1. 1 Application of the SVECHA/QUENCH code to the simulation of the QUENCH bundle tests Q-07 and Q-08 Presented by A.V.Palagin* Nuclear Safety Institute (IBRAE) Russian Academy of Sciences * Currently invited scientist at the JRC-ITU 11th International QUENCH Workshop Forschungszentrum Karlsruhe, October 25-27, 2005

2. 2 Outline 1.Calculations matrix 2.Effective channel approach 3.SVECHA/QUENCH code B 4 C oxidation model 4.Q-07 test simulation 5.Q-08 test simulation 6.Analysis of the results obtained 7.Summary and conclusion

3. 3 Calculations matrix Real testImaginary test* Q-07: B 4 C absorber rodZr cladding/ZrO 2 pellet Q-08: Zry cladding/ZrO 2 pelletB 4 C absorber rod Central rod: *When performing the imaginary test simulation it was assumed that all the test conditions (temperature history, gas mixture composition, etc) were the same as in the corresponding real test.

4. 4 Effective channel approach Since the central rod of the bundle is not heated, its temperature evolution in the course of quench bundle test is completely determined by thermal-hydraulic boundary conditions: temperatures of the surrounding heated rods and shroud and characteristics of the coolant flow. In the case of full-scale simulation of the bundle test the temperatures of the heated rods and shroud are calculated by specifying the electric power time evolution and thus, the boundary conditions for the central rod are determined by the code. In the SVECHA/QUENCH code the thermal boundary conditions for the central rod are predetermined by specifying the temperatures of the “effective channel” inner wall on the basis of experimentally measured temperatures. The inner surface of the effective channel represents the surfaces of the heated rods surrounding the central rod. From the viewpoint of the solution of the heat conduction problem inside the central rod both ways are equivalent. Specification of the boundary conditions on the basis of the experimentally measured temperatures even has certain advantages as it describes the thermal regime around the central rod very close to that in the experiment.

5. 5 Rate controlling mechanisms: 1) Surface reaction kinetics 2) Mass transport in the gas phase inlet H 2 O : Ar H2OH2O H 2, CO 2, CO, B 2 O 3, H 3 BO 3 …. B4CB4C JnJn InIn n = Ar, H 2 O, H 2, CO 2, CO, CH 4, B 2 O 3, H 3 BO 3, HBO 2, H 3 B 3 O 6 outlet u Surface transition layer in gas S/Q code B 4 C oxidation model Schematic representation

6. 6 k = H, O, B, C, i = 0 i = 1 i = 2 n = H 2 O, Ar, CO, … S/Q code B 4 C oxidation model Model implementation in the S/Q code i = N

7. 7 The model considers the following species: Ar, H 2 O, H 2, CO 2, CO, CH 4, B 2 O 3, H 3 BO 3, HBO 2, H 3 B 3 O 6 The model considers surface reaction kinetics and mass transport in the gas phase as rate determining steps of the oxidation process. Linear dependence of the reaction rate on the steam surface partial pressure is derived from quantitative analysis of BOX test results. A full set of independent chemical reactions is considered. Correspondingly, the full set of mass action laws for equilibrium gas reactions either on the surface or in the gas bulk, is used. For non-equilibrium surface reactions a semi-empirical correlation (master equation) for the reaction rate is deduced from the analysis of the BOX Rig test results. The set of chemical equations is supplemented with flux matches for each gas species, which allows self-consistent conjugation of the chemical reaction rate problem with mass transfer problem in the gas phase. S/Q code B 4 C oxidation model The main principles

8. 8 Q-07 test simulation Processing of the Q-07 TC experimental data The numerical procedure of the TC data recalculation includes: smoothening, averaging and interpolation of the temperature curves correction on the basis of the consideration of the oxide scale axial profile

9. 9 Q-07 test simulation Zr/ZrO 2 central rod Oxide layer thickness axial profile of the corner rod B (withdrawn from the bundle at 3090 sec.) compared to the calculated one of the central rod at 3090 sec.

10. 10 Q-07 test simulation Zr/ZrO 2 central rod Measured oxide layer thickness (averaged over the rods, final status), calculated oxide layer thickness of the central rod at 3564 sec. (initiation of cooldown) and calculated oxide layer thickness of the central rod (final status).

11. 11 Q-07 test simulation B 4 C central rod Calculated CO 2, CO and H 2 mass flow rates

12. 12 Q-07 test simulation B 4 C central rod Calculated B 2 O 3, H 3 BO 3 and HBO 2 mass flow rates

13. 13 The calculated mass flow rate values correspond to the temperature of the gas mixture at the bundle outlet. This temperature gradually varied during the test reaching it maximum value of about 2180 K at 3610 sec. Chemical composition of the gas mixture strongly depends on temperature and will be quite different at such high temperatures and at working temperature of GAM300 mass spectrometer (110°C – 120°C) Thus, direct comparison of the calculated results with the GAM300 experimental data is not possible. However, it is possible to compare calculated and experimental data of the total carbide release since the total amount of carbide does not change whatever chemical reactions involving CO2, CO and CH4 take place in the gas mixture under consideration. By definition, Q-07 test simulation B 4 C central rod. Calculation of the carbide production

14. 14 Q-07 test simulation B 4 C central rod. Calculation of the carbide production Experimentally measured and calculated C mass flow rate (in CO 2, CO, CH 4 )

15. 15 Q-08 test simulation Zr/ZrO 2 central rod Oxide layer thickness axial profile of the corner rod (withdrawn from the bundle at 3181 s) compared to the calculated one of the central rod at this time moment.

16. 16 Q-08 test simulation Zr/ZrO 2 central rod Measured oxide layer thickness (averaged over the heated rods) and oxide layer thickness of the central rod (final status), calculated oxide layer thickness of the central rod at 3776 s (initiation of cooldown) and calculated oxide layer thickness of the central rod (final status).

17. 17 Q-08 test simulation B 4 C central rod Calculated CO 2, CO and H 2 mass flow rates

18. 18 Q-08 test simulation B 4 C central rod Calculated B 2 O 3, H 3 BO 3, H 3 B 3 O 6 and HBO 2 mass flow rates

19. 19 Analysis of the results obtained Zr/ZrO 2 central rod

20. 20 Analysis of the results obtained B 4 C central rod Calculated C mass flow rate (in CO 2, CO, CH 4 ) in Q-07 and Q-08 tests. Time scale of the Q-07 data was shifted by 212 s in order to have in line the moments of cooldown initiation (3564 s in Q-07 and 3776 s in Q-08).

21. 21 Analysis of the results obtained B 4 C central rod

22. 22 Analysis of the results obtained B 4 C central rod Heat release due to B 4 C absorber rod oxidation B 4 C oxidation reaction heat effect: 768 KJ per Mole (B 4 C) Zr oxidation reaction heat effect: 300 KJ per Mole (atomic O) Q-07 test Measured amount of released carbide (5.59 g) corresponds to 5.59/12 = 0.466 Moles of oxidized B 4 C Measured amount of released hydrogen (182 g) corresponds to 182/2 = 91 Moles of atomic O consumed (KJ) i.e. 1.3% of the total amount of heat released i.e. 2.9% of the total amount of heat released Q-08 test

23. 23 Summary and conclusions SVECHA/QUENCH code was applied to the simulation of the QUENCH bundle tests Q-07 and Q-08. Four calculations were performed: two simulations of the real tests (Q-07 with B 4 C central rod and Q-08 with Zr/ZrO 2 central rod) and two other ones of the imaginary tests (Q-07 with with Zr/ZrO 2 central rod and Q-08 B 4 C central rod). The simulation was performed within the framework of the ‘effective channel approach’ using the newly implemented B 4 C oxidation model. The experimentally measured temperatures of the heated rods were processed, smoothed and then used as boundary conditions (average temperature field) for the central rod. The evaluated amount of generated hydrogen for the Q-07 test is lower than the experimental one, and for the Q-08 test practically coincide with the experimental data. The difference in the case of Q-07 test may be explained by the fact that due to the higher temperatures, oxidation of some of the bundle components (such as spacer grids, tungsten heaters, molybdenum electrodes) contributed substantially to the total hydrogen amount.

24. 24 Summary and conclusions (continued) Data concerning release rate of B 4 C oxidation products were analysed. Q-08 values are generally smaller than Q-07 ones due to lower temperatures. The dispersion of Q-07/Q-08 values ratio is explained by different temperature histories of the tests. Direct comparison of the calculated release rates of B 4 C oxidation products with the experimentally measured ones is not possible since they correspond to different temperatures. However, this was done for the total carbide release rate which does not depend on temperature value. The calculated carbide release rate generally correlates well with the experimental one but large discrepancies occur during temperature escalation phase. As a result, calculated amount of produced carbide (9.94 g) is higher than the experimental value (5.59 g). The amount of hydrogen produced due to B 4 C absorber rod oxidation is rather small in comparison with the total amount produced due to bundle oxidation. The estimated amount of heat released due to B 4 C oxidation is also small in comparison with the total chemical heat release. Thus we conclude that the main effect of B 4 C central rod on the bundle behavior is connected with liquid B 4 C-Zr and B 4 C-SS eutectics formation, their relocation and flow channels blockage.