ML20212G817
| ML20212G817 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 09/22/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20212G811 | List: |
| References | |
| NUDOCS 9909300128 | |
| Download: ML20212G817 (6) | |
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UNITED STATES E
j NUCLEAR REGULATORY COMMISSION
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t WASHINGTON, D.C. 20665-0001
.....,o SAFETY EVALUATION BY THE OFFtCE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 71 TO FACILITY OPERATING LICENSE NO. NPF-87 AND AMENDMENT NO. 71 TO FACILITY OPERATING LICENSE NO. NPF-89 TXU ELECTRIC COMPANY COMANCHE PEAK STEAM ELECTRIC STATION. UNITS 1 AND 2 DOCKET NOS. 50-445 AND 50-446
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1.0 INTRODUCTION
1 By application dated October 2,1998, as supplemented by letters dated July 27 and August 26, 1999, TXU Electric Company (the licensee) requested changes to the Technical Specifications (TSs) for the Comanche Peak Steam Electric Station (CPSES), Units 1 and 2. The proposed changes would revise the TSs, for CPSES, Unit 1, to define the F* steam generator (SG) tube plugging criteria in TS 5.5.9," Steam Generator (SG) Tube Surveillance Program," and associated reporting requirements in TS 5.6.10," Steam Generator Tube Inspection Report."
The supplemental information provided in letters dated July 27 and August 26,1999, was clarifying in nature and did not change the scope of the October 2,1998, appication and the initial proposed no significant hazards consideration determination.
2.0 BACKGROUND
General Design Criterion (GDC) 14, " Reactor Coolant Pressure Boundary," and GDC 31,
" Fracture Prevention of Reactor Coo l ant Pressure Boundary," of Appendix A to Part 50 of j
Title 10 of the Code of Federal Regulations (10 CFR) state requirements applicable to maintaining adequate structural and leakage integrity for SG tubes. Regulatory Guide l
(RG) 1.121," Bases for Plugging Degraded PWR Steam Generator Tubes," August 1976 (for Comment), provides guidance on acceptable minimum structural scfety margins. The surveillance requirements in the CPSES, Units 1 and 2, TSs require periodic inspection of the SG tubes. If a tube is found to be defective (indications in excess of 40 percent through-wall), it is required to be removed from service.
SG tubes comprise a significant portion of the reactor coolant pressure boundary. Maintenance
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l of this barrier is provided by the integrity of the SG tube wall and the tube-to-tubesheet connection. The connection L 9 tween the tube and the tubesheet is an interference fit made by hard roll expanding or explosit y expanding the tube into a bore through the tubesheet. The inelastically deformed SG tubes are held in place by the elastic springback of the tubesheet.
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2 Undegraded, the tube to-tubesheet joint provides sufficient strength to maintain adequate structural and leakage integrity.
Industry experience has shown that defects have been developed in the tube-to-tubesheet joints. The NRC staff believes that hard rolled tubes having degradation in the joint may remain in service provided that the degradation is below a specified distance from the top of the tubesheet or the roll transistion, whichever is lower, and that the undegraded portion of the SG tube in the joint can maintain adequate structural and leakage integrity under loadings from normal operation, anticipated operational transients, and postulated accident conditions.
The licensee's proposed amendment would allow the use of alternate repair criteria called F*,
for tubes that were hard roll expanded into the tubesheet at CPSES, Unit 1, by specifying an F*
distance within the tubesheet, below which indications of degradation would not affect the structural and leakage integrity of the SG tubes. As a result, hard roll expanded tubes with known degradation below the F* distance in the tubesheet would not require plugging or repair.
The licensee supported its request with a Westinghouse report, WCAP-15004, "F* Tube Plugging Criterion for Tubes with Degradation in the Tubesheet Region of the Comanche Peak Unit 1 Steam Generators," December 1997. The licensee stated that the proposed changes will provide adequate assurance of SG tube integrity because the presence of the tubesheet in conjunction with the hard roll process significantly minimizes the potential for tube structural failure and primary-to-secondary leakage.
CPSES, Unit 1, is a Westingtiouse four-loop pressurized water reactor with Model D4 SGs.
Each SG contains 4578 mill-annealed, inconel 600 tubes. The SGs at CPSES, Unit 1, were fabricated with most of the tubes receiving a full depth hard roll expansion in the tubesheet.
Eddy current data from plants similar to CPSES, Unit 1, have shown a potential for primary water stress corrosion cracking (PWSCC) within the roll expansion portion of the tube in the tubesheet. Using the existing TS tube plugging limit for the length of the tube within the tubesheet may result in many of the tubes being plugged unnecessarily. The F* criteria does not apply to those SG tubes at CPSES, Unit 1, that have been full-depth expanded in the tubesheet using the Westinghouse explosive tube expansion process.
3.0 DISCUSSION In order to demonstrate adequate structural margin for degraded SG tubes, the bases for the
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proposed SG tube repair criterion must address the limiting conditions during normal operation, anticipated operational occurrences, and postulated accident conditions. The margin of failure under normal operating conditions as recommended in RG 1.121 should not be less than three at any tube location. Under postulated accident conditions the margin to failure should not be less than 1.4 percent.
Structural loads imposed on the SG tube-to-tubesheet connections result primarily from the differential pressure between the primary and secondary sides of the tubes and the tubesheet.
The peak postulated loading occurs during a steamline break due to a lowering of the secondary side pressure. However, normal operating loads, including cyclic joint loading from major plant transients (i.e.,'startup/ shutdown) and potential thermal expansion loads, can also be significant. The analysis (WCAP-15004) supporting the licensee's proposed amendment addressed the limiting conditions necessary to maintain adequate SG tube integrity.
Specifically, the tube must not experience excessive displacement relative to the tubesheet.
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3 3.1 Testina to Determine F* Distance The F* criteria provide for sufficient engagement of the tube-to-tubest.eet hardroll such that forces that could be developed during normal or accident conditions would be successfully
. resisted by the elastic preload between the tube and the tubesheet. Below the F* distance, no significant axial SG tube forces will be transmitted further down the tube. Thus, the F* distance
. ill be sufficient to anchor the tube in the tubesheet. The analysis in WCAP-15004 determined w
a distance, designated F* (which is measured from the secondary face of the tubesheet or the bottom of the hardroll, whichever is lower), below which tube degradation of any extent does not necessitate a repair of the affected hard rolled SG tube at CPSES, Unit 1.
In order to evaluate the applicability of any developed criterion for indications within the
' tubesheet, some postulated type of degradation must be considered. For this evaluation, it was conservatively postulated that a circumferential severance of the SG tube could occur at the F*
location. This was accomplished by taking no credit for the undegraded SG tubing located below the F* distance. Implicit in the assumption of a circumferential tube severance is the consideration that degradation of any extent could be demonstrated to be tolerable below the F*
location.
The assessment in WCAP-15004 determined the limiting axial tube loads under normal operating and accident conditions. In addition, the analysis determined the resistive capability of the hardrollinterference fit to axial pullout as a function of the length of hardroll. The F*
distance was determined by equating this to the maximum postulated loads with the appropriate safety factors applied. F* was determined to be equal to 1.13 inches. Verification that this value is significantly conservative was demonstrated by both pullout and hydraulic proof testing of tubes in tubesheet simulating collars.
The licensee's proposed TS changes designate a portion of the tube for which tube degradation in that region does not necessitate remedial action. The area subject to this change is the area below the F* distance. The licensee has proposed an F* distance of 1.13 inches plus an allowance for eddy current measurement uncertainty. The licensee stated that a sound roll expansion of 1.13 inches will ensure safety margins in the tubesheet region of the tube commensurate with that afforded by RG 1.121 for degradation located outside the tubesheet region.
3.2 Limitation of Primarv-to-Secondarv Leakaae
, 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 minimizing the potential for primary-to-
' secondary coolant leakage. With a sufficient length of hardroll, the tube-to-tubesheet connection will not allow any leakage under normal and faulted conditions. Therefore, an acceptable F* distance.must be such that leakage integrity is not jeopardized during all analyzed conditions. Leakage through SG tubes is limited in the plant TSs.
An assessment of the primary-to-secondary leakage is included in WCAP-15004. The licensee evaluated the leakage affects of the F* criteria at operating conditions, postulated accident conditions, and postulated limiting conditions. The licensee concluded that through wall degradation located below the F* distance is not expected to contribute to leakage
- due to the absence of a leak path as demonstrated by the hydraulic proof testing.
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.3.3 Insoection of F* Tubes The F* distance determined in WCAP 15004 is the length of roll expanded tubing necessary to ensure adequate structural and leakage integrity. For field application of the proposed criteria, it is necessary to measure the F* distance using nondestructive examination (NDE) techniques and to verify the absence of detectable indication within this distance. The licensee proposes to
~ inspect tubes utilizing the F* criteria using eddy current methods. A measurement error is introduced and consequently, the F* distance measurements will need to account for the NDE measurement error uncertainty. The NDE error is a function of the entire test system and data analysis. The licensee's methodology for determining the NDE measurement error uncertainty will include testing a representative sample of flawed SG tubes and determining the frequency distribution of the NDE measurement error. The allowance for the NDE measurement error will be based upon a 95 percent confidence value of the 95 percent tolerance limit of the aforementioned frequency distribution. The licensee stated that the acquisition techniques and data analysis guidelines used in the qualification will be equivalent to those used in the outage
' inspection. Consistent outage performance of the analysts will be accomplished by training the
- analyst in the methodology to be applied and by demonstrating that the performance of the analyst lies within the range of uncertainty to be applied to the F* distance measurement. The licensee has committed to describe this NDE uncertainty methodology in the Final Safety Analysis Report.' The commitment is contained in the August 26,1999, supplement to the October 2,1998, application. The NRC staff finds that reasonable controls for the implementation and for subsequent evaluation of proposed changes pertaining to the above regulatory commitment are best provided by the licensee's administrative processes, including the commitment management program. The above regualtory commitment does not warrant the creation of a regulatory requirement (an item requiring prior NRC approval of subsequent changes),
i 4.0 EVALUATION The licensee proposed the following changes in the TS to implement the F* repair criteria.
The following will be added to TS Section 5.5.9, "Stesm Generator (SG) Tube Surveillance Program:"
c.
Steam Generato. F* Tube Insoection (Unit 1 onIv) - In addition to the minimum sample size as determined by Specification 5.5.9.b.,
all F* tubes will be inspected within the tubesheet region. The results of inspections of F* tubes identified in previous inspections j
will not be a cause for additional Inspections per Tables 5.5-1 and i
5.5 2.
TS Section 5.5.9.e, " Acceptance Criteria" would be revised to read as follows:
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Pluaaina Limit means the imperfection depth at or beyond which the tube shall be removed from service and is equal to 40% of the nominal tube wall thickness. This definition does not apply to that portion of the Unit 1 tubing that meets the definition of an F* tube; j
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F* distance (Unit 1 oniv) is the distance of the hardroll expanded portion of a tube which provides a sufficient length of non-degraded tube expansion to resist pullout of the tube from the tubesheet. The F* distance is equal to 1.13 inches, plus an allowance for eddy current measurement uncertainty, and is measured down from the top of the tubesheet, or the bottom of the roll transition, whichever is lower in elevation; k)
F* Tube (Unit 1 oniv) is that portion of the tubing in the area of the tubesheet region below the F* distance with a) degradation below the F* distance equal to or greater than 40%, b) which has no indication of degradation within the F* distance, and c) that remains inservice; and I)
Hard Rolf Expansion (Unit 1 on!v ) is that portion of a tube which has been increased in diameter by a rolling process such that no crevice exists between the outside diameter of the tube and the hole in the tubesheet.
Sections j), k), and I) would bc new sections.
TS Section 5.6.10, " Steam Generator Tube Inspection Report," will be revised to read as follows:
a.
Within 15 days following ine completion of each inservice inspection of steam generator tubes, the number of tubes plugged or designated as an F* tube in each steam generator shall be reported to the Commission; This change will require the licensee to include in its 15-day report to the NRC, the number of tubes designated as F* tubes in each SG.
In addition to the above changes, minor administrative changes were proposed in the TSs to accommodate revised TS numbering.
Based on its review of the licensee's proposed license amendment, the NRC staff has determined that the proposed TS changes will provide adequate assurance of SG tube integrity in that (1) the presence of the tubesheet in conjunction with the hard roll process significantly reduces the potential for SG tube structural failure and primary-to-secondary leakage, (2) the presence of the tubesheet constrains the tube and complements the integrity of the tube by minimizing the amount of deformation a tube can undergo beyond its expanded outside diameter, and (3) the proximity of the tube and tubesheet, due to the hard roll expansion, precludes primary-to-secondary leakage. Therefore, the NRC staff concludes that SG tubes can be left in service with eddy current indications exceeding the depth-based,40 percent through-wall plugging criteria that are below the F* distance provided the SG tube is not degraded within the F* distance, in summary, the proposed F* criteria permit SG tubes to remain in service with degradation in excess of the current plugging limit provided the degradation exists below the F* distance. The l
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F* distance is measured from the secondary face of the tubesheet or the bottom of the hard
- roll, whichever is lower. The staff concludes that the proposed F* criteria is acceptable for
. CPSES, Unit 1, because the licensee has demonstrated, through an acceptable qualification test program, that the F* criteria satisfies GDC 14 and the guidance in RG 1.121. Accordingly, the NRC staff concludes that the TS changes will adequately incorporate the F* criteria into the CPSES, Unit 1, TSs and will provide adequate assurance of SG tube integrity. Therefore, the NRC staff finds the proposed TS changes acceptable.
5.0 STATE CONSULTATION
in accordance with the Commission's regulations, the Texas State official was notified of the proposed issuance of the amendments. The State official had no comments.
6.0 ENVIRONMENTAL CONSIDERATION
The amendments change a requirement with respect to installation or use of a facility
- component located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on such finding (64 FR 59597 dated November 4,1998). The amendments also relate to changes in recordkeeping, reporting, or administrative procedures or requirements. Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9) and (10). Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental
-assessment need be prepared in connection with the issuance of the amendments.
7.0 CONCLUSION
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 I
operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors: A. Keim D. Jaffe
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Date: September 22, 1999 i
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