ML20059A773
| ML20059A773 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 08/09/1990 |
| From: | NRC |
| To: | |
| Shared Package | |
| ML20059A768 | List: |
| References | |
| IEB-88-011, IEB-88-11, NUDOCS 9008230220 | |
| Download: ML20059A773 (7) | |
Text
j, Enclosure-1 Page 1 of 7 l
ENCLOSURE I
R V1 lW 1)F BABC0CK AN) W"Lclx Jun ;RS GROUP al&WOG?
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PRE 55URlIER 5 JRGiJ NE THERMAL 5TRAE FICATION l
- LAW-21!5 DAT ;D MAY 198V i
1NTRODUCT10N 2
Thepressurisersurgeline(PSL)inthepressurizedwaterreactors (PWRs), is a stainless steel pipe, connecting the bottom of the pressurizer vessel to the hot leg of one of the coolant loops. The out i
flow of the pressurizer water is generally warmer than the hot leg flow.
Suchtemperaturedifferential(deltaT)varieswithplantoperation activities and can exceed 400'F during the initial plant heat-up.
Thermal stratification is the separation of hot / cold flow stream in the l
horizontal portion of the PSL resulting in a temperature difference at the top and bottom of the pipe. Since thermal stratification is the direct results of the differences in densities between the pressuriser and the het leg water, the potential for stratification is increased as system delta T increases and as the insurge or outsurga flow decreases.
l Stratification in PSL was found recently and confirmed by data, measured from several PWR plants.
Original design analyses of the surge line aid 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 analysis did not reflect PSL actual thermal condition and potentially may overlook undesirable line deflection and its actual stresses may exceed design limits.
In addition, the striping phenomenon, which is the oscillation of the hot and cold i
stratified boundary and may induce high cycle fatigue to the inner pipe wall, needs also to be analyzed. Thus assessment of stratification l
effects on PSL is necessary to ensure piping integrity and ASME Code l
Section 111 conformance.
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 h9C Bulletin 88-11 for the same concern was also issued on December 20, 1908.
Babcock &Wilcox(B&W),onbehalfoftheB&WOG,hasperformeda genericboundingevaluationreportBAW-2085(Reference 1),whichconsists of presentation material and explanatory text. Additional information was also provided in Reference 2.
The purpose of the report is to:
l a.
Describe the B&WOG program and plans for addressing the surge line thermal stratification and striping issues.
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b.
Pressnt the results of the preliminary work dou to date to justify continued o available. peration (JCO) until the final program results are This is expected to be completed by December 31, 1990.
B&W reported that the thermal striping program has not been completed yet and cnly preliminary results are available. Upon completion of the striping program, a final report will be prepared which will provided the technical basis for the 40 year licensed life of the plants. Comparison of calculated PSL thermal displacements with the Oconee 1 measured data will also be performed in the final report.
The following is the staff's evaluation of B&W efforts and information provided in references 1 and 2.
During the hot functional testing of.the B&W plant at Muelheim-Kaerlich (M-K) in West Germany, the PSL was instrumented and temperature readings were taken during start up which confirmed that stratification existed i
in the PSL. As a result, B&W defined a program to instrument one of the domestic plants to determine magnitude of stratification effects, since the M-K plant is different from the domestic plants in terms of power level, PSL layout, pipe' diameter and thickness.
Later the scope of the program was expanded to include sur movements as well as data for evaluating thermal striping.ge line i
Based on the M-K data, B&W concluded that the Surge Line stratification depends on the following factors:
a Surge Line Flow rate and flow direction.
b Coolingofthefluid(ambientloss).
Boundary thermal conditions (Hot Leg and Pressurizer temperatures).
c 1
SinceallB&Wdomesticplants(7 plants,10 units)havethesamesurge line configuration, with the exception of Davis-Besse, B&W performed two bounding fatigue evaluations:
a.
Oconee 1 b.
Davis-Besse l
The Davis-Besse plant surge line has a long horizontal run from the Hot Leg to a 7.25 foot vertical drop near the center of the surge line span to another long horizontal run of pipe which connects to the bottom of the pressurizer.
Hence the overall run of the' pipe is essentially divided into two horizontal runs by the 7.25 foot vertical section. This i
vertical rise near the center of the surge line will reduce transmission of the stratification gradients downstream.
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Page 3 of 7 In all other plants a 13 foot vertical drop of pipe exists much closer to the hot leg resulting in a very chort horizontal section of 21 inches 4
connecting to the hot leg nozzle. This results in a different surge line routing at Davis-Besse, therefore, two separate fatigue evaluations were performed.
The surge line for each configuration is 10" schedule 140 S.S. pipe.
The Oconee 1 surge line was modeled using the ANSYS finite element computer code.
The loadings consisted of pressure, seismic, deadweight, and thermal expansion, from the original stress report, and were combined l
with the new thermal stratification loads.
The temperature ranges used were obtained from the M-K measured data, since no plant specific data were available at the time of the bounding evaluation, i
l Thermal stratiffcation was assumed to occur over the entire lower horizontal run of pipe.
The maximum pipe top-to-bottom delta T, for the l
three load cases considered, pre-heatup, heatup, and cooldown, was assumed to be 330'F, 422'F and 306"F respectively, and it was based on the maximum system delta T.
Pre-heatup was assumed to occur three times during each cycle.
Stress indices in accordance with ASME III 1977 Edition with Addenda through the Summer of 1979 were used.
The Oconee 1 bounding fatigue evaluation predicted expansion stresses for equation 12 which occurs on the vertical elbow from the pressurizer, is far in excess of the ASME j
i Code allowable limit of 35 "50.1 ksi (or twice the material yield strength), at T=650'F.
EqEation9and13stressesarewithinCode allowables since they are not impacted by thermal stratification.
To take advantage of the material behavior in the inelastic range, B&W substituted the cyclic " strain-hardended" yield strength (S in place of the static yield strength ($ ), and a higher allowable vI)lue of 25 l
was obtained.
The S value is defined as the " strain hardended" yielb l
strengthofthematekialattemperature.
B&W reported that in the final analysis, it is their intent to meet the ASME Code acceptance criteria of section NB-3600.
l The staff disagrees with this approach of using twice the " strain-hardened" yield strength in place of the code specified 35, limit even if this is a preliminary and conservative analysis.
With the exception of Davis-Besse, all other plants have no rigid supports or whip restraints.
Even if the Oconee 1 type plants have no supports which resist thermal motion, there is no indication that snubber / spring travel limitations were considered in the bounding evaluation.
The staff's recomendation is that thermal motion of PSL needs to be considered at the support locations and verified that j
acequate snubber / spring travel exists.
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Page 4 of 7 The fatigue usage factors for Oconee 1 type plants, excluding the striping effects, were calculated at several locations along the surge line, for the thermal stratification load cases, and were combined with those from the stress analysis of record to obtain the total usage factor.
Based on the fatigue life of each part affected by.the stratification effects, the allowable number of heat-up/cooldown cycles was calculated.
Location Allowable number of cycles HotLegnozzle(C.S. portion 270 Hot Leg nozzle (5.5. portion 162 Surgeline(Straightorelbow) 153 Surge line drain nozzle 135 Pressurizernozzle(S.S. portion) 341 Pressurizer nozzle (C.S. portion) 396 l
This indicates that, the worst case plant, Oconee 2 which has experi-enced 96 cycles to date, can withstand an additional 39 cycles to reach the allowable limit of 135 cycles for the Surge line drain nozzle or a C.U.F.=.71(96/135). The.71 factor does not include the effe. cts of j
striping.
Based on the average of 9 cycles / year B&W indicated that an additional 4 years of continued plant operation can be justified.
The Davis-Besse surge line analysis was conducted in a similar manner as the Oconee Unit I analysis.
The design transient inputs for the fatigue analyses were developed by Toledo Edison. Adjustmets were made to the M-K data to account for the differences between M-K and Davis Besse but no i
plant-specific temperature distribution data or surge line displacements were obtained.
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I Thermal stratification was assumed to occur over the full length of the lower horizontal pipe run and each transient was assumed to occur three-times during the busble formation. The staff concludes that further investigation is necessary to justify this assumption for the Davis-Besse type plants. The staff also feels that, depending on the direction of the flow (insurge or outsurge), thermal stratification may occur in the upper l
horizontal run of pipe and that the Davis-Besse unique configuration will experience different stratification conditions than the Oconee 1 type plants.
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For Davis-Besse 1. Impe11 Corporation performed the deflection / stress analysis of the thermal stratification events using the ANSYS computer i
code and the surge line met the 35, limit of the B31.7 Code for equations 10 and 12.
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l-l Page 5 of 7 The 35* limit was based on values from the Certified Material Test Reports (CMTR)
Due to the uncertainty of the relationship between product strenoth l
and the CMTR values, it is obvious that the use of such values for replacing Code allowables is not acceptable.
B&Wperformedtheboundingfatigueevaluationutilizingtheoutputofthe Impe11 analysis. The values excluding the striping ef'ects are as follows:
Location C.U.F.
Hot leg nozzle branchconnection) 0.619 Hot leg nozzle C.S. portion) 0.704 Hot leg nozzle S.S. portion) 0.343 Surgeline(Straightorelbow) 0.063 Pressurizernozzle(S.S. portion) 0.297 Pressurizer nozzle (C.S. portion) 0.634 Based on the average of 3 cycles per year which have been experienced by Davis-Besse during the past 12 years, B&W reported that an additional 5 years of, continued plant operation can be assured.
Subsequent to the bounding evaluations discussed above, temper.ature measurements were obtained during the February 1989 heat-up of Oconee Unit 1, and indicated much smaller pipe top-to-bottom delta T.
As a result of the PSL temperature measurements, the bounding fatigue evaluation was reviewed for consistency. The measurement program initiated at Oconee Unit I determined surgeline temperatures and motions during plant heat up and steady state operation. The temperature differences assumed for the bounding evaluation of delta T=422'F and delta T=330'F were found to envelope the actual delta T's of delta T=280'F and delta T=250'F respectively at most critical locations.
No upsets or complete cooldowns have occurred yet, therefore, measurements for these types of transients are not available to integrate into the bounding evaluation. Temperature data were also obtained at the outside surface of the surgeline and detailed heat transfer analyses were necessary to approximate the conditions at the inside surface to adequately evaluate the striping phenomenon.
To account for the effects of the non-linear temperature profile and the lower pipe top-to-bottom delta Ts, which were obtained by the temperature reasurements, the peak stress ranges calculated in the bounding analysis were refined as follows:
1)
Scaled down in accordance with the lower top-to-bottom delta T's.
2) due to the non-linearity of the temperature profile. pe rotation (Thisfactor Increased by a factor to account for the increased pi is based on a Finite Element analysis comparing actual measured temperature profile versus assumed linear temperature profile).
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4 3)
Added the corresponding thermal striping peak striping peak stress.
Based on the above refinements, the resulting alternating stress range predicted that the worst com>onent for the Oconee Unit 1 PSL drain l'ne l
nozzle can still withstand tie limiting number of 135 heat-up and i
i cooldown cycles, as predicted by the bounding evaluation. Similarly the i
l Davis Besse PSL Hot Leg norrie can still wit 1 stand the limiting number of l
57 heat up cooldown cycles.
l B&W reported that the complete treatment of thermal striping has not been evaluated yet and to account for the thermal stripin[ be utilized. effects to PSL, input from the Oconee Unit 1 measurement program wil.
c Thermocouples were installed around the outside circumference at g locations along the surge line. Heat transfer nr.41ysis will be performed 1
to determine fluid temperature amplitude and period of oscillation at the inside surface of the pipe.
These data will be evaluated and a more detail striping fatigue analysis will be performed based on the evaluation of the Oconee 1 measured data.
1 Results of this effort will be submitted to the staff-in a Topical Report in December 1990.
An interim assessment of the cyclic thermal stresses due to striping has been made. Preliminary results from this striping evaluation indicate that the fatigue impact on the surge line is approxi-mately 10% of the allowable usage factor with the maximum contribution occurring during the early parts of plant heat-up.
Based on the information available from the public domain (BWR feedwater nozzle tests, liquid metal fast Breeder Reactor tests, behavior of theArgonne Nation Laboratory tests, HDR project tests), the oscillatory fluid at the stratified interface was selected. Oconee test data also supports this interim set of characteristics for assessing the striping phenomenon.
CONCLUSIONS Based on our review, we conclude that the information provided by Babcock and Wilcox in references 1 and 2 is inadequate and not entirely acceptable for justifying plant life operation.
The staff concludes that further technical basis to meet the ASME Code acceptance criteria of NB-3600 is required.
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. The approach of using twice the d
- strain hardended" yield strength cr-using the CMTR values in place of the code specif.ied limits may be non-conservative and is not acceptable.
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l b)
All supports, including pipe whip restraints, be considered for the effects of providing thermal constraint in the bounding evaluation.
i All supports, f their capabilities, including clearances and thatincluding pip c) confirmation o they fall within the bounds of the analysis, d)
Comparison of calculated PSL thermal displacements with the Oconee 1
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measured data to demonstrate the validity and conservatism of the l
bounding analysis.
REFERENCES 1)
Babcock and Wilcox report, BAW-2085.
" Pressurizer Surge Line.
Thermal Stratification" dated May 1989.
2)
Babcock and Wilcox letter from Daniel F. Spond to Terence L. Chan 4
4 (NRC),OG-854
" Pressurizer Surge Line Thermal Stratification" cated September 29, 1969.
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