Conclusions from Quench-03 Test Analyses with ICARE2 and MELCOR Codes

1. Conclusions from Quench-03 Test Analyses with ICARE2 and MELCOR Codes Jiří Duspiva Nuclear Research Institute Řež, plc. Nuclear Power and Safety Division Dept. of Reactor Technology 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

2. Outline • Background of Quench analysis in NRI Řež • Quench-03 test analysis with ICARE2(Short summary of 10th QWS presentation) • Comparison to MELCOR 1.8.5 • Improved model of Quench-03 • Comparison to MELCOR 1.8.5 • Regressive application to Quench-01 • Summary and conclusions 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

3. Backgroundof NRI Quench Analyses • First NRI – Quench activities were performed with MELCOR Code • Own MELCOR input model was developed for Quench-01 [1], successfully also applied to Quench-06 [ISP-45 Blind phase] calculations • Quench-03 Calculation was last with MELCOR 1.8.5 [2] • Temperature at onset of reflooding higher than Q-01 and Q-06 • Temperature escalation was not predicted correctly • Strong underestimation of Hydrogen production • Under EU Project SARNET in TPA1/JPA1 (WP9: Early Phase of Core Degradation ST-1 Hydrogen Generation during Core Reflooding) started application of ICARE2 Code with following schedule [3] and [4] • Step 1 – Preparation of model and calculation of Quench-01 test • Step 2 - Sensitivity Study on the change of important parameters • Step 3 – Calculation of Quench-03 test [1] J. Duspiva: Quench-01 Test Calculation with MELCOR Code, CSARP Meeting, May 7-9, 2001, Bethesda, Maryland [2] J. Duspiva: Quench-03 Test Calculation with MELCOR Code, 8th International Quench Workshop, Karlsruhe, Germany, October 29-31, 2002 [3] J. Duspiva: Quench Test Calculations with ICARE2 Code and Comparison with MELCOR Code Results, 10th International Quench Workshop, Karlsruhe, Germany, October 26-28, 2004 [4] J. Duspiva: Quench Test Calculations with ICARE2 Code and Comparison with MELCOR Code Results (Quench-01 and Quench-03 Test Analyses), Report NRI Řež, UJV-12204-T, March 2005 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

4. ICARE2 Input ModelQuench Facility Nodalization • Original model, received from L. Belovsky (ALIAS CZ), was prepared by G. Bandini (ENEA) • Macrocomponents • Unheated rod • 8 heated rods of inner ring • 12 heated rods of outer ring • 3 Corner rods (withdrawal of one corner rod neglected) • Grid spacers and • Shroud with gap above heated zone • One TH channel – FLUID2 type • 41 axial levels • 4 below heated part • 21 in heated part • 16 upper plenum • First test on Quench-01 • Sensitivity Matrix  Improvements 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

5. Quench-01Results of Reference Calculation • Total Hydrogen production predicted correctly  tuned up by external resistivity • Temperature profiles also predicted well until the beginning of reflooding • Heat Balance was checked, based on methodology from ISP-45 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

6. Quench-03Input File Changes • Changes done in comparison with Quench-01 calculation (identical approach as in MELCOR analyses of Quench-01 and Quench-03 tests) Redefinition of (initial and boundary conditions) • Power per ring • Inlet temperatures and mass flow rates (Ar, steam and water) • Initial temperatures in bundle • Time of reflooding beginning • Timestep definition 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

7. Quench-03Hydrogen Production First step in the input modifications was done in tuning up of external resistivity  temperature instability in bottom part of bundle Quench-01 like external resistivity 3.09 m/rod Tuned up external resistivity 2.1 m/rod 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

8. Quench-03Temperature Profile at 2600 s Quench-01 like external resistivity 3.09 m/rod Water injection onset at 2600 s Tuned up external resistivity 2.1 m/rod 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

9. ICARE2 to MELCOR Comparison • Similar behaviour of both analyses for H2 production when all settings from Q-01 are applied in Q-03 (change of initial and boundary conditions) • Also temperature evolutions had a lot of similarities • Relatively good agreement of temperature prediction in lower and middle part of heated zone • No temperature escalation results in underestimation of H2 production [2] J. Duspiva: Quench-03 Test Calculation with MELCOR Code, 8th International QUENCH Workshop, Karslruhe October 29-31, 2002 End of 10th QWS 2004 NRI Contribution Summary 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

10. Identification of Critical PointApplication of ATLAS Postprocessor GRS postprocessorATLAS • ICARE2 results reprocessing by MELCOR At 2600 s • Significant underprediction of temperatures in hot zone • From analyst point of view identified as overestimation of heat losses through shroud in area of hot zone • From Q-03 test point of view identified as “loss of heat removal” 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

11. Modelling Approach Used • More possible ways exist • Definition of material properties was separated for • BZF2-5 • BZF1 • Conductivity of solid material was modified for temperature above 550 K • Final values were defined iteratively based on temperature profiles at onset 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

12. Improved PredictionTemperature Profile at ______ 2500 s 2520 s 2540 s 2560 s 2580 s 2590 s 2600 s 2620 s 2710 s 2640 s 2650 s 2660 s 2670 s 2680 s 2690 s 2699 s 2610 s 2630 s • Reflooding phase observations • Overprediction of temperatures in bottom part • Oscillations of swollen water level • Quench front level remains at bottom • Continuation of calculation is problematic due to unconvergency and too small timestep (< 10-4 s) • Calculation of whole Q-03 was not finished and is not planned • Temperature profiles are in agreement with measured values • Significantly lower temperature of Zirconia fiber layers • All three types of rod are degraded, shroud remains intact Visualization 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

13. Application of New ZF Propertiesto MELCOR Analysis “Identical” figure of temperature profiles with modified ZrO2 Fiber Conductivity At 2600 s • Significant reduction of ZF temperatures as in ICARE2 run • Slightly underpredicted temperatures of hot zone • H2 production underpedicted 22 g • Reason  trick in shroud modelling to allow its oxidation 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

14. Bundle Refloodingin MELCOR Analysis At 2725 s • Reflooding phase observations • Correct prediction of water level • Agreement in temperature drop due to quenching • Rod and shroud degradations are not predicted so intensive as in ICARE2 run • Code stability – no termination of run 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

15. Summary and Conclusions • Application of new postprocessing (using GRS ATLAS tool via MELCOR code) made a possible to identified cause of modelling troubles • Three screens prepared – one of them for both of codes used  direct comparison • Working term of phenomenon identified: “loss of heat removal through shroud in hot zone” • Modelling of this feature was done by changing of Zirconia fiber conductivity • Improved modelling resulted in very good agreement in temperature profiles at the time of water injection onset in ICARE2 analysis • In MELCOR analysis - agreement was not found, but temperature profiles were improved too  trick in shroud oxidation modelling with fixed heat transfer coefficient (COR00011 input row) • ICARE2 calculation of reflooding phase results in unconvergency and timestep reduction, oscillation in swollen water level occurred • MELCOR run was more stable during reflooding phase 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005

16. Summary and Conclusions (2) • Application of ICARE2 code allowed to identify cause, which was not possible to identify with integral code due to its modelling simplifications, but conclusions are relevant to both of codes (new features of MELCOR1.8.6 will allow more direct validation on Quench tests) • Regressive application of Zirconia fiber properties to Quench-01 analysis with ICARE2 code resulted in strong overpediction of temperatures and hydrogen production • Phenomenon occurred in Quench-03 test only, not in Quench-01, so it is not possible to use the same model for analysis of both tests • Final identification and description of phenomenon, which occurred in Quench-03, should be done by experimenters from FZK, only one of possible analytical approaches to model this test was presented • Specificity of Quench-03 results from shroud behaviour • Not objective of Quench program • Unimportant for plant applications  no another analysis are needed • Output from effort is available in Report UJV-12204-T 11th International QUENCH Workshop Karlsruhe, Germany, October 25-27, 2005