ML20136J135

Provides Description of Recently Completed Fracture Toughness Evaluation of 970304 Pressurizer Thermal Transient at San Onofre Unit 2.Pressurizer Remained within Fracture Toughness Acceptance Criteria

ML20136J135
Person / Time
Site:	San Onofre
Issue date:	03/14/1997
From:	Rainsberry J SOUTHERN CALIFORNIA EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
NUDOCS 9703200025
Download: ML20136J135 (9)

Text

{{#Wiki_filter:_ _ _ _ . g soone-o,_ EDISON P.lanager, Plant Licensing An CD/SO\\ /NTIRhf1OV(La Corrpany March 14, 1997 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D. C. 20555 Gentlemen:

Subject:

Docket No. 50-361 f Pressurizer Thermal Transient Fracture Toughness Evaluatlion San Onofre Nuclear Generating Station, Unit 2 } s i As requested by the Management of the NRC Technical Staff, this lett provides a description of the recently completed fracture toughness-luation of the March 4,1997 pressurizer thermal transient at San Onofre Unit This evaluation demonstrates that the pressurizer remained within the fra e_ toughness acceptance criteria of the ASME Code,1989 Edition, no pdd da, " Appendix G. 3 SU E RY: ~ On March 4,1997, the San Onofre Unit 2 pressurizer vessel experienced an accelerated cooldown transient at a rate greater than the 200 F/ hour limit specified in Technical Specification 3.4.3.1. The transient is depicted in. The transient occurred while using the auxiliary spray to collapse the steam bubble and depressurize the pressurizer vessel. This i resulted in an in-surge of colder water (110 F) through the surge line into the hotter pressurizer vessel (430 F). Southern California Edison (Edison) performed an evaluation to demonstrate the ASME Code acceptance criteria and the integrity of the pressurizer vessel and the related components (spray nozzle, surge line, and surge line nozzle) were maintained during the transient. The pressurizer vessel and the related components are ASME Class 1. The Class 1 evaluation of the surge line, surge A~ line nozzle, and the spray nozzle was ieved by comparing this transient and 4 c l the design basis transients consideredw..the design report. It was determined that the design basis transients are enveloping, therefore, the ~' surge line, the surge line nozzle, and the spray nozzle are considered qualified in accordance with the ASME Code, Section III, Subsection NB-3650. The pressurizer vessel was evaluated by Asea Brown Boveri/ Combustion Engineering (ABB/CE) by comparing this event at San Onofre Unit 2 to a similar f event analyzed in CE0G Task 788, and found acceptable. I 9703200025 970314 I PDR ADOCK 05000361 'k' S

PDR, San Onofre Nudeur Generating Station P. O. Box 128 San Clemente, CA 92674-0128 l

714-368-7420

To demonstrate the fracture toughness, the guidance in Appendix G of 10 CFR 50 for fracture toughness requirements for ferritic materials was used to evaluate the pressurizer fracture toughness. To evaluate the fracture toughness it was necessary to choose a bounding location in the vessel. The surge line nozzle was chosen due to the relatively severe temperature transient (430-110 F) due to the reactor coolant system (RCS) water insurge via the surge line which was combined with a large flow rate. This evaluation by AB8/CE was performed according to the general requirements of the ASME Code, Section XI, Appendix A and compared to the acceptance criteria of Appendix G of the same Code. Two nozzle locations were selected for the evaluation: the nozzle forging near the safe end, and the nozzle corner. Axial flaws equal to 10% wall thickness were postulated at these two locations, and the hoop stresses due to temperature were calculated using the finite element method. The 10% wall thickness flaw was chosen based on the size of an allowable surface flaw in accordance with the 1989 ASME Code, Section XI, Table IWB-3514-1 and supported by Edison's recent inservice inspection of the nozzle area, which showed no recordable indication of any flaw. An evaluation was also performed for a 25% wall thickness flaw, in accordance with the ASME Code, Section XI, Appendix G. EVALUATION: Pressurizer Surge Line and the Pressurizer Surge Nozzle In response to NRC Bulletin 88-11, Edison performed an ASME Code Class 1 evaluation of the pressurizer surge line, including the nozzles. It was demonstrated, based on the evaluation results, that the surge line at Unit 2 met all applicable ASME Code requirements for the design life of the plant. As part of the evaluation, thermal transients with a step temperature differential between the pressurizer and the RCS hot leg up to 340 F were considered. Such transients exceed the maximum temperature differential range, 430 - 110 F, that occurred during the March 4, 1997 transient. Furthermore, the March 4,1997 transient occurred over a relatively long period of time rather than occurring in the form of a step change. Therefore, it is concluded that the existing surge line evaluation is enveloping. Pressurizer Vessel The pressurizer vessel was evaluated according to the ASME Code for the March 4, 1997 accelerated cooldown transient by ABB/CE. The evaluation methodology was based on comparing the March 4, 1997 transient with the results of CE0G Task 788, in which several plants were evaluated for accelerated cooldown transients. Some of these plants are similar to San Onofre Unit 2. Three locations were selected for structural / fatigue evaluation: the bottom head support skirt, the heater sleeve, and the water to steam boundary. Pressurizer Surge Nozzle Pressurized Thermal Shock (PTS) Evaluation A fracture toughness evaluation of the pressurizer surge nozzle was performed to assess the effect of the March 4, 1997 transient. This evaluation was performed by ABB/CE according to the general requirements of Appendix A of ASME Code, Section XI and the acceptance criteria of Appendix G. Flaw depth and aspect ratio were assumed based on the ASME Code, Appendix G for an

1 3 i' Document Control Desk 3 acceptable surface flaw (a = 10%t, and 25fst, a/1 = 6). Axial cracks were i postulated at two critical locations of the nozzle. The first location lies j in the nozzle wall between the tapered transition and the safe end, and the j second location is in the nozzle corner (an inclined section across the base of the nozzle). i A thermal-stress analysis was performed to calculate the hoop stress distribution in the nozzle wall at the selected locations, which is allowed in Article A-3100, Appendix A, Section XI of the ASME Code. Distribution of the hoop stresses, across the nozzle wall, is required for the fracture toughness I evaluation. The finite element method was used to perform the stress analysis part of the evaluation. An axisymmetric model was generated which includes the nozzle forging, the stainless steel cladding, the nozzle safe end, and the thermal sleeve. The model also included the lower head of the pressurizer vessel and part of the pressurizer surge line (Enclosure 2). The thermal transient analysis is a calculation of the temperature time history due to a step change in temperature on the inside surface of the i nozzle assembly. The actual drop in temperature during the March 4, 1997 transient occurred over several minutes. Using a step change in temperature is, therefore, conservative. The thermal-stress analysis, based on the results of thermal transient analysis, calculated the time history of the stress at the specified locations in the nozzle. The stress distribution in the nozzle wall was calculated, and the maximum peak stress was used. The stress intensity factor (KI=2 kip + kit) was calculated and compared with the reference stress intensity factor (KIR). KIR was conservatively estimated based on the methodology provided in the NRC Branch Technical Position MTEB 5-2, Fracture Toughness Requirements, paragraph Bl.1.(2) (Enclosure 3). KIC=12 SQRT(CV) was also calculated to be 100 Ksi SQRT(IN) to demonstrate additional margin (

Reference:

David Broek, "The Practical Use of Fracture Mechanics", Kluwer Academic Publishers,1989). CONCLUSIONS: The results of the evaluation were that during the March 4, 1997 pressurizer thermal transient the pressurizer vessel remained within the acceptance criteria of the ASME Code, Section XI, Appendix G. This result is based on performing an ASME Code fatigue evaluation and Pressurized Thermal Shock (PTS) evaluation on the critical locations in the pressurizer. In addition, for the pressurizer nozzle ferritic portion, the temperature at the peak stress time was well above 250 F (Enclosures 4 and 5). In addition, an inservice inspection volumetric examination of the nozzle to vessel and safe end to nozzle welds was performed during the current refueling cycle with no recordable indication.

Document Control Desk 4 If you have any questions or would like additional information, please let me know. Sincerely, 'f r W. u Enclosures cc: E. W. Hershoff, Regional Administrator, NRC Region IV K. E. Perkins, Jr., Director, Walnut Creek Field Office, NRC Region IV J. A. Sloan, NRC Senior Resident Inspector, San Onofre Units 2 & 3 M. B. Fields, NRC Project Manager, San Onofre Units 2 and 3 i

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=- =, +860-285M232 ABB CE NUCLEAR SVCS 712 P09 NAR A3 '97 19:49 i ABB ~~s s ASEA SAoWN BoVER! i e Inter Cffice Correspondence To: J. Ghergurovich March 11,1997 1 k.N N01bp fpfl0 S-PENG-97-006, Rev. 00 j From: S.T. Byme Page 1 of 2

Subject:

San Onofre Unit 2 Pressurizer Surge Nozzle RTm QA Verification Status: COMPLETE An evaluation was performed to determ.ne the reference temperature, RTm of the San Onofre Unit 2 pressurizer surge nozzle. The evaluation used the attached certified material test report for Code No. C-9225-1 (Heat #AV-4584). Le method used is that provided in NRC Branch Technical Position MTEB 5-2, Fracture Toughness Requirements, paragraphs Bl.l.(2) and B.l.1(3)(b). MTEB 5-2 provides a means of determining RTmin cases such as this where the testing requirements were established before the ASME Code requirement for RTm (SectionIII.NB 2300) were issued. No drop weight tests were performed for the surge nozzle. Since the surge nozzle was fabricated using SA508 Class 2 material, Paragraph B1.1.(2)(a) permits the drop weight nil-ductility transition temperature, NDTT, to be estmated as 60T. The other two permissible criteria ofParagraph Bl.l(2) for estimating NDTT could not be satis 5ed by the data contained in the nozzle CMTR. De orientation of the Charpy test specimens is not specified on the CMTR. The material purchase specification, P3C10(b), referenced on the CMTR, requires the Charpy testing to be perforrned per ASTM A370. A370-68 states that unless otherwise speciSed, longitudinal specimens are to ' 'be used. Herefore,it was assumed that the specimen orientation was longitudinal or parallel to the principal working direction in the forging. Paragraph B.1'.l(3)(b) was used to convett the data to the " transverse" orientation. The Charpy impact test results at 107 were: 94,69,129,95,94 and 96 (ft-lbs), all greater than 50 fi-lbs ne Charpy lateral expansion test resuhs at 107 were: 70,61,78,71,72 and 72 mils, a'l greater than 35 mils Therefore, Tcv is estimated per Paragraph Bl.l(3)(b) as 20 T higher than the 107 test temperature, or Tev = 307. RTer is then determmed as the higher of Tev - 60T or ND'IT. Since the estimated NDTT is +607 and Tev 60T is equal to -307, the estmated RTutrr for the pressurizer surge nozzle is +607. STB:tm Attachment

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• DIST=3 I
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=5.5

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