2001 1 : Top-down Approach TRAC-CSAU Understanding of the Best-Estimate Methodology for LB-LOCA Part-I: Top-down Approach and TRAC-CSAU 150 1998 [1] [2] [3] [4,5],, (LB- LOCA) 1 i ( ) ( ), ( ) (Separate Effect Test, SET) (Integral Effect Test, IET), TRAC-CSAU[5] [6] 1 Bottom-up approach 2 Top-down approach
Abstract Since the law[1] for Large Break Loss of Coolant Accident(LB-LOCA) has been revised to allow the use of the Best-Estimate(BE) Methodology, The US Nuclear Regulatory Commission(NRC) issued two independent positions for the old conservative evaluation model[2] and for the BE evaluation model[3] respectively In this paper, by scrutinizing the US regulatory position and the related studies[4,5], it is shown that a consistent regulatory principle is kept in both methodology Following this principle, a methodology can be suggested to select a certain code model or correlation and to quantify their ranges for the concerned important uncertain phenomena or process The key point of the BEmethodology is that the identified important phenomena or processes are to be evaluated to quantify the uncertainties in the LB-LOCA scenario, instead of evaluating the individual models and correlations In doing so, however, a certain model or group of models should be selected and their uncertainties should be quantified But, their role should be understood as the representative parameters for the concerned phenomena or processes Accordingly, the range of the selected uncertainty parameter should be confirmed based on the well-scaled experiments With the above viewpoint, some solutions are suggested for the many questions[6] concerning the TRAC-CSAU[5] methodology 1 TRAC-CSAU,, -1, TRAC-CSAU TPG group[6] TRAC-CSAU, 7,? TPG group ; It is not possible nor necessary to address each potential source of uncertainty in an equal manner One of the strengths of the methodology is its ability, based on expert evaluation of experimental evidence, to prioritize the sources of uncertainty for further analysis and treat them accordingly TRAC-CSAU [5] NUREG-1230[4]?,? LB-LOCA
2 Top-down Approach; LB-LOCA Top-down approach LB-LOCA LB-LOCA LOCA,, LB-LOCA LB-LOCA, top-down approach, 21 Appendix-K Top-down Approach; Appendix-K top-down approach NUREG-1230[4] ; Because extensive directly applicable experimental data did not yet exist in 1972 for use in computer code development or in assessing the predictive capabilities of these codes for key portions of the Light Water Reactor (LWR) response to LOCAs, large uncertainties existed in predictions of these transients Accordingly, sufficiently conservative assumptions were used in developing 5046 and Appendix-K in 1974 to provide assurance that Emergency Core Cooling System(ECCS) criteria would be satisfied even in the unlikely event that worst-case uncertainties prevailed, 2 Appendix-K required features -3 / Appendix-K required features required features / LB-LOCA, Appendix-K required features /
22 Top-down approach LB-LOCA [3] -3 Reg Guide1157, -1-2 -3 Appendix-K, LOCA -4-3 Appendix-K -3-4 -2 NUREG-1230[4] NUREG-1230 ; There can be two method to evaluate a best estimate code In the bottom-up approach, each model and all the closure relations in a code are examined and assessed in a uniform function Sensitivity studies are performed on every single model to assess its effects on calculated results Although this approach is rigorous, it is definitely impractical in view of the number of calculations that would be required In the top-down approach, one identifies significant phenomena that have influence on the overall results for a scenario or for a distinct class of scenarios The capability of the code to calculate these significant phenomena is assessed against test data Finally, sensitivity studies are performed on parameters and/or models that affect the significant phenomena It is evident that the top-down approach has several attractive features, for example, the reduced number of sensitivity calculations However, it still has important shortcomings, that is, it does not offer a method to address the questions related to scaling and to compensating errors among others CSAU method, which removes these and other shortcomings bottom-up approach top-down approach ( -4 ) CSAU top-down approach QA documents SET IET TRAC-CSAU [5] TRAC-PF1[7] 4 LB-LOCA / -1, / ( -2), Appendix-K required features uncertainty parameters, top-
down approach, / Appendix-K / /,,? 3 CSAU TRAC-CSAU phenomena/process identification and ranking, -1 break discharge coefficient, fuel parameters, heat transfer coefficient, minimum boiling temperature, pump, steam binding, ECC bypass, dissolved nitrogen heat transfer coefficient minimum boiling temperature, stored energy and fuel response clad surface heat transfer, [ -1] heat transfer coefficient minimum boiling temperature fuel response, Appendix-K fuel response no return to nucleate, no return to transition boiling, hot channel to be less than one assembly size, FLECHT correlation to be used for 1 in/sec, steam cooling to be used for 1 in / sec required features ( -2 ), Reg Guide Position 42 ( -4 ) SET IET ; Code uncertainty should be evaluated through direct data comparison with relevant integral systems and separate effects experiments at different scales In this manner, an estimate of the uncertainty attributable to the combined effect of the models and correlations within the code can be obtained for all scales and for different phenomena Comparisons to a sufficient number of integral systems experiments from different facilities and different scales, should be made to ensure that a reasonable estimate of code uncertainty and bias has been obtained SET IET code uncertainty bias IET Appendix-K required features
/ TRAC-CSAU heat transfer coefficient minimum boiling temperature 31 heat transfer coefficient fuel response, flow regime map,, TRAC-CSAU Appendix-K required feature TRA-CSAU single heated tube [7], h( t) = h( W ( t), α( t), K, c, t) p h t h t) = h i ( ( W ( t), α( t), K, c,) < p t i 1 < t t i i (heat transfer regime) 1 (t) single tube test[8] h i 2 W α 3
, droplet entrainment, droplet evaporation 4 grid, 5, fuel response,, INEL Post-CHF, SET IET code uncertainty bias IET TRAC-CSAU Reg Guide Reg Guide, power-to-volume THTF[9] THTF,, film boiling transition boiling grid,, distortion FLECHT-SEASET[10] IET Reg Guide
32 TRAC-CSAU heat transfer coefficient 1, PIRT process,,, SET IET,, fuel response, THTF fuel response THTF TRAC-CSAU, break discharge coefficient, break flow, Mraviken[11] Mraviken LB-LOCA critical flow scale / fuel response TRAC-CSAU, Appendix-K decay heat no return to nucleate boiling fuel response THTF SET single tube test TRAC-CSAU SET IET 4 TRAC-CSAU fuel response -1 7,,,
,,, SET IET
1 United States Code of Federal Regulations, Title 10, Section 5046, Acceptance Criteria for Emergency Core Cooling Systems for Light Water Reactors, 1988 2 APPENDIX-K to Part 50 ECCS Evaluation Models, I Required and Acceptable Features of Evaluation Models; Regluations U SNuclear Regulatory Commission 3 Regulatory Guide 1157, Best-Estimate Calculations of ECCS Performance, US Regulatory Commission, May 1989 4 Division of Reactor and Plant Systems, "Compendium of ECCS Research for Realistic LOCA Analysis" NUREG -1230, 1987 5 B Boyack, et, al, "Quantifying Reactor Safety margins" NUREG/CR-5249, 1989 6 G E Wilson, et al, "TPG response to the foregoing letters-to-editor" Nucl Eng and Des V132, pp431, 1992 7 D R Liles et al, "TRAC-PF1/MOD1, An advanced Best-Estimate Computer Program for Pressurized Water Reactor Analysis" NUREG/CR-3858, July 1986 8 R C Gottula et al, "Forced convective, Non-equilibrium, Post-CHF heat transfer experiment data and correlation comparison report" NUREG/CR-3193, March 1985 9 V D Clemens et al, PWR Blowdown Heat Transfer Separate-Effects Program --- Thermal- Hydraulic Test Facility Experimental Data Report for Test 160, NUREG/CR-0730, June 1980 10 M J Loftus, et al, PWR FLECHT-SEASET Unblocked Bundle, Forced and Gravity Reflood Task Data Report, NUREG/CR-1532, September 1981 11 Studsvik Energieknik AB, The Marviken FullpScale Critical-Flow Tests, Final report Vol 1-35, EPRI/NP-2370, December 1982
-1 TRAC-CSAU IMPORTANT PHENOMENA FROM PIRT COMPARISON WITH CODE CAPABILITY KEY CODE PARAMETERS RANGING OF PARAMETERS IMPORTANT UNCERTAINTY PARAMETERS BREAK FLOW MASS FLOW RMS ERROR STORED ENERGY AND FUEL RESPONSE CODE MANUAL GAP CONDUCTANCE FUEL CONDUCTIVITY FUEL CAPACITY INITIAL POWER PEAKING FACTOR AUXILIARY CALCULATIONS GAP CONDUCTANCE FUEL CONDUCTIVITY PEAKING FACTOR USERS GUIDELINES CLAD SURFACE HT HEAT TRANSFER COEF Tmin PUMP 2-PHASE FLOW DEVELOPMENTAL ASSESSMENT MASS FLOW PRESSURE HEAD & TORQUE MULTIPLIER STEAM BINDING ECCS BYPASS MC/QE REPORT LIQUID MASS FLOW EVAPORATION ENTRAINMENT DE-ENTRAINMENT ECC FLOW DIVERSION SERARATE EFFECT TESTS INTERFACIAL DRAG CORE UPPER PLENUM HOT LEG INTERFACIAL DRAG DOWNCOMER NON-CONDENSIBLE GAS PARTIAL PRESSURE DISSOLVED NITROGEN
-1 TRAC-CSAU No Questions on 1 Generality of CSAU 2 Practicality of CSAU 3 Use of Engineering Judgement in application of CSAU 4 Use of Biases in application of CSAU 5 Use of frozen Code Version having Model Deficiency 6 Quantification of uncertainty introduced by user controlled Variations, particularly Nodalization 7 Elimination of many thermal-hydraulic phenomena as sources of Uncertainty 8 Validity of comparing the CSAU results to the world s PCT data bases 9 Validity of the Use of Supplemental Fuel Rods 10 Scalability of Code 11 Statistics -2 Appendix-K Required Features and TRAC-CSAU Phenomena/Processes Required features TRAC-CSAU A Sources of Heat B Fuel Rod Model C Blowdown Phenomena 1 Break Flow a spectrum b discharge model c EOB d Noding near break 2 Frictional Pressure Drop 3 Momentum Equation 4 Critical Heat Flux 5 Post-CHF HTCorrelation 6 Pump Modeling 7 Core Flow Distribution D Post-Blowdown Phenomena 1 Single Failure Criterion 2 Containment Pressure 3 Reflood rate calculation 4 Steam condensation 5 RF HT and steam cooling A Decay Heat; ANS-71 plus 20% B Swelling and rupture Gap conductance C Blowdown Phenomena 1 Break Flow a spectrum b Moody Model c EOB d Split break 2 Realistic Frictional Pressure Drop 3 Momentum Equation 4 No return to Nucleate 5 No return to Transition Boiling 6 Two-phase Pump Data 7 Hot Channel<One Assembly Size D Post-Blowdown Phenomena 1 Single Failure Criterion 2 Minimum Back Pressure 3 High entrainment rate 4 No condensation at pipes 5 FLECHT correlation > 1 in/sec Steam cooling < 1 in/sec Uncertainty Parameters Peaking factor Gap conductance Discharge coeff ECC bypass Tmin Heat Transfer Coeff Pump Degradation Steam binding Tmin Heat Transfer Coeff -3 Summary of Regulatory Guide 1157 Regulatory Position 1; General Attributes in Best-Estimate Calculations Best-Estimate Model; realistic calculation of experiment predict mean value of experiments Unaccounted-for Model; treated as a bias in overall uncertainty do not include the bias in the analysis Range of Models; use within the applicable range if extrapolated, uncertainty evaluation Best-Estimate Code; predict the important phenomena SET and IET; determine overall uncertainty and bias / IET; confirmation of Best-Estimate Code Conservatism; simplification leads little or no effect; only upper bound of model is known Bias is acceptable Regulatory Position 2; Special Considerations for Best-Estimate Calculations NUREG-1230 Compendium guides the Best Estimate Methodology
Uncertainties of features; included in overall uncertainty calculation 21 Basic Structure of Codes 211 Numerical Methods; Noding Sensitivity to be done 212 Correlational Models Regulatory Position 3; Best-Estimate Code Features 31 Initial and Boundary Condition and Equipment Availability 32 Sources of Heat During a LOCA 33 Fuel Rod Parameters 34 Blowdown Phenomena 341 Break Characteristics and Flow /342 ECC Bypass 35 Noding near the Break and ECCS injection Point 36 Frictional Pressure Drop 37 Momentum Equation 38 Critical Heat Flux 39 Post-CHF blowdown Heat Transfer 310 Pump Modeling 311 Core Flow Distribution During Blowdown 312 Post-Blowdown Phenomena 3121 Containment Pressure 3122 Calculation of PB Th for PWR 3123 Steam Interaction with ECC in PWR 3124 Post Blowdown Heat Transfer for PWR 316 Other Features 3161 Completeness BE code should be complete 3162 Data Comparisons Individual Models should be compared with Experiments Regulatory Position 4; Estimation of Overall Calculation Uncertainty 41 General; Definition of Uncertainty; Code Uncertainty; Combined uncertainty accounting the individual models and correlations Overall Calculation Uncertainty; Code Uncertainty Plus Uncertainties from the Various Sources A completely rigorous mathematical treatment is neither practical nor required Approximations and assumptions 42 Code Uncertainty Evaluated by direct comparison with relevant IET s and SET s Separate Uncertainties for Blowdown and Reflood Justification of separate uncertainty treatment 43 Other source of Uncertainty 431 Initial and Boundary Conditions and Equipment availability 432 Fuel Behavior 433 Other Variables 44 Statistical Treatment of Overall Calculation Uncertainty -4 Bottom-up and top-down approach for code evaluation Approach Range of assess Sensitivity range Merits Demerits Bottom-up Single model Every single model rigorous impractical Top-down Phenomena Parameter affecting phenomena economic Scaling error Compensating error