ML20059A229
| ML20059A229 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 08/17/1990 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20059A222 | List: |
| References | |
| IEB-88-011, IEB-88-11, IEIN-88-080, IEIN-88-80, NUDOCS 9008220171 | |
| Download: ML20059A229 (10) | |
Text
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, (NCLO$URE 1 REVitW OF '
i COMBUSTIONENGINEtilnsowwER$ GROUP (CE06)
PRE $$URllER 5 URGE LINE FLOW 5TRATIFICATIOh EVALUATION CEh 357.P JULY 1959 1NTRODUCT10N i
Thepressuritersurgeline(PSL)inthepressurizedwaterreactors (PWRs),isastainlesssteelpipe connectingthebottomofthe pressurizer vessel of the het le ,of the too ant loop. The out flow of the pressurizer water is general warmer than the hot leg flow. Such b teeperature differential.sdelta T varies with plant operational 4
activities and can be as high as 320'F during the initial plant heatup.
Therrial stratification is the separation of the hot / cold flow stream in 1
- 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 betweeh the pressuriter and the hot leg water, 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 loadinn conditions. 'nstead it assumed complete sweep of fluid along the l'ne during insurges or outsurges resulting in uniform therm ! loading at any particular piping location. Such analyses did not reflect PSL actual thermal condition and potentially may overlook undesirable line deflection and its actual stresses may exceed design limits. In which is the oscillation of the hot addition, the striping and cold stratified phenomenon, 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 A$ME Code Section 111 conformance.
STAFF IVALUATION Since stratification in PSL is a generic concern to all PWRs, en NRC Information Notice 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. CombustionEngineeringonbehalfoftheCombustionEngineerIng Owners Group (CEOG), has performed a generic bounding evaluation report, CEN 387-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, 9008220171 9oog37
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' A).. p'lant monitoring and update of design transients.
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l As a result of the INP0 Safety Evaluation Report, which was issued in i September 1987 and identified concerns associated with the stratified i flow in the P5L, the CE0G initiated surge line temperature collection '
dataat[ ). Concurrently with this effort. [
l e i data on PSL at [) initiated efforts also J. '
forThis the collection was later folded of. temperature into the CE0G effort. inaddition,['., ~'
. )alsocollecied similar data for . , after the CEOG Task ' Reduction ,'
and analysis of P[ressurizer Surge Line d)ata collected from CE06 plants
- had comenced, and submitted them to Cosbustion Engineering for review and comparison with the data already collected from the first two CE0G plants.
Withtheexceptionof[ ),whichwasabletoretain the temperature distribution date 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. l )
obtained displacement readings also, in addition to temperature.
The Owners Group is going to decide on a proposed task to collect data duringthenextcooldownatboth[ . ] and [ .
The staff requests that monitoring should continue fo,r a full cycle.J.Data should be obtained and evaluated to determine whether the observed thermal transients are bounded by the transients assumed.
Due to similar design features of all the CE0G 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 CE0G p5Ls are similar in layout. They consist of a It" (except for
[a vertical drop f]om the pressurizer to the horizontal run of pipe and awhich is a r
verticaldroptothehotlegnozzle(exceptfor[ ]whichisat a60'verticalangledrop).
, A review of tne data, which measures pipe well outside temperature I
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 or an outsurge of the pressurizer. Therefore emphasis was movements in,'given to these transients for evaluation. Surge line I
pipemovementsmeasuredatthree) locations.. , were calculated and compared to The deflections predicted by the analysis model were based on a stratified flow model with a pipe top-to-bottom delta T=320'F. The actual l
I measured top-to-bottomdata deltacollected T=181'F anat [, d when the fluid inside the pipe),wereobtainedd l approximated a uniform temperature distribution model. Even though the
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' anslysis modelinside predicted the same general the fluid conditions the pipe were not sim lar. shake as the measured dataT
. further investigation and/or comparisons are required to predict P51.
displacement behavior.
The data obtained from all three plants recorded outside pipe wall 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 i
The stratified flow model assumed the hot (pressurizer temperature) fluid in the upper half of the pipe, and t h cold (hot leg temperature) fluid in the lower half of the pipe, with a sharp interface in between. During the outsurge it was assumed that flow occurred in the up>er portion of the pipe only, while during the insurge it was assumed tint flow occurred ;
in the lower portion of the pipe only. For a given transient, a flow i 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 was divided into a finite number of water layers to approximate a 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 unifc.rm temperature gradient. It was assumed that flow occurs at the full pipe cross section during an outsurge or an insurge. During a given i transient, a flow rate was calculated based on the pressurizer level change vs. time plots and a heat transfer coefficient was then determined.
Based on the above coefficients and using the in house CEMARC computer code,a2-Dfiniteelementmodelwasdevelopedtodeterminetheinside pipe wall temperature distribution for both the stratified flow and the J uniform temperature gradient models. The temperatures at selected nodes ;
were calculated and compared with the thermocouple data. The uniform I temperature distribution model more closely approximated the measured results. This indicates that it does not appear to be a sharp hot / cold l interface, and it is more likely that there is some mixing of the hot and cold fluids with a uniform temperature gradient from top to bottom of I pipe. Changes were made to the stratified flow model to better match the measured data. These changes tended to better match the measured data 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 striping, stress and fatigue evaluations utilizing the stratified flow model.
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.. }j, ,ASME Code compliance for Stress and Fatinue. :
- 1) Code Compliance in $ tress (!nelastic Analysis),
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- Each plant specific surge line was reanalyzed by the SUPERP!pt computer code using a bounding generic stratified flow loading. !
Elastic analyses were performed on the plant specific piping layout and !
j support configuration for each plant, considering that tse maximum delta T l t
for a given transient, occurs along the entire horizontal length of '
pipe. These results were used to choose a specific surge line for the
, bounding inelastic analysis. The elastic analyses pred' cted stress intensity levels in excess of the 35 allowable limit of the ASME Code '!
Section 111, NB 3600, equation 12. Thusaninelasticshakedownanalysis I was performed as per NB 3228.4 to determine if after a few cycles of load !
application, racheting and progressive inelastic deformation ceases, i However, the P$l, nozzle moments were calculated from the $UPERPIPE elastic analysis.
A$ME 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 inherently included in the analysis.
L The $UPERPIPE computer code was used to performed the initial dlastic I analysis, which considered thermal effects of the stratified flow over the !
l entire horizontal length of pipe, for delta T*32'F, delta T=90'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 l stratified flow effects were investigated. !
l l a) 1.ocal stress due to temperature gradient in the pipe wall.
I l b) Thermal gradient stress across pipe wall due to transient condition.
l c) Thermal pipe bending moment generated by the restraining effects of s supports.
( Actual support stiffnesses were used considering a a 2' limit of spring motion, beyond which springs will act as rigids.' The maximum movement based on delta T=320'F, pipe top to bottom stratified flow, was l calculatedfor( )and[ "~
),bothat location H2.
The staff feels that since no plant specific support data and displacement limitations were considered further evaluations are
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required In addition,to itjustify is the the sta [ ff's opin on i that the assumption on spring 1inelast l motion may not be conservative, in that upward movement of a spring
' which exceeds it's travel range will cau,se the spring to unload and redistribution of stresses will occur.
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The[> ]PSLconfieurationwaschosenfortheinelasticevaluation,
,.' since it predicted the higf.est stress levels under the elastic analysis.
While esci line will behave differently under a given stratified flow loading, it was concluded that the surge line with the hi i
stresses will provide en upper bound for all other lines.ghest This elastic 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 most critIcallocation.
i Material properties as T*650'F were used consideting the strain hardening behavior of the material. The stress strain curve used was developed by Combustion Engineering based on the ASME code minimum yield stress value l and plastic strain.
l '
Three For fatigue complete evaluations cycles of heatup,imum principal strain range the max values wer calculatedfromthemaxImumandminimumprincipalstrains. The maximum positive principal strain was calev14ted for three cycles and based on extrapolated the decreasingto rate beofless than strain 25 after increases 500 with heatup/cooldown additional cycles. cycles,The 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.
Changes in plastic strains showed some decrease 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 tie A$ME Section 111 code the value of it was obtained from the High Temperature Code Case N47 andItwasusedasaguideforthemaximumpositiveprincipalstrain limit. The staff concluded that the use of 25 strain limit in this case '
needs further , justification. .
I 1), Code compitance in Fatigue.
To determine stresses at the inside face of the pipe well due to fluid a 1-D oscillation finite elementatanalysis the interface of the hot was performed. toinput The coldassumptions boundaryused (strping),in i this analysis were based on the measured data from the CE0G plants and
- other information available in the public domain. ThethermalstrIping
Page 6 of 7 model considered the hot fluid at the pressuriser temperature, the cold fluid at the Hot Leg temperature, and a sharp interface with no mixing of the hot and cold fluid. A cowtooth fluid estillation was assumed to occur across the interface region, l'
Results indicated that fatigue damage due to stripin compared to all the other causes of fati stratification,thermaltransientsetc.)guedamage,gisinsignif .
si.e. static thermal The CC report indicated that based on the stress levels calculated, an infinite nun 6er of allowable i cycles exist and thermal stri ing is not a concern. Since maximum stress
! duetostripIngoccursatthekol/coldinterface,whichisnearthe o horizontal axis of the pipe, and maximum stress due to fatigue occurs at i
, 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 n l should be provided for the use of a fraction of the striping amplitude.
In addition, data based on measurement outside the l for the purpose of defining the striping phenomena. pipe may be inconclusive l
Analysisforeclicoperation(fatigue)wasperformed in addition to the shakedown anal sis. Using the results of the inelastic analysis, the maximum princi al total strain range which occurs from shakedown analysis was multiplied by one half the elastic modulus to determine the equivalent asperNB3228.4(c). This maximum strain range alternating occurs af ter cycle stress,3, and this value was assumed for the rer41ning cycles.
,For the first two heatup.cooldown cycles, the larger of cycle 1 and 2 strain range was used. .
The cumulative usage factor for this generic bounding analysis was '
determined to be 0.21 for 1. The maximum cumulativa 1 usage f actor, when the eeff[ct of the 2' disp'acement i
limitation was !
considered,was0.36for[1
- 0. The staff feels that further evaluation is required to justify the [ ]inelasticanalysisis the worst case. I 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 associated with the stratification effects for continued plant operation until final resolution of the Bulletin 88-11 is issued. This is scheduled to be completed by the end of 1990 and should also address the ,
Code acceptance criteria of ASME NB-3600. '
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.
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. 3 page 7 of 7 b) All supports, including pipe ship restraints, be considered for the effects of providing any additional constraints to the Surge Line, in the plant specific or the bounding pipe stress evaluation. !
c) All supports, including pipe whip restraints, require plant specific confirmation of their capabilities including clearances, and that they fall within the bounds of the anal sis, d) JustifytheI J inelastic analysis as the worst case for i stressandfatiguefora11CEOGplants, including [- . J.
e) Justify PSL displacement behavior predicted by the analysis model i and the use of a fraction of the striping amplitude. i REFERENCES
- 1. CombustionEngineeringReportCEN387-p(proprietary)." Combustion Engineering Owners Group pressuriser surge line flow stratification evaluation." July 1989,
- 2. Draf t meeting minutes of the NRC audit on September 25 and 26,1989 regarding the CEOG Report CEN 387 P MPS 89 1048, dated October 17, 19E9.
Page 1 of 3 ENCLOSURL t 1
5taff review of the CE responses regarding the NRC Audit
' on 5e stember 3 an0 F6.1959 Reft ctos :teport c J.357 P MF5-sy-1048 '
fated October L7. 1989.
Section 2.0
- 1) The staff requests that monitoring should continue for a full fuel cycle. Data should be obtained and evaluated to determine whether the '
l observed thermal transients are bounded by the transients assumed.
I
- 2) The staff feels that further investigation is required to predict PSL displacement behavior, considering the stratification effects. The deflection predicted by the analysis model were based on a stratified flow model with a pipe top-to bottom delta T*320'F. The actual measured datacollectedat[,
1wereobtainedduringapipetop-tobottom delta T=181*F and when the fluid inside the pipe approximated a uniform l temperature distribution model. Even though the analysis model predicted the same general shape as the measured data, the fluid conditions inside the pipe were not similar. ,
)
- 3) Closed.
- 4) Closed.
- 5) Closed.
- 6) Closed.
- 7) Closed.
- 8) The staff requests that further investigation is required to demonstrate that strains were stabilized af ter the three heatup/cooldown cycles and that progressive distortion does not exist. It is required to demonstrate that the decreasing rate of plastic strain will approach zero and the l peak value will not exceed a maximum strain acceptance criteria of 2. 1 The staff feels that the inelastic analysis will be accepted as !
Justification for Continued Operation and that the ASME Code acceptance criteria of section NB 3600 equations 9 14 need to be satisfied, as required by the Bulletin. I l
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- 9) Closed.
- 30) Closed.
- 11) The staff feels that all supports, including pipe whip restraints, !
beconsideredfortheeffectsofprovidingadditionalconstraints i in the plant specific or the bounding eva untion, j l 12) The staff feels that all supports, including pipe whip restraints requireplantspecificconfirmationoftheircapabilities, int 19 ding 1
clearances, and that they fall within the bounds of the analysis. ;
Section 3.0
- 1) Closed.
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- 2) Closed.
- 3) Closed. J
- 4) Closed. .I
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- 5) Closed. '
- 6) Closed. .
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Table 2.7-1. Closed Table 3.2.2 4. See response of Section 2.0 Item 2. f Table 3.4.3 2. Closed.
Table 3.6.2 1. Closed. !
Table 3.6.3 2. The staff feels that further evaluation is required to i justifythe[. inelastic analysis as the worst case. The i maximumcumulativeusage)factorfor[ionsareco]nsid effects of the 2" displacement limitat is 0.36 when the Figure 3.1.2 5. Closed.
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Section 4.0 1)~ No specific review was performed. '
- 2) See response of Section 2.0 Item 8. ,
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- 3) No specific review was performed.
Questions during meeting. i Closed '
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