ML20056A334
| ML20056A334 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 07/25/1990 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML13302B807 | List: |
| References | |
| IEB-88-011, IEIN-88-080, NUDOCS 9008060400 | |
| Download: ML20056A334 (10) | |
Text
C-E Enclosure- .
, Page 1 of 7-t- .
, ENCLOSURE 1 REVIEW OF COMBUSTION ENGINEER 1Ns OWNERS BROUP (CE06)
PRES $URlIER 5 URGE LINE FL0ef 5TRATIFICATION EVALUATION GEh 357-P JULY 1959 -
_1NTRODUt110H-Thepressurizersurgeline(PSL)inthepressurizedwaterreactors (PWRs),isastainlesssteelpipe connectingthebottomofthe pressuriter vessel of the hot le ,of the coo ant loop. The out flow of the pressurizer water is general warmer than the hot leg flow. Such temperature differential (delta varies with plant operational activities and can be as high as 320'T dvring the initial plant heatup.
Thermal stratification is the separation of the hot / cold flow stream in the horizontal portion of the PSL resulting in temperature differences at the top and bottom of the pipe. Since thermal stratification is the direct result of the difference in densities between the pressurizer and the hot leg watst, the potential for stratification is increased as system delta T_ increases and as the insurge or outsurge flow decreases.
Stratification in PSLs was found recently and confirmed by data measured from several PWR plants. ,
Original design analyses did not include any stratified flow loading conditions. Lnstead it' assumed complete sweep of fluid along the l'ne during insurges or outsurges resulting in uniform thermal loading at any 4 particular piping location. Such analyses did not reflect PSL actual thermal condition and potu tielly may overlook undesirable line .
deflection and its actual 9 ;sses may exceed design limits. In addition, the striping phenomenon which is the oscillation of the het and cold stratified boundary, may, induce high cycle fatigue to the inner pipe wall, needs also to be analyzed. Thus assessment of stratification effects on PSLs is necessary to ensure piping integrity and ASME Code Section 111 conformance.
STAFF EVA).UATION Since stratification in PSL is a generic concern-to all PWRs. en NRC Information Notice 88-80 was issued on Octcher-7 Bulletin 1988.
88-11 for the same concern was also issu.1988 ed on and then an RAC December 20 CombustionEngineeringonbehalfoftheCombustionEngineerIng Owners Group (CEOG), has performed a generic bounding evaluation report.
CEN 3B7-P (Reference 1), which documents the results of the PSL stratification effects. The following is the staff's evaluation of the Combustion Engineering efforts and information provided in the report.
788"%888R388386 0 ,
( :o
l Page 2 of 7 A) Plant monitoring and update of design transients.
As a result of the 1NPD safety Evaluation Report, which was issued in September 1987 and identified concerns associated with the stratified flow in the PSL, the CEDG initiated surge line temperature collection data at [ .
Concurrentlywiththiseffort.[. '
data on PSL at [) initiated ef) orts also for the collection of temp f '
CE0G effort. Inaddition,[... ). This was later folded into the similar data for
~~ .
also collected and analysis of P(ressurizer Surge 0 Line d)ata collected fro had comenced and submitted them to Combustion Engineering for review -
and comparison, with the data already collected from the first two CE06 plants..
Withtheexceptionof[ ),whichwasabletoretain the temperature distribution data only after the bubble was formed in the pressurizer, the other two plants were able to retain the tem >erature distribution data during heatup and until normal operation. '
obtained displacement readings also, in addition to temperature.
)
The Owners Group is going to decide on a proposed task to collect data and during)thenextcooldownatboth(The cycle.
staff requests that monitoring should ,
Data should be obtained and evaluated to determine whether the observed therm 1 transients are bounded by the transients assumed.
Due to similar design features of all the CEOG plants (10 plants, 15 units), the data obtained were deemed adequate and CE0G met with NRC staff on February 13, 1989, to discuss the scope of the " Task' and how the Bulletin's requirements will be addressed.
All CEOG PSts are similar in layout. They consist of a 12' (except for
[ >
a vertical drop r f]om the pressurizer' to the horizontal run of pipe an,d aw vertical drop to the hot le a 60' vertical angle drop).g nozzle (except for [ ]wsichisat A review of the data, which measures pipe wall outside temperature variation with time, indicated that the largest surge line top to-bottom temperature differentials were similar for the three plants and caused either by an insurge tr an outsurge of the pressurizer. Therefore-
,movements emphasis in was i,
given to these transients for evaluation. Surge line pipe movements measured at three) locations.. , were calculated and compared to actual The deflections predicted by the analysis model were based on a g
stratified flow model with a pipe top to-bottom delta T=320*F. The actual measureddatacollectedat[,dwhenthefluidinsidethepipe),wereobtained top-to-bottom delta T=181'F an approximated a uniform temperature distribution model. Even though the i
Page 3 of 7 the fluid conditions inside the pipe w0re not similar. analysis The staff feels tmodel predicte further investigation and/or comparisons are required to predict PSL displacement' behavior.
The data obtained from all three plants recorded outside pipe well surface temperature distribution about the longitudinal and.circumferential axis of the pipe. In order to determine fluid conditions for the design basis events at the inside surface of the pipe wall, a 2-D finite element heat transfer analysis of the pipe cross section was performed.
Two bounding analytical heat transfer models with various inside fluid conditions were developed, with an attempt to reproduce the recorded outside pipe wall surface temperature distribution.
1)Astratifiedflowmodel
- 2) A uniform temperature gradient model Thestratifiedflowmodelassumedthehot(pressurizertemperature) fluid ;
intheupperhalfofthepipe,andthecold(hotlegtemperature) fluid -l i
in the lower half of the pipe, with a sharp interface in between. During the outsurge it was assumed that flow occurred in the upper portion of the pipe only, while during the insurge it was assumed that flow occurred in the lower portion of the pipe only. For a given transient a flow rate was calculated based on tie pressurizer level change vs., time plots, and a heat transfer coefficient was then determined For the uniform temperature gradient model the pipe cross sectional area wasdividedintoafinitenumberofwaterlayerstoapproximatea continuous temperature gradient. The uppermost layer was considered the hot fluid (pressurizer temperature), and the lowest layer was considered the cold fluid (hot leg temperature), with the intermediate layers having a uniform temperature gradient. It was assumed that flow occurs at the full pipe cross section during an outsurge or an insurge. During a given 4
transient, a flow rate was calculated based on the pressurizer level change vs. time plots and a heat transfer coefficient was then determined.
Eased on the above coefficients, and using the in-house CEMARC computer code, a 2-D finite element model was developed to determine the inside i
pipe wall temperature distribution for both the strat.ified flow and-the uniform temperature gradient models. The temperatures at selected nodes '
were calculated and compared with the thermocouple data. The uniform
" temperature distribution model more closely approximated the measured results. This indicates that it does not appear to be a sharp hot / cold i interface, and it is more likely that i,ius S some mixing of the hot and
' cold fluids with a uniform t uperature gradient from top to bettom of pipe. Charges were matt to the stratified flow model i,0 better match the measured data. These changes tended to better match the measu"ed data )
i for the outside pipe wall temperature distribution, but CE could not explain why these would be valid assumptions. Since a unique solution could not be derived, assumptions were used for the thermal s triping, stress and fatigue evaluations utilizing the stratified' flow model.
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i
' , page d of 1 c, ASP.E Code compliance for Stress and Fatieve.
- 1) Code Compliance in Stress (Inelastic Analysis).
Each plant specific surge line was reanalyzed by the SUPERPIPE computer code using a bounding generic stratified flow loading. Elastic analyses were performed on th plant specific piping layout and L support configuration for each plant. considering that the maximum delta T for a pipe. given transient, occurs along the entire horizontal length of These results were used to choose a specific surge line for the bounding inelastic analysis. The elastic analyses predicted stress intensity levels in excess of the 35 allowable limit of the ASME Code 4 Section 111 NB 3600 equation 12. Ibus an inelastic shakedown analysis ' was performe.d as per,NB 3228.4 to determine if after a few cycles of load application, racheting and progressive inelastic deformation ceases. However, the PSL nozzle moments were calculated from the SUPERPIPE elastic analysis. ASP.E Code stress indices were used for each pipe component for the plant specific elastic analyses. The bounding inelastic analysis was based on a Finite Element shell model and therefore, the stress indices were inherentlyincludedintheanalysis. The SUPERPIPE computer code was used to performed the initial dlastie-analysis, which considered thermal effects of the stratified flow over the entire horizontal length of pipe, for delta T=32'F, delta TagD'F and delta T=320'F. For each structural model a uniform fluid temperature loading and a stratified flow loading were, applied. Three types of ; stratified flow effects were investigated. s) local stress due to temperature gradient in the pipe wall. b) Thermal gradient stress across pipe wall due to transient condition. c) Thermal pipe bending moment generated by the restraining effects of supports. Actual support stiffnesses were used considering a a 2" limit of spring motion, beyond which springs will act as rigids.' The maximum moveinent based on delta T=320'F pipe top to bottom stratified flow, was calculatedfor[ ] and [ ), both at location H2. ~" ! The staff feels that since no plant specific support data and. displacement limitations were considered, further evaluations are-required to justify the [ )inelasticanalysisastheworstcase. In addition it is the staff's o. motion may n,ot be conservative in pinion that that the assumption on spring which exceeds it's travel range, will cau,se the spring to unload andupward movement of redistribution of stresses will occur.
.a j
Page 5 of 7 The{ since it predicted the highest stress levels under the elastic an While each line will behave differently under a given stratified flow loading, it was concluded that the surge line with the hi stresses will provide an upper bound for all other lines.ghest elastic This was verified by the fact that the most highly stressed region is the same location for both the elastic and the inelastic evaluation. For this line, the elbow under the pressurizer was determined to be the aest-critical location. i Material properties as T=650'F were used considering the strain hardening behavior of the material. Combustion and Engineering based en the ASME code minimum yield stress va plastic strain. Three complete cycles of hes'Jp, steady 5 tate and cooldown were analyzed. For fatigue evaluations, the maximum principal strain range values were calculated from the maximum and minimum principal strains. The maximum positive principal strain was calculated for three cycles and extrapolated the decreasing ' rateto ofbe less strain than 25 increases withafter 500cyclet. additional heatup/cooldown based on cycles,T analysis results demonstrated that the first cycle undergoes significant permanent strain with subsequent cycles having smaller accumulation. The strain range from the first two cycles was considered in the fatigue analysis with the strain range from the third cycle used for the remaining 498 cycles. Review of Fig. 3.6.2-8 and Fig. 3.6.2-9 of the report could not clearly demonstrate that strains were stabilized after the three heatup/cooldown cycles and that progressive distortion does not exist. thanges in plastic strains showed some 3ecrease with each cycle but the staff concluded that additional investigation was required to demonstrate that the decreasing rate of plastic strain will approach zero. Since there are no maximum strain limits prescribed in the ASME Section 111 code, the value of 21 was obtained from the High Temperature Code Case M47 and limit.it was used as a guide for the maximum positive principal strain The staff needs further concluded that the use of 25 strain limit in this case
, justification. .
]), Code Compliance in Fatigue.
To determine stresses at the inside face of the pipe wall due to fluid oscillation at the interface of the hot to cold boundary (strping), a 1-D finite element analysis was performed. The input assumptions used in this analysis were based on the measured data from the CEOG plants and other information available in the public domain. ThethermalstrIping 1
Page 6 of 7 model considered the hot fluid at the Pressurizer temperature, the cold fluid at the Hot Leg temperature, and a sharp interface with no mixing of 1 the het occur andthe across cold fluid. region. interface A sawtooth. fluid oscillation was assumed to
~
Results indicated that fatigue damage due to stripin compared to all the other causes of fati stratification,thermaltransientsetc.)guedamage.gisinsignificantwhe (i.e. static thermal The CE report indicated that based on the stress levels calculated, an infinite ausber of allowable cycles exist and thermal striping is not a concern. due to striping occurs at the hot / cold interface, which is near theSince maximum stress horizontal axis of the pipe and maximum stress due to fatigue occurs at the top and bottom of the pipe, these stresses do not occur at the same location and are not additive. The staff feels that further investigation should be provided for the use of a fraction of the striping amplitude. - In addition data based on measurement outside the for the purp,ose of defining the striping phenomena. pipe may be inconclusive Analysis for cyclic operation (fatigue) was performed in addition to the shakedown analysis. Using the results of the inelastic analysis th. maximum principal total strain range which occurs from shakedown, analysis was multiplied by one half the elastic modulus to determine the equivalent alternatingstress,asperNS3228.4(c). This :sximum strain range occurs af ter cycle 3, and this value was assumed for the remaining cycles. j For the strain first was range two used. heatup.cooldown cycles, the larger of cycle 1 and 2 The cumulative usage factor for this generic bounding analysis was determined to be 0.21 for usage factor, when the effe[ct of the a 2" displacement limitation was3. T considered,was0.36for[L evaluation is required to justify the [0. The staff feels that further
]inelasticanalysisis the worst case.
CONCLUSION Based on our review we conclude that the information provided by CombustionEngineerInginReferences1and2isnotadequatetojustify continued operation for the 40 year plant life. However, the staff believes that there is no immediate or short term safety concerns until final resolution of the Bulletin 88 11 isThis issued. is associated with the
' Code acceptance criteria of ASP.E NB-3600. scheduled to be completed by Concerns that the staff has are the following:
a) The ASME code acceptance criteria of Section NB-3600 Equations 9-14 need to be satisfied as applicable. m e
l page 7 of 7
' -r b) All supports including pipe ship restreints, be considered for the effects of pr,oviding any additional constraints to the Surge Line, in the plant specific or the bounding pipe stress evaluation.
1 c)- All supports, including pipe whip restraints, require plant specific I confirmation of their capabilities, including clearances, and that they fall within the bounds of the analysis. 1 d) inelastic analysis as the worst case for l Justify stress and thefat [ igue for a]ll CE0G plants, including , ).[- ' e) Justify PSL displacement behavior predicted by the analysis model and the use of a fraction of the striping amplitude. , REFEREh'CES 3. Combustion Engineering Report CEN 387-P (Proprietary). ' Combustion Engineering Owners Group pressurizer surge line flow stratification evaluation." July 1989. 2. Draf t meeting minutes of the NRC audit on September 25 and 26,1989 regarding the CE0G Report CEN 387-P MPS 89-1048, dated October 17, 1989. i e t l l. i 4 _ _ _ _ _ _ _ __ _ _ . - .--...-. - - - - - - - - - - - ~ '~
Pape 1 of 3
, ENCLOSURI 2
$taff review of the CE responses regarding the NRC Audit on Neptember 15 ene 26. 1959 Ref:- cgos Report 6 ;n.187.r Mrs.sp.1048-dated Detober , .7. 1989.
Section 2.0 2) The staff requests that monitoring should continue for a full fuel cycle. Data should be obtained and evaluated'to determine whether the observed thermal transients are bounded by the transients assumed.' 2) The staff feels that further investigation is required to predict PSL . displacement behavior, considering the stratification effects. The 1 flow model with a pipe top-to bottom delta T=320'F. deflection predicte ! datacollectedat[. The actual seasured delta T=181'F and when the fluid inside the pipe approximated a uniform temperature distribution model. N j thepipe the same weregeneral shape as the eaa.ured data, the fluid conditions-inside-E not similar. i I
- 3) Closed.
- 4) Closed.
- 5) Closed.
- 6) Closed.
- 7) Closed.
B)- < The staff requests that further investigation is'requ' ired to demonstrate that progressive distortion does not exist.that strains were stabilized afte , that the decreasing rate of plastic strain will approach zero and theIt is' required to i The staff feels that the inelastic analysis will-be accepted.aspea
' Justification for Continued Operation and that the ASME Code acceptance criteria ofbysection required NB 3600' equations 9-14 need to be satisfied, as the Bulletin.
u 1 m 9
I,'- I. Page 2 of 3
, 9). Closed.
- 10) Closed.' 1
- 11) The staff feels that all supports, including pipe whip restraints, '
be considered for the effects of providinp additional constraints in the plant specific or the bounding eva uation. I 12) The staff feels that all supports, including pipe whip restraints i require plant specific confirmation of their capabilities int 19 ding clearances, and that they fall within the bounds of the analysis. Section 3.0
- 3) Closed.
- 2) Closed.
- 3) Closed.
- 4) Closed.
- 5) Closed.
- 6) Closed.
Table.2.7-1. Closed i Table 3.2.2-4. See response of Section 2.0 item 2. - Table 3.4.3-2. Closed. Table 3.6.2 1. Closed. Tablejustify 3.6.3 the
- 2. The staff feels that further evaluation is required to .
maximumcumu[lativeusage)factorfor[ inelastic analysis-as the worst ca effectsofthe2"displacementlimitationsareco]nsidered.is 0.36 when the Figure 3.1.2 5. Closed. 1 e 4 n
(' . .
,. i Page 3 of 3 Section 4.0
- 1) No specific review was performed.
- 2) See response of Section 2.0 Item 8.
- 3) No specific review was performed.
1 1 Questions during meeting. Closed 1 l l l l 1
- l l
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