ML20055H092
| ML20055H092 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 06/25/1990 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20055H093 | List: |
| References | |
| NUDOCS 9007250192 | |
| Download: ML20055H092 (10) | |
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4 Page 1 of 7 ENCLOSURE 1 i
REVIEW 0F COMBUSTION ENGINEERTI T OWNERS GROUP (CE06)
PRESSURIZER 5 URGE LINE F,0W 5TRATIFICATION EVALUATION CEN 337 P JULY 1959 i
INTRODUCTION Thepressurizersurgeline(PSL)inthepressurizedwaterreactors (PWR$),isastainlesssteelpipe connecting the bottom of the pressurizer vessel of the hot le,of the coo ant loop. The out flow of I
i thepressurizerwaterisgeneral warmer than the hot leg flow. Such l
temperature differential Ldelta T varies with plant operational activities and can be as high as 320'T during 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 water, the potential for stratification is increased as f
i 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.
.nstead 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 end its actual stresses may exceed design limits. In-addition, the striping phenomenon, induce high cycle fatique which is the oscillation of the hot and cold stratified boundary, may pipe wall, needs also to be analyzed. Thus assessment o' stratification 1
effects on PSLs is necessary to ensure piping integrity and ASME Code Section 111 conformance, l
l STAFF EVALUATION L
Since stratification in PSL is a generic concern to all PWRs, an NRC Information Notice 88-80 was. issued on October 7 1988 and then an NRC Bulletin 88-11 for the same concern was also issu,ed on December 20 1
1988. CombustionEngineeringonbehalfoftheCombustionEngineerIng OwnersGroup(CEOG),hasperformedagenericboundingevaluationreport, 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.
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Page 2 of 7 A)
Plant monitoring and update of design transients.
As a result of the INPO $afety Evaluation Report, which was issued in September 1987 and identified concerns associated with the stratified flow in the PSL, the CE06 initiated surge line temperature collection l
l dataat[
).
Concurrently with this effort. [
dataonPSLat[)initiatedeffortsalsoforthecollectionoftemperature i
J. This was later folded into the CEOG effort.
Inaddition,[~..
)alsocollected similar data for
., after the CE06 Task ' Reduction and analysis of P[ressurizer Surge Line d)ata collected from CEOG p had comenced, and submitted them to Combustion Engineering for review t
and comparison with the data already collected from the first two CE0G plants.
1 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.
[
]
j obtained displacement readings also, in. addition to temperature.
The Owners Group is going to decide on a proposed task to coliect data during the next cooldown at both [
1and[
cycle.). The staff requests that monitoring should continue fo,r a full 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 det.ned adequate and CE06 met with NRC i
staff on February 13, 1989, to discuss the scope of the ' Task' and how the Bulletin's requirements will be addressed.
All CEOG PSLs are similar in layout. They consist of a 12' (except for
[
whichisa10")stainlesssteelschedule160 pipe with averticaldropfr]omthepressurizertothehorizontalrunofpipean,da verticaldroptothehotlegnozzle(exceptfor[
]wiichisat a 60' vertical angle drop).
l A review of the data, which measures pipe wall outside temperature variation with time, indicated that the lartlest surge line top-to-bottom temperature differentials were similar for she three plants and caused either by an insurge or an outsurge of the pressurizer. Therefore
. emphasis was given to these transients for evaluation. Surge line movementsin[
pipemovementsmeasuredatthree) locations.., were calculated and compared to actu i
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 measured data collected at [, d when the fluid inside the pipe),wereo top-to bottom delta T=181'F an approximated a uniform temperature distribution model.
Even though the
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I Page 3 of 7 analysis model predicted the same general shape as the measured data, the fluid conditions inside the pipe were not similar.
The staff feels that further investigation and/or comparisons are requirnd to predict PSL i
displacement behavior.
I The data obtained from all three plants recorded outside pipe wall surface
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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 i
transfer analysis of the pipe cross section was performed.
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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 1)Astratifiedflowmodel
- 2) A uniform temperature gradient model i
Thestratifiedflowmodelassumedthehot(pressurizertemperature) fluid in the upper half of the pipe, and the cold (hot leg temperature) fluid l
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 I
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 the 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 l
was divided into a finite number of water layers to approximate a i
continuous temperature gradient. The uppermost layer was considered the hot fluid (pressurizer temperature) and the lowest layer was considered thecoldfluid(hotlegtemperature},withtheintermediatelayershaving a uniform temperature gradient.
It was assumed that flow occurs at the i
full pipe cross section during an outsurge or an insurge.
During a given transient, a flow rate was calculated based on the pressurizer level change vs. time picts and a heat transfer coefficient was then determined.
Eased 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 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 interface, and it is more likely that there is some mixing of the hot and i
cold fluids with a uniform temperature gradient from top to bottom of 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, l-E
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e page 4 of 7 B),
A5ME Code compliance for Stress and Fatigue.-
1)
Code Como11ance in Stress (inelastic Analysis).
Each plant specific surge line was reanalyzed by the $UPERP!PE computer code using a bounding generic stratified flow loading.
Elastic analyses were performed on the plant specific piping layout and support configuration for each plant, considering that the maximum delta T for a 11pe. 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 section !!! NB.3600 equation 12. Thusaninelasticshakedownanalysis was performe,d as per,NB 3228.4 to determine if after a few cycles of load
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application, racheting and progressive inelastic deformation ceases.
However, the PSL nozzle moments were calculated from the $UPERPIPE elastic analysis.
ASP.E Code stress indices were used for each pipe component for the plant specific elastic analyses.
Finite Element shell model andThe bounding inelastic analysis was based on a inherentlyincludedintheanalysis,therefore, the stress indices were The SUPERPIPE computer code was used to performed the initial dlastic i
analysis, which considered thermal effects of the stratified flow over the entire horizontal length of pipe, for delta T=32'F delta Tag 0'F and delta T=320'F.
For each structural model loading and a stratified flow loading were, a uniform fluid temperature applied. Three types of stratified flow effects were investigated, a) 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 t l' 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 calculatedfor[
] and [
),bothat location H2.
The staff feels that since no plant specific support data and displacement limitations were considered, further evaluations are required to justify the [
) inelastic analysis as the worst case, In addition motion may n,ot be conservativeit is the staff's o, pinion that the assumption on spring i
in that which exceeds it's travel' range, will cau,se the spring to unload andupward movement of redistribution of stresses will occur.
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Page 5 of 7 4
The(
since it predicted the' highest stress levels under the elastic an i
While each line will behave differently under a given stratified flow loading, it was concluded that the surge line with the hi i
stresses will provide an upper bound for all other lines.ghest elastic This was i
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 critical location.
Material properties as T=650'F were used considering the strain hardening behavior of the material. The stress strain curve used was developed by Combustion Engineering based on the A5ME code minimum yield stress value and plastic strain.
Three complete cycles of heatup, steady state and cooldown were analyzed.
For fatigue evaivations, 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 4
extrapolated to be less than 21 af ter 500.heatup/cooldown cycles based on i
the decreasing rate of strain increases with additional cycles.
analysis results demonstrated that the first cycle undergoes significant permanent strain with subsequent cycles having smaller accumulation. The l
strain range from the first two cycles was considered in the fa.tigue analysis with the strain range from the third cycle used for the remaining 4g8 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-the ASME Section !!!
code, the value of 2% was obtained from the High Temperature Code Case N47 and it was used as a guide for the maximum positive principal strain limit. The staff concluded that the use of 25 strain limit in this case needsfurtherjustification.
M Code Compliance in Fatigue.
l To determine stresses at the inside face of the pipe wall due to fluid finite element analysis was performed. oscillation at the interface a 1-0 The input assumptions used i
this analysis were based on the measured data from the CE0G plants and other information available in the public domain.
ThethermalstrIping
t page 6 of 7 model considered the hot fluid at the pressurizer temperature, the cold i
f191d at the Hot Leg temperature, and a sharp interface with no mixing of the hot and cold fluid. A sawtooth fluid oscillation was assumed to j
occur across the interface region.
Results indicated that fatigue damage due to stripin stratification,thermaltrar.sientsetc.)guedamage.gisinsignifica compared to all the other causes of fati si.e. static thermal The CE report indicated that based on the stress levels calculated, en infinite number of allowable i
cycles exist and thermal striping is not a concern.
duetostripIngoccursatthehot/coldinterface,whichisneartheSince maximum stress horizontal axis of the pipe and maximum stress due to fatigue occurs at thetopandbottomoftheVIpe,thesestressesdonotoccuratthesame location and are not additive. The staff feels that further investigation should be provided for the use of a fraction of the striping emp11tude.
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 (fati ve) was performed in addition to the shakedown analysis. Using the resu ts of th inelast c analysis the maximum principal total strain range which occurs from shakedown, analysis
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was multiplied by one half the. elastic modulus to determine the equivalent alternating stress as per NB 3228.4 (c occurs af ter cycle,3, and this value was). This maximum strain range
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assumed for the remaining cycles.
For the first two heatup-cooldown cycles, the larger of cycle 1 and 2 j
strain range was used.
The cumulative usage factor for this generic bounding analysis was determinedtobe0.21for[
.).
The maximum cumulative usage factor, when the effect of the 12" displacement limitation was considered,was0.36for[t evaluationisrequiredtojustifythe[). The staff feels that further
]inelasticanalysisis i
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 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.
1 Concerns that the staff has are the following:
a)
The ASME code acceptance criteria of Section NS-3600 Equations g-14 need to be satisfied as applict,ble.
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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 fall within the bounds of the analy, sis,inc1Nding clearances, and that they d)
Justify the (
inelastic analysis as the worst case for stress and fatigue 4%r a)ll CE06 plants, including [-
).
e)
Justify PSL displacement bahavior predicted by the analysis model and the use of a fraction of the striping amplitude.
REFERENtts 1.
CombustionEngineeringReportCEN387-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 CEOG Report CEN 387-P MPS-89-1048, dated October 17, 1989.
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4 I
nr
1 page 1 af 3-ENCLOSURE 2 Staff review of the CE responses recording the IIRC Audit-on Neptember 75 ene 26.
959 Ref t Lt05 Report 0 LN 357-P MPu-tiv.1048 e
datedOctogr, 7, 1959.
Section 2.0 1)
The staff requests that monitoring should continue for a full fuel-cycle.
I observed thermal transients are bounded by the transients ass 2)
The staff feels that further investigation is required to predict pSL-displacement behavior -considering the stratification effects. The
-de' W. tion predicted by the analysis model were based on a stratified
- ~w rodel with a pipe top-to-bottom delta"T 320'F The actual measured
=
di.t. collected at g delta T=181'F ana H e the fluid inside the pipe approximated ~a uniform temperature distrihnien model.
the same general' shape as the measured data, the fluid conditions insi the pipe were not similar.
3)
Closec.
4)
Closed.
5)
Closed.
I
- 6)
Closed.
i 1
7)
Closed.
1 B)
The staff requests that further investigation is required to demonstrate that progressive distortion does not exist.that strains were stabilized a that the decreasing rate of plastic strain will approach zero and the-It is required t The staff feels that the inelastic analysis will be accepted as:
3 Justification for Continued Operation and that the ASME Code acceptance
' i criteria'of section NB-3600 equations S 14 need to be satisfied, as required by the Bulletin.
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'page 2 pf 3 9)- Closed.
10)- Closed;
- 11) ~ The ' staff feels that all supports, including pipe whip restraints, be considered for the effects of providina additional constraints in the plant specific or the bounding evahation.
- 12) The staff feels that all supports, including pipe whip restraints require plant specific confirmation of their capabilities, including
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clearances, and that they fall within the bounds of the analysis.
Section 3.0 1)
Closed.
2)
Closed.
3)
Closed.
4)
Closed.
5)
Closed.
6)
Closed.
Table 2.7-1. Closed Table 3.2.2-4. See response of-Section 2.0 Item 2.
1 Table 3.4.3-2. Closed.
Table 3.6.2-1. Closed.
Tab 1r 3.6.3-2. The staff feels that further evaluation is required to e
justify the maximumcumu[lativeusage]factorfor[ inelastic analysis as the worst ca effectsofthe2"displacementlimitationsareco)sidered._is,0.36.when the n
Figure 3.1.2-5.
Closed.
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Page 3 of 3 i
Section'4.0L 1).
No specific review was performed.
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- 2)-
See response of Section 2.0 Item 8.
3)
No specific review was performed.
1 Questions during meeting.
Closed e
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b.
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