ML020100071
| ML020100071 | |
| Person / Time | |
|---|---|
| Site: | Nine Mile Point  |
| Issue date: | 01/16/2002 |
| From: | Tam P NRC/NRR/DLPM/LPD1 |
| To: | Conway J Constellation Nuclear |
| References | |
| TAC MB2797 | |
| Download: ML020100071 (12) | |
Text
January 16, 2002 Mr. John T. Conway Site Vice President Nine Mile Point Nuclear Station, LLC P.O. Box 63 Lycoming, NY 13093
SUBJECT:
NINE MILE POINT NUCLEAR STATION, UNIT NO. 2 (NMP2) -
REINSPECTION PLAN FOR CORE SHROUD WELD H4 (TAC NO. MB2797)
Dear Mr. Conway:
By letter dated October 30, 2000, the NRC staff conveyed its review results of NMP2s Refueling Outage 7 (RFO7) core shroud ultrasonic reexamination, concluding then that it can be operated without repairing weld H4 for one fuel cycle. By letter dated September 14, 2001, Niagara Mohawk Power Corporation (NMPC) submitted its reevaluation of core shroud weld H4, taking into account benefits from the implementation of noble metal chemistry application and hydrogen water chemistry. NMPC demonstrated through an analytical flaw evaluation, using modified crack growth rates, that NMP2 can be operated without repair of weld H4 for two fuel cycles of operation after RFO7 (i.e., without reinspection during RFO8).
On November 7, 2001, NMPCs ownership interest and operating license in NMP2 were transferred to Nine Mile Point Nuclear Station, LLC (NMPNS), thus allowing NMPNS to possess, use and operate NMP2. By letter dated November 20, 2001, NMPNS requested that the NRC continue to review and act on all requests previously submitted by NMPC before the transfer, and to consider such requests as if they had been originally submitted by NMPNS.
Accordingly, the staff continued its review of the subject submittal.
The NRC staff completed its review of the subject submittal and found that the flaw evaluation meets the intent of the rules in Section XI of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code). Since the safety factors associated with the detected cracks are greater than 2.77 specified in the ASME Code, the NRC staff concludes that NMP2 can be operated without repair of weld H4 for two fuel cycles after RFO7. The staffs review results are set forth in the enclosed safety evaluation.
Sincerely,
/RA/
Peter S. Tam, Senior Project Manager,Section I Project Directorate I Division of Licensing Project Management Office of Nuclear Reactor Regulation Docket No. 50-410
Enclosure:
Safety Evaluation cc w/encl: See next page Mr. John T. Conway January 16, 2002
ML020100071 OFFICE PDI-1\\PM PDI-1\\LA PDI-1\\SC NAME PTam SLittle**
LRaghavan*
DATE 1/11/02 1/11/02 1/11/02
DATED: January 16, 2002 NINE MILE POINT UNIT 2 - REINSPECTION PLAN FOR CORE SHROUD WELD H4 PUBLIC PDI R/F JZwolinski\\TMarsh SLittle PTam Acting Section Chief OGC ACRS PD plant-specific file BPlatchek, RI SSheng WKoo LLois cc: Plant Service list
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REEVALUATION OF CORE SHROUD WELD H4 NINE MILE POINT NUCLEAR STATION, LLC NINE MILE POINT NUCLEAR STATION, UNIT 2 (NMP2)
DOCKET NO. 50-410
1.0 INTRODUCTION
By letter dated October 30, 2000, the NRC staff conveyed to Niagara Mohawk Power Corporation (NMPC) its review results of core shroud ultrasonic reexamination performed during NMP2s Refueling Outage 7 (RFO7). The NRC staff concluded then that the unit can be operated without repairing weld H4 for one fuel cycle based on a bounding crack growth rate (CGR) of 5x10-5 inch/hour. By letter dated September 14, 2001, NMPC submitted its reevaluation of core shroud weld H4, taking into account benefits from the implementation of noble metal chemistry application (NMCA) and hydrogen water chemistry (HWC). NMPC intended to demonstrate, through a revised analytical flaw evaluation using modified assumptions on crack geometry and modified CGRs, that NMP2 could be operated without repair of weld H4 for two fuel cycles of operation after RFO7 (i.e., without reinspection during RFO8 ).
On November 7, 2001, NMPCs ownership interest and operating license in NMP2 were transferred to Nine Mile Point Nuclear Station, LLC (NMPNS), thus allowing NMPNS to possess, use and operate NMP2. By letter dated November 20, 2001, NMPNS requested that the NRC continue to review and act on all requests previously submitted by NMPC before the transfer, and to consider such requests as if they had been originally submitted by NMPNS.
Accordingly, the staff continued its review of the subject submittal.
2.0 EVALUATION 2.1 Licensees Evaluation 2.1.1 Crack Growth Rate The licensee used a CGR of 2.2x10-5 inch/hour for Cycle 8 to reflect water chemisty with a conductivity of 0.15 µS/cm and an electrochemical potential of +200 mV. Since NMCA and hydrogen injection were implemented at NMP2 during fuel Cycle 8, the licensee used a reduced CGR of 1.1x10-5 inch/hour for Cycle 9, as permitted by an NRC safety evaluation (SE) dated December 3, 1999 (see Reference 3). For the length direction, however, the licensee used the bounding CGR of 5.0x10-5 inch/hour for both cycles.
In the subsequent flaw evaluation for weld H4, the licensee employed its distributed ligament length (DLL) flaw evaluation methodology (BWRVIP-20) to conduct the evaluation. The DLL Enclosure methodology is capable of analyzing a core shroud weld with multiple flaws using either the limit load or the linear elastic fracture mechanics (LEFM) analysis. Since the predicted fluence at the end of Cycle 9 for weld H4 exceeds the LEFM threshold fluence of 3.0x1020 n/cm2 as specified in Topical Report BWRVIP-01, Revision 1 (dated March 1995, and also referred to as GENE-523-113-0894, Rev. 1, BWR Core Shroud Inspection and Evaluation Guidelines; approved by the NRC staff on June 16, 1995), the licensee has performed both limit load and LEFM evaluations for weld H4.
2.1.2 Flaw Evaluation - Limit Load Analysis In its limit load analysis, the licensee assumed through-wall flaws for regions where the inside diameter (ID) surface fluence at the end of Cycle 9 exceeds 3.0x1020 n/cm2. The initial crack depth in all other regions with detected cracks or regions with uninspected cracks was assumed to be the maximum detected flaw depth of 0.78 inch. Using this initial crack depth and the above-mentioned CGRs for Cycle 8 and Cycle 9 for 16,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> each, the licensee estimated that the final crack depth is 1.42 inches at the end of Cycle 9. This estimate included a ultrasonic testing (UT) uncertainty of 0.108 inch. The final crack lengths between flaws were calculated internally by the DLL program in a similar manner. The results indicated that the safety factor at the most limiting location of weld H4 is 4.98 based on a stress intensity, Sm, of 14.4 ksi and primary membrane and bending stresses of 0.328 ksi and 1.19 ksi for the normal and upset loading condition. Since the calculated safety factor is greater than the Code-specified safety factor of 2.77, the licensee concludes that the structural margin requirements at weld H4 are satisfied through the end of Cycle 9.
2.1.3 Flaw Evaluation - LEFM In the LEFM analysis, the licensee assumed essentially the same multiple crack geometry that was used in the limit load analysis except that the through-wall regions now correspond to locations with ID surface fluence at the end of Cycle 9 exceeding 5.0x1020 n/cm2. The final crack geometry was developed using the same crack growth approach as described in Section 2.1.2 above. The licensees LEFM methodology was based on a fracture mechanics model of multiple flaws with uniform depth, which is consistent with that in a previous submittal dated July 9, 1998, for the evaluation of the core shroud examination results of RFO6. The LEFM results indicated that the calculated safety factor was 3.05 for a KIc value of 150 ksi(in)1/2, exceeding the Code-specified value of 2.77. Based on the results from the limit load and the LEFM analyses, the licensee concludes that weld H4 meets the structural margin requirements for continued operation after RFO7 for two fuel cycles. To demonstrate that weld H4 has more margin than that was indicated by the LEFM results, the licensee performed an additional analysis based on elastic-plastic fracture mechanics (EPFM), and found that the corresponding safety factor for EPFM was 4.0.
2.1.4 Neutron Fluence (See NRC staffs evaluation in Section 2.2.4) 2.2 NRC Staffs Evaluation 2.2.1 Crack Growth Rate The licensees CGR of 2.2x10-5 inch/hour for Cycle 8, reflecting water chemistry with a conductivity of 0.15 µS/cm and an electrochemical potential of +200 mV, and CGR of 1.1x10-5 inch/hour for Cycle 9, reflecting the implementation of NMCA and hydrogen injection, are acceptable because they are in accordance with the staffs SE on Topical Report BWRVIP-14 (see Reference 3). According to this SE, the above-mentioned CGRs are only applicable to components which had experienced fluences less than 5.0x1020 n/cm2. The licensee simplified the weld H4 crack geometries for the limit load and LEFM analyses so that these CGRs could be applied. For the length direction, it is conservative to use the bounding CGR of 5.0x10-5 inch/hour for both cycles.
In order to use the reduced bounding crack growth rates for flaw evaluation, certain conditions must be met. To use the CGR of 2.2 x 10-5 inch/hour, the necessary conditions are specified in BWRVIP-14 and the staffs SE (Reference 3). The use of a CGR of 1.1 x 10-5 inch/hour is allowed when an effective HWC or NMCA program is maintained. The criteria for an effective HWC or NMCA program are identified in Open Item 3.2 of the staffs SE dated January 30, 2001 (Reference 4). The staffs conclusion is based on the assumption that these conditions or criteria are met, especially that the measurements would indicate that the electrochemical potential at the weld H4 location is below -230 mV, or the hydrogen vs. oxygen molar ratio is 4:1 and above.
2.2.2 Flaw Evaluation - Limit Load Analysis By assuming through-wall flaws for regions where the ID surface fluence of weld H4 at the end of Cycle 9 exceeds 3.0x1020 n/cm2, the licensees approach satisfied not only the fluence condition for using the reduced CGRs, but also the condition for employing the limit load analysis to assess the structural integrity weld H4. Further, the licensees approach of considering the uninspected regions as flaws of the maximum detected flaw depth is appropriate. The staffs SE dated August 20, 2001 (see Reference 5) accepted this approach for plants with normal water chemistry. Although Topical Report BWRVIP-63 only discussed axial welds, the staff determined that this approach applies to circumferential welds also, because the technical basis for the staffs prior approval for the axial welds is independent of the crack orientation, and the stresses due to pressure are much less for the circumferential welds. The rest of the licensees limit load analysis is similar to that in the previous submittal and is similarly acceptable. Since the calculated safety factor is greater than the Code-specified safety factor of 2.77, the staff agrees with the licensees conclusion that the structural margin requirements at weld H4 are satisfied through the end of Cycle 9.
2.2.3 Flaw Evaluation - LEFM In the LEFM analysis, the licensee assumed essentially the same multiple crack geometry that was used in the limit load analysis except that the through-wall regions now correspond to locations with ID surface fluence at the end of Cycle 9 exceeding 5.0x1020 n/cm2. By assuming through-wall flaws for regions where the ID surface fluence of weld H4 at the end of Cycle 9 exceeds 5.0x1020 n/cm2, the licensees approach satisfied the fluence condition for using the reduced CGRs. It should be mentioned that this assumed geometry contains regions of fluences (at the end of Cycle 9) greater than 3.0x1020 n/cm2, and, therefore, triggered the licensee to perform an evaluation using LEFM. The licensees LEFM methodology has already been reviewed and accepted by the staff as indicated in the SE dated October 30, 2000. Since the current LEFM results indicated that the calculated safety factor of 3.05 exceeds the Code-specified value of 2.77, the staff concludes that weld H4 meets the structural margin requirements for continued operation of NMP2 after refueling outages for two fuel cycles. The licensees EPFM results, which show additional margin, are another positive factor to the staffs determination.
2.2.4 Neutron Fluence The structural strength of the remaining ligament in weld H4 is a function of neutron fluence.
The staff reviewed the acceptability of the azimuthal variation of the proposed fluence value as measured in the 3-degree surveillance capsule and calculated in Reference 7.
Two types of dosimeters were used by the licensee: Fe-54 (n, p) Mn-54 and Cu-63 (n, a) Co-
- 60. In addition, for verification, a scrapping from a Charpy test bar was also used. The samples were weighed, counted and handled in a standard manner. The licensees transport calculation used the DORT Code (Reference 8) and the BUGLE-96 cross section library (Reference 9). Neutron scattering was treated with a P3 approximation and angular discretization was modeled with an S8 approximation.
(R, ) and (r, z) calculations were performed for an eighth geometry which included the 3-degree capsule and the jet-pump and pump risers in the downcomer. The calculation extended through the vessel thickness to the reactor cavity. The water void fraction was input in segments as a function of (r,, z). The Code, the cross sections, the approximations and the number of mesh points satisfy the guidance in Regulatory Guide (RG) 1.190. The results of the calculation for the capsule center flux (E > 1.0 MeV) is 2.7x108 n/cm2 s. This value compared favorably with similar calculations performed by others.
The licensees dosimeter results appear inconsistent (Reference 7 states that the position of the dosimeters within the capsule was uncertain). Assumptions of the possible locations yield calculated/experimental activation values within and outside the 20 percent value stated in RG 1.190. To obtain more information, a scrapping was obtained from a Charpy sample, but the axial location of the Charpy sample and the location of the scrapping was also uncertain. While the mean values are within reasonable limits of each other, the uncertainty in the location of the dosimeters, the scrapping, and the Charpy sample makes it difficult to establish the value of the uncertainty derived from the measured dosimetry. For the purposes of the crack growth rate, the licensee states that a 2 uncertainty is added to the calculated fluence value for the end of Cycle 9. In addition, the Cycle 7 contribution to the fluence was calculated for a cycle length of 664.3 effective full-power days (EFPDs) while the actual run was about 545 EFPD.
Comparing the fluence conservatism with the addition of the 2 (about 32%) and the Cycle 7 short run to the results of the dosimetry, the staff concludes that the values of the azimuthal fluence distribution used for the core shroud weld H4 structural analysis are conservative and, thus, are acceptable.
3.0 CONCLUSION
The NRC staff has completed its review and found that the licensees flaw evaluation meets the intent of the rules in Section XI of the ASME Code. Since the safety factors associated with the detected cracks are greater than 2.77 specified in the ASME Code, the staff concludes that NMP2 can be operated without repair of the H4 weld for two fuel cycles after RFO7. The staffs conclusion is based on the assumption that the conditions or criteria mentioned in Section 2.2.1 are met, especially that the measurements would indicate that the electrochemical potential at the weld H4 location is below -230 mV, or the hydrogen vs. oxygen molar ratio is 4:1 and above.
4.0 REFERENCES
1.
Letter from R. B. Abbott of NMPC to NRC, Reinspection Plan for the Core Shroud Weld H4," dated September 14, 2001.
2.
Letter from P. Tam of NRC to J. H. Mueller of NMPC, Nine Mile Point Nuclear Station Unit No. 2 - Safety Evaluation of Core Shroud Inspection Results (TAC No. MA9057), dated October 30, 2000.
3.
TR-105873, BWR Vessel and Internals Project, Evaluation of Crack Growth in BWR Stainless Steel Internals (BWRVIP-14), issued by Electric Power Research Institute (EPRI), March 1996. (NRC staff safety evaluation dated December 3, 1999).
4.
TR-108705, BWR Vessel and Internals Project, Technical Basis for Inspection Relief for BWR Internal Components with Hydrogen Injection (BWRVIP-62), issued by EPRI, December 1998. (NRC staff safety evaluation dated January 30, 2001).
5.
TR-131170, BWR Vessel and Internal Project, Shroud Vertical Weld Inspection and Evaluation Guidelines (BWRVIP-63), issued by EPRI, June 1999. (NRC staff safety evaluation dated, August 20, 2001).
6.
TR-114232 BWR Core Shroud Inspection and Flaw Evaluation Guidelines, issued by EPRI, November 1999.
7.
Letter from R. B. Abbott of NMPC to NRC, dated March 8, 2001, transmitting Topical Report MPM-1200676 Nine Mile Point Unit 2, 3-Degree Surveillance Capsule Report MPM Technologies, Inc., dated December 2000.
8.
TORT-DORT-PC, Two-and Three-Dimensional Discrete Ordinates Transport Version 2.7.3, " Radiation Safety Information Computation Center, Computer Code Collection CCC-543, Oak Ridge National Laboratory, Oak Ridge TN, June 1996.
9.
BUGLE-96, Coupled 47 Neutron, 20 Gamma Ray Group Cross Section Library Derived from ENDF/B-VI for LWR shielding and Pressure Vessel Dosimetry Applications, Radiation Safety Information Computation Center, DLC-185, Oak Ridge National Laboratory, Oak Ridge TN, March 1996.
Principal Contributors: S. Sheng W. Koo L. Lois Date: January 16, 2002
Nine Mile Point Nuclear Station Unit No. 2 cc:
Regional Administrator, Region I U. S. Nuclear Regulatory Commission 475 Allendale Road King of Prussia, PA 19406 Resident Inspector Nine Mile Point Nuclear Station P.O. Box 126 Lycoming, NY 13093 Mr. John V. Vinquist, MATS Inc.
P.O. Box 63 Lycoming, NY 13093 Supervisor Town of Scriba Route 8, Box 382 Oswego, NY 13126 Mr. Paul D. Eddy Electric Division NYS Department of Public Service Agency Building 3 Empire State Plaza Albany, NY 12223 Charles Donaldson, Esquire Assistant Attorney General New York Department of Law 120 Broadway New York, NY 10271 C. Adrienne Rhodes Chairman and Executive Director State Consumer Protection Board 5 Empire State Plaza, Suite 2101 Albany, NY 12223-1556 Mark J. Wetterhahn, Esquire Winston & Strawn 1400 L Street, NW.
Washington, DC 20005-3502 Mr. Robert E. Denton President Nine Mile Point Nuclear Station, LLC 1997 Annapolis Exchange Parkway Suite 500 Annapolis, MD 21401 Mr. Raymond L. Wenderlich Senior Constellation Nuclear Officer Responsible for Nine Mile Point Nine Mile Point Nuclear Station, LLC P.O. Box 63 Lycoming, NY 13093 James M. Petro, Jr., Esquire Counsel Constellation Power Source, Inc.
111 Market Place Suite 500 Baltimore, MD 21202 Mr. William M. Flynn, President New York State Energy, Research, and Development Authority Corporate Plaza West 286 Washington Avenue Extension Albany, NY 12203-6399