ML20112B074
| ML20112B074 | |
| Person / Time | |
|---|---|
| Site: | Farley  |
| Issue date: | 05/20/1996 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20112B070 | List: |
| References | |
| NUDOCS 9605220152 | |
| Download: ML20112B074 (7) | |
Text
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UNITED STATES y
NUCLEAR REGULATORY COMMISSION f
WASHINGTON, D.C. 200eH001
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SAFETY EVALVATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 110 TO FACILITY OPERATING LICENSE NO. NPF-8 SOUTHERN NUCLEAR OPERATING COMPANY. INC.
JOSEPH M. FARLEY NUCLEAR PLANT. UNIT 2 DOCKET NO. 50-364
1.0 INTRODUCTION
By letter dated April 23, 1996, the Southern Nuclear Operating Company, Inc.,
et al. (the licensee), submitted a request for changes to the Joseph M. Farley Nuclear Plant, Unit 2, Technical Specifications (TS).
The requested changes would allow steam generator tubes to remain in service with bands of axial degradation in the tube sheet region, for the remainder of Cycle 11, provided sufficient non-degraded tubing remains to satisfy the L*-type criteria restrictions established by the licensee.
2.0 BACKGROUND
Surveillance Requirement 4.4.6.4.11 of TS 3/4.4.6, "F" Distance,*" requires a tube to be repaired if this F* distance is not satisfied.
The F distance is the distance of the expanded portion of a steam generator tube which provides a sufficient length of undegraded tube expansion to resist pullout of the tube from the tubesheet.
For Farley, this distance equals to 1.79 inches and is measured down from the top of the tubesheet or the bottom of the roll transition, whichever is lower in elevation. As stated in TS 4.4.6.4.12, an F tube is a tube that (1) has degradation equal to or greater than 40 percent below the F* distance, (2) has no indication of greater than or equal to 20 percent of nominal wall thickness within the F* distance, and (3) remains in service. The licensee reported that as a result of the misapplication of the TS requirements, six tubes were left in service that have indications that do not satisfy the above F criteria.
By a letter dated April 23, 1996, the licensee requested that the NRC exercise discretion not to enforce compliance with the actions required in the TS on the basis of proposed L*-type criteria.
The L*-type criteria would allow those tubes that do not satisfy the F criteria to remain in service.
In a second letter dated April 23, 1996, the licensee submitted a proposed TS change that utilized an L*-type criteria that is limited to the remainder of Cycle 11 which is scheduled to end by the fall of 1996.
The L*-type criteria are more restrictive and conservative than the more generic L criteria proposed TS change that was submitted by the licensee in a letter dated April 22, 1996.
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On April 22, 1996, the staff verbally approved, and by a letter dated April 25, 1996, the staff officially notified tne licensee,that it was l
exercising discretion not to enforce compliance with the F criteria for the l
ppriod from April 23, 1996, until the issuance of a license amendment for the L -type criteria.
Farley Unit 2 uses three Westinghouse 51 Series steam generators.
The tubes l
are hard rolled with full expansion in the tubesheet. The tubes have an i
outside digmeter of 7/8 inch with a wall thickness of 0.050 inch.. The l
proposed L -type criteria defines the L* length as the length of the expanded portion of the tube into the tubesheet from the bottom of the rolled transition or the top of the tubesheet, whichever is lower, and has been determined to be 1.45 inches.
For Farley Unit 2, the proposed TS L*-type criteria define the L* tube as (1) a tube with degradatipn equal or greater than 40% below the L* length and not degraded within the degradation below the L,L length; (2) the eddy current indication of length must be determined to be the result of cracks with an orientation no greater than 15 degrees from axial; (3) the L criteria l
shallbelimiteptoamaximumof600tubeendspersteamgenerator;(4) tubes qualifying as F tubes are not classified as L* tubes; (5) a minimum of 3.1 inches of the tube into the tubesheet from the top of tubesheet or bottom of the rolled transition, whichever is lower, shall be inspected using rotating pancake coil eddy current technique or an inspection method shown to give equivalent or better information on the orientation and length of axial cracks; (6) a minimum aggregate of 2.07 inches of sound roll expansion; (7) a maximum crack length of 0.39 inches; and (8) a maximum of 5 distinct indications with a' single band of tube degradation.
3.0 EVALUATION 3.1 Discussion General Design Criterion (GDC) 14 of Appendix A to 10 CFR Part 50 requires that the reactor coolant pressure boundary be designed, fabricated, erected, and tested so as to have an extremely low probability of abnormal leakage, of rapidly propagating failure, and of gross rupture.
Draft Regulatory Guide (RG) 1.121 provides guidance on an acceptable method for establishing repair limits for steam generator tubes.
Surveillance requirements in the plant TS require periodic inspections of steam generator tubes.
If a tube is found to have degradation in excess of the plugging limits (i.e., 40 percent through-wall), it is required to be repaired or removed from service.
Steam generator tubes comprise a significant portion of the reactor coolant pressure boundary. Maintenance of this boundary is provided by the integrity of the steam generator tube wall and tube-to-tubesheet joint.
The joint between the tube and tubesheet is an interference fit constructed by roll expanding the tube into the bore of the tubesheet.
The tubes are inserted
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into the bore of the tubesheet followed by seal welding at the primary face of the tubesheet. Step rolls are then performed on the tube to fully expand the tube in the bore of the tubesheet. The tubes are restrained in the bore of l
the tubesheet by the elastic springback of the tubesheet.
The tube-to-l tubesheet joint provides sufficient strength to maintain adequate structural and pressure boundary integrity.
l The industry experience has shown that defects have developed in the tube-to-tubesheet joints by various degradation processes. The staff believes that some tubes having degradation in the joint may remain in service because the i
reinforcing effect of the tubesheet on the undegraded portion of the tube would maintain structural and leakage integrity. However, an undegraded portion of the tube must be available to maintain adequate structural and leakage integrity under loadings from normal operation, anticipated t
operational occurrences, and postulated accident conditions.
RG 1.121 recommends that the margin of safety against tube rupture under normal i
operating conditions should not be less than three for any tube location where defects have been detected. For postulated accidents, RG 1.121 recommends that the margin of safety against tube rupture be consistent with the margin of safety determined by the stress limits specified in NB-3225 of Section III of the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME).
l The staff has previously approved two alternative tube repair criteria, F* and i
L, at other PWR facilities to allow tubes with degradation within the tubesheet to remain in service provided the, degradation within the tubes has been shown to meet certain criteria. The F criteria allow degradeu tubes to remain in service when defects exist at a specified distance below the bottom of the roll transition.
The L* criteria permit certain defe:ts to exist within the F* distance provided additional conditions are satisfied.
Structural loads imposed on the tube-to-tubesheet joint primarily result from the differential pressure between the primary and secondary sides of the l
tubes.
The peak postulated loading occurs during a main steamline break due l
to a lowering of the secondary side pressure. However, normal operating I
loads, cyclic loading from transients (e.g., startup/ shutdown), and potential thermal expansion loads can also be significant.
The analysis supporting these alternative criteria must address all loading conditions necessary to maintain adequate integrity af the tube-to-tubesheet joint.
The elastic preload between the tube and tubesheet not only prevents pullout of the tube from the tubesheet, but also provides a leak tight barrier l
minimizing the potential for primary to secondary coolant leakage. With sufficient length of undegraded hardroll, the tube-to-tubesheet joint will not allow any leakage under normal and faulted conditions. Under the proposed L -
type criteria, the licensee has to demonstrate that leakage integrity of the l
joint is maintained under all analyzed conditions.
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l W;w i f l' 3.2 Structural ~Intearity of the L*-Tvoe Criteria The licensee developed the L*-type criteria for the 7/8-inch diameter tubes in the Farley steam generators using a combination of analyses and adjustment of test data from an L program that was developed for 3/4-inch diameter tubes.
The licensee considered various loads that would affect the structural integrity of the degraded tubes.
The primary load considered was the maximum pullout loads under the normal operation and faulted-(feed line break) conditions. The licensee applied a safety factor of 3 as recommended by RG 1.121 to the axial load under the normal operating conditions. A safety factor.of 1.43 was applied to the axial load for the feed line break case.
The licensee also considered axial compression loads caused by large break loss-of-coolant accident and bending and torsion loads about the tube vertical axis. The licensee stated that significant margin exists between tube collapse strength and the limiting secondary-to-primary pressure differential.
The licensee used appropriate safety factors recommended by RG 1.121 in determining the pullout load of the degraded tubes.-
In addition, the F* criteria approved for Farley Unit 2 was developed on the assumption that the portion of the tube below the F* distange does not contribute to the tube-to-tubesheet joint integrity. The L -type criteria allow tubes with a small number of short predominantly axial cracks to remain in service provided a minimum aggregate of approximately 2 inches of sound j
roll existed below the bottom of the roll transition. This limited degradation would be expected to have an insignificant effect on the structural integrity of the joint within the inspected region.
Based.on this criteria in comparison with the staff's detailed review and evaluation of the structural integrity tests performed for the F criteria, the staff concludes that the L,-type criteria for Farley Unit 2 provide adequate assurance of structural integrity.
3.3 Leakaae Intearity of the L*-Tvoe Criteria The licensee stated that the L* 1ength of the sound roll expansion is suffipient to preclude significant leakage from tube degradation located below the L distance. The existing TS leak rate requirements and accident analysis.
assumption; tubes does occur. remain unchanged in the unlikely event that significant leak from the L Any leakage from the tube within the tubesheet at any elevation in the tubesheet is fully bounded by the existing steam generator tube rupture analysis included in the Farley Nuclear Plant, Final Safety Analysis Report. A conservative leakage allowance for each L tube is provided to determine the impact of the L*-type criteria on offsite radiological doses in the event of a postulated double ended guillotine break of the main steamline outside of containment. The licensee determined that the primary-to-secondary leakage will result in offsite doses that are a small fractionofthe10CFRPart100 guidelines. The licensee stated that the j
proposed L criteria do not adversely impact any other previously evaluated l
design basis accident.
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- The staff has reviewed F* 1eakage testing data from steam generator tubes with undegraded hardroll which indicated no leakage occurred.
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undegraded hardroll exists in the six tubes that the proposed L -type criteria are being applied to.
Based on this comparison, for the degradation in the six tubes govered by this amendment, any leakage resulting from the use of proposed L -type criteria is acceptable.
The staff concludes that the proposed L*-type criteria are acceptable for l
Farley Unit 2 for the limited number of tubes affected bayed on the adequacy of the structural and leakage integrity provided by the L -type criteria.
Thereforethestaffhasdeterminedthatthelicensee'sproposedTSchangethat incorporates the L -type criteria into the Farley Unit 2 TS for the duration of the present operating cycle is acceptable.
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4.0 EXIGENT CIRCUMSTANCES
l The Commission's regulations, 10 CFR 50.91(a)(6), contain provisions for issuance of amendments when the usual 30-day public notice period cannot be met. One type of special exception is an exigency. Under.such circumstances,
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the Commission notifies the public in one of two ways:
by issuing a Federal' Reaister notice providing an opportunity for hearing and allowing at least 2 weeks for prior public comments, or by issuing a press release discussing the proposed changes, using local media.
In this case, the Commission used the first approach.
l The licensee, on April 22, 1996, identified six tubes that were left in service which did not satisfy,the. F* distance requirements of TS.4.4.6.4.11.
.Non-compliance with this TS required the licensee to declare the steam l
generators inoperable and to take action within one hour to be in hot standby l
within the next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> in accordance with TS 3.0.3.
The licensee promptly l
notified the NRC and requested that the NRC exercise its enforcement-l discretion. The licensee proposed a revision to-the TS, as stated in its April 23, 1996, submittal, which, if granted, would bring the plant into compliance with its TS for the remainder of the operating cycle.
Based on the information provided by the licensee, the staff concluded that continued operation was acceptable and granted a Notice of Enforcement Discretion (N0ED) to avoid an undesirable transient as a result of forcing compliance with the TS. The NOED was granted until the prompt issuance of the proposed license amendment.
Accordingly, pursuant to 10 CFR 50.91(a)(6), the Commission has determined that an exigent situation exists in which it must act before~the expiration of the 30-day comment period to bring the plant into compliance with its TSs.
5.0 FINAL NO SIGNIFICANT HAZARDS CONSIDERATION
DETERMINATION The Commission's regulations in 10 CFR 50.92(c) state that the Commission may make a final determination that a license amendment involves no significant hazards consideration if operation of the facility in accordance with the amendment would not:
(1) involve a significant increase in the probability or l
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possibility.of a new or different kind of accident from any accident i
i previously evaluated; or-(3) involve a significant reduction in a margin of-j
- safety.
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- Operation of the facility in accordance with the proposed amendment will not j
l involve a significant increase in the probability or consequences of an l
accident previously evaluated. As discussed in Section 3.0 of this SE, the L*
j criterion provides adequate assurance of structural integrity to prevent tube pullout or collapse.,In addition, the licensee assumed conservative leakage allowances for each L tube to assure that the primary-to-secondary leakage j
would not result in offsite doses that exceed 10 CFR Part 100 guidelines l
during a postulated double ended guillotine break of the main steamline i
j outside of containment.
Operation of the facility in accordance with the proposed amendment will not i
create the possibility of a new or different kind of accident from any accident previously evaluated.
The implementation of the L criterion does not provide a mechanism that would result in an accident initiated outside of l
the tubesheet.
Verification of the L* criterion prevents tube displacement of any magnitude and postulated axial cracks existing a minimum of 0.5 inch from j
either the bottom of the roll transition or top of-the tube sheet, whichever F
is lower, from migrating out of the tubesheet.
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Operation of the facility in accordance with the amendment will not involye a J
j significant reduction in a margin of safety. The implementation of the L l
criterion maintains the integrity of the tube bundle commensurate with the
- guidance in draft Regulatory Guide 1.121 under normal and accident conditions.
In addition, the L length has been verified by testing that sufficient roll expansion length is available to preclude significant leakage and to provide structural integrity during normal and postulated accident conditions.
Based upon the above considerations, the staff concludes that the amendment meets the three criteria of 10 CFR 50.92.
Therefore, the staff has made a final determination that the proposed amendment does not involve a significant hazards consideration.
6.0 STATE CONSULTATION
In accordance with the Commission's regulations, the State of Alabama official was notified of the proposed issuance of the amendment.
The State official had no comments.
7.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes the surveillance requirements. The NRC' staff has determined that the amendment involves.no significant increase in the amounts, and no significant change in the types, of any effluents that may be released
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1 offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has made a final finding that l
the amendment involves no significant hazards consideration. Accordingly, the.
l amendment meets the eligibility criteria for categorical exclusion set forth l
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of this amendment.
8.0 CONCLUSION
i The Commission has concluded, based on the considerations discussed above, that (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
l Principal Contributor:
J. Tsao l
Date:
May 20,1996 1
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