ML102430126

Request for Additional Information Relief Request ISI-3-31(TAC No. ME3961 and ME3962)

ML102430126
Person / Time
Site:	San Onofre
Issue date:	08/31/2010
From:	Hall J Plant Licensing Branch IV
To:	Conklin L Southern California Edison Co
Hall, J R, NRR/DORL/LPL4,301-415-4032
Shared Package
ML102430125	List: ML102430120 ML102430126
References
TAC ME3961, TAC ME3962
Download: ML102430126 (4)

Text

OFFICE OF NUCLEAR REACTOR REGULATION REQUEST FOR ADDITIONAL INFORMATION THIRD TEN-YEAR INSERVICE INSPECTION INTERVAL RELIEF REQUEST ISI-3-31 FLAW EVALUATION OF HIGH ENERGY SCHEDULE 10S EMERGENCY CORE COOLING SYSTEM PIPING SAN ONOFRE NUCLEAR GENERATING STATION, UNITS 2 AND 3 SOUTHERN CALIFORNIA EDISON DOCKET NUMBERS: 50-361 AND 50-362 By letter dated May 19, 2010, (Agencywide Document Access and Management System Accession Number ML101440381) Southern California Edison (the licensee) proposed contingency alternative evaluation criteria as shown in Relief Request ISI-3-31 for temporary acceptance of flaws in certain high energy Class 2 and 3 Emergency Core Cooling System (ECCS) schedule 10s piping at San Onofre Nuclear Generating Station (SONGS), Units 2 and 3. The licensee proposes to follow the requirements of ASME Code Case N-513-2, except that the proposed alternative in Relief Request ISI-3-31 will be applied to piping with a higher operating temperature and with a smaller diameter than permitted by the code case. The enclosure to the licensee's May 19, 2010 letter contains the information provided in support of Relief Request ISI-3-31. To complete its review, the NRC staff requests the following additional information.

1. On page 2 of the enclosure, last sentence, the licensee stated that, "- [t]he maximum operating temperatures listed above [i.e., the temperatures in Table 1 in the relief request] were developed during initial plant design and construction. These are not design temperatures and they contain margin above the maximum operating temperature determined in the analysis of record-.". The second sentence is confusing. (1) Explain whether the maximum operating temperatures determined in the analysis of record are different from the maximum operating temperatures in Table 1. (2) If design temperatures are higher than the maximum operating temperature, explain why design temperatures were not used in the flaw evaluation.

2. On page 6 of the enclosure, Table 2 provides allowable lengths of 100% through-wall circumferential flaws for various pipe sizes in the subject ECCS piping. However, it is not clear where the allowable circumferential flaw lengths come from. In Appendix A of Attachment 1 to the enclosure, various tables and figures present allowable lengths based on the limiting of the length calculated by the combined stress criteria and membrane stress criteria. For example, Figures 8-1 and 8-2 in Attachment 1 provide allowable flaw lengths vs. bending stresses. Table 8-1 provides a summary of allowable flaw lengths based on membrane stress criteria. These data feed into Table 2. (1) Clarify exactly which tables or figures provide the allowable circumferential flaw lengths. For example, explain why data in Tables A7-1, A7-2, A11-1, A11-2, A13-1, A13-2, A14-1, and A14-2 are not shown in Table 2. (2) The allowable flaw lengths for pipe diameter sizes 8 to 24 inches in Table 2 can be traced to the tables and calculations in Appendix A. However, the allowable flaw lengths for pipe diameter size 2.5, 4, and 6 inches in Table 2 cannot be traced to the tables and calculations in Appendix A. Provide the source of the allowable flaw lengths for pipe diameter sizes 2.5, 4, and 6 inches.

3. In Table 2 on page 6 of the enclosure: (1) Clarify whether the allowable flaw lengths in Table 2 are the initial or final flaw lengths. (2) If Table 2 provides the allowable final flaw length (i.e., the length at the time of the ASME Code repair), the licensee needs to provide the allowable initial flaw length and the crack growth rate for various degradation mechanisms. In addition, the licensee needs to demonstrate that the flaw detected, including its growth in service, will not exceed the allowable length before the ASME Code repair is made. (3) If Table 2 provides the allowable initial flaw length (i.e., flaw length at the time of detection), the staff has the following concerns. Many of the allowable circumferential flaw lengths in Table 2 exceed 50% of the outside diameter circumference of the pipe. Some of the allowable lengths are as high as 62% of the outside diameter circumference of the pipe as shown in Item 2 of Table 2. Also, Tables A7-1 and A7-2 show allowable lengths of 71.8% and 76.2% of the outside diameter circumference of the pipe. The staff is concerned that a leaking circumferential flaw of any length is permitted under the relief request to remain in service when its crack growth is not known. Even if the crack growth rate is known, the licensee needs to consider measurement uncertainty of the flaw length and uncertainty in the crack growth rate when demonstrating that the flaw detected plus its growth to the end of the operating cycle will not challenge the structural integrity of the pipe. Depending on crack growth rates, some allowable circumferential flaw lengths in Table 2 may not be conservative and may be inappropriate. The licensee needs to address the staff's concerns if Table 2 provides the allowable initial flaw length.

4. On page 8 of the enclosure, the last sentence, Item 1 states that an "Immediate Operability Determination [will be prepared] based upon visual characterization of the indication and operating experience with the degradation mechanisms of this piping (stress corrosion cracking or cyclic fatigue failure)." Licensees may perform their immediate operability determination based on visual inspection, as long as the degradation mechanism is readily discernable from a visual examination or is determined based on substantial operating experience with the identified degradation mechanism in the affected system. However, it is expected that licensees perform a volumetric NDE examination following the visual examination to support their prompt operability determination. Therefore, discuss whether a volumetric examination will be performed following the initial visual examination or justify how a visual examination can characterize flaw size accurately, especially for the portion of the flaw that is embedded in the pipe wall or in the inside diameter of the pipe.

5. On page 9 of the enclosure, Item 2 states that the prompt operability determination will be based on non-destructive examinations of flaws. Discuss the exact NDE method(s) that will be used per Item 2.

6. On page 9 of the enclosure, Section 6, Duration of Proposed Alternative: (1) The licensee stated that the proposed alternative will apply for the duration of the third 10-year inspection interval which ends on August 17, 2013 for both SONGS Units 2 and 3. Provide the cycle number and approximate dates for the remaining cycles from now to August 2013 for each unit. (2) If a flaw is detected during a refueling outage (e.g., in 2011), clarify whether the subject relief request applies. That is, if a flaw is detected during a refueling outage, discuss whether the degraded pipe will be repaired or replaced according to the ASME Code, or if the flaw will be allowed to remain in service for the following operating cycle. If continued operation with the flaw is anticipated, provide justification for not making the repairs during the refueling outage.

7. The fluid inside of some the pipes identified in the relief request would be at 275°F with a pressure of 275 psi. (1) Discuss how the leakage from the flaw(s) will be managed to protect plant personnel from the steam/water jet exiting a 100% through-wall flaw. (2) Discuss the potential maximum leak rates from these flaws. (3) Some of the flaw sizes in the proposed relief request could have large leak rates that could potentially generate unacceptable offsite radiological doses for those piping segments that contain radioactive coolant. Discuss the impact on offsite dose if an accident were to occur that results in a release of radionuclides into the atmosphere when the flaws in these pipes are left in-service.

Regarding Attachment 1, Calculation M-DSC-445:

8. On page 22, second paragraph, the licensee stated that the allowable circumferential flaw lengths were calculated based on the limit load and elastic plastic failure mechanic [fracture mechanics] methods. However, the allowable axial flaw lengths were calculated using the limit load method only. The staff notes that the limit load method is less conservative than elastic plastic fracture mechanics method for the welds that are made with submerged arc welding (SAW) and shield metal arc welding (SMAW). Explain why the allowable axial flaw lengths were not calculated using the elastic plastic fracture mechanics method for the welds that were made with SAW and SMAW.

9. On page 26, Section 5.1.3: (1) Explain why the flaw evaluation does not include loading from emergency conditions as part of the design basis.

10. On pages 27 to 29; Section 5.3 discusses an eight-step procedure to calculate pipe stresses. The explanation of the eight-step procedure is confusing. It seems that the pipe stresses resulting from each of the applied loads (e.g., deadweight, thermal, seismic) can be obtained individually and directly from the original pipe stress analyses for each affected pipe without using the eight-step procedure. Explain why the eight-step procedure is needed.

11. On page 30, last sentence, the licensee stated that "- [e]quation 5-12 will be used in this calculation to verify that the allowable flaw length for circumferential flaws will be bounding for axial oriented flaws-" The allowable axial flaw lengths are presented in Table 8-2 (on page 40). The allowable circumferential flaw lengths are presented in Table 8-1 and Figures 8-1 and 8-2. From these tables and figures, the allowable circumferential flaw lengths are longer than the allowable axial lengths. However, in terms of permitting flaws to remain in service, the shorter, not longer, allowable flaw length should be bounding and should be used. It appears that the allowable circumferential flaw lengths in Table 2 of the relief request will be used to disposition circumferential flaws and allowable axial flaw lengths in Table 3 of the relief request will be used to disposition axial flaws. Therefore, explain why the circumferential flaw lengths which are longer than the axial flaw lengths are bounding as the licensee stated above.

12. On page 40, it appears that for the 24-inch size pipe, there is a large difference (large ratio) between the allowable circumferential flaw length for SAW and SMAW and allowable flaw length for TIG and wrought pipe. This large difference is evident in Tables A2-1 vs. A2-2, A6-1 vs. A6-2, and A10-1 vs. A10-2. However, for the small diameter pipe this difference (ratio) is small as shown in Tables A3-1 vs. A3-2 for the 10-inch and 16-inch pipes. The staff understands that the difference may be caused by the application of the Z factors to the material property of SAW and SMAW, but not to the material property of TIG and wrought pipe. (1) Discuss why a large difference (ratio) exists between the allowable flaw length for the SAW weld vs. TIG weld for the large diameter pipe but not for the smaller diameter pipe. (2) Discuss why a large difference (the ratio) exists between the allowable flaw length for SAW and SMAW and the allowable flaw length for TIG and wrought pipe for the 6-inch diameter pipe as shown in Tables A12-1 and A12-2, but not for other small size pipe.

13. (1) Discuss the nondestructive examination requirements (inspection method and frequency) for the flaw(s) that will remain in service under the relief request. (2) Discuss how the crack growth will be monitored during operation for the flaw that will remain in service. (3) Discuss actions that will be taken if the actual crack growth exceeds the crack growth used in the flaw evaluation.

14. Based on the flaw evaluation in Attachment 1, it appears that the relief request applies to planar flaws. Discuss whether the relief request would be applicable to non-planar flaws due to wall thinning.

15. Clarify if the relief request (i.e., the allowable lengths for through wall circumferential and axial flaws) applies to flaws that are embedded (subsurface) in the pipe wall.