ML20086A631
| ML20086A631 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 06/27/1995 |
| From: | Richard Anderson NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GL-95-03, GL-95-3, NUDOCS 9507030319 | |
| Download: ML20086A631 (8) | |
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Northern States Power Company Prairie Island Nuclear Generating Plant 1717 Wakonada Dr. East Welch, Minnesota 55o89 June 27, 1995 Generic Letter 95-03 U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Response to Generic Letter 95-03 Circumferential Crackine of Steam Generator Tubes The attachment to this letter provides our response to Generic Letter 95-03, "Circumferential Cracking of Steam Generator Tubes".
In this letter we have made a new Nuclear Regulatory Commission commitment which is indicated in italics.
Please contact Jack Leveille (612-388-1121, Ext. 4662) if you have any questions related to this letter.
xd Roger 0 Anderson 3
Director Licensing and Management Issues c: Regional Administrator - Region III, NRC Senior Resident Inspector, NRC NRR Project Manager, NRC J E Silberg
Attachment:
Response to Generic Letter 95-03, Circumferential Cracking of Steam Generator Tubes
- 30007, 9507030319 950627
'i' PDR ADDCK 05000282 l
P PDR i
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t RESPONSE TO GENERIC LETTER 95-03 CIRCUMFERENTIAL CRACKING OF STEAM CENERATOR TUBES j
i
1.0 INTRODUCTION
Recent nondestructive examination of the steam generator tubing at the Maine Yankee Nuclear Plant has identified a large number of circumferential indications at the top of the tubesheet region.
These most recent inspection findings, coupled with previously documented inspection results regarding circumferential cracking have led to the issuance.of NRC Generic Letter 95-03, "Circumferential Cracking of Steam Generator Tubes" on April 28, 1995. The information detailed herein, will address the requested actions of the Generic Letter 95-03 as they pertain to Westinghouse designed and manufactured steam generators in general and specifically to Prairie Island Nuclear Generating Plant.
1.1 Historical Circumferential Degradation Locations Available historical information shows that for some Westinghouse plants, circumferential cracking has been detected in the tubesheet region tube expansion transitions from expanded to unexpanded tube, at the Row 1 and 2 U-bend tangent points, and at one plant (two twin units), at dented tube support plate intersections.
The main focus of this response will be to address tubesheet region expansion transition cracking, since this was the primary reason for the issuance of the generic letter. Other circumferential crack initiation sites will be addressed in the following section due to their limited numbers of field indications detected and limited number of tubes which can be affected (specifically small radius U-bends and dented tube l
support plate intersections).
j 1.2 Circumferential Degradation Evaluation of Small Radius U-bends and Tube Support Plates (TSPs)
The incidence of circumferential indications at the Row 1 and 2 U-bend tangent points has not been significant. Some plants have administratively decided to preventively plug the Row 1, and in some plants Rows 1 and 2 tubes, i
Additional plants have applied U-bend heat treatment in this region and have effectively recovered tubes previously preventively plugged.
Prairie Island routinely inspects rows 1 and 2 with rotating i
A leakage event occurred in 1987 which resulted in a steam generator tube rupture due to high cycle fatigue at a dented top tube support plate.
Pursuant to NRC Bulletin 88-02, all domestic Westinghouse steam generators with carbon steel tube support plates have been analyzed for the potential to experience high cycle fatigue at this location using a methodology accepted by the NRC.
In cases where the analysis indicated that fatigue usage could i
exceed 1.0, the tube was either plugged and stabilized or plugged using a leak limiting sentinel plug. Three conditions must be present for high cycle fatigue at the top tube support plate; denting, lack of anti-vibration bar (AVB) support, and locally elevated steam velocities due to nonuniform AVB insertion depths.
l Attachment l
Jun. 27, toes Pase 2 Anti-vibration bars were replaced in 1986 and no tubes required plugging by NRC Bulletin 88-02 at Prairie Island, l
An apparent inspection transient event occurred first in 1992 and involved the l
apparent identification of circumferentially oriented indications at the top of tubesheet region in a plant with partial depth roll expansion.
Degradation was detected using the bobbin probe, and denting was also associated with many of the indications. Many of the indications had large voltage distorted bobbin indications, which is uncharacteristic of circumferential degradation.
Several tubes were pulled from the steam generator and destructively examined.
The corrosion morphology was found to be closely spaced axial degradation and cellular degradation, as opposed to circumferential as suggested by RPC.
In a cellular morphology closely spaced axial degradation and circumferential1y oriented degradation can link and form a patch-like structure.
The significant axial components of the cellular morphology were attributed to the distorted bobbin indication components. Tube pull specimens which were burst tested all produced axial burst openings, which suggests that the axial degradation dominated the morphology.
Prairie Island has performed a 100%
b ~> bin and 100% RPC inspection of the top of tubesheet region of all steam
'rators; Unit 1 in 1994 and Unit 2 in 1995.
I Circumferential cracking at dented TSP intersections has been detected at one plant (two twin units).
The steam generators experiencing this phenomenon have been replaced.
This phenomenon has not been detected at other units.
This plant also operated at higher temperatures than most other units.
In many Westinghouse plants, an augmented top of tubesheet region inspection program is conducted on a cycle to cycle basis. Many Westinghouse plants have had all hot leg tubes inspected at the top of tubesheet region using the RPC probe and continue to do so on a cycle to cycle basis.
Currently available probes, coupled with properly implemented calling criteria and techniques have been demonstrated to be sufficicct to identify circumferential indications in the tubesheet region. Recognizing the potential susceptibility to cracking in the expansion transition regions, many Westinghouse units have implemented shotpeening or rotopeening of tha expansion transitions to enhance the resistance of this region of the tube bundle to primary water stress corrosions cracking (PWSCC).
This remedial measure, when implemented prior to commercial operation, can be effective in mitigating the effects of PWSCC.
Prairie Island did not take this measure because the units had already been placed in service.
Collectively, the items discussed above and further detailed on the following pages provide justification for the continued operation of Prairie Island.
2.0 OPERATING EXPERIENCE WITH CIRCUMFERENTIAL CRACKING FOR THE U.S.
POPULATION OF SIMIIAR WESTINGHOUSE STEAM GENERATORS This section lists Westinghouse steam generators with Alloy 600 mill-annealed tubing with a partial depth hardroll expansion process.
The Prairie Island Plant uses Westinghouse Model 51 steam generators. The steam generators at Prairie Island use Alloy 600 mill-annealed tubing with GL9503. DOC
p Attachment Jae 27, 1995 Pge 3 partial depth hardroll expansion.
The nominal tube OD is 0.875-inch OD x 0.050 inch nominal wall thickness.
3.1 Partial Depth Hardroll Plants Plants with partial depth hardroll expanded tube to tubesheet joints represent the earliest units to ccme on line, some with over twenty years of operational i
experience and all but one with at least twenty calendar years of operation.
The incidence of circumferential cracking at the roll transition in these plants has been negligible.
Circumferential indications in the transition region has been reported at Indian Point Unit 2 in March of 1995. While circumferential indications were reported, it is believed that these indications are due to closely spaced axial cracking.
In 1992, a large number of indications of either a circumferential or cellular nature as determined by eddy current inspection were reported at Cook Unit 1.
Tube pull examination data indicated that the degradation morphology was that of OD initiated cellular corrosion.
In cellular corrosion, axial cracking dominates the morphology. Short, circumferential crack components of lesser depth than the dominant axial cracking can cause linkage between the axial cracks. This degradation was located at the top of the tubesheet region in non-expanded tubing, and has been attributed to localized denting.
3.1.1 Pulled Tube Examination Results Kewaunee
]
In 1990, two tubes were removed from the Kewaunee plant for the examination of significant indications at the top of tubesheet region (in nonexpanded tubing) and within the nonexpanded tube lengths within the tubesheet region. No PWSCC was detected at the roll transition regions upon destructive examination.
t Insignificant (2 to 3% deep) OD degradation was detected in the roll transition region.
Cook Unit 1 i
Four tubes were removed from Cook Unit 1 in 1992 in support of the voltage based IPC and also to determine the nature of tube degradation at the top of tubesheet region. One tube, R12 C29, circumferential1y separated at the top of tubesheet elevation during the tube pull.
Further investigation of this tube and the remaining three showed the cracking pattern to be axially dominated cellular corrosion. The remaining three top of tubesheet elevations were burst tested.
Burst pressures were 5625 psi, 7400 psi and 8100 psi. All burst specimens developed axial burst openings. The burst macrocracks were all comprised of multiple microcracks.
The extent of the burst cracks for all samples extended from about 0.2" above the top of the tubesheet to about 1 inch below the top of the tubesheet.
In situ, indications of similar morphology, if continued to experience corrosion, would have been provided additional burst capability due to the proximity of the tubesheet. No roll transition degradation was reported. NDE data indicated that these signals were associated with denting at the top of the tubesheet.
In the case of R12 C29, a 37 volt dent was reported.
For each of these indications, either GL9503. DOC
l Attachment J ee 27, 1995 l
rose a distorted bobbin indications (DI) or RPC indications or circumferential1y oriented or a narrow band of closely spaced axial cracking or obliquely angled cracking was detected.
l Prairie Island 3.1.2 EOC Structural Limit Crack Angle Calculations Despite the apparent lach of circumferential indications in partial depth roll expanded plants, the folicuing EOC structural limit values are provided for information only.
Such limits would apply at other regions of the tube, not only at the expansion transition.
In partial depth roll expanded plants, tube burst is essentially precluded by the presence of the tubesheet.
Even if it were postulated that a circumferential indication were to propagate to separation, the thin gap between the tube OD and tubesheet hole ID would limit the amount of leakage encountered.
The tubesheet hole to tube OD gap represents approximately 0.018 a
inch flow area, and further hydraulic resistance would be provided by the thin gap length of approximately 18 inches from a postulated separation location to the top of the tubesheet.
In the case of the North Anna plug top I
release event, estimated primary to secondary leak rate was less than 80 gpm.
The flow area through the plug expander is approximately 0.092 inch *, and, based on this flow rate and area, it is reasonable to assume that leakage from a postulated circumferentially separated partial depth roll expanded tube separated at the top of the roll transition would be much less than the makeup j
capacity of the plant, and tube rupture is not a potential, regardless of the extent of a EOC circumferential1y oriented indications.
Further flow restriction would be provided by tubesheet bow effects and sludge accumulation in the tube to tubesheet crevice.
Despite these considerations, EOC structural limits for circumferential degradation are provided in the table below. The data used in this table was developed for 7/8 inch OD WEXTEX tubes, and are considered a conservative application to 7/8 inch OD partial depth roll expanded tubing.
In this program, the crack simulation was performed by slitting tube samples using an EDM process. A sealing bladder with thin reinforcing foil was used to prevent premature bladder extrusion through the EDM slit. EDM crack simulations and subsequent burst testing were performed for single throughwall cracks, segmented crack networks, and complex crack networks (50% OD degradation in the non-throughwall areas).
In the steam generator, lateral support provided by the tubesheet restrains bending of the tube during pressurization and would provide additional margins to these data, which were developed using modeling of nominal TSP gap lateral restraint. Additional burst pressure capability would be provided if the tube is axially constrained by the TSP due to corrosion product buildup for plants with drilled hole carbon steel TSPs, by axial constraint provided in the
(
tubesheet region by corrosion product accumulation in this area. The burst I
pressure correlations were developed based on burst tests with lateral but not axial restraint.
oL9503. DOC l
a 4
Attachment June 27, 1995
=
Pue 5 c
Utilizing the burst correlations developed from EDM data and analytical models, the structural limits for throughwall circumferential indications were developed as given for the crack models in the following table. The burst pressure data were adjusted to account for lower tolerance limit material properties.
7/8 Inch Tubing EOC Structural Limits for Circumferentially Oriented Degradations Single Single TW Crack Segmented Throughwall with 50% Degraded Throughwall Crack
[
Crack Model Ligament Model l
3(deltaP) - 4500 psi 210*
210' 264' 3(deltaP) - 4300 psi 226' 226*
269' SLB(deltaP) - 2560 psi 321*
283' 318*
The single throughwall crack model is applicable to both ID or OD degradation.
The segmen'ed model is more typical of PWSCC. The throughwall plus 50% deep model was developed to represent 360* indications found for ODSCC.
3.1.3 inspection Methodologies and Adequateness A steam generator inspection of the Prairie Island Unit 2 steam generators was just completed (June 8,1995).
This inspection included a full length 100%
bobbin coil examination and a 100% examination of the hot leg tubesheet region from tube end hot to 3 inches above the tubesheet of all inservice tubes in both steam generators using a Rotating Coil Probe which contained three different coils.
These coils were a 0.115 inch pancake coil, a 0.080 inch pancake coil for discrimination of inside versus outside diameter signals and the Plus-Point coils. A similar probe was used to inspect most of the Rows 1 and 2 U-bends.
This probe was also used to resolve distorted signals called by the bobbin probe inspection.
Four hundred nineteen tubes contained single or multiple axial indications at the Roll Transition Zone. One tube contained axial indications at the U-bend.
In all cases in the U-bends and tubesheet regions, no circumferential indications were found. All tubes with axial indications were either plugged or repaired under the F-star Alternate Repair Criteria with Additional Roll Expansion.
During the Unit 1 outage, May 1994, both Unit 1 steam generators were inspected.
This inspection included a full length 100% bobbin coil examination and a 100% examination of the hot leg tubesheet region from tube end hot to 2 inches above the tubesheet of all inservice tubes in both steam j
generators using a conventional 3-coil Rotating Pancake Coil Probe with the pancake coil size at.0.115 inch. Three hundred nineteen sleeved tubes were examined with the "I"-coil probe. Thirty-two tubes contained single or multiple axial indications at the Roll. Transition Zone.
In all cases, U-bends and tubesheet regions, no circumferential indications were found.
3.1.4 Tube Integrity Evaluations Performed By Westinghouse GL9503. DOC l
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c Attachment June 27, 1995 Fase 6 Due to the minimal, if any, circumferential1y oriented indications in the roll transitions of partial depth roll expanded plants, Westinghouse has not performed any plant specific tube integrity evaluations for this region. The recently reported indications detected at Indian Point Unit 2 in 1995 are believed to be bands of closely spaced axial cracks as opposed to circumferential1y oriented degradation.
3.1.5 Individual Plant Tube Integrity Comparisons The past two inspection programs at Prairie Island have been performed consistent with the EPRI and industry guidelines regarding calling criteria guidelines and initial sample inspection size such that any structurally significant circumferential indications have been identified, except that the most recent inspections at Prairie Island did not indicate circumferential indications in the partial depth roll expansion region.
Considering the lack of relevant indications, it is difficult to establish a growth rate for this region.
Since there are limited operating experiences of circumferential indications in partial depth roll expanded plants, it is reasonable to assume that growth rates for this region are negligible.
It would be considered conservative to assume that postulated growth rates for these tubes would be l
on the order of the detection threshold, approximately 50% as a bounding j
value. When considering these growth rates and when factoring in the industry j
accepted detection thresholds for throughwall circumferential degradation, no l
indications would be expected at Prairie Island which would challenge tube integrity at the end of the current operating cycle. Tube EOC structural integrity limits for partial depth roll plants are provided for information only. As previously discussed, the geometric configuration of partial depth
)
roll expansions essentially preclude large leakage events from expansion transition regions in partial depth roll expanded plants.
Bounding leakage from postulated large circumferential indications in partial depth roll expansions would be expected to be less than the normal makeup capacity, even for a postulated circumferential1y separated tube.
l Furthermore, future inspection plans for Prairie Island call for 1008 l
inspection of the hot leg tubesheet region using rotating coils with Plus-
[
Point or equivalent for the next two steam generator inspections for each l
unit. At that time, we will evaluate the plans for continuing inspections.
l Following this inspection program, and considering the lack of field data regarding circumferential indications for partial depth roll expanded plants, EOC throughwall circumferential indications of lengths and depths which would l
challenge tube integrity would not be expected.
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3.2 Historical Leakage Events in Westinghouse Steam Generator Tubes from Circumferential1y Oriented Tube Degradation 3.2.1 Non-sleeved Tubes Several leakage events have been attributed to WEXTEX region cracking and to U-bend tangent point leakage. Low level (<20 gpd) leakage was detected during the operational cycle prior to the 1987 inspection outage at North Anna Unit 1.
Other low level leakage events may have occurred in some 3/4 inch full depth hardroll expansion plants. Several small (less than 20 gpd) events have GL9503. DOC
1
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Attachment J oe 27. 1995 Page 7 occurred in outer Row U-bends (McGuire Unit 1).
Also, a Row 1 leaker of approximately 200 gpd was found at Catawba Unit 1 in 1988.
Both these events had dent signals associated with the cracking.
3.2.2 Sleeved Tubes The only domestic incidents involving primary to secondary leakage from circumferentially oriented degradation in sleeved tubes has occurred in tubes sleeved by Babcock and Wilcox Corporation using the kinetically welded sleeve.
At the Trojan plant, large leakage, estimated to be 2 to 3 gallons a minute i
was detected during startup from a non-scheduled outage.
The operating period from sleeve installation to detection of the leak was approximately 7 months.
The leakage came from a tube which was determined to have received an inadequate heat treatment.
Two kinetically sleeved tubes developed primary to secondary leaks at the McGuire plant.
In both cases, the leakage was attributed to circumferential cracking of the parent tube.
The cause of the cracking was eventually determined to be related to large residual stresses introduced to the tube as a result of the sleeve installation process. Operating periods from sleeve installation to time of leakage were less than two operating cycles.
All sleeves installed at Prairie Island are Combustion Engineering sleeves.
These sleeves were inspected with the "I"-coil in May 1994. No indications-due to corrosion were observed.
4.0 Defense in Depth Assessment Points 4.1 Partial Depth Roll (PDR)
Partial depth roll expanded plants are different from full depth expanded plants should postulated severe circumferential1y oriented degradation occur in the expansion transition.
The elevation of the postulated de5radation is approximately 18 inches below the top of the tubesheet, and as such, this distance would prevent tube axial misposition such that full steam generator tube rupture release rates would not be anticipated.
Sludge accumulation in the tube to tubesheet crevice region would also act to restrict any potential leakage.
Excluding the presence of sludge in the crevice, the gap between the postulated separated tube and tubesheet is limited.
For a postulated tube separation with a 17 inch crevice (1 inch of tube axial displacement is assumed), the expected primary to secondary leak rates for primary to secondary pressure differentials of 1500 and 2600 psi would be expected to be less than the normal makeup capacity of the plant. Also, tubesheet bow effects would act to close any available gap between the tube and tubesheet tube hole.
This applies also to tubes which meet the F-star Alternate Repair Criteria.
GL9503. DOC