ML20059M041

Identifies No short-term Safety Concerns Re Thermal Stratification,Per 890531 Response to NRC Bulletin 88-011, Presurizer Surge Line Thermal Stratification

ML20059M041
Person / Time
Site:	Point Beach
Issue date:	09/25/1990
From:	Samworth R Office of Nuclear Reactor Regulation
To:	Fay C WISCONSIN ELECTRIC POWER CO.
References
IEB-88-011, IEB-88-11, TAC-72155, TAC-72156, NUDOCS 9010030002
Download: ML20059M041 (3)

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September 25, 1990 Docket Nos. 50-266 DISTRIBUTION:

and 50-301 Docket: Files! NRC & Local PDRs POIII-3 r/f DCrutchfield JZwolinski JHannon RSamworth PKreutzer Mr. C. W. Fay, Vice President- OGC-WF1 ACRS(10)

Nuclear Power Department EJordan PDIII-3 Gray Wisconsin Electric Power Company 231 West Michigan Street, Room 308 Milwaukee, Wisconsin 53201

SUBJECT:

NRC BULLETIN N 98-11. " PRESSURIZER SURGE LINE THERMAL STRATIFICATION"

- EVALUATION Ot h.STINGHOUSE DWNERS GROUP BOUNDING ANALYSIS (TAC NOS. 72155 /cHD 72156)

By letter dated May 31, 1989, Wisconsin Electric Power Company responded to

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Item 1.b of NRC Bulletin No. 88-11. " Pressurizer Surge Line Thermal Stratifi-cation." The letter stated that a bounding analysis performed by the Westing-house Owners Group (WOG) which is applicable to the Point Beach Nuclear. Plant Units' I and 2 indicated that the pressurizer surge line (PSL) may not satisfy the ASME Section III Code criteria for the life of the 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.

The staff has completed its review of the WOG bounding analysis and concludes that there are no short-term safety concerns associated with the thermal stratification effects for a duration of ten additional heatup/cooldown cycles of continued plant operation. A copy of the evaluation that was sent to the WOG is enclosed for your information.

The staff will assess the conformance of the PSL to applicable Codes and regulatory requirements for 40 year plant life when the WOG report regarding Item 1.d of the Bulletin is submitted.

Sincerely, OriginalSigned By:

Robert B. Samworth, Project Manager Project Directorate III-3 Division of Reactor Projects - III, IV, V and Special Projects Office of Nuclear Reactor Regulation

Enclosure:

As stated.

cc w/ enclosure:

See next page DOCUMENT NAME: 0 W si h215s But. 88-11 Of fice: PD/ D II-3 n

LA/P4 I-3 Surname: 'PXpeu zer / /g M/P0lII-3 {

RSamworth/tg JHannon f()

.Date: t) /j3 /90 7/tfl90 9010030002 900925 f/)f/90 i\\

PDR ADOCK 05000266 \'

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. .' September 25, 1990 Docket Nos. 50-266 DISTRIBUTION:

T. and 50-301 Docket Files NRC & Local PDRs 9 PDIll-3 r/f DCrutchfield JZwolinski JHannon RSamworth PKreutzer Mr.-C. W. Fay, Vice President OGC-WF1 ACRS(10)

Nuclear Power Department EJordan PDIII-3 Gray Wisconsin Electric Power Company 231 West Michigan Street, Room 308 Milwaukee, Wisconsin 53201

SUBJECT:

NRC BULLETIN NO. 88-11. " PRESSURIZER SURGE LINE THERMAL STRATIFICATION"

- EVALUATION OF WESTINGHOUSE OWNERS GROUP BOUNDING ANALYSIS (TAC NOS 72155 AND 72156)

By letter dated May 31, 1989, Wisconsin Electric Power Company responded to Item 1.b of NRC Bulletin No. 88-11. " Pressurizer Surge Line Thermal Stratifi-cation." The letter stated that a bounding analysis performed by the Westing-house Owners Group (WOG) which is applicable to the Point Beach Nuclear Plant Units I and 2 indicated that the pressurizer surge line (PSL) may not satisfy the ASME Section III Code criteria foe the life of the plant, taking inte account the effects of thermal stratification. The analysis did, however, state that the integrity of the PSL was adequate to just'.fy continued operation of the facility for several years.

Tha staff has completed its review of the WOG bounding analysis and concludes that there are no short-term safety concerns associated with the. thermal 1 stratification effects for a duration of ten additional heatup/cooldown cycles 1 of continued plant operation. A copy of the evaluation that was sent to the .

WOG is enclosed for your information.

i The staff will-assess the conformance of the PSL to-applicable Codes and regulatory requirements for 40 year plant life when the WOG report regarding l Item 1.d of the Bulletin is submitted. 1 1

Sincerely, OrbinalSigned By: j Robert B. Samworth, Project Manager Project Directorate III-3 Division of Reactor Projects - III, IV,.V and Special Projects Office of Nuclear' Reactor Regulation

Enclosure:

As stated cc w/ enclosure: b See'next page DOCUMENT NAME: 72155/72156 BUL. 88-11 Office: LA/PQI41-3 M/PDIII-3 PD/ DIII-3 i Surname: 'PKr6uiz'er RSamworth/tg JHannon Date: - $ /j a /90 7/t[/90 p/Jf/90 i

Mr. C. W. Fay: Point Beach Nuclear Plant Wisconsin' Electric Power Company Unitt 1 and 2 cc: Ernest L. Blake,' Jr.

Shaw, Pittman,- Potts ant _Trowt ridge 2300 N: Street, N.W. i Washington, DC 20037 Mr. Gregory J. Maxfield, Manager Point Beach Nuclear Plant Wisconsin Electric Power Company 6610 NL: lear Road Two Rivers,-Wisconsin 54241 Town Chairman Town of Two Creeks Route 3 Two Rivers, Wisconsin 54241- _i Chairman 1

Public Service Commission of Wisconsin j Hills Farms State Office Building i Madison, Wisconsin 53702 Regional Administrator, Region III U.S. Nuclear Regulatory Commission Office of Executive Director  !

for Operations 799 Roosevelt Road Glen Ellyn, Illinois 60137 i

Resident Inspector's Office U.S. Nuclear Regulatory Commission l 6612 Nuclear Road ,

1 Two Rivers, Wisconsin 54241-l 1

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'f Page 1 of 8 j ENCLOSURE 1 NRR REVIEW OF WESTINGHOUSE OWNERS GROUP (WOG)

BOUNDING EVALUATInN FOR PRESSURIZER SURGE LINE THERMAL STRATIFICATION WCAP-12277 4

INTRODUCTION The pressurizer aurge 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  ;

I 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.

Since thermal stratification is the direct result of the differences in-densities between the pressurizer water and the hot leg water, the  !

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 seural PWR plants.

i, Original design analyses did not include 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  ;

e 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 fat 1gue to the inner pipe wall and needs also to be analyzed. Thus assessment of stratification effects on I'SL _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 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, 1988.

-l Westinghouse, on behalf of the Westinghouse Owners Group (WOG), has performed a generic bounding evaluation report WCAP-12277 (Reference 1).

This report provides the technical basis for the generic justification for .i continued operation (JCO), for each of the WOG plants and constitutes j compliance with the requested action 1.b of Bulletin 88-11. Plants which have discovered any gross discernable distress during performance of the a 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. The following is,the staff's evaluation of the Westing-  :

house's efforts and information provided in the report.

l Prior to:the issuance of the Bulletin, WOG implemented a program to address-the issue of the surge line thermal stratification. The program 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. The thermal monitoring data obtained considered outside wall temperatures at i different location around the pipe and along-the axis of the pipe vs.

time, vertical end lateral displacements at various locations along the ,

pipe vs. time, and various plant parameters vs. time, from existing plant instrumentation and control sensors. In some cases the data were based l

-not only on plant heatup but also on operation and plant cooldown conditions. The specific analyses included redefinition of revised thermal ~ transients considering stratification effects and evaluation of pipe stress and fatigue usage factors. The overall analytical approach (+

used inbyallthe detail these NRCc:.ses staff.have been consistent and has been reviewed in '

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 t bounding evaluation was performed to justify continued operation for at i least ten (10) additional heatup/cooldown cycles.

l The bounding-evaluation is essentially a demonstration of the applicability

[ of the plant specific and the monitoring results to the remaining WOG .

plants. All plant-specific analyses completed to-date, have demonstrated-a 40 year life of the surge line  !

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.Two sets of parameters were defined.

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l a) Pararreters which affect severity of thermal stratifica- '

tion (i.e thermal hydraulic and operational effecte)

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Page 3 of 8 b) Parameters which affect PSL response to thermal strati-fication (i.e. structural effects).

. 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 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 analysis of the pertinent thermal hydraulic, operational and structural parameters. Currently 22 separate monitoring 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 monitored in each group.

Parameters which have a significant effect from a thermal hydraulic stand point are pipe inside diameter vnd slope. Plants falling outside the resulting bounding criteria of p!pe ir. side 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 dte to a i 20% change in pipe size and slope. The staff. agrees with Westinghoc e's efforts and methodology for monitoring, updating and assessing PSL for the stratification condition.

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:

Entrance angle to the hot leg nozzle Mid line riser 1

Length of the longest straight run of pipe Type of in-line component Presence of whip restraints Number of vertical rigid supports

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i 4- J Page 4 of 8 Two plants.have longest straight ..: lengths which are lessLthan 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-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 terperature, (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 s'

stratification which include Finite Element structural analysis of each specific. configuration under stratified conditions.

The five plants for which detailed plant-specific analyses performed and evaluation reports submitted.to NRC, are as follows:

Seabrook (see WCAP-12151-and Suppl. 1, and WCAP-12305) South Texas Units 1 and 2 (see WCAP-12067 Rev 1 and Suppl. 1) Vogle Unit 2 (see WCAP-12132, WCAP-12199 and WCAP-12218) Beaver Valley Unit 2 (see WCAP-12093 and i Supp1's 1 and 2) Comanche Peak Unit 1 (see WCAP-12248 and Suppl. 1) t Based on these reviews Westinghousi concluded that a shorter horizontal s

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 - J distribution of the bending moment due to stratified loading.' In addition I middle line risers will also t6nd to reduce the stratification effects.

The stratification induced global bending of the surge line was calculated i

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usirg 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 eierent model. Finite Element l' models were used to calculate local stresses due to top-tobottom non-linear thermal gradients in the PSL. Five-(5) hot-to-cold interface locations were analyzed-using eleven (11) cases of thermal stratification, to-calculate piping response undser all required loading conditions, reflecting temperatures differences up to 320'F. Other cases we,e obtained by interpolation. Westinghouse reported that their best estimate analytical results compared favorably with measured displacements data observed during monitoring.

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In two of the analyses, a rigid vertical support was removed. In one case  ;

it was shown that the support was not required, 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 ratio of equation 12 of ASME III section NB-3600, is less than 1.0. The critical location for stress is usually the safe end weld of the nozzla connecting the surge line to the primary loop hot leg. In one case only I

it was determined to be at the reducer. This seem to be a unique case  :!

since no other utility within the WOG has a mid line reducer in the 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 tne approaches used by the licensee for performing PSL reanalysis.  ;

To account for the thermal striping effects to PSL, flow model test' results, performed for the Liquid Metal Fast Breeder Reactor primary 'oop and for the Mitsubishi Heavy Industries Feedwater Line cracking, were reviewed to establish the boundary condition. These test results were t

used to define striping oscillation data, amplitude and frequencies, for evaluating high cycle fatigue. Portions of PSL which experience stratification and striping were defined based on measured results.

- Westinghouse teported that considering AT attenuation with time, and a frequency o' 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.

weld. A.' surfaceThe worst case element was do' ermined to be the butt film coefficient of 500 BTU /hr-sq. ft *F was used and it t

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 L coef ficient of 500 BTU /hr-sq. ft. *F and attenuation of AT is questionable, but at this time no other better number exists 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.

With the thermal transients redefined, new fatigue usage factors were calculated. To' determine the new fatigue usage facters, the more detail techniques of ASME III NB-3200 were employed. Due to the non-axisymmetric nature of stratification loading, 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  ;

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Page 6 of 8 points were calculated using the WECEVAL program. Stresses were intensified.

by "K" factors to account for worst case concentration in the piping 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. For that case a 14x16 reducer existed in the surge line and the CUF at that location was determined to 0.94. The CUF included contributions from both global bending and local effects of stratifi- {

cation considering the nonlinear step change top to bottom temperature l distribution and striping. I

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i Stresses. resulting from primary loading such as pressure, dead weight, and l seismic are not affected by thermal stratification loading and typically 'l have a minor effect on the calculation of the fatigue usage factor. The .l contribution'of seismic loadings to the cumalative usage factor from the plant specific analyses was reported to be 18%. All of the detailed plant specific analyses assum',3 the occurrence of twenty operational earthquakes. l The staff agrees with the approaches used by Westinghouse for calculating the usage factor. '

Westinghouse repor.ad that the PSL fatigue life is primarily depended upon the number of heatup and cooldown cycles rather than the years of operation.

The worst case years of operation at any WOG plant is.28.5 years. The a worst case number of heatup cooldown cycles is 75 and occurs at a different- '

plant.

Based on the combination of these two worst case values an

" Operating Life Factor" (OLF) of 0.44 is obtained which indicates -that no 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 4 attributed to age and an 83% was attributed to fatigue with a 20% of the +

83% value attributed to striping.

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, ; -, s Page 7 of 8 CONCLUS10NS Based on our review, we conclude that the information provided by Westinghousefin references 1 and 2, is comprehensive and acceptable. i

' Westinghouse on behalf of the Owners Group had made acceptable efforts to i

' -provide technical basis for the licensee'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 term safety concerns associated with the stratification effects for 10' additional heatup/ .

cooldown cycles of continued plant operation. However, each of the WOG l plants should submit a JC0 using this report as the basis. We will assess if the surge line in each plant meet the code acceptance criteria for the l i

40 year plant life when additional generic analyses based on plant grouping  !

is performed by; Westinghouse for the Owners group.

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, Page 8 of 8-REFERENCES l

1. Westinghouse Report WCAP-12277 (Proprietary), and WCAP-12278 (Non-Proprietary).. " Westinghouse Owners Group bounding evaluation for Pressurizer surge Line thermal stratification," June 15, 1988.

2, Viewgraphs by Westinghouse. Presentation to NRC on May 23 and 24.

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