ML19270H852
| ML19270H852 | |
| Person / Time | |
|---|---|
| Issue date: | 11/06/1979 |
| From: | Levine S NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| RIL-067, RIL-67, NUDOCS 8001040138 | |
| Download: ML19270H852 (16) | |
Text
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UNITED STATES
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NUCLEAR REGULATORY COMMISSION
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E WASHINGTON, D. c. 20555
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p NOV 0 1983 MEMORANDUM FOR: Harold R. Denton, Director Office of Nuclear Reactor Regulation FROM:
Saul Levine, Director Office of Nuclear Regulatory Research
SUBJECT:
RESEARCH INFORMATION LETTER # 67 - REFLOODING OF SIMULATED PWR CORES AT LOW FLOW RATES This Research Information Letter describes the cooling of electrically heated rods during bottom flooding. The information presented is considered appli-cable to the evaluation of emergency cooling system performance in pressurized water reactors.
1.0 Introduction This letter summarizes results and analyses of bottom flooding ex::eriments conducted at constant inlet flooding rates.
The primary information base has been provided by the PWR Full Length Emergency Cooling Heat T ansfer (FLECHT) experiments which are continuing with separate effects and system effects tests in FLECHT SEASET.
Supplementing this data base are significant results from several other programs including Semiscale and programs conducted in Germany by KfK and KWU.
The goals of this ongoing research are:
A.
To study heat transfer at the low reflood rates (less than one inch per second) that have been predicted to occur in LPWRs and that require restrictive modeling assumotions by Ap;:endix K cf 10 CFR 50; 3.
To provide a clearer understanding of reficed flow egires and heat transfer mechanisms, and to develop best estimate metnocs for predicting heat transfer coefficients, quench front velccities, liquid carryover, cladding temperatures and other features of reflood behavior; and C.
To examine the effects of geometry and system parameters sucn as bundle bloc 4 ages, grid spacers, fuel array design, and loop and steam generator operating conditions.
The information available to date provides a substantially imoroved understanding of emergency core cooling, especially with respect to Items A and 3, and allows the calculation of more realistic peak cladding tempera-tures. The FLECHT SEASET program will study particularly the questions listed in Item C.
Typical FLECHI results are reported in this letter and the ca a from other crograms are presented in order to provide perspective on tne effects of test facility and heater rod cesign.
A summary of the current precictive caca-oilities for reflood behavior, as orovided by both computer coces and empirical correlations, is also included.
IW 1681 270 so 01o40
Harold R. Denton 2
The :LECHT tests utilized electrically heated rods with nominal external dimensions equivalent to those employed in standard 15 x 15 PWR nuclear fuel arrays. Axial heater profiles peaked near the midplane (' cosine-shaped') and peaked near the top (' skewed-shaped') were employed. More than 200 reflooding experiments were conducted with a systematic variation of inlet flooding rate, system pressure, rod power, and other important parameters. The test section was well-instrumented, providing data for calculation of heat transfer coefficients and detailed mass distributions.
The information presented is considered applicable to 10 CFR 50.46, caragraphs (a)(1), (c)(2) and to Appendix K of Part 50, paragraphs ID3 and IDS.
Subsection 105 allows use of reflood heat transfer coefficients based on the unblocked FLECHT results when flooding rates are one inch per second or nigher.
For lower flooding rates, steam cooling and blockage must be considered:
"During refill and during reflood when reflood rates are less than one inch per second, heat transfer calculations shall be based on the assumption that cooling is only by steam, and shall take into account any flow blockage calculated to occur as a result of cladding swelling or rupture as such blockage might affect both local steam flow and heat transfer."
FLECHT SEASET will address these broader issues and will provide data needed for a reevaluation of Appendix K.
2.0 Summary Significant conclusions supported by reflooc research at constant inlet floccing rates include the following:
A.
Exceriment Results for Low Floodina Rates 1.
Substantial heat transfer is available below 2.5 centimeters (1 inen) per second in unblocked bundles.
Cooling by dispersed droplet flow is observed even for reflood rates of 1 centimeter per second.
Appendix K is conservative in requiring that only cooling by steam can be considered at low flooding rates.
2.
Evidence to date from heater rod experiments indicates that reflooding behavior of stainless steel-clad rods is conservative with respect to reflooding of zircaloy-clad rods.
Conduction models predict that conservatism is due to differences in cladding thermal properties and that it may be enhanced by the existence of a thermal resistance as caused by a design gap or by moderate ballooning.
5.
Analytical Predictions of Refloco. Behavior 1.
Basic aspects of reflood behavior including quencn front velocity, liquid carryover rate, void fraction, and heat transfer coefficients 1681 2L7I
Harold R. Denton 3
can be better predicted by semiempirical or empirical correlations than by phenomenologically-basec calculations.
2.
Early temperature behavior including tne cladding temperature increase during reflood and the time that the maximum temperature occurs can generally be well predicted by :nenomenologically-based codes.
Later temperature behavior, heat transfer in the dispersed droplet flow regime, and the quench time cannot yet be descri:ed well by mechanistic models.
These conclusions are supported by:
Parametric studies in the FLECHT program and results of other U.S. anc German reflood tests, and predic-tions using empirical correlations ard computer codes.
These topics are discussed in Appendices A through C anc are highlighted in Sections 2.1 and 2.2.
2.1 Experiment Results for Low Floodino Rates 2.1.1 Parametric Effects Found in FLECHT Tests The effectiveness of emergency coolirg is indicated by studies of cladding temperatures, heat transfer, rod quer.cr.ir.g times anc bundle mass effluent fraction.
These variables have been studied extensively in FLECHT as a function of the parameters listed in Table I.
Many of :ne basic tests that were performed with a cosine-shaced axial power profile were repeated with a skewed-shaped axial power profile.
Variation of the parameters has qualitatively predictable e'ffects on reflooding behavior:
A.
Higher inlet flooding velocity, pressure, or inlet subccoling cause a lower cladding peak temperature rise, a higher quench front velocity, and a shorter quench time; and S.
Higher power, initial cladding temperature, or housing temperature cause a lower quench front velocity, a longer quench time, and (except for the case of initial temperature) a higner temperature rise.
Increases in cladding temperature are terminated by cooling due to cispersed droplet flow.
A significant quantity of licuit in croplet form exists above the quench front which aics in cooling tne a::er rod eleva-tions.
FLECHT movies indicate typical droplet s1:es of 1 - 2 mm, requiring steam velocities on the order of 5 m/sec for entrainment.
Heat transfer.
coefficients are determined by the amount of water in the vacor, and both heat transfer and water content generally increase with ;uencn front velocity.
E'xcept for the highest ve'ocities, vapor su:erheat is found to increase sharply with distance above the tuench front ir tne central portion of the rods.
i681 272
Harold R. Denton 4
Figure i shows that the sensitivity of peak cladding temperature to flooding rate increases as the flooding rate decreases, and there is not a dramatic deterioration in heat transfer at 2.5 centimeters per second.
2.1.2 Results of Other Refloodina Exceriments The primary FLECHT data for full length stainless steel rods agree well or are conservative in comparison with results of other experimental programs.
The experiments compared include tests with shorter rods in the Semiscale Facility (INEL), tests using a very large bundle at KWU (Erlangen, Germany), tests with unpressurized zircaloy-clad rods in the FLECHT facility, tests with pressurized zircaloy-clad rods at KfK (Karlsruhe, Germany), and tests with partially blocked channels in FLECHT.
A.
Semiscale.
In spite of substantial geometrical cifferences between Semiscale and FLECHT, the same parametric trends are found, and the magnitudes of the cladding midplane peak temperature rise and the quench front velocity for coccarable test conditions are very similar.
S.
KWU.
The KWU tests using a 340 rod bundle resultec in similar heat IFansfer coefficients, and all differences can be explained in terms of differences in test design and conduct.
C.
KfK.
The experiments with zircalcy-clad heater rocs by KfK and in TGCHT each resulted in higher quench front velocities than found for stainless steel-clad rods tested under similar conditions.
The difference appears to be ennanced when the zircaloy cladding balloons, and may approach the theory-predicted enhancement fact'or of two in sucn cases as suggested by both theory and experiment (see Appendices B, C).
C.
FLECHT Blockaoe Tests.
The FLECHT tests with plate blockage resulted in cetter cooling immediately dcwnstream of the blockage than experi-enced in the unblocked tests.
These experiments were nonconclusive, however, due to the unrealistic blockage geometry and the limited parameter range (including limited flow bypass area).
2.2 Analvtical Predictions of Reflood Behavior Based on FLECHT Results
? ecictions of cladding temperature rise, quench front velocity, and liquid carryover to the upper plenum are especially important for assessing safety
.argins during reflood. Modeling approacnes tend to emphasi:e either (1) the fuel rod side and heat conduction, or (2) the fluid side and heat convection.
The former approach can result in closed-form analytical expressions for key varia':les, while the latter generally involves empirical mocels or
- hencmenologically-based computer codes.
FLECHT data have played a key role in ceveloping or assessing models of each type.
1681 273
Harold R. Denton 5
2.2.1 Conduction-Contro11ino Mocels Simple models are available for cuench front velocity and liquid carryover and tend to be of the conduction-controlling type.
The " classical" conduction model for quench front.'elocity (Appendix C) lacXs a convec-tion made of heat transfer and assumes that no heat transfer occurs aoove the quench front.
This is more conservative than Appendix K regulations which allow heat transfer to steam a:ove the quench front and it may be used to determine a bound for reflood behavior.
Figure 2 shows the FLECHT low and nigh flooding rate data as a function of the dimensionless temperature parameter that controls the quench front velocity in conduction models.
Also snown is the quench front velocity as predicted by the classical one-ci er sional conduction equation with boundary conditions of a constant neat uansfer coefficient in the wet region and zero heat transfer in tne dry aegion.
These boundary condi-tions are only slightly more conservative t.. an would exist under steam w
cooling restrictions and result in sestantially lower predicted velocities than found in the experiments.
The calculation must be considered to be semiquantitative since it depends on an " effective wall t' Tkness," which is somewhat uncertain (Appendix C).
2.2.2 Convection-Controllino Models 2.2.2.1 Empirical Models Empirical FLECHT correlations f:r heat transfer coefficients and quench times have been generatec (Appendix D) which are highly successful in fitting their data base. When these correlations are emolayed in evaluation model co.puter codes, the peak cladding temoerature is sucstantially lo-er than obtained by assu ing that cooling is cue to dry steam.
A: plication of the FLECHT correlation to other geometries or to an ex:arded carameter range has uncertain valicity, however, due to its hign cegree of empiricism, providing rationale for development of more phenomenologically-based models.
2.2.2.2 Phenomenolooical Models A number of best-estimate, phen:cenologically-based coces having varicus degrees of complexity have been cevelocec including RELAP.",
REFLUX 2, FLOOD 4, SUPERH (all C../RSR-sponsored) anc UCFLCOD (EPR -
sponsored).
The reflood heat transfer and entrainment mocel para-meters in the systems code RELAP4/M006 were determined by a parametric study and comparison with FLECHT data.
Good agreement with data was obtained by variation of these parameters, and quicelines for their choice were established.
Consistently reliable quicelines c:ula not be found, however, indicating a need for further modeling work on refloco heat transfer.
The single enannel core coces, REFLUX 2, FLG004 anc SUPERH, were developec to crovide a mechanistic descriction of the reflood phenomena scecifically 1681 274
Harold R. Denton 6
observed in FLECHT.
Figure 3 compares the predictions of REFLUX 2 for a FLECHT test in a best-estimate calculation and in a calculation that only allows dry steam cooling above the quench region. This calculation must be considered qualitative in nature due to its many modeling assumptions (Appendix C).
It is also important to note that there is no steam cooling recuirement in licensing calculations for reflood rates above one inch per second, and the actual steam cooling penalty below one inch /second is subject to details of each vendor's modeling approach.
Phenome-nological modeling of heat transfer above the quenen front has not yet reached a satisfactory state, although the calculation of cladding temperature increase and turnaround time by REFLUX 2, FL0004, SUPERH, RELAP4, or UCFLOOD is of ten good.
2.2.2.3 Semiemoirical Models A number of semiempirical mocels have been developed for the prediction of quench front velocities during reflooding at constant inlet flooding rates (Appendix C).
A successful model developed within RSR empitys initial system parameters rather than transient thermal-hydraul.,: conditions and constitutes a simple tool for making pretest estimations of reflood velocity, quench time and mars carryover.
The correlation was developed using both low and high flooding rate data from the FLECHT program and is given by:
N
~
u
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(
)
(la) q in I init - sat where u = quench front velocity q
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= inlet flooding velocity in Tmin = minimum film b iling temperature T
= saturation tem:erature sat Tinitial = initial cladding temperature The dimensionless number (r) in Equation (la) is given by:
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(.l b )
f p,
"fg v v UP 1681 275
Harold R. Denton 7
It is approximately proportional to initial local heat flux (q) and inversely proportional to pressure.
A higher number by Equation (lb) tends to indicate greater turbulence in the froth region, enhanced droplet entrainment and reduced quench front velocity.
Figure 4 shows a comparison of measured and predicted velocities for the FLECHT experiments. The cata base includes a wide range of test parameters: ' inlet velocities of 1 - 45 cm/s, peak rod power of 1.7 - 4.6 kW/m, pressures of 0.1 - 0.6 MPa, initial peak cladding temperatures of 140 - Il00'C, inlet subcooling of 72 - 90*C, and both cosine and " skewed" axial power profiles. The average percent discrepancy between the measured and predicted velocities is 20%.
The model provides good predictions of quench front velocities for other facilities.
Semiscale tests performed with system parameters similar to those of FLECHT tests resulted in similar actual and predicted quench front velocities, in spite of different core lengths of 1.7 and 3.7 m.
The reficed rate for zircaloy-clad rods in LOFT is nearly twice that predicted by Equation 1 for stainless steel-sneathed heater rods.
This enhancement factor is due only to differences in rod design and is expected according to conduction model theory and the results of small-scale experiments (such as those sponsored by EPRI at UCLA).
The mass carryover rate fraction for low flooding rates, exclusive calculated of fall back, is given by unity linus the ratio u /u,"be mace by 4
by Equation (1).
Refinements to this calculation can application of Yeh's void fraction correlation or by the treatment of Sun and Juffey (Appendix C).
Details of this calculation are given in Appendix C.
Further results and analyses are presentec in Appendices A - D as outlined in Figure 5.
Nomenclature is described in Appencix E.
3.0 Discussion Results of the FLECHT program have been widely exposed to technical review through documentation and presentation at numerous meetings, including the Water Reactor Safety Research Information meetings.
NRR staff utilize FLECHT data and consult with RES heat transfer staff when making licensing decisions.
The general consensus is that the FLECHT prcgram has been conducted well and tnat the data are valuable and important for consideration in the licensing of pressuri:ed water reactors.
The FLECHT program has served as a model for programs in other countries and has produced benefits in addition to an understanding of Icw flocc rate behavior and a cata base for model cevelo: ment.
All reactor vencors have utili:ed FLECHT data to improve tneir refico: models.
Com:: uter coce 1681 276
Harold R. Denton 8
cevelopment groups at RSR-sponsored laboratories have made extensive use of the data, including the unique FLECHT measurements of steam temperature above the quench front.
These measurements demonstrated the thermal nonequilibrium nature of the fluid, and have allowed more detailed model assessments.
The program also provided valuable flow visualization data and demonstrated its importance in the interpretation of recorded measurements.
In addition it was demonstrated that differential pressure cells could be used to calculate void fractions in the test section and mass cistributions throughout the system.
An understanding of scaling effects, blockage effects, and system behavior will be gained in the FLECHT SEASET tests.
Original system effects tests (FLECHT-SET) verified the parametric trends observed in FLECHT (Section 2.1.1).
Those tests also indicated the importance and complexity of steam generator effects and upper plenum design on system behavior.
Such behavior must be understood for the analysis of gravity feed reflood (with continuously varying reflood rates) and will be examined in the FLECHT SEASET program.
Other areas of continuing research involve:
(1) the investigation of reflood behavior for Zircalcy-clad rods; and (2) analytical modeling efforts.
Reflooding with zircaloy-clad rods is now being investigated in out-of pile tests at the German KfK Rebeka Facility.
Parametric reflood studies are also being planned for late 1980 in the NRU reactor at Chalk River, Canada, as part of the RSR fuel behavior research program. Analytical efforts are continuing by many organizations, including RSR in-house correlation work giving quench velocity and time as a simple function of physical variables, anc Westinghouse work to somewhat simplify the accurate FLECHT heat transfer correlation by using elevation above the quench front rather than time as an indeoendent variable.
4.0 Recommendation The results presented above are recommenced for consideration in the applica-tion and appraisal of evaluation models for reficed heat transfer.
The following points are of particular nnte:
A.
The low flooding rate (less than 1 inch per second) FLECHT data can be used in the preparation of a future Appendix K revision; and B.
Simple tools can be used to evaluate vendor calculations of reflood behavior, such as the RSR correlation for cuench front velocity and the uccer plenum carryover fraction, anc exist'ng phencmenologically-based codes for the increase in peak cladding tem::erature during reflood.
1681 277
Harold R. Denton 9
For further clarification or evaluation of these results, L. B. Thompson or Y. Y. Hsu of the Separate Effects Research Branch of the Division of Reactor Safety Research may be contacted.
M Saul Levine, Director Office of Nuclear Regulatory Research 1.
Appendix A 2.
Appendix B 3.
Appendix C 4
Appendix 0 5.
Appendix E 1681 278
10 TABLE I RANGE OF PARAMETERS IN THE FLECHT EXPERIMENTS Parameter Rance (SI Units)
Rance (British Units)
Inlet Flooding Rate 1.0 - 46 cm/sec 0.4 - 18 inch /sec System Pressure 0.1 - 0.62 MPa 15 - 90 psia Peak Pcwer 0.7 - 4.6 kW/m 0.2 - 1.a kW/ft Initial Cladding Temperature 150 - 1200*C 300' - 2200*F Co:lant Inlet Subcooling 9 - 105'C 16 - 189'F L: cal Channel Area Elockage 0 - 100 Percent Same Surdle Area Blockage 0 - 30 Percent Same Decay :ower ANS + 20% -- A*iS-155 Same Ax'ai Nwer Profile cosine, skewed Same Bundle Radial Power Profile
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Ficure 1 C:m arison of Measured ar.c Fredicted Quench Front Velocities for FLECHT Tests with Ple-Velocities of i to 15 cm/s.
1681 283
15 Figure 5.
Accendices to t*e Reficed RIL*
A::endix A Westinghouse FLECHT Results Facility De.scription Program and Procedure II.
Effects of Inlet Velocity and etner Parameters on Reflooding Behavior III.
IV.
Comparison of Tests with Cosine and Skewed Axial Power Profiles V.
Heat Transfer Information VI.
Mass Entrainment Information Correlations 1.
Heat Transfer and Quench ire 2.
Effect of Variable Floodirg Rate A::endix B_
Results of Other Refloeding Ex;eriments and Ccmparison with FLECHT Data I.
Semiscale Constant Floeding Rate Ex:eriments KWU Constant Flooding Rate Exceri.ents II, FLECHT Tests Using Unpressurized Reds with Zircaloy Cladding III.
IV.
KfK Tests Using Pressurized Rocs with Zircaloy Cladding V.
FLECHT Flow Bicckage Tests 3.::encix C Analytical Predictions of Ther al-Fydraulic Sehavior During Reflood I.
Convection-Controlling Models 1.
FLECHT Correlations 2.
REFLUX a.
Models b.
Comparison of REFLUX Predictions with Data Conservative Predictiers by REFLUX c.
d.
Prediction of Peak Ci a::ing Tem:eratures 3.
RELAP4 II.
Conducticn-Centrolling Models fer Quench Front Velocity a.
One-Dimensional Models b.
Two-Dimensional Models c.
Multi-Region Model Semi-Em:irical Approach for Quen:n Front Velocity Murao Ecuation for Hign Ficoding Rates a.
b.
RSR Correlation for "i;h and Low Flooding Rates IV.
Conclusions A:cendix D Tqe FLECHT Ouench Time anc Heat Transfer Cerrelations Correlations for Quencn Time 1681 294
16 Figure 5 (Continued)
II.
Correlations for Heat Transfer Coefficient 1.
The 'FLECHT' Correlation 2.
The Z-Z Correlation Accendix E Nomenclature
- Available on request from S. Decker, Division of Reactor Safety Research, U. S. Nuclear Regulatory Comission, Washington D. C. 20555.
1681 qsg
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