas identified in sketch Nos. 3 and 5 as shown in the May 4, 2011, submittal. Sketch NO.5 also identifies the area affected by the shadowing effect of the non-fused sleeve interface (the triple point). The licensee showed the examination coverage provided by each of the UT transducers that will be used in sketch Nos. 4 through 8 in the May 4, 2011, submittal. The resulting area that cannot be examined by any of the examination angles is estimated to be less than 5% of the total examination volume. The NRC staff finds that although the licensee will not be able to achieve 100% coverage, the licensee will be able to achieve the "essentially 100% coverage" which is defined as greater than 90% coverage in ASME Code Case N-460. The NRC staff was concerned about the reliability and capability of UT detecting a triple point anomaly of such a small flaw (0.05 inches in depth). In response to RAI Question 14, the licensee responded that the ultrasonic examination technique uses an immersion probe from the nozzle bore containing six transducers to generate refracted beam angles of 0 degree L, 45 degree L, and 70 degree L in the weld material. The 45 degree L beams are generated in four directions; two axial and two circumferential, each looking in opposing directions. The 70 degree L beam is only applied in one axial direction looking up. The transducer frequency is 7.5 MHz and results in a short wavelength beam capable of detecting small flaws with high resolution. The selection of the beam angles is based on detecting and characterizing the most probable welding manufacturing flaws likely to exist with this welding process. Scanning is performed using an automated scanner to move the probe in a raster pattern to examine the area of interest that includes the weld and heat affected zones. Representative mockups have been fabricated and notches as small as 0.025 inches located at the triple point have been demonstrated to be detected and characterized.
- 10 The licensee stated that mock-up coupons were routinely cross-sectioned and metallographically examined in the areas of the weld identified by UT as the location of maximum signal amplitude. The triple point anomalies were thus measured at these locations and then correlated to the UT data. Sufficient quantities of mock-ups were metallographically examined to statistically verify the accuracy of the UT data prior to process qualification. During process qualification, there were no triple point anomalies found to be greater than 0.05 inches by either metallographic examination or UT. The licensee stated that if UT detected a triple point anomaly that exceeds 0.05 inches, the weld in the case cited would be removed and replaced. In response to RAI Question 18 regarding future lSI, the licensee stated that the lSI requirements for the new heater sleeve, lower level nozzles, and welds will consist of performing a Class 1 pressure test each refueling outage. The licensee stated further that this inspection will be conducted in conjunction with the existing pressure test of reactor coolant system pressure boundaries. This pressure test is performed while the unit is in Mode 3 with the unit at normal operating temperature and pressure. The NRC staff notes that the licensee needs to follow the ASME Code, Section XI, IWB-5000, to perform its pressure test. A part of the pressure test requirement is that the licensee is required to perform visual examination of the heater sleeve/lower level nozzles. The visual examination will verify the structural integrity of the repaired nozzles. The NRC staff finds that the licensee's proposed PT and UT and associated acceptance criteria for flaw disposition provide an acceptable level of quality and reliability as that of Code Case N-638-1 paragraphs 4.0(b) and 4.0(e). Therefore, the proposed examinations are acceptable. 4.5 Corrosion Analysis The proposed nozzle repair would result in a small gap (approximately 1 inch as discussed above) inside the pressurizer nozzle penetration between the new weld of the replacement half nozzle and the bottom of the original heater sleeve. This 1-inch gap will cause low alloy steel of the pressurizer bore (vessel shell) to be exposed to primary coolant. A similar situation also exists with the modification performed on the lower level nozzles. The gap may cause corrosion of the low ally steel in the pressurizer bore. By letter dated September 19, 2011, the licensee discussed the evaluation of potential corrosion associated with the pressurizer heater sleeve and lower level nozzle repairs. The corrosion evaluation calculates the amount of material loss due to general (uniform) corrosion over time under both at power (low oxygen) and shut down (moderate oxygen) conditions. The licensee used the expected rates of corrosion to determine that the calculated material loss of the exposed low alloy steel would be insignificant over a 40-year period. The licensee determined that galvanic corrosion, hydrogen embrittlement, stress-corrosion cracking and crevice corrosion are not a concern for the low alloy steel of the pressurizer shell that is exposed to primary coolant. Similarly, the licensee determined that general corrosion, galvanic corrosion, crevice corrosion, and hydrogen embrittlement were also determined not to be a concern for the replacement stainless steel components. Stress corrosion cracking of the replacement stainless steel components was also determined not to be a concern because two of the three necessary synergistic elements of stress-corrosion cracking (susceptible material and aggressive environment) are minimal or not present. Although some residual stress will likely be present adjacent to the weld of the stainless steel components, this will not likely cause crack initiation or propagation because the other two
- 11 necessary elements (susceptible material and aggressive environment) are minimal or not present. The NRC staff finds the licensee's corrosion evaluation acceptable based on comparing it with the corrosion studies submitted by other licensees regarding the half nozzle repairs and operating experience of half nozzle repairs at other nuclear plants.
5.0 CONCLUSION
On the basis of information submitted, the NRC staff concludes that the proposed alternative in Relief Request RR-PZR-01 will provide an acceptable level of quality and safety for the repair of the pressurizer heater sleeve and lower level nozzle penetrations. Therefore, pursuant to 10 CFR 50.55a{a)(3)(i), the staff authorizes the use of Relief Request RR-PZR-01 for the fourth 10-year I SI interval at the Calvert Cliffs Nuclear Power Plant, Unit No.1. The staff notes that the repairs performed in accordance with the provisions of this relief request may remain in place for the design life of the modifications. All other ASME Code, Section XI requirements for which relief was not specifically requested and approved in this relief request remain applicable, including third-party review by the Authorized Nuclear Inservice Inspector. Principal Contributor: John Tsao, NRR Date: December 9, 2011
ML113360526
======~~=========r===========91 OFFICE LPL 1-1/PM NAME DPickett DATE 12/8/11 12/08 111 Lupoid via mail dated 11 129/11 LPL1-1/BC NSalgado (JKim for) 1219/11}}