ML20235Y328
| ML20235Y328 | |
| Person / Time | |
|---|---|
| Site: | Seabrook  |
| Issue date: | 02/28/1989 |
| From: | Chang K, Ellis G, Swamy S WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML19302D655 | List: |
| References | |
| WCAP-12152, NUDOCS 8903140071 | |
| Download: ML20235Y328 (23) | |
Text
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WESTINGHOUSE PROPRIETARY CLASS 3 WCAP-12152 ASSESSMENT OF THERMAL STRATIFICATION FOR THE SEABROOK UNIT 1 PRESSURIZER SURGE LINE l
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February 1989 l
1 B. J. Coslow E. L. Cranford W. H. Bamford B. F. Maurer M. A. Gray Verified by:
Verified by: M em7
/5. M K. G. Chang G
Approved by:
M Approved by:
G. R. Ellis, Manfger
- 5. 5. fillusamy, Manager Structural Engineering &
Structural Materials Piping Technology Engineering Work Performed Under Shop Order NGXP-961 WESTINGHOUSE ELECTRIC CORPORATION Nuclear and Advanced Technology Division P.O. Box 2728 Pittsburgh, Pennsylvania 15230-2728 3064a/02070010 8903140071 890307 DR ADOCK 050 3
TABLE OF CONTENTS Soction
- Title Page
1.0 INTRODUCTION
1-1 2.0 THERMAL TRANSIENT ASSESSMENT 2-1 2.1 Heatup and Cooldown Procedures 2-1 2.2 Preliminary Monitoring Program and Results 2-1 2.3 Conclusions for Thermal Transient Assessment 2-2
3.0 ASSESSMENT
OF PIPING LAYOUT 3-1 3.1 Seabrook Layout 3-1 3.2 Comparison with Analyzed Plants 3-1 3.3 Conclusions on Assessment of Piping Layout 3-2 4.0 FEASIBILITY OF DEMONSTRATING ASME CODE ACCEPTABILITY 4-1 4.1 Pipe Stress 4-1 4.2 Support Load 4-3 4.3 Fatigue Life 4-3 4.4 Fatigue Crack Growth 4-4 5.0 OVERALL SAFETY ASSESSMENT 5-1 5.1 Current Status of the Seabrook Unit 1 5-1 5.2 Probability of Surge Line Failure 5-2 5.3 Plant LOCA Design Basis 5-3 5.4 NRC Involvement 5-3 5.5 WDG Pressurizer Surge Line Stratification Program 5-4 6.0 PROGRAM DESCRIPTION FOR DETAILED PLANT SPECIFIC ANALYSIS 6-1 7.0
SUMMARY
AND CONCLUSION 7-1 mu.mona
SECTION
1.0 INTRODUCTION
The purpose of this report is to present the results of a study performed to estimate the effect of thermal stratification on the Seabrook Unit 1 pressurizer surge line. This report is intended to assess the feasibility of demonstrating compliance with the ASME Code, considering stratification offects, prior to performing a detailed plant specific analysis.
This feasibility assessment is based on a comparison of the Seabrook surge line to that of several other plants for which detailed plant specific analyses have been performed by Westinghouse, and by a limited Seabrook-specific finite element structural analysis and thermal monitoring test.
The conclusions reached in this study are based upon this comparison, as well as the judgement of engineers expseienced in the evaluation of surge line stratification.
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m..mem o 11
SECTION 2.0 THERMAL TRANSIENT ASSESSMENT This section presents an assessment of the expected thermal transient severity for the Seabrook surge line as compared with transients developed by Westinghouse for use in evaluating surge line stratification.
The transients developed by Westinghouse are based on a review f operating procedures, including heatup and cooldown practices, and evaluation of thermal stratification monitoring data from several Westinghouse plants.
2.1 Heatup and Cooldown Procedures fn the evaluation of surge line thermal transients considering thermal stratification and striping, the most critical factor related to operating procedures is the (
ja.c.e Westinghouse performed a review of the Seabrook heatup and cooldown procedures which included discussions with plant operating personnel.
[
3a,c.e 2.2 Preliminary Monitoring Program and Results A preliminary monitoring program was established at Seabrook Unit 1.
This program utilizes one set of circumferentially located temperature detectors approximately fifteen (15) feet from the RCL safe end. One temperature detector is also located on the vertical riser below the pressurizer.
These locations are shown in figure 1.
This arrangement is sufficient to determine if thermal stratification is present in the Seabrook surge.line.
l sspuomow to 2-1 l
WESTINGHOUSE PRoPalETARY Vertical displacement sensors are located approximately eighteen (18) feet from the RCL safe end and nine (9) feet from the vertical riser below the pressurizer.
These locations are also shown in figure 1.
Monitoring data has been received from a recent heatup.
The data confirms the presence of thermal stratification. The maximum temperature difference (
ja,c.e This value is consistent with that observed at other plants, and is enveloped by the existing transients developed by Westinghouse. (
3a,c.e 2.3 Conclusions for Thermal Transient Assessment e
Based upon a review of the Seabrook operating procedures and limited thermal stratification monitoring data, Westinghouse considers that the expected thermal transient severity for the Seabrook surge line will not exceed that used to perform plant specific analyses to date.
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m. mom in 2-2
WESTINGHOUSE FRoFRlrTARY SECTION
3.0 ASSESSMENT
OF PIPING LAYOUT This section presents the results of a comparison between the Seabrook surge line layout and that of several plants for which a plant specific analysis has been performed.
s 3.1 Seabrook Layout The Seabrook surge line layout is shown in figure 1.
The piping size is 14 inch schedule 160.
3.2 Comparison With Analyzed Plants Experience with the analysis of thermal stratification has indicated that surge line layout [
)a,c.e The Seabrook surge line layout contains two vertical rigid restraints as shown in figure 1.
Plant specific analyses have been performed on two plants (
la,c.e For Seabrook, however, the effect of two vertical rigid restraints will be less severe as outlined below and in section 4.
Pipe stresses in the Seabrook surge line are expected to be less sensitive to the presence of two rigid vertical restraints. [
la.c.e At Seabrook, a 5-D bend exists between these two supports (all curved pipe portions of the Seabrook layout are 5-D bends). Stresses in a 5-0 bend will be lower than those in a welded elbow. (
la,c.e The Seabrook plant has no reducer in the surge line.
nu.moru in 3-1
WESTINGHOUSE PROPRIETARY Support load capacity for the two rigid vertical restraints on the Seabrook surge line is much greater than for the [-
.]a,c,e Both of the Seabrook rigid vertical supports are very substantial fabricated structures.
Another important factor regarding surge line layout is (
ja c.e [
]a,c.e therefore, the Seabrook surge line layout compares favorably to other plants in this regard.
Additionally, except for one case, all plant specific analyses have been done on
. surge lines with (
Ja,c.e This is consistent with the Seabrook surge line.
3.3 Conclusions on Assessment of Pipino Layout Based solely on a comparison of layout and geometry between the Seabrook surge line and those for which a plant specific analysis has been performed, it is judged that pipe expansion stresses under stratification will be higher than original design, but should still meet the limits of the ASME Code.
Based upon a review of suppcrt and whip restraint drawings, it is judged that support capacities will not be exceeded and that other whip restraints will not impede vertical motion due to stratification.
l w.mme io 3-2
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WESTINGHOUSE PROPRIETARY l
l l
SECTION 4.0 FEASIBILITY OF DEMONSTRATING ASME CODE ACCEPTABILITY 4.1 Pipe Stress l
l In order to assess the potential of meeting the ASME Section III Code limits, a structural model of the Seabrook surgeline was developed using the ANSYS general purpose finite element computer code.
The model was constructed using (
Ja.c.e to reflect the layout of straight pipe, 5-D bends and field welds as shown in figure 1.
Also included were the two rigid vertical supports and one rigid horizontal lateral support near the pressurizer.
(
3a,c.e For the stratified condition, (
Ja,c.e These temperature distributions were established from the bounding transients, developed by Westinghouse, as discussed in section 2.0.
From the bounding transients set, [
la,c.e cases of stratification temperature distribution had been defined. These [
Ja,c.e cases develop global thermal expansion moment loadings in the pipe for the range of stratification conditions postulated by the bounding transient set.
From analyses of plants with layouts similar to Seabrook, the maximum range of thermal expansion stress (ASME equation 12) was found to be (
3a,c.e The results of the ANSYS analysis showed piping loads to be maximum at ('
la c.e An expansion stress range between (
3a,c e resulted in an equation 12 stress value of 53 ksi, compared to an allowable stress (3 S,) value of 55.7 ksi, [
la c.e This stress was calculated using a stress index (C ) value derived from (
2 3a,c.e NSes/02244010 4q
WESTINGHOUSE FRzPRIETARY In addition, the expansion stress range was checked at all other intermediate locations along the surge line. Stresses at straight pipe and weld locations were well within the 3 S, limit. The stresses at the bends were also below 3 S, using standard ASME stress indices, and could also be reduced using the more detailed stress index calculations for curved pipe allowed by the Code.
i At the pressurizer nozzle, the stratification case loads exceeded the conservative allowable values given in the equipment specification.
- However, the pipe stress at the safe end weld was found to be within the 3 S, allowable for this case.
It is, therefore, judged that this load will be acceptable when l
a detailed stress analysis of the pressurizer nozzle is performed.
1 l
Other pipe stress requirements include primary stress and equation 13 stress.
Primary stresses will not be affected by thermal stratification, so that the previous design analysis is still valid. Also, equation 13 stresses from the previous design analysis will not be affected at locations without a structural discontinuity, since equation 13 only considers pressure, mechanical loads (which are unchanged), and stress due to differential expansion at a structural discontinuity.
[
la.c.e For the hot leg nozzle, squation 13 stress has been qualified for the postulated stratification l
transient loadings on other plants who.e nozzles have a more severe structural discontinuity than the Seabrook hot leg nozzle. Therefore, it was judged that equation 13 stress limits will be satisfied.
l In light of the above comparison, although structural analyses for all j
stratification cases were not performed, it is judged that detailed analysis of l
the Seabrook surge line will result in pipe stresses below ASME allowable j
limits. The fatigue usage. factor assessment for external loading and transient loadings is presented in section 4.3 4-2
WESTINGHOUSE PROPRIETARY 4.2 Support Loads For the stratification case chosen, the loads on the two vertical rigid supports, 49-SG-01 and 49-SG-04, were 42 kips and 10 kips, respectively.
For 49-SG-01, the support has been analyzed to maximum loads of 90 kips in the +Y direction and 111 kips in the -Y direction. It is therefore judged that sufficient margin exists at 49-SG-01 to account for the increase to 42 kips due to thermal stratification load.
For 49-SG-04, the previous design analysis load was 18 kips, which is greater than the load for the stratifica-tion case. Also, horizontal loads and displacements are generally less than in the previous design analysis.
Displacements were also compared between the stratification case and the design analysis case. The maximum vertical displacement (stratified case) was 0.83 inches compared to 0.6 inches, and horizontal displacements were less than those from the previous analysis. Vertical displacements from the stratification case were found to be less than available clearances at whip restraints.
Based on these comparisons, it is judged that a detailed analysis will not necessitate support modifications.
l 4.3 Fatigue Life For other plants analyzed, ASME fatigue usage factors have been calculated considering the phenomenon of thermal stratification and thermal striping at l
various locations in the surge line.
Total stresses included (
Ja,c.e The total stresses for all transients in the bounding set were used to form combinations to calculate alternating stresses and resulting fatigue damage in the manner defined by the Code. Of this total. stress, the stresses in the 14 inch schedule 160 pipe due to ['
Ja,c.e The stress due to global expansion moments will be different for the Seabrook configuration.
m.mme in 43
WESTINGHOUSE PROPRIETARY
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The maximum fatigue usage factors for other plants with similar pipe size (14 inch Schedule 160) and layout occurred at (
Ja,c.e The most significant contribution to
]a c.e was the stress due to global expansion the usage factor at the [
moment. For an elbow with equation 12 stress just below the 3 S, allowable limit, a usage factor much less than the code allowable of 1.0 was calculated.
Therefore, since the Seabrook surge line has (
la.c.e, with an expansion stress less than that of the elbow previously analyzed and qualified in similar locations, it is judged that the usage factor limit for straight pipe and bends in the Seabrook surge line would be met if a detailed fatigue evaluation is performed.
For the [
la.c.e a usage factor less than 1.0 was achieved for a [
Ja.c.e with a more severe structural discontinuity and with equation 12 stress just below the 3 S, limit.
Therefore, it is judged that the Seabrook (
Ja,c.e would be shown to be acceptable for fatigue if a detailed analysis is performed.
For the pressurizer nozzle, [
]a,c.e The effect of loads higher than those obtained for the stratification case for Seabrook is presently being evaluated for a similar plant. Although the evaluation is not yet complete, preliminary indications are that the pressurizer nozzle will be acceptable for the full 40 year plant life.
It is therefore judged that the Seabrook pressurizer surge nozzle will be able to withstand the cyclic loading from thermal stratification and striping.
l 4.4 Fatigue Crack Growth Fatigue crack growth calculations have been performed as part of a detailed ovaluation of surge line stratification for several Westinghouse plants.
Standard ASME Section XI methods were used.
Initial flaw sizes were selected based on Section XI inspection detection toleran:es.
m..mone in 44
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WinmetGMOUDE PROPRIETARY Experience indicates that maximum fatigue crack growth is dependent upon (.
Ja.c.e For an initial flaw size of 10% of the wall thickness, all previous analyses of stratified surge lines have demonstrated final'(40 year) crack sizes of less than 60% of the wall thickness.
Local stratification stresses, for the Seabrook surge line, will be [
Ja,c.e Global bending stresses, based on results from the ANSYS analysis described in section 4.1, will be somewhat less than values calculated for other surge lines.
8t is therefore judged that a detailed fatigue crack growth evaluation of the Seabrook surge line would demonstrate that a 10%-of-wa11 initial flaw will grow to less than 50% of tha wall thickness in 40 years.
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WESTINGHOUSE r.toPnlETARY _
SECTION 5.0 OVERALL SAFETY ASSESSMENT St was first reported in INPO SER 25-87 that temperature measurements at a German PWR indicated thermal transients different than design. Recent measurements at several domestic PWR's have indicated that the temperature difference between the pressurizer and the hot leg results in stratified flow in the surge line, with the top of the flow stream being hot (pressurizer temperature) and the bottom being colder (hot leg temperature). The top-to-bottom temperature difference can reach 250*F to 300'F in certain modes of operation, particularly Modes 3, 4, or 5 during heatup and cooldown.
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Surge line stratification causes two effects:
o Global bending of the pipe different than that predicted in the original design.
s o
Fatigue life of the piping can be reduced due to the global and local stresses from stratification and striping.
Westinghouse has performed an assessment of the Seabrook Unit 1 pressurizer surge line, with regard to stratification loading conditions. The results of our assessment are discussed in the following sections.
5.1, Current Status of the Seabrook Unit 1 westinghouse has performed an assessment of the surge line, including the effects of stratification loading, in accordance with the ASME Code Section XII which is the design code of record for Seabrook. The assessment included i
maximum uniform thermal expansion, per the original design basis analysis, as l
well as the maximum predicted flow stratification condition.
The results of this assessment are reported in sections 2 through 4 of this report.
ms mowo 51
WEsTIN2 HOUSE PR2PRIETARY The results of the walkdown performed after hot functional testing by New Hampshire Yankee personnel indicated no signs of distress in the supports.
Also, there was no indication of any crushed insulation or other signs of abnormal pipe movements.
The design of the Seabrook Unit 1 surge line is quite similar to the surge lines of other Westinghouse plants. The layout includes 'several bends to allow for thermal expansion of the line. The materials are SA376 Type 316 and SA376 Type 304 stainless steel. Operation of the plant relative to para-meters affecting the surge line (primarily the maximum temperature difference between the pressurizer and the reactor coolant loops) is also similar to other plants. Therefore, the acceptable analysis results that are being cbtained from detailed stratification evaluations on other plants are indicative of a favorable fatigue situation at Seabrook Unit 1.
5.2 Probability of Surge Line Failure Non-destructive examinations (NDE) have been performed on the Seabr6ok Unit 1 surge line, using current ASME Section XI criteria. The NDE results have identified no cracks or indications in the surge line.
In addition, there have been no instances in the industry of any cracks or flaws reported in the surge line of any Westinghouse PWR.
Stratification may cause crack growth rates which differ from current calculations. However, recognizing that specific calculations have not yet been performad, Westinghouse fracture mechanics experience indicates the substantial margin between a detectable leakage crack and an unstable crack will remain.
Seabrook Unit 1 operation to date has shown no signs of distress with the surge line. The Seabrook surge line configuration is nearly identical to that of another Westinghouse plant which has been in service for about 12 years. A comparison of the two configurations is shown in figure 2.
For purposes of comparison, the Seabrook layout is shown here in mirror-image.
The older plant's surge line has been walked down and found to have no signs of distress.
mu.mu io 5-2
WESTINGH!uSE PROPRIETARY Furthermore, this line has been inspected and found to be free of indications.
This result gives further confidence that the Seabrook Unit 1 surge line will operate reliably.
Based on the similarity of the Seabrook Unit 1 surge line loadings and materials to the many other plants that Westinghouse has been involved with, and the fact that no indications have been found during the inspections of the surge line to date, it would be highly unikely that a through wall crack would be generated. Furthermore, even if such a crack were to develop, experience indicates that the crack would remain stable and would not cause catastrophic failure of the surge line.
5.3 Plant LOC!. Design Basis The Seabrook plant design is basod, in part, on the pot.tulation of breaks in high energy piping, including breaks in the pressurizer surge line.
(The dynamic offects of pipe rupture of the reactor coolant loop piping are excluded from the c e'gn basis as a result of the NRC's acceptance of LBB for the Seabrook Unit 1.) As a result of this design basis, pipe whip restraints have been installed, and the effects of pipe break on containment design, instrumentation qualification, and ECCS performance have been addressed.
Even in the extremely remote event that a V 1 double ended break should occur in the surge line, the resultant effocts of such a break have been accounted for in the design of the plant.
5.4 NRC Involvement Westinghouse has had a number of meetings with the NRC concerning surge line stratification involving several other Westinghouse plants. The stratifica-tion phenomenon and its potential effect on piping integrity have been discussed. As a result of these meetings, as well as NRC meetings with the three PWR Owners Groups, the NRC issued Information Notice 88-80, " Unexpected Piping Novement Attributed To Thermal Stratification," dated October 7, 1988.
Increased fatigue usage is the primary thrust of this Notice.
5-3
WESTINGHOUSE PROPRIETARY More recently, the NRC has issued Bulletin 88-11 " Pressurizer Surge Line Thermal Stratification," December 20, 1988, identifying actions to be taken by licensees.
a)
Conduct visual inspection - walkdown b)
Update stress and fatigue analysis to account for stratification c)
Install monitoring, as necessary It is mentioned in the bulletin that licensees may obtain monitoring data through collective efforts with other plants of similar design.
5.5 WOG Pressurizer Surge Line Stratification Program
!n October 1988, the Westinghouse Owners Group (WOG) authorized a program to perform a generic evaluation of surge line stratification in Westinghouse PWR's. This program will evaluate the design and operation of Westinghouse units with respect to the surge line, and determine the factors which could significantly contribute to thermal stratification. The key objectives of this program are :
o Collect and summarize plant physical and operational data for all WOG plants.
o Develop a generic Justification for Continued Operation, o
Categorize and prioritize plants and parameters o
Estimate the extent of fatigue damage, o
Provide short term and long term recommendations.
l The schedule for completion of the total program is estimated to be approxi-I mately (14) months with an estimated completion date of January,1990. The program has been presented to the NRC, and they are in agreement with the approach. The program has been structured to be responsive to the NRC Bulletin 88-11.
m.. mom io 5-4
WESTINGHOUSE PROPRIETARY SECTION 6.0 PROGRAM DESCRIPTION FOR DETAILED PLANT SPECIFIC ANALYSIC l
A detailed plant specific surge line stratification analysis will be l
performed. This analysis will satisfy the requirements of NRC Bulletin 88-11.
The basic outline of this analysis will include the following tasks as required:
o Plant specific monitoring and data collection during low power testing o
Heat transfer, local thermal and component stress analysis o
Develop revised transient set o
Define piping loads o
Fatigue usage factor analysis
WESTINGHOUSE PROPRIETARY SECTION 7.0
SUMMARY
AND CONCLUSION A study was performed to assess the feasibility of demonstrating ASME Code compliance of the Seabrook pressurizer surge line under thermal stratification loading. The assessment is based on comparison of the Seabrook surge line to i
several other plants which have undergone detailed analyses by Westinghouse and a limited plant specific piping structural analysis.
In addition, Westinghouse PWR's have a significant number of years of operating experience. There has never been a reported failure or flaw indication as a result of NDE of the surge line at any of these plants. The Seabrook Unit 1 surge line is similar in design, materials, and operation to these other plants.
Westinghouse's extensive fracturc mechanics experience in fatigue crack growth and LBB technology suggests a very small likelihood of a through wall crack resulting in leakage in the surge line. The likelihood of a full double ended break would be even more remote.
Based on the >tudy summarized in this report, it is the judgement of Westinghouse that a detailed plant specific analysis of the Seabrook Unit 1 surge line, considering thermal stratification and striping loading, will demonstrate compliance to all applicable ASME Code requirements, maanm ie 71
WESTINGHOUSE PROPRIETARY
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WESTIN2k;JtJtt PROPfutTARY a,c.e l
l Figure 2.
Comparison of Seabrook Surgeline Layout with that of a Similar }{
Plant nu. mon. io 7-3
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l ENCLOSURE 3 TO NYN-89023 PIPING ISOMETRIC DRAVING NO. SURC-WOO 49
1 OVERSIZE DOCUMENT PAGE PULLED SEE APERTURE CARDS
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APERTURE CARD /HARD COPY AVAILABLE FROM QECORDS AND REPORTS MANAGEMENT BRANCH 1
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I ENCLOSURE 4 TO NYN-89023 WESTINGHOUSE LETTER NO. CAW-89-025, PROPRIETARY INFORMATION NOTICE, AFFIDAVIT CAW-88-129 1
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