1. The 5th International Conference "Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 UNIVERSITÀ DI PISA DIPARTIMENTO DI INGEGNERIA MECCANICA, NUCLEARE E DELLA PRODUZIONE VIA DIOTISALVI 2, 56100 PISA Gruppo Ricerca Nucleare S. Piero a Grado COMPARATIVE STUDY OF THE TH-SYS CODES PERFORMANCES IN PREDICTING EXPERIMENTS PERFORMED IN VVER-1000 SIMULATOR A. Del Nevo, F. D’Auria

2. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 2 LIST OF CONTENT  FRAMEWORK OF THE ACTIVITY  OBJECTIVE OF THE ACTIVITY  PROCEDURE FOR CODE APPLICATION AND ASSESSMENT  THE COMPUTATIONAL TOOLS  PSB-VVER NUMERIC SIMULATIONS  QUALITATIVE ACCURACY EVALUATION  QUANTITATIVE ACCURACY EVALUATION  CONCLUSIONS

3. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 3 FRAMEWORK OF THE ACTIVITY PROJECT TACIS 2.03/97: Development and qualification of accident management (AM) procedures for the VVER1000/320 Balakovo NPP (2004–2006)  THE KEY-ELEMENTS OF THE PROJECT  The AM status in VVER-1000 safety technology  The ‘creation’ of the PSB-VVER EXP database  The Ideal Test Matrices (considering ATWS, RIA and ‘all-over-the world’ ITF experiments)  15+1 ‘long-lasting’ PSB-VVER-1000 experiments (12 committed)  The demonstration of SYS-TH code capabilities  The qualification of computational tools (code, nodalisation, BIC)  Addressing the Scaling Issue  Calculating the Uncertainty  User Qualification and Training  The proposal of AM strategy for VVER-1000  Putting the bases for ‘preserving the expertise’  Final Report  Final Meeting  Electronic database (exp-data, input decks, output, comparison exp/calc)

4. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 4 FRAMEWORK OF THE ACTIVITY  The reference facility is the PSB-VVER ITF, simulator of VVER- 1000 (V-320 design) NPP, operated at EREC  The AM TM includes 12 experiments  Loss Of Coolant Accident (LOCA) – 5  Loss Of Feed Water (LOFW) – 3  Station BlackOut (SBO) – 1  PRImary to SEcondary side leak (PRISE) – 1  Main Steam Line Break (MSLB) – 1  Natural Circulation (NC) – 1  The experiments performed in the Project were actually 16  The AM TM (12)  1 test for the demonstration of repeatability of the experimental results  3 single variant experiments.

5. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 5 FRAMEWORK OF THE ACTIVITY INSTITUTIONS INVOLVED IN THE PROJECT  University of Pisa (UNIPI)  Electrogorsk Research and Engineering Center (EREC)  Gidropress (GP)  RRC-Kurchatov Institute (RRC-KI) Several other Key Experts were involved in the Project SCIENTISTS INVOLVED IN THE ANALYTICAL SIMULATIONS & ANALYSES  UNIPI - A. Del Nevo, F. D’Auria, G.M. Galassi, W. Giannotti, M. Cherubini, D. Araneo, N. Muellner  EREC - O.I. Melikhov, Y. Parfenov, V.I.Melikhov, I.V.Elkin, A. Kapustin  GP - M. Bykov, E. Lisenkov, M. Zakutaev  RRC-KI - A. Suslov, A. Andryushchenko, A. Drush, L. Gilvanov

6. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 6 OBJECTIVE OF THE ACTIVITY GENERAL OBJECTIVES RELATED WITH THE APPLICATION OF THE TH-SYS AGAINST PSB- VVER EXPERIMENTS  Support the design of the experiments executed in PSB-VVER ITF (TACIS 2.03/97 Projects);  To demonstrate (the availability of) acceptable prediction capabilities for existing codes to simulate transients in easter NPP  Qualifying computational tools used for performing safety analyses focused on the development of AM procedures  To compare the performances of different TH-SYS codes used for Best Estimate analyses (i.e. Cathare2, Relap5/Mod3.3, Relap5-3D©, Athlet, Korsar, etc…). This comparison is based on the application of the tools used at UNIPI in the procedure for code assessment  General comparison between the codes (time evolutions) – NOT PRESENTED  The qualitative accuracy evaluation based on the analysis of the Relevant Thermohydraulic Aspect (RTA)  The quantitative accuracy evaluation performed with the FFT-BM  To contribute in providing relevant post-tests results usable for the development of the CIAU procedure for CATHARE2 and RELAP5 code

7. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 7 THE PROCEDURE FOR CODE APPLICATION AND ASSESSMENT  The activity aimed at the qualification of computational tools was based on the following steps:  Facility configuration  Check of the experimental Boundary and Initial Conditions (BIC)  Check of the experimental data  Input deck preparation and documentation  Steady state conditions  Transient analysis

8. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 8 THE COMPUTATIONAL TOOLS  UNIPI PSB-VVER nodalization by CATHARE2/V1.5B – pre/post- test phases  UNIPI PSB-VVER nodalization by RELAP5/Mod3.3 & RELAP5-3D©/v2.2.4 – pre/post test-phases  EREC PSB-VVER nodalization by CATHARE2/V1.3L – pre/post- test phases  EREC PSB-VVER nodalization by RELAP5/Mod3.3 – pre-test phase  GIDROPRESS PSB-VVER nodalization by KORSAR/v1.1 – pre/post-test phases  GIDROPRESS PSB-VVER nodalization by TRAP – pre-test phase  RRC-KI PSB-VVER nodalization by ATHLET/Mod 2.0 Cycle A – post-test phase.

9. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 9 PSB-VVER NUMERIC SIMULATIONS

10. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 10 PSB-VVER NUMERIC SIMULATIONS

11. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 11 OVERVIEW OF THE POST-TESTS ANALYSES  A comparison between the results obtained by the codes and their applications to the transients of the test matrix was performed.  Different approaches were considered for this comparison in order to evaluate not only the performance of the codes and their capabilities to predict the phenomena but also the UNIPI methodology for code assessment.  These steps were performed in several subsequent phases  Comparison and discussion of the results, considering the homogeneous quantities such as: pressures, mass flow rates, integrated mass flow rate, mass inventories, fluid temperatures, structure (typically the core) temperatures, levels, pressure drops and powers (core, heaters, etc).  The figures of the results time trends (more than 50) were depicted for each test (at least CATHARE2 and RELAP5). Visual judgment of the calculation results quality and of the code capability to correctly reproduce the experiment.  The list of the resulting events or Time Sequence of Events (TSE)  Detailed tables (Qualitative accuracy evaluation) were prepared reporting the Phenomenological Windows (Ph.W.), the Relevant Thermal-hydraulic Aspects (RTA) and the related parameters. These were used to evaluate if the calculation reproduced all the main aspects/phenomena of the experiment and if these aspects/phenomena were well identified and characterized.  The quantitative accuracy was evaluated using the specific tool developed at UNIPI: the FFTBM

12. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 12 QUALITATIVE ACCURACY EVALUATION  The QUALITATIVE ACCURACY evaluation is based upon a systematic procedure consisting in the identification of phenomena and of the RTA. It derives from a visual observation of the experimental and predicted trends. The technical judgments envisaged are:  the code predicts qualitatively and quantitatively the parameter (Excellent – the calculation result is within experimental data uncertainty band);  the code predicts qualitatively, but not quantitatively the parameter (Reasonable – the calculation result shows only correct behavior and trends);  the code does not predict the parameter, but the reason is understood and predictable (Minimal – the calculation result is not within experimental data uncertainty band and sometimes does not have a correct trend);  the code does not predict the parameter and the reason is not understood (Unqualified - calculation result does not show correct trend and behavior, reasons are unknown and unpredictable). The parameters characterizing the RTA (i.e., SVP = Single Valued Parameter, TSE = parameter belonging to the Time Sequence of Events, IPA= Integral Parameter and NDP = Non Dimensional Parameter) give an idea of the amount of the discrepancy.

13. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 13 QUALITATIVE ACCURACY EVALUATION TEST #8

14. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 14 QUALITATIVE ACCURACY EVALUATION

15. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 15 QUALITATIVE ACCURACY EVALUATION

16. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 16 QUALITATIVE ACCURACY EVALUATION

17. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 17  A comparative tables highlighted the performance of the codes Cathare2 and Relap5 in predicting the RTAsCathare2 Relap5  Main conclusions regarding the comparison of the qualitative accuracy evaluations:  The judgments on PRZ behavior are generally excellent or reasonable in all transients in the CATHARE2 and in RELAP5 simulations.  Number of PORV opening (pressurizer behavior) in the high pressure transient (i.e. LOFW) always more than in the experiment.  The analysis of the SG behavior is also often “minimum” in the number of the BRU-A valves opening and closure during the high pressure transient (both codes).  A general faster pressure increase is observed in SG behavior following the main steam line closure (“reasonable” or “minimum”).  Primary mass distribution has been judged correctly simulated in almost all transients for CATHARE2 code  The dryout have been correctly predicted by the code, even if it should be noted that the timing of the occurrence of this phenomenon was generally anticipated. This occurred in particular in the post test of the experiment N. 11 and 12 conducted with RELAP5 code.  The occurrence of the loop seal clearance has been reasonable predicted by the codes. In particular, the main difference was the prediction of the loop where the clearance occurred QUALITATIVE ACCURACY EVALUATION

18. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 18 QUANTITATIVE ACCURACY EVALUATION  The tool (FFT-BM) has been developed at University of Pisa and is based on the use of the Fast Fourier Transform.  The requirements that this tool for accuracy evaluation fulfills are:  at any time of the transient this function should remember the previous history;  engineering judgment should be avoided or reduced;  the mathematical formulation should be simple;  the function should be non-dimensional;  it should be independent upon the transient duration;  compensating errors should be taken into account (or pointed out);  its values should be normalized.  20-25 parameters selected between a list of 27 are used for the application of the method depending upon the transient and the availability of the experimental measurements: pressures, levels, pressure drops, cladding and fluid temperatures, mass inventory or injected and power.

19. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 19 QUANTITATIVE ACCURACY EVALUATION  The FFT-BM provides a measure of the accuracy of the calculation, in particular:  AA is the relative magnitude of the discrepancy  WF factor characterizes the kind of error (…at low or high frequencies)  Two limits are envisaged in the UNIPI procedure for code assessment: AA pp =0.1; AA tot =0.4 ‘VERY POOR/ UNACCEPTABLE’ (AA)tot > 0.7 ‘poor’ 0.5 < (AA)tot < 0.7 ‘good’ 0.3 < (AA)tot < 0.5 ‘very good’ (AA)tot < 0.3

20. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 20 QUANTITATIVE ACCURACY EVALUATION BY R5 & C2 Relap5 Cathare2

21. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 21 COMPARISON AMONG CODE RESULTS BY FFTBM  COMPARATIVE ANALYSIS:  General capability of all codes to predict the scenarios of the Project, notwithstanding the peculiarities of each code (i.e. numerical scheme, availability of cross junction, possibility to simulate the throttle of the break, etc).  The user effect has been identified as the relevant key issue in the application of the TH-SYS codes.

22. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 22 OVERALL QUANTITATIVE ACCURACY EVALUATION

23. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 23 OVERALL QUANTITATIVE ACCURACY EVALUATION

24. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 24 CONCLUSIVE REMARKS  CODE ASSESSMENT, AIMED AT CONFIRMING THE CAPABILITIES OF TH-SYS CODES AGAINST AM TM CARRIED OUT AT EREC TEST FACILITY PSB-VVER, WAS PERFORMED  FOUR TH-SYS CODES (CATHARE2, RELAP5, ATHLET AND KORSAR) WERE APPLIED WITH A DIFFERENT EXTENT AND WITH DIFFERENT PURPOSES BY FOUR INSTITUTIONS: UNIPI, GIDROPRESS, EREC AND RRC-KI  THE COMPARATIVE ANALYSIS OF THE RESULTS BRINGS TO THE FOLLOWING CONSIDERATIONS:  The codes assessment confirmed that all codes were in general capable to predict the TH scenarios, involving the simulation of AM actions, with satisfactory results, notwithstanding the peculiarities of each code (i.e. numerical scheme, availability of cross junction, possibility to simulate the throttle of the break)  Some general minor discrepancies between the experimental evidences and the code simulations were identified, highlighted and justified  The differences in the simulation of the PRZ valve cycling  A faster pressure increase was observed in SG behavior following the main steam line closure.  General longer coupling between primary and secondary side pressure during the transient scenarios

25. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 25 CONCLUSIVE REMARKS  …FOLLOWING CONSIDERATIONS (CONT’ED):  The analysis demonstrated that the FFT-BM is a suitable tool to measure the accuracy of the results.  The primary pressure threshold foreseen in the UNIPI methodology for the high pressure transients involving cycling of valves was highlighted to be too restrictive.  The user effect, well known in literature, was confirmed to be a key issue in the application of the TH-SYS codes.

26. Gruppo Ricerca Nucleare S. Piero a Grado The 5 th Int. Conf. “Safety Assurance of NPP with WWER” FSUE OKB “GIDROPRESS”, Podolsk, Russia 29 May-1 June, 2007 26 THANKS