ML20199A701
| ML20199A701 | |
| Person / Time | |
|---|---|
| Site: | Byron, Braidwood  |
| Issue date: | 01/16/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20199A683 | List: |
| References | |
| NUDOCS 9801280060 | |
| Download: ML20199A701 (5) | |
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4 UNITED STATES 1
- NUCLEAR RESULATORY COMMIS810N -
WASHINOToN, D.c. 2006 4001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO EXEMPTION FROM REQUIREMENTS OF 10 CFR 50.tl0 COMMONWEALTH EDISON COMPAtg BYRON STATION. UNITS 1 AND 2. AND BRAIDWOOD STATION. UNITS 1 AND 2 DOCKET NOS, STN 50454. STN 50-455, STN 50 456 AND STN 50-457
1.0 INTRODUCTION
By letter dated April 3,1997, as supplemented by letter dated June 19,1997, Commonwealth Edison Company (Comed), the licensee for Byron Station, Units 1 and 2, and Braidwood Station, -
Units 1 and 2, requested that the NRC exempt these units from the application of the 1989 Edition of the American Society for Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code),Section XI, Appendix G (1989 methodology) as required by Title 10 of the Code of Federal Regulations, Part 50 (10 CFR Part 50) Section 55a, Section 60, and Appendix G. As an attemative, Comed proposed to use the version of ASME Code,Section X!, Appendix G, found in the 1996 Addenda to the ASME Code (1996 methodology). When compared to the 1989 methodoiogy, the 1996 methodology would permit Comed to usw lower stress intensity factors for determining the applied stress intensity from pressure and thermal stresses and to set the low-temperature overpressure protection (LTOP) system pressure setpoint so that system pressure does not exceed 110 percent of that required by the pressure-temperature (P-T) limits.
2.0 Ll0ENSEE'S DETERMINATION The NRC has established requirements in 10 CFR Part 50 to protect the integrity of the reactor coolant systr.1 pressure txmdary. As one of the requirements,10 CFR Part 50, Appendix G, requires that P-T limits be it etablished for reactor pressure vessels (RPVs) during normal operation and vessel hydrostatic testing. In particular,10 CFR Part 50, Appendix G, Section IV.2.o., requires that these limits must be "at least as conservative as limits obtained by following the methods of analysis and the margins of safety of Appendix G of Section XI of the ASME Codec' 10 CFR 50.55a requires that any reference to the ASME Code,Section XI, in 10 CFR Part 50, Appendix G,' refers to the 1989 Edition of the Code unless otherwise noted.
10 CFR 50.60, which broadly addresses the establishment of criteria for fracture prevention, states that " proposed altomatives to the described requirements in Appendices G and H of this part or portions thereof may be used when an exemption is granted by the Commission under
$50.12." Therefore, Comed determined that application of the 1996 methodology in lieu of the 1969 methodology approved by the staff in the regulations would require en exempticn.
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In Comed's initial letter, this exemption was requested under the special circumstances given in 10 CFR 50.12(a)(2)(iii). The provisions of this section state that an exemption may be granted when " compliance (with the regulation) would result in undue hardship or other costs that are significantly in excess of those contemplated when the regulation was adopted, or that are ENCLOSURE 2 9801290060 990116 PDR ADOCK 05000454 P
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1 2-l sigrdficantly in excess of those incurred by others similarty situated...." Comed contended that application of the 1969 methodology would result in undue hardship because of a narrowing of
- the cperational window betwoon the vessel P-T limits or LTOP system setpoint and the minimum reactor coolant system pressure required by reactor coolant pump operation. The NRC staff,
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although noting that this reduction in operational flexibility was real,' did not concur that such a i.
reduchon constitutes an undue hardship for these particular facilities. The staff based this finding on the fact that the RPV materials for these units exhibit a relatively low nil-ductility reference temperature (RT,or) value through end-of-license when compared with other "similarly situated" licensees.: A higher RT,er value results in more restrictive vessel P-T limits. Also, no attempt was made in the initial submittal to quantify whether the costs were "significantly in
- exuoss of inose contemplated when the regulation was adopted...."
c By letter dated June 19,1997, Comed amended its application to cite 10 CFR 5012(a)(2)(ii) as the special circumstance for requesting this exemption.10 CFR 50.12(a)(2)(ii) states that special circumstances are present whenever " application of the regulation injoe particular j
F circumstances...is not necessary to achieve the undertying purpose of the rule." Comed explained that "the 1996 Addenda.... solutions better characterize the conditions for irradiated vessels in tne low temperature region where the thermal stresses ar,d allowable pressure are low." Comed alsc noted that conservatisms beorporated into ASME Code Section XI, Appendix G,' include: (1) the 6:1 aspect ratio one-quarter of :he vessel wall thickness flaw, (2) a factor of 2 on the membrane stress intensity factor, (3) the determination of material toughness from a reference curve based on dynamic and crack arrest data, and (4) margins on the materials' adjusted reference temperature based on Regulatory Guide 1.99, Revision 2, remain intact in the 1996 methodology. Therefore, Comed concluded that application of the 1996 methodology would als<. meet the underlying intent of the regulations--namely, to protect the
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integetty of the RPV from nonductile failure.
3.0 STAFF EVALUATION
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Initiallyithe staff examined the regulatory bases for Comed's axemptien request. The staff agreed with Comed's determination that an exemption from 10 CFR Part 50, Appendix G, would be required for applying the 1996 methodology, since it would likely produce results less conservative than the 1989 methodology. Further, the staff examined Comed's rationale to support the exemption request and the staff concurred that an examination of this alterr.. ave method should demonstrate that application of the 1996 methodology would also meet the underlying intent of the regulations. Therefore, requesting the exemption under the special c!rcumstances of 10 CFR 50.12(a)(2)(ii) was appropriate. The staff then examined the 1996 methodology and the bases for the changes made in the ASME Code to confirm whether or not its application would meet the undertying intent of the regulations.
To begin, the staff compared critical features of tne 1989 and 1996 methodologies-using parameter values that were expected to be approximately correct for ths Byron and Braidwood RPVs-in order to examine the magnitude of the changas to be expected by use of the 1996 methodology. The staff intends to review the application of the 1996 methodology in detail when Comed submits it as part of an update of the facilities' P-T limits. The differences in the two methodologies are apparent in the determination of the stress intensity factor multiplier, M., the
_ formulations for determining the contribution of the thermal stress intensity term, K., from the cooldown rate or temperature gradient, and the provision for setting the LTOP system pressure -
setpoint so that system pressure does not exceed 110 percer,; of the pressure required by the P-T limit curves. However, this provision on the LTOP system pressure setpoint was previously m
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approved for Byron and Braidwood through the NRC's approval of Comed's use of ASME Code
- Case N-514. Therefore, although it represents a change from the 1989 methodology, it is not a i
i change from the current licensing basis for the facilities and will not be addressed further in this
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safety evaluation.
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- stress intensity factor multiplier, %. For a rypical-to-conservative applied stress-to yield stress The staff first examined the effect of the difference in the way the methodologies determine the f
ratio of 0.7 and a vessel well thickness of 8.5 inches (typical of the Byron and Braidwood t
vessels), Figure G-22141 in the 1989 Code gave a value of M,,, = 2.87; the calculational methods of the 1996 Addenda gave M,,, = 2.70. This change would result in approximately a l
6 percent reduction in the applied stress intensity and a corresponding shift in the allowable l:
pressure at a given temperature in the nwoonservative direction. The staff has concluded that L
this difference is minor when compared to the explicit conservatisms incorporated into Appendix G of the ASME Code,Section XI, (as listed in Sec4on 2.0 of this SE) and that the
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chan9es in the M,,, factor are supported by the work performed by J.A. Keeney and T.L. Dickson J
at Oak Ridge National Laboratory (ORNL) for the NRC (Reference 1), A. Zahoor (Reference 2),
and 1.S. Raju and J.C. Newman (Reference 3),
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The at aff then examined the changes to the determination of K, incorporated into the 1996 1
l nk*thodology. The staff limited its examination of this issue to cooldown transients, in which 1
F significant tensile thermal stresses can be developed on the inside diameter of the RPV. As noted in Standard Review Plan Section 5.3.2, " Pressure-Temperature Limits," the staff has j
approved the use of the information and methodology from Welding Research Council (WRC) 4
- Bulletin 175 for the evaluation of applied ther nal stress intensities due to various cooldown rates.
The staff determined in this review that within the uncertainties in using the graphs from WRC L
Bulletin 175, the functional form given for K, in G-2214.3(a) for the cooldown transient is an -
equivalent methodology.
The staff also accepts that the additional metiiodology for the heatup transient K, and the altemate methodology making use of the detailed thermal stress distribution given in G-2214.3(b) are supported by the work of J.A. Keeney and T.L. Dickson for the NPC (Reference 1), A. Zahoor (Reference 2), and I.S. Raju and J.C. Newman (Reference 3). The staff, therefore, concludes l
. that the methodology given in the 1996 Add:nda is acceptable for catermining K. However, the staff must still review and approve details regarding the application of the 1996 methodology (for example, the method chosen for determining thermal stresses as an input to the G-2214.3(b) procedure, if used). These details will be reviewed along with the updated P T limits or with the Pressure-Temperature Limits Report (PTLR) that incorporates the 1996 msthodology into the a
facilities' licensing basis, f
4.0 CONCLUSION
The staff, having been involved in the consensus body development of the 1996 Addenda to Section X! of the ASME Code and having reviewed the major changes Letween the 1989 S4.ction
- XI, Appendix G, and 1996 Section XI, Appendix G, methodologies for this review, concludes that
.the use of the 1996 methodology would meet the underlying intent of 10 CFR 50.60 and 10 CFR
- Part 50, Appendix G,'and is, therefore, acceptable. The staff accepts that the explicit cor'servatism incorporated within the 1996 Addenda to the ASME Code,Section XI, Appendix G methodology will ensure that the RPV for which this methodology is used will be protected from non-ductile fa' lure. The staff further concludes tSat, since application of the 1989 methodology i
i poses no' undue hardship for Byron and Braidwood, special circumstances as defined in 10 CFR K
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4 50.12(a)(2)(ii) exist. The staff has reviewed Comed's request and approves the use of the 1996 rnethodology in lieu of the 1989 methodology currenty required in 10 CFR 50.60 at Byron and Braidwood.
Principal Contributoc M. Mitchell Date:
January 16, 1998 2
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-5 5,0 REFERENCES
- 1. J.A. Keeney and T.L mckson, " Stress-Intensity-Factor Innuence Cc. hts for Axially Oriented Semielhptical inner-Surface Flaws in Clad Pressure Vessels (R/t=10),"
ORNUNRC/LTR 93/33, Revision 1. Septwnber 30,1995.
- 2. A. Zahoor, Ductile Fracture Handbook. Volume 3, put,lishwd jointly by EPRI (NP 6301-D) and Novatech (N14-3), January 1991.
- 3. l.C. Raju and J.C. Newman, Jr., " Stress intensity Factors for intomal and Extemal Surface Cracks in Cylindrical Vessels," Joumal of Pressure Vessel Technoloov. Volume 104, pp 283-288, November 1982.
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