ML20059C417
| ML20059C417 | |
| Person / Time | |
|---|---|
| Site: | Wolf Creek  |
| Issue date: | 08/23/1990 |
| From: | Pickett D Office of Nuclear Reactor Regulation |
| To: | Withers B WOLF CREEK NUCLEAR OPERATING CORP. |
| References | |
| IEB-88-011, IEB-88-11, TAC-72184, NUDOCS 9009050009 | |
| Download: ML20059C417 (2) | |
Text
N August 23, 1990
,D:cket N). 50-482 DISTRIBUT::0N 0ocket-F19e' DPickett 2
NRC PDR EJordan Mr. Bart D. Withers Local PDR EPeyton President and Chief Executive Officer PDIV-2 R/F OGC Wolf Creek Nuclear Operating Corporation' PDIV-2 Plant File ACRS (10)
Post Office Box 411 DCrutchfield JWiebe, RIV P'irlington, Kansas 66839 BBoger
. ear Mr. Withers:
SUBJECT:
NRC BULLETIN NO. 88-11, " PRESSURIZER SURGE LINE THERMAL STRATIFI-CATION" - EVALUATION OF WESTINGHOUSE OWNERS GROUP BOUNDING ANALYSIS (TAC NO. 72184)
By letter dated May 31, 1989 (WM 89-0158), the Wolf Creek Nuclear Operating Corporation responded to Item 1.b of NRC Bulletin No. 88-11. " Pressurizer Surge Line Thermal Stratification." The letter stated that a bounding analysis performed by the Westinghouse Owners Group (WOG) for which the Wolf Creek Generating Station.is applicable, indicated that the pressurizer surge line l
(PSL) may not satisfy the ASME Section III Code criteria for the life of the l
plant, taking into account the effects of thermal stratification.
The analysis did, however, state that the integrity of the PSL was adequate to justify continued operation of the facility for several years.
I The staff has completed its review of the WOG bounding analysis and concluded that there are no short-term safety concerns associated with the thermal
. stratification effects for a duration of 10 additional heatup/cooldown cycles of continued plant operation.
A copy of-the evaluation which was sent to the WOG is enclosed for your information.
The staff will assess the conformance of the PSL to applicable Codes and regu-l latory requirements for 40 year plant life when the WOG report regarding l
Item 1.d of the Bulletin is submitted.
Sincerely, OriginalSigned By:
Douglas V. Pickett, Project Manager Project Directorate IV-2 Division of Reactor Projects - III, IV, V and Special Projects Office of Nuclear Reactor Regulation
Enclosure:
i As stated 4
cc w/ enclosure:
9009050009 900823 See next page PDR ADOCK 05000482 Q
PDC OFC
- PDIV-2/LA
- PDIV-2/ M
- PDIV-2/D II DATE $8AQSV90
$8/23/90
$8 _
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......a 0FFICIfRgCORDCOPY DocumenPNam'd: " NRC BULLETIN NO. 8811 0\\
Mr. Bart D. Withers.
cc w/ enclosure:
Jay Silberg, Esq.
Mr. Gary Boyer, Plant Manager Shaw, Pittman, Potts & Trowbridge Wolf. Creek Nuclear Operating Corporation 1800 M Street, NW P. O. Box 411 Washington, D.C.
20036 Burlington, Kansas 66839 Mr. Chris R. Rogers, P.E.
Regional Administrator, Region IV Manager, Electric Department U.S. Nuclear Regulatory Commission Public Service Commission 611 Ryan Plaza Drive, Suite 1000 P. O. Box 360 Arlington, Texas 76011 Jefferson City, Missouri 65102 Mr. Otto Maynard, M6aeger Regional Administrator, Region III Regulatory Services U.S. Nuclear Regulatory Commission Wolf Creek Nuclear Operating Corporation 799 Roosevelt Road P. O. Box 411 Glen Ellyn, Illinois 60137 Burlington, Kansas 66S39 Senior Resident Inspector U. S. Nuclear Regulatory Commission P. O. Box 311 Burlington, Kansas 66839 Mr. Robert Elliot, Chief Engineer Utilities Division Kansas Corporation Commission 4th Floor - State Office Building Topeka, Kansas 66612-1571 Office of the Governor State of Kansas-Topeka, Kansas 66612 Attorney General 1st Floor - The Statehouse Topeka, Kansas 66612 Chairman, Coffey County Commission Coffey County Courthouse Burlington, Kansas 66839 Mr. Gerald Allen Public Health Physicist Bureau of Air Quality & Radiation Control Division of Environment Kansas Department of Health and Environment Forbes Field Building 321 Topeka, Kansas 66620
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Page 1 of 8 ENCLOSURE 1 NRR REVIEW 0F i
WESTINGHOUSE OWNERS GROUP (WOG)
BOUNDING EVALUATION FOR PRESSURIZER SURGE LINE THERMAL STRATIFICATION WCAP-22277 l
l INTRODUCTION
- The pressurizer surge line (PSL) in the pressurized water reactors (PWRs),
is a stainless steel pipe, connecting the bottom of the pressurizer vessel to the hot leg of the coolant loop.
The out flow of the pressurizer water is generally warmer than the hot leg flow.
Such temperature differential (AT) varies with plant operation activities and can be as high as-320*F during the initial plant heat up. Thermal stratification is the separation of cold flow stream in the horizontal portion of the PSL resulting in temperature difference at the top and bottom of the pipe.
l Since thermal stratification is the direct result of the differences in densities between the pressurizar water and the hot leg water, the
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l potential for stratification is increased as system AT increases,and as the insurge or outsurge flow decreases.
Stratification in PSL was found recently and confirmed by data measured from several PWR plants.
Original design analyses did not ilyclude any stratified flow loading conditions.
Instead it assumed complete sweep of fluid along the line during insurges or outsurges resulting in uniform thermal loading at any particular piping location. Such analyses did not reflect PSL actual thermal condition and potentially may overlook undesirable line deflection I
and its actual high stresses may exceed design limits.
In addition, the striping phenomenon, which is the oscillation of the hot and cold stratified boundary, may induce high cycle fatigue to the inner pipe wall i
L and needs also to be analyzed. Thus assessment of stratification effects l-on PSL is necessary to ensure piping integrity and ASME Code Section III conformance.
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STAFF EVALUATION Since stratification in PSL is a generic concern to all PWRs an NRC E
Information Notice No 88 80 was issued on October 7,1988, and then an NRC Bulletin 88-11 for the same concern was also issued on December 20, 1888.
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Westinghouse, on behalf of the Westinghouse Dwners Group (WDG), has performed a generic bounding evaluation report WCAP-12277 (Reference 1).
t This recurt provides the technical basis for the generic justification for continust coeration (JCO), for each of the WOG plants and constitutes compliance with the requested action 1.b of Bulletin 88-11.
Plants which have discesered any gross discernable distress during performance of the walkdown, as requested by Bulletin's action 1.a,. should report findings and specify corrective actions in their JCO, in addition to that provided in this report.
house's efforts and information provided in the report.The following Prior to.the issuance of the Bulletin, WOG implemented a program to l
address the issue of the surge line thermal stratification. The program t
consisted of plant specific analysis covering five plants and a review of thermal monitoring data from eight plants.
Westinghouse had instrumented PSLs and collected data for verifying stratification conditions.
l The thermal monitoring data obtained considered outside wall temperatures at different location around the pipe and along the axis of the pipe vs..
l time, vertical and lateral displacements at various locations along the l
pipe vs. time, and various plant parameters vs. time, from existing pitst instrumentation and control sensors.
In some cases the data were based not only on plant heatup but also on operation and plant cooldown conditions.
The specific analyses 'scluded redefinition of revised thermal transients considering stratification effects and evaluation of pipe stress and fatigue usage fact 9rs.
The overall analytical approach used in all these cases have been consistent and has been reviewed in detail by the NRC staff.
The evaluation concluded that a single bounding analysis was not feasible.
Due to the variations in design, Westinghouse could not define a single envelope case to justify the 40 year life of the surge line, therefore a bounding evaluation was-performed to justify continued operation for at least ten (10) additional heatup/cooldown cycles.
The bounding evaluation is essentially a demonstration of the applicability of the plant-specific and the monitoring results to the remaining WDG plants.
All plant-specific analyses completed to-date, have demonstrated a 40. year life of the surge line Two sets of parameters were defined.
a)
Parameters which affect severity of thermal stratifica-tion (i.e thermal hydraulic and operational effects) l 1
1 Page 3'of 8 J
b)
Parameters which affect PSL response to thermal strati-fication (i.e. structural effects).
j The range of the parameters for the plants analyzed and/or monitored was used to establish the bounding criteria and to enable an individual plant by plant comparison.
To expand the data base for a wider range of PSL configurations.
Westinghouse recommended additional plant monitoring based on plant similarities (grouping), and surge line physical, design and operational parameters of all 55 domestic Westinghouse plants.
Plants with parameters
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not within the range of the current monitoring database were recommended for additional monitoring.
Ten (10) different groups were identified for the 55 domestic Westinghouse plants for data collection, review and b
analysis of the pertinent thermal hydraulic, operational and structural i
parameters.
Currently 22 separate monitorir;g programs are either completed, in process, or being planned.
About 40% of the plants falling in one group with the remaining 60% of the Westinghouse PWR's divided among the nine groups.
When this program is completed, it will provide. sufficient monitoring data with at least one plant mordtored in each group.
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Parameters which have a significant effect from a thermal hydraulic stand point are pipe inside diameter. and slope. Plants falling outside the resulting bounding criteria of pipe inside diameter of 7" to 15.4" and pipe average slope of O' to 1.44' degrees, were recommended for monitoring.
The range of the parameters was expanded by 1 20% in determining a bounding criteria to which other plants were compared.
It was concluded that a bounding evaluation which is based on enveloping techniques, will not have a significant effect on the thermohydraulic behavior due to a 1 20% change in pipe size and slope. The staff. agrees with Westinghouse's efforts and rnethodology for monitoring, updating and assessing PSL for the stratification condition.
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Thermal hydraulic evaluations using higher slope and smaller diameter pipe are expected to reduce the stratification effects.
Eleven plants have an average slope higher and one plant has a pipe size smaller than that allowed by the criteria.
Some parameters judged to be relatively significant for the structural effects are:
P Entra6ce angle to the hot leg nozzle j
Mid line riser Length of the longest straight run of pipe L
Type of in-line component Presence of whip restraints L.
Number of vertical rigid supports i
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Page 4 of 8 Two plants have longest straight run lengths which are less than the minimum in the criteria, and four plants have welded lugs or trunnion attachments to PSL.
Since none of the analysis to date has included evaluation of welded attachments, which may increase thermal stresses due i
to structural discontinuities, this condition falls outside the criteria and the following has been recommended.
a)
Inspection of these welds during walkdown b)
Evaluation of these attachments on a plant-specific basis.'
The five plant specific analysis consisted of three parts: (1) global effects on stresses, moments, displacements, and support reaction loads, based on both axial and radial variations in the pipe metal temperature, (2) local stresses due to thermal gradient, and (3) local stresses and effects to fatigue due to thermal striping.
The global and local stresses in items (1) and (2) above were superimposed to obtain total stresses.
In addition to the detailed plant specific analysis for the five plants, twelve (12) plants have completed interim evaluations of the surge line stratification which include Finite Element structural analysis of each-I specific configuration under stratified conditions.
The five plants for which detailed plant-specific analyses performed and evaluation reports submitted to NRC, are as follows:
l-Seabrook (see WCAP-12151 and Suppl.1, and WCAP-12305) south Texas Units 1 and 2 (see WCAP-12067 Rav 1 and Suppl. 1) Vogle Unit 2 (see WCAP'12132, WCAP-12199 and WCAP-12218) Beaver Valley Unit 2 (see WCAP-12093 and Supp1's 1 and 2) Comanche. Peak Unit 1 (see WCAP-12248 and Suppl. 1)
Based on these reviews Westinghous concluded that a shorter horizontal length will result in lower loads since the the surge line will experience less vertical deflection and it will tend to to result in a more uniform distribution of the bending moment due to stratified loading.
In addition middle line risers will also tend to reduce the stratification effects.
The stratification induced global. bending of the surge line was calculated using ANSYS computer code.
Although a 320'F step temperature change was assumed for stratification through out the surge line, the changes were linearized in ANSYS using conventional pipe element model.. Finite Element L
models were used to calculate local stresses due to top-tobottom non-linear l
thermal gradients in the PSL.
Five (5) hot-to-cold interface locations were analyzed using eleven (11) cases of thermal stratification, to L
calculate piping response under all required loading conditions, reflecting temperatures differences up to 320*F.
Other casos were obtained by interpolation. Westinghouse reported that their best estimate analytical j
l results compared favorably with measured displacements data observed during monitoring.
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Page 5 of 8 In two of the analyses, a rigid vertical support was removed. In ane case 1
it was shown that the support was not requireo and in the other case it was replaced with a snubber and a spring. The PSL was subsequently re qualified and found acceptable.
Stress summary results from the five plant specific analyses performed to date indicates that the primary plus secondary stress intensity range i
ratio of equation 12 of ASME III section NS-3600, is less than 1.0.
j The i
critical location for stress is usually the safe end weld of the nozzle connecting the surge line to the primary-loop hot leg.
In one case onl it was determined to be at the reducer. This seem to be a unique ce:: y since no other utility within the WDG has a mid line reaucer it, de PSL.
Stresses were intensified by "K" factors, to account for the worst case concentration for all piping elements in the PSL. The staff agrees with
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the approaches used by the ifcensee for performing PSL reanalysis.
l To account for the thermal striping effects to PSL, flow model test results, performed for the Liquid Metal Fast Breeder Reactor primary loop and for the Mitsubishi. Heavy Industries Feedwater Line cracking, were reviewed to establish the boundary condition.
These test results were l
used to define striping esci11ation data, amplitude and frequencies, for evaluating high cycle fatigue.
Portions of PSL which experience stratification and striping were defined based on measured results.
Westinghouse reported that consioering AT attenuation with time, and a frequency of.30 HZ, a usage factor of less than.20 was determined as the worst case due to striping alone, even when the stresses were intensified by "K" factors.
The worst case element was determined to be the butt weld.
A surface film coefficient of 500 870/hr-sq. ft
'F was used and it was based on a flow rate of 90 gpm;which was assumed to be constant throughout all striping analysis. Although the data used in the assess-ment were obtained from scale test model which showed that the frequency can range from.10-10 HZ. the staff agreed that the stresses will be higher with the lower-frequency and the.30 HZ. average frequency is justified.
However the thermal striping potential due to a film coefficient of 500 STU/hr-sq. ft.
- F and attenuation of AT is questionable, but at this time no other better number axists and therefore this represents the best judgement.
If other information will be available, based on the ongoing efforts by EPRI or possible future NRC research work, it will be utilized and further assessments will be made for assessing the striping effects to PSL.
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With the thermal transients redefined, new fatigue usage factors were calculated.
To determine the new fatigue usage factors, the more detail techniques of ASME III Nis-3200 were employed.- Due to the non-axisymmetric nature of stratification Icading, stresses due to all loadings were obtained from Finite Element analysis and then combined on a stress compo-nent basis.
Five (5) levels of thermal stratification at five worst case l'
i st Page 6 of 8 points were calculated using the WECEVAL program.
by "K" factors to account for worst case concentration in the pipingStr elements.-
Westinghouse reported that a cumulative usage factor (CUF) of 0.73 was determined.to be the worst case at the' hot leg nozzle safe end location for all but one case.-
line and the CUF at that location was determined to 0.94.For that 1
contributions from both global bending and local effects of stratiff-The CUF inclu distribution and striping. cation considering the nonlinear step cha seismic are not affected by thermal stratification have a minor effect on the calculation of the fatigue usage factor.
1 plant specific analyses was reported to be 18%. contribution The I
All of the detailed plant y
specific analyses assumed the occurrence of twenty operational earthqua The staff agrees with the approaches used by Westinghouse for calc i
the usage factor.
1 Westinghouse reported that the PSL fatigue life is 4
The worst case years of operation at any WOG plant is 28.5 years.
j worst case number of heatup cooldown cycles is 75 and occurs at a 'different The plant.
Based on the combination of these two worst case valu'es an i
" Operating Life Factor" (0LF) of 0.44 is obtained which indicates that no l
more than 50% of the operating life has been used at any Westinghouse plant to date.
For the generic case of a CUF = 1.0 a 17% value was 83% value attributed to striping. attributed to age and an 83% wa l
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page 1 of g CONCLUSIONS i
i Bssed on our review, we conclude that the infomation provided by k'estinghouse in references 1 and 2, is comprehensive and acceptable.
Westinghouse on behalf of the Owners Group had made acceptable efforts to provide technical basis for the Itcensee's' JC0 as indicated in the requested actions of the NRC Bulletin 88-11, item 1.b.
The staff' believes that there is no immediate or short tem safety concerns associated with the stratification effects for 10 additional heatup/
cooldown cycles of continued plant operation.
However, each of the WDG plants should submit a JC0 using this report as the basis.
We will assess if the surge Ifne in each plant meet the code acceptance criteria for the 40 year plant life when additional generic analyses based on plant Grouping l
1s performed by Westinghouse for the Owners group.
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-REFERENCES 1.
Westinghouse Report WCAP-12277 (Proprietary), and WCAP-12278 (Non-Proprietary).
" Westinghouse Owners Group bounding evaluation for Pressurizer surge Line thermal stratification," Jur.a 15, 1988.
. 2.
Viewgraphs by Westinghouse.
Presentation to NRC on May 23 and 24.
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