ML20077N066
| ML20077N066 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 01/09/1995 |
| From: | Wadley M NORTHERN STATES POWER CO. |
| To: | |
| Shared Package | |
| ML19311B656 | List: |
| References | |
| NUDOCS 9501130284 | |
| Download: ML20077N066 (19) | |
Text
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UNITED STATES NUCLEAR REGULATORY COMMISSION NORTHERN STATES POWER COMPANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NO. 50-282 50-306 REQUEST FOR AMENDMENT TO OPERATING LICENSES DPR-42 6 DPR-60 LICENSE AMENDMENT REQUEST DATED JANUARY 9, 1995 Northern States Power Company, a Minnesota corporation, requests authorization for changes to Appendix A of the Prairie Island Operating License as shown on the attachments labeled Exhibits A, B, C, D, E and F.
Exhibit A describes the proposed changes, reasons for the changes, safety evaluation and a significant hazards evaluation.
Exhibits B and C are copies of the Prairie Island Technical Specifications incorporating the proposed changes.
Exhibits D and E are reports supporting the requested changes.
Exhibit F is a Westinghouse Electric Corporation affidavit for withholding of proprietary information.
This letter contains no restricted or other defense information.
NORTHERN STATES POWER COMPAh"f M O S N u h h 9>-t By Mlke*Wadley Plant Manager Prairie Island Nucle r Generating Plant Onthish ay of M M fore me a notary public in and for said County, ' personally ppeared MTke Wadley, Plant Manager, Prairie Island Nucler.r Generating Plant, and being first duly sworn acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof, and that to the best of his knowledge, informati n, and belief the statements made in it are true and that it is not inter o e fo ay.
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leMCIA K. LeCORE NOTAMREXMANNESOTA HBOWWCOUNTY
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- j
Exhibit A Prairie Island Nu lear Generating Plant License Amendment Request Dated January 9,1995 u
Evaluation of Proposed Changes to the Technical Specifications Appendix A of Operating License DPR-42 and DPR-60 3
Pursuant to 10 CFR Part 50, Sections 50.59 and 50.90, the holders of Operating Licenses DPR-42 and DPR-60 hereby propose the following changes to Appendix A, J
Technical Specifications:
- 1. F* Steam Generator Tube Repair Criteria Backcround This amendment request proposes a change to Technical Specification (TS) 4.12, " Steam Generator Tube Surveillance", to incorporate a revised
]
acceptance criterion for steam generator tubes with degradation in the tubesheet roll expansion region.
This criterion for. steam generator tube acceptance was developed by Westinghouse Electric Corporation and is known as F* ("F-Star"). This criteria was developed to avoid. unnecessary plugging of steam generator tubes, l
The purpose for F* is to provide an alternate plugging criteria for the roll expanded region and to support the repair method of additional roll j
expansion above the existing roll expansion region for tubes with Stress Corrosion Cracking near the Roll Transition Zone region or below.
The steam generators at Prairie Island are Westinghouse Model 51 steam J
generators with low temperature mill-annealed Alloy 600 tubing. In the
)
tubesheet region, the tubing has a hard roll expansion only in the lower
)
2.75 inches.
Currently there have been no eddy current indications in either unit which would meet the F* criteria. The only indications which have been found in the original roll expanded region are located at the roll transition zone which is at.he top of the nominal 2.75 inch roll expanded region.
These indications are representative of Primary Water Stress Corrosion Cracking (PWSCC) at the Roll Transition Region. Roll transition Zone PWSCC was first identified at Prairie Island in Unit 2 in March 1989.
Progression of this damage form mechanism has been slow, so far, with 50 tubes affected in Unit 1 and 12 tubes in Unit 2. There have also been some short axial indications just above the expanded region due to secondary side stress corrosion cracking which would be candidates for repair by additional roll expansion. In anticipation of increasing numbers of tubes affected by stress corrosion cracking in the lower tubesheet region, the F* criteria is requested to provide an alternative to plugging or sleeving tubes in the future.
Following approval of the F* criteria, Prairie Island will use the plant modification process (per 10 CFR 50.59) to implement the F*
criteria and the additional roll expansion repair.
The additional roll expansion process will be qualified to meet the requirements of WCAP-14225.
Exhibit A Page 3 el18 Eddy current uncertainty will be addressed by the plant modification process. The current methodology proposed is the use of a special eddy current probe containing two sets of coils, possibly rotating pancake coils and bobbin coils, with a known distance between the coils to reduce axial i
position uncertainties. A special inspection calibration standard will be fabricated incorporating F* distances.
The existing Technical Specification tube repairing and plugging criteria apply throughout the tube length, but do not take into account the reinforcing effect of the tubesheet on the external surface of the tube in the roll expansion region.
The presence of the tubesheet will constrain the tube and will complement its integrity in that region by precluding tube deformation beyond its expanded outside diameter. The resistance to both tube rupture and tube collapse is significantly strengthened by the tubesheet.
In addition, the proximity of the tubesheet significantly affects the leak behavior of throughwall tube cracks in this region.
No significant leakage relative to plant Technical Specification limits is to be expected from application of the F* criterion.
The F* methodology and determination of the F* distance are included in
-WCAP 14225, entitled "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." This report, prepared by Westinghouse Electric Corporation, is contained in Exhibit D.
A non-proprietary version of the report is included in Exhibit E.
Proposed Channes Technical Specification 4.12, " Steam Generators Tube Surveillance",
describes the inservice inspection program used to demonstrate steem J
generator operability.
In order to utilize the F* repair criteria, several l
i items in Technical Specification 4.12 must be revised. A brief description of the proposed revisions is provided below.
The specific wording changes to the Technical Specifications are shown in Exhibits B and C.
1
- 1. Proposed New Technical Specification 4.12.B.3 The proposed new Specification 4.12.B.3 would add a requirement to inspect the F* distance in the roll expanded region of all tubes which have had the F* criterion applied.
New Specification 4.12.B.3 would i
also allow the roll expanded region of these tubes to be excluded from the requirements of Technical Specification 4.12.B.2.a.
Old j
Specification 4.12.B.3 is being renumbered to 4.12.B.4.
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- 2. Proposed Chances to Technical Specification 4.12.D.1.f l
The current definition of " Repair / Plugging Limit" in Specification i
l 4.12.D.1.f is being modified to note that the 40% repair / plugging limit l
l does not apply to the portion of the tube in the tubesheet below the F*
I l
distance for F* tubes provided the tube is not degraded within the F*
distance.
I ExWbh A hge 3 tf l8
- 3. Proposed New 1echnical Specifications 4.12.D.1.1 and k New Specifications 4.12.D.1.j and 4.12.D.1.k provide definitions for the F* distance and an F* tube.
- 4. Proposed New Technical Specification 4.12.E.4 The proposed new Specification 4.12.E.4 adds a requirement to report ts, the NRC the results of inspections performed under Technical Specification 4.12.B for all tubes that have defects below the F*
distance and were not plugged.
Justification Using existing Technical Specification tube plugging criteria, many tubes experiencing only minor degradations would have to be repaired or removed from service. However, with the analyses described in this submittal and l
WCAP 14225, it can be shown that tube plugging or repair is not required in many cases to maintain tube bundle integrity. WCaP-14225 was developed by Westinghouse specifically for Prairie Island and provides F* criteria for j
the Prairie Island steam generators.
The basis for steam generator tube surveillance and plugging / repair is to ensure that the structural integrity of the tubes is maintained.
The F*
criteria was developed to allow for an alternative to tube plugging or l
sleeving for indications which occur in the tubesheet area.
The F*
l criteria defines a length of undegraded expanded tube in the tubesheet which is sufficient to maintain any potential leakage (resulting from cracks occurring further down in the tubesheet) to well belov tha Technical Specification limit and Safety Analysis assumptions. The F* criteria were premised on the fact that the tubesheet provides reinforcement of the expanded portion of the tube, provides resistance to tube rupture and collapse, and limits leakage of through wall cracks. WCAP-14225 describes in detail the analysis and testing performed to demonstrate acceptability of the F* criteria.
The proposed Technical Specification change is requested to provide Prairie Island with an alternative for dispositioning steam generator tubes degraded in the tubesheet region. The proposed amendment offers several benefits including:
l
- 1. Reduced occupational radiation exposure that would otherwise he incurred by plant workers involved in tube plugging or sleeving operations, l
- 2. Minimizing the loss of margin in the reactor coolant flow through the j
steam ganerator in LOCA analyses,
- 3. Avoiding loss of margin in reactor coolant system flow and therefore assisting in assuring that minimum flow rates are maintained in excess l
of that required for operation at full power, t
t i
i
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l Exhibit A Page 4 cf l8 l
l l
- 4. Reduction in the length of plant outages and the time that the steam l
generators are open to the containment environment during an outage as a result of the reduction in the amount of tube plugging or sleeving
- required, l
S. Less complex eddy current inspections because of fewer sleeved tubes.
- 6. Reduced tube plugging and sleevtus will contribute to longer steam i
generator life.
Safety Evaluation l
Introduction The amendment has been proposed to address eddy current indications of tube degradation which can occur in the original or additional roll expanded portion of the tubes within the tubesheet in the steam generators at Prairie Island Units 1 and 2.
These steam generators were fabricated with a 2.75 inch partial depth roll expansion in the tubesheet.
Interpretation of eddy current data from the Prairie Island Units and similar plants has shown a potential for primary water stress corrosion cracking (PWSCC) within the roll expanded and roll transition regions of the tube in the tubesheet. Using existing Technical Specification tube plugging criteria for the length of the roll expanded region of the tube within the j
tubesheet, many of the tubes with potential indications would have to be l
sleeved or removed from service.
l It can be shown that tube plugging or sleeving is not required in many such cases to maintain steam generator tube integrity. The proposed amendment would revise Technical Specification Section 4.12 specifying an F* distance within the tubesheet below which indications of degradation would have no impact on the determination of integrity of a steam generator tube. As a t
result, steam generator tubes with degradation below the F* distance in the tubesheet region would not require sleeving or plugging but could be repaired by installation of additional roll expansion to meet the F*
criterion. The proposed change 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 tube failure and/or leakage within the tubesheet area when compared to the free span portion of the tube. The presence of the tubesheet provides j
for constraint of the tube, and the tubesheet 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 region as that afforded by Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes", for degradation located outside the tubesheet region.
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Exhibit A Page 5 of 18 The proposed amendment would preclude occupational radiation exposure that l
would otherwise be incurred by plant workers involved in tube plugging or sleeving operations. The proposed amendment would also avoid loss of margin in reactor coolant system flow and therefore assist in assuring that minimum flow rates are maintained in excess of that required for operation l
at full power. Reduction in the amount of tube plugging or sleeving l
required can reduce the length of plant outages and reduce the time that l
the steam generator is open to the containment environment during an l
outage.
l
\\
The possibility of tube repair by sleeving should not be considered a l
reason to exclude use of this proposed tubesheet plugging criterion, but j
should be considered one of the options used to address degradation in the j
expanded region of the tube. The disadvantages of tube plugs noted above also apply to some extent to sleeves.
I Evaluation 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 axial forces will be transmitted further down the tube, and that length of tubing, the F* distance, will be l
sufficient to anchor the tube in the tubesheet.
In order to determine the l
F* distance for application in Westinghouse Model 51 steam generators, a l
testing program was conducted to measure the elastic preload of the tubes in the tubesheet.
The proposed F* distance provides for sufficient engagement of the tube-to-tubesheet hardroll such that pullout forces that could be developed during normal or accident operating conditions would be successfully resisted by the elastic preload between the tube and tubesheet.
An axial length of roll expansion equal to the F* distance at the top of j
the roll expansion of the tube into the tubesheet provides sufficient j
structural integrity to preclude pull out of the tube due to pressure j
effects, even after assuming that the tube has experienced a complete i
circumferential separation at or below the bottom of the F* distance. This l
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*,
i Exhibit A Page 6 cf 18
{
The proposed change designates a portion of the tube for which tube 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* 1ength has been determined to be 1.07 inches (not including eddy current uncertainty).
Sound roll expansion of 1.07 inches will satisfy all applicable recommendations of Regulatory Guide 1.121, with regard to tube burst capability, l
l As described above, the F* criterion requires a minimum length of hardroll i
engagement below the bottom of the roll transition.
For Prairie Island, an l
F* distance of 1.07 inches has been proposed. The presence of the elastic l
preload presents a significant resistance to flow of primary-to-secondary l
or secondary-to-primary water for degradation which has progressed fully through the thickness of the tube wall.
In effect, no leakage would be i
expected if a sufficient length of hardroll is present.
Because of the difficulty in accurately sizing stress corrosion crack indications, the proposed Technical Specifications 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.
l The relationship between the tubesheet region leak rate at most limiting l
postulated accident (feedline break) conditions relative to that for normal 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 alto increases the tube to tubesheet loading.
For an initial location of a leak source a distance eater 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 result of the increased tube to tubesheet loading associated with the increased differential pressure. Thus, for a circumferential indication wichin 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 Technical Specification leakage limit which l
is consistent with accident analysis assumptier.s.
Steam generator tube integrity must be maintained under the postulated loss l
of coolant accident condition of secondary-to-primary differential l
pressure.
Based on tube collapse strength characteristics, the constraint l
provided to the tube by the tubesheet gives a margin between the tube collapse strength and the limiting secondary-to-primary differential pressure condition, even in the presence of circumferential or axial indications.
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Exhibit A Page 7 of 18 Conclusions In conclusion, Northern States Power believes there is reasonable assurance that the health and safety of the public will not be adversely affected by the proposed Technical Specification changes.
Determination of Sinnificant Hazards Considerations The proposed changes to the Operating License have been evaluated to determine whether they constitute a significant hazards consideration as required by 10 CFR Part 50, Section 50.91 using the standards provided in Section 50.92.
This analysis is provided below:
1.
The proposed amendment will not involve a significant increase in l
the probability or consecuences of an accident previous 1v evaluated.
l The supporting technical and safety evaluations of the subject criterion i
demonstrate that the presence of the tubesheet will enhance the tube integrity in the region of the hardroll by precluding tube deformation beyond its initial expanded outside diameter.
The resistance to both tube rupture and tube collapse is strengthened by the presence of the tubesheet in that region.
The results of hardrolling of the tube into the tubesheet is an interference fit between the tube and the tubesheet.
Tube rupture cannot occur because the contact between the tube and tubesheet does not permit sufficient movement of tube material.
The radial preload developed by the rolling process will secure a postr' 'ted separated tube end witnin the tubesheet during all plant conditions In a similar manner, the tubesheet does not permit sufficient move-..nt of tube material to permit buckling collapse of the tube during postulated LOCA loadings.
The F* 1ength of roll expansion is sufficient to preclude tube pullout I
from tube degradation located below the F* distance, regardless of the extent of the tube degradation. The existing Technical Specification leakage rate requirements and accident analysis assumptions remain unchanged in the unlikely event that significant leakage from this region does occur.
As noted above, tube rupture and pullout is not expected for
)
tubes using the F* criterion. Any leakage out of the tube from within the i
tubesheet at any elevation in the tubesheet is fully bounded by the existing steam generator tube rupture analysis included in the Prairie Island Plant USAR.
For plants with partial depth roll expansion like Prairie Island, a postulated tube separation within the tube near the top of the roll expansion (with subsequent limited tube axial displacement) would not be expected to result in coolant release rates equal to those i
assumed in the USAR for a steam generator tube rupture event due to the limited gap between the tube and tubesheet.
The proposed plugging criterion does not adversely impact any other previously evaluated design basis accident.
Leakage testing of roll expanded tubes indicates that for roll lengths approximately equal to the F* distance, any postulated faulted condition primary to secondary leakage from F* tubes would be insignificant.
Exhibit A Pase s er is 2.
The proposed amendment will not create the possibility of a new or different kind of accident from any accident previously analyzed.
Implementation of the proposed F* criterion does not introduce any significant changes to the plant design basis.
Use of the criterion does not provide a mechanism to initiate an accident outside of the region of the expanded portion of the tube.
Any hypothetical accident as a result of any tube degradation in the expanded portion of the tube would be bounded by the existing tube rupture accident analysis. Tube bundle structural integrity will be maintained. Tube bundle leaktightness will be maintained such that any postulated accident leakage from F* tubes will be negligible with regard to offsite doses.
3.
The proposed amendment will not involve a significant
' tion in the marzin of safety.
The use of the F* criterion has been demonstrated to anintain the integrity of the tube bundle commensurate with the requirements of Reg Guide 1.121 (intended for indications in the free span of tubes) and the primary to secondary pressure boundary under normal and postulated accident conditions. Acceptable tube degradation for the F* criterion is any degradction indication in the tubesheet region, more than the F*
distance below the bottom of the transition between the roll expansion and the unexpanded tube.
The safety factors used in the verification of the strength of the degraded tube are consistent with the safety factors in the ASME Boiler and Pressure Vessel Code used in steam generator design.
The F* distance has been verified by testing to be greater than the length of roll expansion required to preclude both tube pullout and significant leakage during normal and postulated accident conditions.
Resistance to tube pullout is based upon the primary to secondary pressure differential as it acts on the surface area of the tube, which includes the tube wall cross-section, in addition to the inner diameter based area of the tube.
The leak teuting acceptance criteria are based on the primary to secondary leakage limit in the Technical Specifications and the leakage assumptions used in the USAR accident analyses.
Implementation of the tubesheet plugging criterion will decrease the-number of tubes which must be taken out of service with tube plugs or repaired with sleeves.
Both plugs and sleeves reduce the RCS (reactor coolant system) flow margin; thus, implementation of the F* criterion will maintain the margin of flow that would otherwise be reduced in the event of increased plugging or sleeving.
Based on the above, it is concluded that the proposed change does not result in a significant reduction in margin with respect to plant safety as defined in the USAR or the Technical Specification Bases.
Based on the evaluation described above, and pursuant to 10 CFR Part 50, Section 50.91, Northern States Power Company has determined that operation of the Prairie Island Nuclear Generating Plant in accordance with the proposed license amendment request does not involve any significant hazards considerations as defined by NRC regulations in 10 CFR Part 50, Section 50.92.
Exhibit A Page 9 cf 18 Environmental Assessment Northern States Power has evaluated the proposed changes and determined that:
1 1.
The changes do not involve a significant hazards consideration, l
2.
The changes do not involve a significant change in the types or significant increase in the amounts of any effluents that may be released j
offsite, or 3.
The changes do not involve a significant increase in individual or cumulative occupational radiation exposure.
Therefore, the proposed Technical Specification changes would not result in a significant radiological environmental impact.
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Exhibit A Page 10 ef 18
- 2. L* Steam Generator Tube Repair Criteria Backcround This amendment request proposes a change to Technical Specification (TS) l 4.12, " Steam Generator Tube Surveillance", to incorp tate a revised l
acceptance criterion for steam generator tubes with degradation in the tubesheet roll expansion region. This criterion for steam generator tube acceptance was developed by Westinghouse Electric Corporation and is known as L* ("L-Star"). This criteria was developed to avoid unnecessary plugging l
of steam generator tubes.
l l
l The purpose for L* is to provide an alternate plugging criteria for the roll expanded region and to support the repair method of additional roll expansion above the existing roll expansion region for tubes with Stress Corrosion Cracking near the Roll Transition Zone region or below.
The steam generators at Prairie Island are Westinghouse Model 51 steam generators with low temperature mill-annealed Alloy 600 tubing. In the tubesheet region, the tubing has a hard roll expansion only in the lower l
2.75 inches.
Currently there have been no eddy current indications in either unit which would meet the L* criteria. The only indications which have been found in the original roll expanded region are located at the roll transition zone which is at the top of the nominal 2.75 inch roll expanded region. These indications are representative of Primary Water Stress Corrosion Cracking (PWSCC) at the Roll Transition Region. Roll transition Zone PWSCC was first identified at Prairie Island in Unit 2 in March 1989.
Progression of this damage form mechanism has been slow, so far, with 50 tubes affected in Unit 1 and 12 tubes in Unit 2. There have also been some short axial indications just above the expanded region due to secondary side stress corrosion cracking which would be candidates for repair by additional roll expansion. In anticipation of increasing numbers of tubes affected by stress corrosion cracking in the lower tubesheet region, the L* criteria is requested to provide an alternative to plugging or sleeving tubes in the future.
Following approval of the L* criteria, Prairie Island will use the plant modification process (per 10 CFR 50.59) to implement the L*
l criteria and the additional roll expansion repair. The additional roll l
expansion process will be qualified to meet the requirements of WCAP-14225.
Two eddy current issues will be addressed during implementation:
l
- 1) An advanced inspection method is required to implement L*.
Currently, that method will be the rotating pancake coil eddy current probe.
- 2) Eddy current uncertainty will be addressed by the plant modification process.
The current methodology proposed is the use of a special eddy current probe conti ning two sets of coils, possibly rotating pancake coils and bobbin coils, with a known distance between the coils to I
reduce axial position uncertainties. A special inspection calibration standard will be fabricated incorporating L* distances.
l t
Exhibit A hge ll of 18 The existing Technical Specification tube repairing and plugging criteria apply throughout the tube length, but do not take into account the reinforcing effect of the tubesheet on the external surface of the tube in the roll expansion region.
The presence of the tubesheet will constrain the tube and will complement its integrity in that region by precluding tube deformation beyond its expanded outside diameter. The resistance to both tube rupture and tube collapse is significantly strengthened by the tubesheet. In addition, the proximity of the tubesheet significantly affects the leak behavior of through wall tube cracks in this region. No significant leakage relative to plant Technical Specification limits is to be expected.
I The L* methodology and cetermination of the L* distance are included in WCAP 14225, entitled "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 " This report, prepared by Westinghouse Electric Corporation, is contained in Exhibit D.
A non-proprietary version of the report is included in Exhibit E.
Proposed Channes l
Technical Specification 4.12, " Steam Generators Tube Surveillance",
describes the inservice inspection program used to demonstrate steam generator operability.
In order to utilize the L* repair criteria, several items in Technical Specification 4.12 must be revised. The proposed l
changes provide requirements for L* which are similar to the type of requirements imposed for utilization of F*.
A brief description of the l
proposed revisions is provided below. The specific wording changes to the l
Technical Specifications are shown in Exhibits B and C.
- 1. Proposed New Technical Specification 4.12.B.3 The proposed new Specification 4.12.B.3 would add a requirement to inspect, in the roll expanded region, all tubes which have had the L*
l criteria applied. New Specification 4.12.B.3 would also allow the roll expanded region of these tubes to be excluded from the requirements of Technical Specification 4.12.B.2.a.
Old Specification 4.12.B.3 is being renumbered to 4.12.B.4.
l l
- 2. Proposed Chances to Technical Specification 4.12.D.1.f l
l The current definition of " Repair / Plugging Limit" in Specification i
4.12.D.1.f is being modified to note that the 40% repair / plugging limit l
does not apply to the portion of the tube in the tubesheet below the L*
l distance for L* tubes provided the tube is not degraded within the L*
l distance.
- 3. Proposed New Technical Specifications 4.12.D.1.1 and m New Specifications 4.12.D.1.1 and 4.12.D.l.m provide definitions for the L* distance and an L* tube.
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l Eshibit A Page 12 of 18 l
- 4. Proposed New Technical Specification 4.12.E.4 The proposed new Specification 4.12.E.4 adds a requirement to report to the NRC the results of inspections performed under Technical Specification 4.12.B for all tubes that have defects below the L*
distance and were not plugged.
l Justification Using existing Technical Specification tube plugging criteria, many tubes experiencing only minor degradations would have to be repaired or removed from service. However, with the analyses described in this submittal and WCAP 14225, it can be shown that tube plugging or repair is not required in many cases to maintain tube bundle integrity. WCAP-14225 was developed by l
Westinghouse specifically for Prairie Island and provides L* criteria for l
the Prairie Island steam generators. The propo -d L* criteria were l
evaluated for the four tube modes recommended Regulatory Guide 1.121 for three steam generator conditions (normal operations, feedline break and loss of coolant).
The basis for steam generator tube surveillance and plugging / repair is to ensure that the structural integrity of the tubes is maintained.
The L*
l criteria was developed to allow for an alternative to tube plugging or sleeving for indications which occur in the tubesheet area.
The L*
l criteria defines a length of undegraded expanded tube in the tubesheet l
which '.s sufficient to maintain any potential leakage (resulting from l
crackr. occurring further down in the tubesheet) to well below the Technical Specification limit and Safety Analysis assumptions.
The L* criteria were premised on the fact that the tubesheet provides reinforcement of the expanded portion of the tube, provides resistance to tube rupture and collapse, and limits leakage of throughwall cracks. WCAP-14225 describes in detail the analysis and testing performed to demonstrate acceptability of the L* criteria.
The proposed Technical Specification change is requested to provide Prairie Island with an alternative for dispositioning steam generator tubes degraded in the tubesheet region.
The proposed amendment offers several benefits including:
I
- 1. Reduced occupational radiation exposure that would otherwise be incurred by plant workers involved in tube plugging or sleeving operations, 1
- 2. Minimizing the loss of margin in the reactor coolant flow through the steam generator in LOCA analyses,
)
i j
- 3. Avoiding loss of margin in reactor coolant system flow and therefore assisting in assuring that minimum flow rates are maintained in excess I
of that required for operation at full power,
- 4. Reduction in the length of plant outages and the time that the steam generators are open to the containment environment during an outage as a result of the reduction in the amount of tube plugging or sleeving
- required, I
l 1
Exhibit A Page 13 of 18 l
- 5. Less complex eddy current inspections because of fewer sleeved tubes, and
- 6. Reduced tube plugging and sleeving will contribute to longer steam generator life.
l Safety Evaluation Introduction The amendment has been proposed to address eddy current indications of tube degradation which can occur in the roll expanded portion of the tubes i
within the tubesheet in the sterm generators at Prairie Island Units 1 and 2.
These steam generators were fabricated with a 2.75 inch partial depth l
roll expansion in the tubesheet.
Interpretation of eddy current data from the Prairie Island Units and similar plants has shown a potential for primary water stress corrosion cracking (PWSCC) within the roll expanded l
portion of the tube in the tubesheet. Using existing Technical l
Specification tube plugging criteria for the length of the roll expanded region of the tube within the tubesheet, many of the tubes with potential indications would have to be repaired or removed from service.
It can be shown that tube plugging or sleeving is not required in many such cases to maintain steam generator tube integrity. The proposed amendment would revise Technical Specification Section 4.12 specifying an L* distance within the tubesheet below which certain types of degradation would have no impact on the determination of integrity of a steam generator tube; As a result, steam generator tubes with degradation below the L* distance in the tubesheet region would not require repair or plugging.
The proposed change will provide adequate assurance of steam generator tube integrity because l
the presence of the tubesheet in conjunction with the hardroll process l
significantly reduces the potential for tube failure and/or leakage within the tubesheet area when compared to the free span portion of the tube.
The presence of the tubesheet provides for constraint of the tube, and the l
tubesheet complements the integrity of the tube by minimizing the amount of l
deformation a tube can undergo beyond its expanded outside diameter. The l
proximity of the tube and tubesheet, due to the hardroll expansion, leads to limiting the amount of primary-to-secondary leakage.
The L* criterion provides a similar level of protection for tube degradation in the-tubesheet region as that afforded by Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes", for degradation located outside the tubesheet region.
The proposed amendment would preclude occupational radiation exposure that would otherwise be incurred by plant workers involved in tube plugging or sleeving operations. The proposed amendment would also avoid loss of margin in reactor coolant system flow and therefore assist in assuring that minimum flow rates are maintained in excess of that required for operation l
at full power. Reduction in the amount of tube plugging or sleeving required can reduce the length of plant outages and reduce the time that the steam generator is open to the containment environment during an outage.
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F i
Exhibit A i
Page 14 c.f 18 l
t The possibility of tube repair by sleeving should not be considered a reason to exclude use of this proposed tubesheet plugging criterion, but should be considered one of the options used to address degradation in the expanded region of the tube.
The disadvantages of tube plugs noted above also apply to some extent to sleeves.
l Evaluation l
l Tube plugging criteria have been developed for indications of tube degradation in the tube expansion region below the transition of the I
mechanically expanded /unexpanded portions of the tube.
The F* criterion, discussed in Part 1 of this amendment request, represents a length, designated F*,
of continuous roll expansion in the tubesheet such that tube pullout would not occur during either normal operation or postulated accident condition loadings. The implicit assumption of a circumferential severance of a tube in the development of the F* criterion permitted the conclusion that degradation of any extent or orientation within the tubesheet below the F* distance is acceptable during normal and postulated accident conditions. This very conservative assumption results in a distance that is longer than necessary to provide a limit to significant leakage in excess of the Technical Specification allowances and Safety Analysis assumptions.
To address some of the indications occurring at an elevation too high in the tube to meet the F* criterion, an additional alternative plugging l
criteria, designated the L* criteria, is proposed.
The L* criteria defines a length, L*, of undegraded expanded tube which, even in a worst case t
scenario, is sufficient to maintain (well below the Technical Specification limit and Safety Analysis assumptions) any potential leakage. Use of the L* criteria requires that the condition of the degradation below the L*
distance be assessed.
For those tubes in which the degradation below L* is determined to be axial or near axial (not greater than 30 degrees) cracking, the degraded tube provides sufficient structural strength to preclude pullout of the tube and it may remain in service without repair or plugging.
The approach taken in developing the L* criteria was to build on the fundamental basis of the F* criterion.
The minimum required engagement length, L*,
of roll expansion to preclude significant leakage under normal operation and postulated accident loading conditions was determined to be 0.50 inches (This value does not include an allowance for eddy current elevation measurement uncertainty).
This L*
distance is measured from the bottom of the transition between the expanded and unexpanded portions of the tube.
In order to evcluate the L* criterion concept for indications within the tubesheet, an evaluation of the strength of degraded tubes was made.
Based on plant operation and laboratory experience the configuration of any cracks, should they occur, is initially axial.
For axial or nearly axial indications in the tubesheet region, the tube end remains structurally intact minimizing any potential for tube pullout.
The strength of tubes with axial or near axial cracks has been evaluated using analysis and testing.
It has been shown that tube integrity is maintained in a tube which has at least 1.34 inch (plus eddy current uncertainty) of sound
ExMMt A Page 15 of 18 expanded tube below the bottom of the roll transition with no more than two bands of axial (30 degrees or less from tube axis) degradation separating the sound portions and with sound tube in the first 0.5 inch from the bottom of the roll transition.
L* tubes should be inspected with an advanced inspection technique in the top two inches of the roll expanded region to adequately characterize the degradation. A maximum of 600 tube ends per unit may use L*. When there is no degradation in the uppermost 1.07 inches (plus NDE uncertainty) below the bottom of the roll transition, the F* criterion applies and the tube end would not be counted in the 600 tube limit.
The L* engagement length determination was derived from preload, tube pullout, hydraulic proof (pressure), and leak testing done to develop the F* criteria. Test results from 3/4 inch tubing have been shown to be applicable to 7/8 inch tubing and have been incorporated in this analysis.
An evaluation consisting of analysis and testing programs was conducted to verify that the strength of tubes with axial or near axial cracks in the roll expansion region is greater than that required to resist pullout forces during normal operation and postulated accident loading conditions.
An additional program of tests was done to verify that a roll expansion with the length of L* is sufficient to significantly restrict leakage during normal operating and postulated accident condition loadings. The leak testing done to validate the L* distance used holes drilled through the tube to simulate the ends of axial cracks.
The F* leak testing had used a less sophisticated method for simulating tube degradation using a 1
circumferential cut through the tube.
The acceptance criteria for the leak testing was based on maintaining the total leakage through the L* distance to less than the primary to secondary leakage limit in the Technical Specifications. To provide operational flexibility, the acceptance criteria was determined using a fraction of the Technical Specification limit.
This value was divided by a number of tuoes larger than the number of tubes expected to use the L* criteria.
The WCAP allows L* to be applied to 600 tube ends per steam generator. The results of the L* leak testing compared favorably with the acceptance criteria.
For normal operating pressure differential, primary to secondary leakage for an L* value of 0.5 inch was negligible. The leak testing included tests of lengths shorter than 0.5 inch to demonstrate that the function of leak rate versus length of sound expansion is not near a threshold value in the region of the L* length chosen.
The use of the L* criteria could not affect the probability of occurrence of any other accident which originates from conditions outside the steam generator.
The limiting of the total leakage from L* tubes to less than the Technical Specification limit will assure that the consequences of any analyzed accident are not increased by the use of the L* criteria. The use of this criteria could not cause the steam generator or any other equipment l
important to safety to malfunction. Existing tube rupture analyses bound the effects of any hypothetical failure of the tube due to the use of the L* criteria and use of the L* criteria does not result in the possibility of an accident different from those previously analyzed.
The margin of safety is not reduced and is provided by the safety factors implicit in the use of the ASME Code to analyze the structural integrity of the tubes, the safety factors included in the recommendations of Regulatory Guide 1.121, i
l EdnWtA Page 16 of 18 I
and the margin represented by the difference in the size of a crack sufficient.to exceed Technical Specification leak limits / Safety Analysis assumptions and the minimum size of crack required to result in tube rupture or exceed analysis assumptions in the steamline break analysis.
On the basis of the evaluation above and as further. detailed in WCAP-14225, it is determined that tubes with tube degradation which can be categorized as axial or near axial cracking (not greater than 30 degrees) within the tubesheet region below the L* distance (defined as 0.5 inches, excluding
]
eddy current uncertainty) can be left in service. Tubes with tube i
degradation which is located a distance of less than L* below the bottom of the transition between the expanded and unexpanded tubes or the top of the tubesheet, whichever.is lower, will be removed from service by plugging or repairing in accordance with Technical Specification requirements.
Conclusions In conclusion, Northern States Power believes there is reasonable assurance that the health and safety of the public will not be adversely affected by the proposed Technical Specification changes.
Determination of Siznificant Hazards Considerations The proposed changes to the Operating License have been evaluated to i
determine whether they constitute a significant hazards consideration as l
required by 10 CFR Part 50, Section 50.91 using the. standards provided in Section 50.92.
This analysis is provided below:
)
- 1. The proposed amendment will not involve a significant increase in the probability or consecuences of an accident previousiv evaluated.
(
The presence of the tubesheet enhances steam generator tube integrity in the region of the hardroll by precluding tube deformation beyond its initial expanded outside diameter. The resistance to both tube rupture and tube collapse is ' strengthened by the presence of the tubesheet in that region. The result of the hardroll of the tube into the tubesheet i_
is an interference fit between the tube and the tubesheet.
Tube rupture cannot occur because the contact between the tube and tubesheet does not permit sufficient movement of tube material.
In a similar manner, the tubesheet does not permit sufficient movement of tube material to permit buckling collapse of the tube during postulated LOCA loadings.
The type of degradation for which the L* criteria has been developed (cracking with an axial or near axial orientation) has been found not to significantly reduce the axial strength of a tube. An evaluation including analysis and testing has been done to determine the strength reduction for axial loads with simulated axial and near axial cracks; This evaluation provided the basis for the acceptance criteria for tube degradation subject to the L* criteria.
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Exhibit A hge 17 ef 18 The length of roll expansion above L* is sufficient to preclude significant leakage from tube degradation located below the L*
distance. The existing Technical Specification leakage rate requirements and accident analysis assumptions remain unchanged in the unlikely event that significant leakage from this region does occur.
As noted above, tube rupture and pullout is not expected for tubes using the alternate plugging criteria.
Any leakage out of the tube from within the tubesheet at any elevation in the tubesheet is fully bounded by the existing steam generator tube rupture analysis included in the Prairie Island Updated Safety Analysis Report. The proposed alternate plugging criteria do not adversely impact any other previously evaluated design basis accident.
- 2. The proposed amendment will not create the possibility of a new or different kind of accident from any accident previously analyzed.
Implementation of the proposed alternate tubesheet tube plugging criteria does not introduce changes to the plant design basis. Use of the criteria does not provide a mechanism to result in an accident outside of the region of the tubesheet expansion. Any hypothetical accident as a result of any. tube degradation in the expanded portion of the tube would be bounded by the existing tube rupture accident analysis.
- 3. The proposed amendment will not involve a significant reduction in the marzin of safety.
The use of the alternate tubesheet plugging criteria hr.s been demonstrated to maintain the integrity of the tube bunelle commensurate with the requirements of Reg. Guide 1.121 for indicatians in the free span of tubes and the primary to secondary pressure boundary under normal and postulated accident conditions. Acceptable tube degradation for the L* criteria is any degradation indication with axial or nearly axial cracking in the tubesheet region, more than the Le distance below the bottom of the transition between the roll expansion end the unexpanded tube.
For tubes with axial or nearly axial cracks the strength of the tube relative to an axial load would not be reduced below the strength required to resist potential axial loads.
The safety factors used in the verification of the strength of the degraded tube are consistent with the safety factors in the ASME Boiler and Pressure Vessel Code used in steam generator design. The L* distance j
has been verified by testing to be greater than the length of roll expansion required to preclude significant leakage during normal and l
postulated accident conditions.
The leak testing acceptance criteria j
are based on the primary to secondary leakage limit in the Technical Specifications and the leakage assumptions used in the USAR accident analyses.
I
EaMbit A Page 18 of 18 Implementation of the proposed tubesheet plugging criteria vill decrease the number of tubes which must be taken out of service with tube plugs or repaired with sleeves. Both plugs and sleeves reduce the RCS flow margin, thus implementation of the alternate plugging criteria will maintain the margin of flow that would otherwise be reduced in the event of increased plugging or sleeving.
Based on the above, it is concluded that the proposed change does not result in a reduction in a loss of margin with respect to plant safety as defined in the Updated 4
Safety Analysis Report or the bases of the Technical Specifications.
Based on the evaluation described above, and pursuant to 10 CFR Part 50, Section 50.91, Northern States Power. Company has determined that operation of the Prairia Island Nuclear Generating Plant in accordance with the proposed license amendment request does not involve any significant j
hazards considerations as defined by NRC regulations in 10 CFR Part 50, j
Section 50.92.
Environmental Assessment Northern States Power has evaluated the proposed changes and determined that:
- 1. The changes do not involve a significant hazards consideration,
- 2. The changes do not involve a significant change in the types or significant increase in the amounts of any effluents that may be released offsite, or
- 3. The changes do not involve a significant increase in individual or cumulative occupational radiation exposure.
Therefore, the proposed Technical Specification changes would not result in a significant radiological environmental impact.
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