ML20059L634

Advises That No short-term Safety Concerns Noted Per 890530 Response to NRC Bulletin 88-011 Re Thermal Stratification

ML20059L634
Person / Time
Site:	Beaver Valley
Issue date:	09/20/1990
From:	De Agazio A Office of Nuclear Reactor Regulation
To:	Sieber J DUQUESNE LIGHT CO.
References
BV-90-010, BV-90-10, IEB-88-011, IEB-88-11, TAC-72110, NUDOCS 9009270196
Download: ML20059L634 (4)

Text

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.. September 20, 1990

• bocket,No.50-334 S,erial No. BV-90-010 Mr. J. D. Sieber, Vice President Nuclear Group Duquesne Light Company P. O. Box 4 Shippingport, Pennsylvania 15007

SUBJECT:

BULLETIN 88 PRES 7URIZER SURGE LINE THERMAL STRATIFICATION:

EVALUATION OF WESTINGHOUSE OWNERS GROUP BOUNDING ANALYSIS (TAC 72110)

Duquesne Light Company responded to Item 1.b of NRC Bulletin 88-11, Pressurizer Surge Line Thermal Stratification, by letter dated May 30, 1989.

That response provided a Justification for Continued Operation (JCO) which referenced Westinghouse iopical Report WCAP 12277 (which was submitted separately by the Westinghouse Owners Group (WOG)). The JC0 concluded that it would be acceptable for Beaver Valley, Unit 1 to continue power operation for at least several years even though thermal stratification in the surge line introduces thermal transients not considered in the design. The JC0 and the Topical Report together form the bounding analysis to meet the intent of Item 1.b of Bulletin 88-11.

The staff has completed its review of the WOG bounding analysis and has con-cluded that there are no short-term safety concerns associated with thermal stratification effects for the duration of 10 additional heatup/cooldown cycles of continued operation. The staff's Safety Evaluation supporting this conclusion is enclosed A copy of the evaluation also has been sent to the WOG.

The staff will assess the conformance of the pressurizer surge line to applicable codes and regulatory requirements for the 40 yev plant life when the WOG report regarding item i.d of the bulletin is submitted.

Sincerely, original signed by Albert De Agazio Albert W. De Agazio, Sr. Project Manager Project Directorate I-4 Division of Reactor Projects - I/II Office of Nuclear Reactor Regulation

Enclosure:

Safety Evaluation cc w/ enclosure:

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Mr. J. D. Sieber, Vice President Nuclear Group Duquesne Light Company P. O. Box 4 Shippingport, Pennsylvania 15007

SUBJECT:

BULLETIN 88 PRESSURIZER SURGE LINE THERMAL STRATIFICATION:

EVALUATION OF WESTINGHOUSE OWNERS GROUP BOUNDING ANALYSIS (TAC 72110)

Duquesne Light Company responded to Item 1.b of NRC Bulletin 88-11, Pressurizer Surge Line Thermal Stratification, by letter dated May 30, 1989.

That response provided a Justification for Continued Operation (JCO) which referenced Westinghouse Topical Report WCAP 12277 (which was submitted separately by the Westinghouse Owners Group (WOG)). The JC0 concluded that it would be acceptable for Beaver Valley, Unit I to continue power operation for at least several years even though thermal stratification in the surge line introduces thermal transients not considered in the design. The JC0 and the Topical Report together form the bounding analysis to meet the intent of ltem 1.b of Bulletin 88-11.

The staff has completed its review of the WOG bounding analysis and has con-cluded that there are no short-term safety concerns associated with thermal stratification effects for the duration of 10 additional heatup/cooldown cycles of continued operation. The staff's safety Evaluation supporting this conclusion is enclosed A copy of the evaluation also has been sent to the WOG.

The staff will assess the conformance of the pressurizer surge line to applicable esdes and regulatory requirements for the 40 year plant life when the WOG report regarding item 1.d of the bulletin is submitted.

Sincerely, Albert V. De Agazio, Sr.Pr ject Manager Project Directorate I-4 Division of Reactor Projects I/II Office of Nuclear Reactor Regulation

Enclosure:

Safety Evaluation cc w/ enclosure:

See next page l

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Mr. J. Sieber Beaver Valley-Power Station ,

Duquesne Light Compoay Units 1 & 2 cc:

Jay E. Silberg, Esquire Bureau of Radiation Protection-Shaw, Pittman, Potts and Trowbridge Pennsylvania Department of 2300 N Street N.W. Environmental Resources Washington, DC 20037 ATTN: R. Janati Post Office Box 2063 Nelson Tonet, Manager Harrisburg, Pennsylvania 17120 Nuclear Safety Duquesne Light Company Mayor of the Borrough of P. O. Box 4 Shippingport Shippingport, Pennsylvania 15077 Post Office Box 3 Shippingport, Pennsylvania 15077 Commissioner Roy M. Smith Regional Administrator, Region I k West Virginia Department of Labor U.S. Nuclear Regulatory Commission Building 3, Room 319 475 Allendale Road Capitol Complex King of Prussia, Pennsylvania 19406 Charleston, WV 25305 -

John D. Borrows Resident Inspector Director, Utilities Department U.S. Nuclear Regulatory Commission Public Utilities Commission Post Office Box 181 180 East Broad Street Shippingport, Pennsylvania 15077 Calumbus, Ohio 43266-0573 Director, Pennsylvania Emergency Management Agency Post Office Box 3321 Harrisburg, Pennsylvania 17105-3321 S

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DISTRIBUTION

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NRR REVIEW 0F WESTINGHOUSE OWNERS GROUP (WOG) j D BOUNDING EVALUATION FOR PRESSURIZER SuaGE LINE t D

THERMAL STRATIFICATION  ;

WCAP-12277 '

, INTRODUCTION

? The pressurizer surge line (PSL) in the pressurized water reactors (PWRs)..

is a stainless steel pipes connecting the bottom of the pressurizer vessel 1 b to the hot leg of the coolant loop. The out flow of the pressurizer water s is generally warmer than the. hot-leg flow. Such temperature differential <

h (c?) varies with plant operation activities and can be as high as 320*F ,

during the-initial-plant heat ep. Thermal stratification .is the "

i separation of cold flow stream in the horizontal portion of the PSL ,

resulting in temperature difference at the top and bottom of the pipe. j Since thermal stratification is the direct result of-the differences in '

1 densities between the pressurizer water and the hotfleg' water, the l potential for stratification is increased as system AT increasesland as-the insurge or outsurge flow decreases. Stratification in PSL was found 1 recently and confirmed by data measured from several PWR plants.

Original design analyses did not include any stratified flow loading.

conditions. Instead it assumed complete sweep of fluid along the line l during insurges or outsurges--resulting in uniform thermal loading at any '

particular piping locstion. Such analyses did not reflect PSL actual thermal condition and potentially may overlook undesirable line deflection -

and its actual high stresses may exceed design limits. In addition, the ,

striping phenomenon, which'is the oscillation of the hoc and cold 1 i

stratified boundary, may inducs.high cycle fatigue a the inner pipe wall and needs also to be analyzed. Thus assessment of stratification effects- i on PSL is necessary to ensure piping integrity and ASME Code Section III-  ;

conformance. '

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Page 2 of 8 STAFF EVALOATION ,

Since stratification in PSL is a gev rit concere to all PWRs an NRC Information Notice No 88-80 was itwN co Mtobv 1,1988, and then an NRC i Bulletin 88-11 for the same concern m Osu issued on December 20, 1988. .

Westinghouse, on behalf of the Westir.gh e n N ners Group (WOG), has performed a generic boundbg evaluativ m; ort WCAP-12277 (Reference 1).

This report provides the technical bas!. for tre generic justification for

• continued operation (JCO), for each of the WOG plants and constitutes compliance with the requested action 1.b of bulletin 88-11.1 Plants which.

heve discovered any gross discernable distress dering performance of the. i walkdown, as requested by Bulletin's action 1.a. should report findings and in this specify report. corrective actions in their JCO, in addition to. that provided

_The following is the staff's evaluation of the Westing-house's efforts and information provided in the report. >

Prior to the issuance of the Bulletin, WOG implemented a program to

' address the issue of the surge line thermal stratification. The-program l

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. Tim i

thermal monitoring data obtained considered outside wall temperatures at L

different location around the pipe and along the axis of the pipe vs.

- time, vertical and lateral displacements at various locations along the pipe vs. time, and various plant parameters vs. time, from existing plant l

instrumentation and control sensors. In some cases the data were based not only on plant heatup but also on. operation and plant cooldown l conditions. The specific analyses included redefinition of revised p'

' thermal transients considering stratification effects and evaluation of pipe stress and fatigue usage factors. The overall analytical approach used inbyallthe detail these NRC casesstaff.have been consistent and has been reviewed in j-The evaluation concluded that a single bounding analysis was not= feasible.

E 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 moMtoring results to the remaining WOG plants All plant-specific analyses completed to-date, have demonstrated l

s 40 year life of the surge line l

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Two-sets of parameters were defined.

o .a) Parameters which affect severity of thermal stratifica-tion (i.e thermal hydraulic and operational effects)

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.. 1 Page 3 of 8 I 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 by to comparison.

plant establish the bounding criteria and to enable an individual plant 7

To expand the data base for a wider range of PSL configurations, j 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 recossended 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, ooerational and structural-parameters.

in process, orCurrently. 22 separate menitoring programs are either completed, being planned. About 40% of the plants falling in one group with the remaining 60% of the Westinghouse PWR's divided among the I nine groups. When this program is completed, it will provide sufficient monitoring data with at least one pient monitored in each group.  ;

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 0* 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

insignificant-effect on the thermohydraulic behavior due to a i 20% change pipe size and slope.  ;

The staff: agrees with Westinghouse's efforts and methodology for monitoring, updating and assessing PSL for the 3 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. '

j Some parameters judged to be relatively significant for the structural I effects are: ,

Entra6ce 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 I

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t Two plants- have longest straight run lengths which are less than the -

V 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. '

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The five plant specific analysis consisted of three parts: (1) global i

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 loca1' stresses.

L j in items (1) and (2) above wer= :9perimposed to obtain total stresses. -In l, addition to the detailed plant speciffa analysis fra the five plants, twelve (12) plants-have completed interim

• valuations of the surge line L stratification which include Finite Element structural analysis of each-L specific configuration under stratified conditlons. <

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 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

• 1ess vertical deflection and it will tend __to to-result in a'more uniform 1 di Gribution of the bending moment due to stratified loading. .Ii1 addition '

middle line risers will also tend to-reduce the stratification effects. .

The stratification induced global bending of the surge line:was calculated d 1

L' 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 i

models were used to calculate local stresses due to top-tobottom non-linear i

thermal gradients in the PSL. Five (5) hot-to-cold interface locations c-were analyzed using eleven (11) cases of-thermal stratification, to calculate piping response under all required loading conditions, reflecting  ;

temperatures differences up to 320*F. Other cases were obtained by I g

interpolation. Westinghouse reported that their best estimate analytical results compared favorably with measured displacements data observed l during monitoring.

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Page 5 of 8 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 subsequent 13 re qualified and found acceptable. ,

Stress summary results from the five plant-specific analyses performed te, date indicates that the primary plus secondary stress intensity range ratio of equation 12 of ASME III section N8-3600, 1s-less than 1.0. The M 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 only 't it was determined to be at the reducer. This seem to be a unique case i'

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 the approaches used by the licensee for perform'., , PSL reanalysis, q

To account for the thermal striping effects to PSL, v sodel test '

results, performed for the Liquid Metal Fast Breeder kew r primary loop and for the Mitsubishi Heavy Industries Feedwater Line cratm ing, were reviewed to establish the boundary condition. These' test revults were 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 reported that considering AT attenuation with time, and a-frequency of .30 HZ, a usage-factor of less'than .20 was' determined as the  !

-by "K" case worst dre to striping alone, even when the stresses were intensified factors.

1 weld. A surface The worst case element was determined to be the-butt

~ film coefficient of 500 BTU /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- L ment weee 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.naverage frequency is justified. J However the thermal striping potential due to a film coef ficient of 500 BTU /hr-sq. ft. 'F and attenuation of AT is. -1 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 beeffects striping utilizedtoand PSL.further assessments will be made for assessing the i i

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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 i

. techniques-of ASME'III NB-3200 were employed.

j Due to the non-axisymmetric-1 nature of stratification loading, stressac 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 e

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• 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 l

op for all but one case.

. tine and the CUF at that location was determined toThe 0.94.For that case a 1 ,

Jf ' contributions from both global bending and local effects of stratifi--

CUF included gli d cation considering the nonlinear step change top to bottom temperature distribution and striping. <

Stresses resulting from primary. loading such as pressure, dead weight, and 1

seismic are not affected by thermal stratification loading and typically c

• have a minor effect on the calculation of the fatigue usage factor. The '

W contribution of seismic loadings to the cumulative usage factor from the plant specific analyses was reported to be 18%. All of the detailed plant s l >

specific analyses assumed the occurrence of twenty cperational earthquakes.- r b

The the usage staff agrees factor. with the approaches used by Westinghouse for calculating 1 l'

s 1

- Westinghouse reported 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 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 i l- " Operating Life Factor" (OLF) 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 attributed to age and an 83% was attributed to fatigue with a 20% of the L 83% value attributed to striping.

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CONCLUS10NS' A

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Based on our review, we conclude that the information provided by >

. Westinghouse 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 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 ters safety concerns  ;

associated with the stratification effects for 10 additional heatup/. .,

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couldown cycles of continued plant operation. However, each of the h0Gl ,

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 <

40 year plant life when additional generic analyses based on plant grouping l is performed by Westinghouse for the Owners group.

PRINCIPAL CONTRIBUTORS: S. Hou ,

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REFERENCES 1.

Westinghouse- Report WCAP-12T77 (Proprietary), and WCAP-12278 (Non-Proprietary). " Westinghouse Owners Group bounding evaluation for Pressurizer surge Line thermal stratification,"' June ~ 15,1988. i

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

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