ML20083M757
| ML20083M757 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island, Fort Calhoun  |
| Issue date: | 05/15/1995 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20083M755 | List: |
| References | |
| NUDOCS 9505220042 | |
| Download: ML20083M757 (9) | |
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UNITED STATES
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.. j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 30866 0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NOS. 118 AND 111 TO FACILITY OPERATING LICENSE NOS. DPR-42 AND DPR-60 NORTHERN STATES POWER COMPANY (NSP)
PRAIRIE ISLAND NUCLEAR GENERATING PLANT. UNIT NOS. 1 AND 2 DOCKET NOS. 50-282 AND 50-306
1.0 INTRODUCTION
By letter dated January 9,1995, as supplemented February 7, March 15, March 27, April 3, and April 20, 1995, the licensee requested approval of amendments to the Technical Specifications (TSs) for the Prairie Island Nuclear Plant. The amendments would revise TS Section 4.12 to specify an F*
distance within the tubesheet below which indications of degradation would not affect the integrity of a steam generator tube. As a result, tubes with-degradation below the F* distance in the tubesheet would not require sleeving or plugging but could be repaired by installation of additional roll expansion to meet the F* criterion.
The licensee supported its requests with a Westinghouse report WCAP-14225, "F*
and L* Plugging Criteria for Tubes with Degradation in the Tubesheet Roll Expansion Region of the Prairie Island Units 1 and 2 Steam Generators,"
(Proprietary). To support its repair methodology, the licensee also supplied a report by Combustion Engineering, CEN-620-P, Revision 00 " Series 44 & 51 Design Steam Generator Tube Repair Using A Tube Rerolling Technique,"
(Proprietary).
Although the licensee requested TS changes regarding L* criteria, the staff's safety evaluation (SE) will address only those changes proposed for the F*
criteria.
A separate SE covering the L* criteria will be issued at a later date.
The March 15, March 22, April 3, and April 20, 1995, letters provided updated TS pages and clarifying information in response to NRC's requests for additional information.
This information was within the scope of the original application and did not change the staff's initial proposed no significant hazards consideration determination.
The licensee stated that the proposed changes will provide adequate assurance of steam generator tube integrity because the presence of the tubesheet in conjunction with the hardroll process significantly reduces the potential for NbbN NNN82 P
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tube failure and/or leakage within the tubesheet when compared to the free span portion of the tube. 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. The proximity of the tube and tubesheet, due to the hardroll expansion, limits the amount of primary-to-secondary leakage. The F* criterion provides a similar level of protection for tube degradation in the tubesheet as that afforded by Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes," for degradation located outside the tubesheet region.
2.0 DISCUSSION AND EVALUATION 2.1 Licensee's Evaluation Enaaaement Distance Determination The licensee determined a distance, designated F* (and identified as the F*
criterion), below the bottom of the roll transition for which tube degradation of any extent does not necessitate remedial action, e.g., sleeving or plugging. This criterion would be used in determining whether or not repairing or plugging of full depth hardroll expanded steam generator tubes is necessary for potential degradation which has been detected in that portion of the tube which is within the tubesheet.
The F* criterion provides for sufficient engagement of the tube-to-tubesheet 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.
In order to evaluate the applicability of any developed criterion for indications within the tubesheet, some postulated type of degradation must necessarily be considered.
For this evaluation it was postulated that a circumferential severance of a tube could occur.
However, implicit in assuming a circumferential severance may occur is the consideration that degradation of any extent could be demonstrated to be tolerable below the location determined acceptable for a postulated condition.
Tubes are installed in the steam generator tubesheet by a hardrolling process which expands the tube to bring the outside surface into intimate contact with the tubesheet hole. The roll process and roll torque are specified to result in a metal-to-metal interference fit between the tube and the tubesheet.
When the tubes have been hardrolled into the tubesheet, any axial loads developed by pressure and/or mechanical forces acting on the tubes are resisted by frictional forces developed by the elastic preload that exists between the tube and the tubesheet.
For some specific length of engagement of the hardroll, no significant forces will be transmitted further down the tube, and that length of tubing, the F* distance, will be sufficient to anchor the tube in the tubesheet.
In order to determine the F* distance for application in Westinghouse Model 51 steam generators, a testing program was conducted to measure the elastic preload of the tubes in the tubesheet.
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4 An axial length of roll expansion equal to the F* distance at the top of the roll expansion of the tube into the tubesheet provides sufficient structural integrity to preclude pull out of the tube due to pressure effects, even after assuming that the tube has expanded a complete circumferential separation at or below the bottom of the F* distance. This same axial length of roll expansion of the tube into the tubesheet provides a barrier to leakage during all plant conditions for through wall cracking of the tube in the expanded region below F*.
The proposed change designates a portion of the tube for which tube j
degradation of a defined type does not necessitate remedial action. As noted above, the area subject to this change is in the original or additional expanded portion of the tube within the tubesheet of the steam generators.
The F* length has been determined to be 1.07 inches (not including eddy current uncertainty). Sound roll expansion of 1.07 inches will satisfy all applicable recommendation.; of Regulatory Guide 1.121 with regard to tube burst capability.
Limitation of Primary-To-Secondary leakaae As described above, the F* criterion requires a minimum length of hardroll engagement below the bottom of the roll transition.
For Prairie Island, an F*
distance of 1.07 inches has been proposed. The presence of the elastic preload presents a significant resistance to flow of primary-to-secondary or secondary-to-primary water for degradation which has progressed fully through the thickness of the tube wall.
In effect, no leakage would be expected if a sufficient length of hardroll is present. Because of the difficulty in accurately sizing stress corrosion crack indications, the proposed TSs require that no indications of cracking can be present within the F* distance in tubes to which the F* criterion is applied.
This requirement has the effect of preventing the start of a leak path.
l The issue of leakage within the F* region up to the top of the roll transition includes the consideration of postulated accident conditions. The i
relationship between the tubesheet region leak rate at most limiting postulated accident (feedline break) conditions relative to that for normal i
plant operating conditions has been assessed. For the postulated leak source within the roll expansion, increasing the differential pressure on the tube wall increases the driving head for the leak; however, it also increases the tube to tubesheet loading.
For an initial location of a leak sour:e a distance greater than F* below the bottom of the roll transition, the feedwater line break pressure differential results in an insignificant leak rate relative to that which could be associated with normal-plant operation.
This is a resu of the increased tube to tubesheet loading associated with the increased differential pressure. Thus, for a circumferential indication i
within the roll expansion that is left in service in accordance with the F*
pull-out criterion, any leakage under accident conditions would be less than that experienced under normal operating conditions. Therefore, any leakage under accident conditions would be less than the existing TS leakage limit which is consistent with the accident analysis assumption.
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Tube Intearity Postulated Limitina Conditions The final aspect'of the evaluation is to demonstrate tube integrity under the postulated loss-of-coolant accident (LOCA) condition of secondary-to-primary differential pressure. A review of tube collapse strength characteristics indicates that the constraints provided to the tube by the tubesheet give a margin between tube collapse strength and the limiting secondary-to-primary differential pressure condition, even in the presence of circumferential or axial indications.
2.2 Staff Evaluation.
The staff has reviewed similar proposals from other licensees.
For consistency in our reviews, the staff compared the information in WCAP-14225 to that in WCAP-13970, "F* Tube Plugging Criterion for tubes with Degradation in the Tubesheet Roll Expansion Region of the D.C. Cook Unit 1 Steam Generators." The report was almost identical insofar as the F* criterion. The staff had reviewed Westinghouse WCAP-13970 in its review of a proposal by the licensee for D.C Cook. Our SE for D.C. Cook found the proposed revision acceptable based on WCAP-13970.
Similarly, we find the supporting report acceptable for the licensee for Prairie Island.
However, certain concerns have risen since we completed our evaluation for D.C. Cook, and the staff requested additional information from the licensee in a telephone call on January 27, 1995, and a letter on March 8, 1995. The licensee addressed our requests for information as follows:
Oualification of additional roll exoansion f
In its letters dated January 9, and February 7, 1995, the licensee stated that i
it will implement rerolled tubesheet joint expansions to obtain new leak tight rerolled joints for tube repair. The technology was qualified by CE. The details were set forth in the above-mentioned CE report.
t One factor that can affect the integrity of the repair is the presence of sludge.
In its qualification testing the licensee addressed the adverse effects that sludge could have.
Sludge from Zion was used to simulate possible sludge conditions between the tube and tubesheet hole.
In its March 15, 1995, letter, the licensee also addressed why the original qualification tests should remain valid despite moving the F* distance to a new location by rerolling, a location containing sludge which could affect test results, stating:
During qualification testing... tubes were rerolled both with and without sludge between the tube and tubesheet collar...both with wet and dry baked sludge. All tests provided sufficient tube pull out restraint, however, the baked on sludge resulted in minor leakage.
Because of this minor leakage, tubes with reroll torque traces representative of hard sludge will not be left in service.
Evaluation of the torque trace provides the basis for acceptance or rejection of the new roll region.
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The staff finds the licensee's approach is consistent with ensuring repair integrity in full conformance with the above-mentioned testing and analyses.
The staff finds the licensee adequately addressed leakage in rerolled tubes based on testing and analysis.
After installing the rerolled joints, the licensee will examine them with eddy current.
It will install plugs or sleeves if the reroll process is not successful or if steam generator tubes are unacceptably degraded from the process.. The staff finds these solutions acceptable to ensure tube integrity and in accordance with industry practice.
Leakaoe Concerns The staff asked the licensee to address the ramifications of having more than one alternate repair criterion that allows leakage.
The total leakage should continue to be within the allowed limits under accident conditions.
In its March 15, 1995, letter, the licensee addressed the maximum postulated accident leakage from cracks allowed to remain in service under the F*, L*, and other criteria, such as the interim repair criteria. The licensee has revised the bases for TS 4.12 to incorporate the following statement:
When more than one Alternate Repair Criteria are used, the summation of leakage from all tubes left in service by all repair criteria must be less than the allowable leakage for the most limiting of those Alternate Repair Criteria.
The staff finds the licensee will ensure leakage limits are not exceeded with its proposed statement.
Tube Lockup Issue In its letter dated March 15, 1995, the licensee addressed the effects of tube support plate lockup on F* with a letter from Westinghouse.
Westinghouse found the forces acceptable, stating the following:
The locking, considered as postulated at this plant, occurs during operation. Therefore, the axial force on locked tubes is essentially zero during operation.
For a single tube, locking at the lowest tube support plate, the axial force on the tube is approximately 220 lbs., tensile, during shutdown.
For the case which is most likely to be encountered, when locking occurs, i.e., locking of more than one tube, the tensile, per-tube force reduces significantly from the single-tube value.
For instance, for ten locked, adjacent tubes, the force reduces to approximately 24 lbs. at the shutdown condition.
Therefore, because the maximum locked-tube forces occur only during shutdown, there is no need to add them to the "3 Delta P" normal operation load of 2973 lbs. which was used to calculate the F* length in Ref. 2 [of Westinghouse's March 1, 1995 ltr (WCAP-14225)].
The 220 lbs. locked tube
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force will be easily accommodated by the F* length of the tube joint during the. shutdown condition. At this condition, i.e., without the beneficial, pressure-tightening and thermal growth mismatch contributions to joint strength, the joint can accommodate approximately 1987 lbs. of i
axial force. This is approximately nine times the force that can be exerted on the joint by a single, locked tube..
The staff finds these solutions acceptable because they are in accordance with
. industry procedures.
The staff accepts the Westinghouse analysis.
Re-examination l
In ~its letters dated February 7 and March'15,1995, the licensee addressed staff concerns about re-examination of F* tubes for_ the first two cycles and 1
described its examination method.
The licensee stated that new TS 4.12.3 provides the criteria to inspect all F*
'1 tubes in the roll expansion region.
In addition, the licensee states that it will perform inspections with rotating pancake coil or equivalent for the first two cycles of implementation of F*.
The staff finds these solutions acceptable because they are in accordance with industry procedures.
Proposed TS Chanaes The licensee proposed the following changes in the TS to implement the F*
criterion.
1.
Prooosed New TS 4.12.B.3 The proposed new TS 4.12.B.3, would add a requirement to inspect the F*
distance in the roll expanded region of all tubes that have had the F*
criterion applied.
New TS 4.12.B.3, would also allow the roll expanded region of these tubes to be excluded from the requirements of TS 4.12.B.2.a.
The previous TS 4.12.B.3 is being renumbered to 4.12.B.4.
2.
Proposed Chanaes to TS 4.12.D.1.f The current definition of " Repair /olugging Limit" in TS 4.12.D.I.f is being modified to note that the 40% repair / plugging limit does not apply to the part of the tube in the tubesheet below the F* distance for F*
tubes, if the tube is not degraded, (i.e., no indications of cracks) within the F* distance.
3.
Proposed New TS 4.12.D.1..i and TS 4.12.D.I.k New TS 4.12.D.1.j and TS 4.12.D.I.k provide definitions for the F*
distance and an F* tube.
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ProDosed New TS 4.12.E.4 Proposed new TS 4.12.E.4 adds a requirement to report to the NRC the results of inspections performed under TS 4.12.8 for all tubes that have defects below the F* distance and were not plugged.
5.
Proposed Revision to Bases 4.12 The Bases have been revised to incorporate changes in the TS.
The staff has reviewed the TS changes in the licensee's submittal and finds them acceptable. These TSs provide acceptable implementation of the F*
criteria as analyzed in the Westinghouse Report WCAP-14225 and CE Report, CEN-620-P, Revision 00.
Based on a review of the licensee's submittal, the staff concludes that tubes can be left in service with eddy current indications of pluggable magnitude that are below the F* distance provided the tune is not degraded within the F* distance. The F* distance is 1.07 inches (not including eddy current uncertainty) from the bottom of the hardroll transition toward the bottom of the tube sheet.
The staff concludes that the proposed TS changes on steam generators surveillance requirements, Section 3/4.4.12, as detailed in the licensee's submittal of January 9,1995, as supplemented February 7, March 15, March 22, April 3, and April 20, 1995, are acceptable and may be incorporated in amendments to the operating licenses for Prairie Island Generating Plant, Units No. I and 2, in accordance with 10 CFR 50.90 and 50.92.
The staff will complete its review of the L* criteria and issue its SE at a later date.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Minnesota State official was notified of the proposed issuance of the amendments.
The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendments change surveillance requirements. 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 (60 FR 14023). The amendments also change reporting or recordkeeping requirements. Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9) and (c)(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.
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5.0 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 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 Contributor:
M. Banic Date: May 15, 1995 5
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6.0 REFERENCES
1.
Westinghouse Electric Corporation, WCAP-14225, "F* AND L* Tube Plugging Criteria for Tubes with Degradation in the Tubesheet Roll Expansion Region of the Prairie Island Units 1 and 2 Steam Generators," December 1994.
(Proprietary information. Not publicly available.)
2.
Westinghouse Electric Corporation, WCAP-14226; "F* AND L* Tube Plugging Criteria for Tubes with Degradation in the Tubesheet Roll Expansion Region of the Prairie Island Units 1 and 2 Steam Generators," December 1994.
(Non-proprietary) 3.
Combustion Engineering, Inc., CEN-620-P, Rev. 00, " Series 44 & 51 Design Steam Generator Tube Repair Using a Tube Re-Rolling Technique," Final Report, March 1995.
(Proprietary Information. Not publicly available.)
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