Development of Model of Zr-based Claddings Oxidation in Air and Application to Air Ingress Experiments

Transcription

1 Development of Model of Zr-based Claddings Oxidation in Air and Application to Air Ingress Experiments 1 IBRAE, Moscow (RU) VASILIEV A.D. 1 ABSTRACT The oxidation of zirconium-based claddings in the air behaves in a different way compared to oxidation in steam. First, the heat effect of the chemical reaction of oxidation in air is approximately two times larger than in steam. Second, the kinetics of oxidation in air is non-parabolic (approximately linear, that is more strong) in contrast to parabolic kinetics of zirconium oxidation in steam. Third, an important factor is the diffusion of oxygen atoms through very porous (transparent) nitride layers formed in -Zr layer. It resulted in strong hydrogen release during reflood phase in QUENCH-16 test. The role of nitride formation during the air ingress phase seems to be very important especially if oxygen starvation phase is present. Fourth, it is necessary to treat adequately the mass transfer of oxygen atoms in gas mixture containing nitrogen, hydrogen, argon. This effect influences the time moment of oxygen starvation. Fifth, it is important to calculate heat effect of nitriding (formation of ZrN) which is not small. IBRAE developed a special model of Zr-based alloys oxidation considering abovementioned processes. A numerical scheme was realized to determine layer boundaries relocation and layer transformations in claddings. Some analytical solutions are given. The SOCRAT Russian computer code is used for calculations of air ingress tests like QUENCH-1, QUENCH-16 and PARAMETER-SF4. It is shown that not-typical high hydrogen generation during reflood in QUENCH-16 and PARAMETER-SF4 may be explained by enhancement of oxygen diffusion coefficients due to formation of porous ZrN layer in cladding. 1 INTRODUCTION The lessons learned from severe nuclear accidents at Three Mile Island ([1]), US, 1979) Chernobyl ([2]), USSR, 1986) and Fukushima ([3], Japan, 211) showed the very high importance of accident management measures to prevent the evolution of design basis accident to beyond design basis accident (BDBA) and to mitigate the consequences of the latter. The deep realistic understanding of hydraulic, mechanical and chemical processes taking place under accident conditions is necessary, in particular, under LOCA (Loss Of Coolant Accident) nuclear power plant accident sequence conditions with air ingress and water reflood. Regarding this, the experimental and computational investigation of LOCA representative scenario with air ingress and water flooding as accident management measure will help in more thorough understanding of processes and phenomena relevant of accident sequences and improving of models implemented into reactor accident computer codes. Session Severe accident scenarios and codes 1/26

2 Both small-scale ([4], [5], [6]) and large-scale tests ([7], [8], [9]) showed that the oxidation of zirconium-based claddings in air differs in much extent from the oxidation in steam. For example, those tests [4] revealed that at temperature range of about 9-11 K (which seems to be connected with the so-called breakaway mechanism) the air oxidation kinetics is linear instead of parabolic one. PSI developed a special model [1] to take into account the transition from parabolic to linear oxidation kinetics. Integral experiments PARAMETER-SF4 [8] (in NPO LUCH, Russia) and QUENCH-16 [9] (in KIT, Germany) clearly manifested that there are some additional features of oxidation in air like nitrides formation under oxygen starvation conditions and drastic hydrogen generation enhancement during reflood at respectively low temperatures of K which is much lower than Zr melting point. All this makes this phenomenon such intriguing issue as well as so unlike oxidation in steam. Concerning the unexpectedly high hydrogen spike at reflood, the reasonable explanation was proposed (for example in [9]) that it was due to formation of nitride layers which are very transparent for oxygen atoms diffusion in a cladding. This is why nitriding phenomena [6] are of large importance in the problem considered. In light of stated above, it was not surprising that the benchmark on air ingress experiments QUENCH-1 and QUENCH-16 [11] showed that a majority of computer codes cannot adequately predict oxygen starvation and subsequent strong oxidation at reflood in QUENCH-16. Analogously, the attempts to calculate the hydrogen spike at reflood in PARAMETER-SF4 also failed. Basically, severe accident computer codes use parabolic oxidation correlations both for steam and air oxidation. SOCRAT computer modelling code [12] developed in IBRAE uses solution of oxygen diffusion equations [13] instead of parabolic correlations. However, the SOCRAT basic version V3 has some simplifications like assumption of linear oxygen concentration profile in cladding and it does not take into account nitriding effect. This is why a special model of Zr-based claddings oxidation in air is developed in this paper. It is shown that not-typical high hydrogen generation during reflood in QUENCH-16 and PARAMETER-SF4 may be explained by enhancement of oxygen diffusion coefficients due to formation of porous ZrN layer in cladding. 2 MODELING OF HYDROGEN GENERATION DURING REFLOOD OF NITRIDED CLADDINGS 2.1 Hydrogen Generation Features in QUENCH-16 and PARAMETER-SF4 Hydrogen generation in experiment QUENCH-16 [9] took place only at 3 phases (see Table 1): small amount at phase 1, moderate production at phase 2 and huge amount at phase 5. The temperature at phase 1 was low enough to produce a considerable amount of hydrogen: about 5 g. About 1 g of H 2 was generated during pre-oxidation phase 2. Then at phase 4 (air ingress phase) there was no H 2 generation due to absence of steam in the medium. However, this phase 4 prepared conditions for upcoming tremendous H 2 spike at reflood phase (see later Figure 29) to result in total hydrogen production of 143 g. A similar picture was observed in the PARAMETER-SF4 experiment to result in cumulative hydrogen release of 17 g (see later Figure 27). Session Severe accident scenarios and codes 2/26

3 Table 1: Phases of QUENCH-16 experiment Main parameters Phase Fuel assembly (FA) temperature K Environment Heating rate, K/s Time, s 1. FA preliminary heating-up in steam-argon flow Steam-argon mixture (argon/steam flow rate is 3/3.4 g/s) Stabilization of main parameters and FA preoxidizing 3. FA cool-down 4. Air ingress Steam-argon mixture (argon/steam flow rate is 3/3.4 g/s) Steam-argon mixture (argon/steam flow rate is 3/3.4 g/s) Air-argon mixture (air/argon flow rate is.2/3 g/s) Bottom flooding of the assembly (when the assembly reached the temperature T max 187 K) Till complete cooling of the assembly Water (flow rate of 53 g/s per assembly) Role of Nitride Formation under Starvation Conditions It is well known that the oxidation of Zr-based alloys both in steam and vapour is determined by diffusion of oxygen atoms in three-layer system of ZrO 2, -Zr(O) and -Zr formed in a cladding. Zirconium nitrides are formed in cladding in contact to air atmosphere. As it was shown in the reference [14], the rate of ZrN formation strongly depends on the type of cladding s external layer (ZrO 2, -Zr(O) or -Zr). It is interesting that the parabolic constant of nitride formation is most considerable in the case of -Zr(O) layer. The ZrO 2 layer plays a role of a protective layer. Hence, if the cladding external surface layer is ZrO 2 then the nitride layer formation is very weak. In contrary, if -Zr(O) layer is in contact to air, then we encounter a very fast formation of nitride layer. From the literature [15] it is also known that in the case of local oxygen starvation conditions, the ZrO 2 layer will diminish with formation of -Zr(O) precipitates inside oxide or even disappear totally. It means that the oxygen starvation enhances the probability of ZrN intensive formation leading to massive appearance of yellow nitride zones. It was clearly manifested in QUENCH-16 experiment (Figure 1). Zirconium nitride and oxide have Session Severe accident scenarios and codes 3/26

4 different densities, and ZrO 2 layer becomes porous. As a result, in ZrO 2 and -Zr(O) layers oxygen diffusivity coefficients will be drastically enhanced. Also, it is necessary to take into account that heat effect of nitriding (formation of ZrN) is considerable [6]. It influences temperature behaviour in FA, especially at middle elevations. Figure 1: QUENCH-16 cladding view at elevation 75 mm at the end of air ingress phase [16] 2.3 Analytical Model to Solve Diffusion Equations under Air Ingress Conditions Our goal is to develop a diffusion model in three-layer system and to illustrate enhanced oxidation in the case of ZrN formation. To calculate the diffusion in the cladding we consider the schematic shown in Figure 2. A similar approach with three-layer system was presented recently in the reference [2]. ZrN ZrO 2 d -Zr(O) x a -Zr Figure 2: Schematic of diffusion problem in a three-layer system Session Severe accident scenarios and codes 4/26

5 We are looking for a solution of following diffusion equations in each layer: c t O co D x x, x d( t) (1) where c O, dimensionless, is the molar oxygen concentration; d(t) is the cladding thickness depending on time t; the diffusion coefficient D, cm 2 /s, is a function of layer name (ZrO 2, -Zr(O), -Zr) and temperature T, K: D D (T ), x ( t) d D D (T ), ( t) x ( t) a D D (T ), ( t) x d( t) b d a d a The diffusion coefficients for all three layers are available in literature [15, 17-19] RT D d 8.67e cm 2 /s 482 RT D a 1.54e cm 2 /s 282 RT D b.263e cm 2 /s where T is a temperature, K; R=1.987 cal/mole; indexes d, a, b denote ZrO 2, -Zr(O) or - Zr respectively. These quantities will be as following for the temperature T=14 K: D d = m 2 /s D a = m 2 /s D b = m 2 /s So, the diffusion coefficient is maximal for oxide ZrO 2 layer and minimal for metallic - Zr(O) layer. The solution of Equation (1) for c O will be x / 4Dit x 2 y i ( x, t) e dy erf 2 4Dit Session Severe accident scenarios and codes 5/26

6 So, for each of three layers the solution takes a form: Ad Bd d x, t Aa Ba a x, t Bb Bb b x, t 2 where A d, conditions. A a, B d, B a, Bb are constants which should be found from boundary There are several complications to the original statement of a problem: a) Discontinuity of oxygen concentration at layer interfaces Boundary conditions for c O are as following (see Figure 2): ZrO 2 : c O = d/o (external interface), c O = d/ (internal interface); -Zr(O): c O = /d (external interface), c O = / (internal interface); -Zr: c O = / (external interface), c O = (internal interface). These boundary conditions are applied in the case of unlimited oxygen supply at the outer cladding interface. In reality the situation is possible when the quantity of oxygen available in gas mixture is limited. Then we encounter the case of oxygen starvation. The external boundary condition should be switched to diffusion flux condition. The interface oxygen concentrations are available in the literature: Cd / g g/cm 3 [18] C d / x 1.511(2 ) / 2 g/cm 3, x=.15 [18] 112 C / d 1.12 g/cm 3, T 1487 K [19] exp 6748 / T C / d 1.12 g/cm 3, T 1487 K [19] exp 631/ T 4.46 T C / g/cm 3 [13] T C / g/cm 3, T 1373K [13] T C /.649 g/cm 3, T 1373K [13] Session Severe accident scenarios and codes 6/26

7 For example, for temperature T=14 K we have: d / O.667, d /. 662 / d.134, /. 38 /.12 b) Discontinuity of oxygen concentration derivative at layer interfaces This circumstance is taken into account using the following boundary conditions: D d x x d D a x x d d / d / d D a x x a Db x x a / / a c) Cylindrical geometry of a problem Taking into account cylindrical geometry we will have t D r r r r d) Change of density at layer interfaces Because three substances ZrO 2, -Zr(O) and -Zr have different densities there is a convective term in the equations like last term in the following equations: D t x C t x We need to apply the new variables instead of old ones: y x 2CD t t Then we have t CD, t y x y 2 2 x y, 2 2 Session Severe accident scenarios and codes 7/26

8 2 1 2 D y Let us introduce a new function ( ) D d D The following equations is valid for the function : t 2 D( ) D( ) 2 r r r (2) The function () has a very simple expression if each layer (ZrO 2, -Zr(O) or -Zr) has a uniform diffusivity. This is a reasonable approximation. Then we will have:, / /, / / ( D a / D ) / / b, / / d ( D / D ) / / d / a b, / d d / D / D ) D / D / / d / ( a b d / d b, d / d / O For the solution of Equation (2) the ordinary uniform numerical mesh is used (Figure 3). When we will get it will be very easy to transform it back to. For calculation of second derivative on length coordinate the following operator [21] is used: 1 Di h 1 i1 h i D i i h i1 where h is the length step. So, we will get a solution at a new time using approximation n1 i n i n1 i (1 ) n i Session Severe accident scenarios and codes 8/26

9 where n is current time index and n+1 is the next time index; is semi-implicitness parameter. Let us now illustrate the ability of this model to predict the oxidation behaviour after initial pre-oxidation phase (lasted e.g. 4 s) during the air ingress phase (lasted between 4 and 48 s). The numerical solution for function is very easy due to its continuity and presented in Figure 4. Figure 5 presents the O concentration profile in a cladding. In Figure 6 the numerical solutions for interface fronts ZrO 2 --Zr(O) and -Zr(O)--Zr are presented for temperature T=14 K. The analytical parabolic approximation in the following forms t, a t d K d K a is also included in the graph. This analytical solution coincides with numerical one at initial phase, because the unlimited oxygen appliance is proposed at this phase. Later, the limitation of oxygen is postulated in this example. For time interval 4-48 s the total oxygen starvation is given (i.e. 8 s starvation duration as for the QUENCH-16 test). After 48 s we propose again unlimited oxygen appliance (water flooding). Two cases are considered: the unchanged diffusion coefficients of ZrO 2, -Zr(O) after 48 s and enhanced (by a factor of two) diffusion coefficients which simulates the influence of nitride formation. The factor of two is chosen in the analysis because the volume fraction of nitrides in ZrO 2 layer in the experiment was of the order of.5 (Figure 1). Figure 7 clearly shows that the hydrogen generation rate is much more pronounced in a second case (approximately three times larger). Moreover, if we take into account the temperature escalation induced by this augmentation of oxidation, then the hydrogen generation rate would be even more considerable. It seems to be a main reason for drastic oxidation rate observed in QUENCH-16 (see the next chapter)..4 d (t) a (t) Function ZrO2 Zr(O) Zr Number of mesh Figure 3: Numerical mesh to solve diffusion equations in a three-layer cladding Session Severe accident scenarios and codes 9/26

10 .4.3 t = 1 s t = 5 s t = 1 s t = 4 s t = 42 s Function Coordinate, m Figure 4: Function at different times for test problem Oxygen molar concentration t = 1 s t = 5 s t = 1 s t = 4 s t = 42 s Coordinate, m Figure 5: Oxygen molar concentration at different times for test problem Session Severe accident scenarios and codes 1/26

11 .8.6 ZrO2-alfaZr front ZrO2-alfaZr front analytical alfazr-betazr front alfazr-betazr front analytical Coordinate, m Time, s Figure 6: Dynamics of inter-phase fronts in test problem Hydrogen production, relative values Ordinary diffusion K=1 Enhanced diffusion K= Time, s Figure 7: Hydrogen production in test problem. Phases: 1- pre-oxidation, 2 O starvation, 3 - flooding Session Severe accident scenarios and codes 11/26

12 3 APPLICATION OF THE DEVELOPED MODEL TO INTEGRAL AIR INGRESS EXPERIMENTS The QUENCH and PARAMETER facilities (Figures 8 to 11) are designed for studies of respectively the PWR and VVER fuel assemblies behaviour under conditions simulating design basis, beyond design basis and severe accidents. Important parameters of air ingress phase influencing the nitride formation effect on hydrogen production during reflood (see Table 2) are: ZrO2 - the zirconium dioxide layer thickness which is determined by pre-oxidation phase characteristic temperature and pre-oxidation duration (t time of oxygen starvation initiation in a test); t - the time duration of oxygen starvation phase which determines the time interval of nitride formation phase; T - the cladding temperature which determines the rate of nitride formation; A gas mass flow rate at air ingress phase; Re characteristic Reynolds number of gas mixture in a channel; c i mass concentration of gas i. Table 2: Basic parameters of Q-1, Q-16 and SF4 experiments Experiment Q-1 SF4 Q-16 c argon c nitrogen c oxygen A noncond, g/s Re t, s t, s ZrO2, m Tests differ in decisive manner in two aspects: Very thick protective ZrO 2 layer in QUENCH-1 compared to QUENCH-16 and PARAMETER-SF4 Very short oxygen starvation phase in QUENCH-1 compared to QUENCH-16 and PARAMETER-SF4. All three experiments are calculated by SOCRAT code on the basis of single approach (analogous nodalization, all models without change, identical input files and model parameters in Q-1/Q-16 calculations. The nodalization schemes of the QUENCH and PARAMETER test facilities for the SOCRAT/V3 computer code are presented in Figures 12 and 13. Session Severe accident scenarios and codes 12/26

13 Figure 8: Schematic representation of QUENCH test section facility Figure 9: Cross-section of QUENCH-16 test bundle (2 heated, 4 corner rods). Consecutive numbers of rods are indicated Session Severe accident scenarios and codes 13/26

14 Figure 1: Schematic representation of PARAMETER test section facility Figure 11: Cross-section of PARAMETER-SF4 test bundle (18 heated rods) Session Severe accident scenarios and codes 14/26

15 Figure 12: SOCRAT nodalization for QUENCH-1 and QUENCH-16 calculations un heated rod 1 group 1 h eated rod 6 group 2 heated rod 12 group 3 coolant water exit ZrO 2 isolation 1 group 4 CORE hydro channel cooling jacket 4 water inlet steam inlet 3 2 argon inlet 1 air inlet bottom reflood Figure 13: SOCRAT nodalization for PARAMETER-SF4 calculations Session Severe accident scenarios and codes 15/26

16 The total electric power in tests QUENCH-1, PARAMETER-SF4 and QUENCH-16 is presented in Figures 14, 16, 17. The maximum total electric power in QUENCH-16 was about 11.5 kw as shown in Figure 17. The calculated and experimental bundle temperatures at high elevations versus time for experiments are presented in Figures 15, 16, 17. The basic thermal parameters of experiments QUENCH-16 are reasonably reproduced by the code. Figure 14: QUENCH-1 total electric power Figure 15: QUENCH-1 temperature at level 85 mm Session Severe accident scenarios and codes 16/26

17 6th European Review meeting on Severe Accident Research (ERMSAR-213) 1 T2.3_9 SOCRAT Power Power, W Temperature, K Time, s Figure 16: PARAMETER-SF4 total electric power and temperature at level 1 mm TFS 17/13 SOCRAT Power Electric Total 16 Temperature, K Power, W Time, s Figure 17: QUENCH-16 total electric power and temperature at level 95 mm (the numbers of test phases are indicated) Session Severe accident scenarios and codes 17/26

18 The air ingress phase (Figure 17, phase 4) was a very important phase of QUENCH-16 experiment. The oxidation of zirconium claddings in the air behaves in different way in comparison to oxidation in the steam. First, the heat effect of the chemical reaction of oxidation in the air is approximately two times larger than in the steam. Second, the kinetics of oxidation in the air is non-parabolic (approximately linear, that is more strong) in contrast to parabolic kinetics of zirconium oxidation in the steam. This explains partially the tendency to reach the highest temperatures at medium elevations (above elevation of 4 mm) during the air ingress phase. This fact is in contrast to oxidation in the steam (without air) tests where the highest temperatures were reached definitely at highest elevation 95 mm from the bottom of heated region. The calculated consumed part of oxygen at middle elevations is presented in Figures 18 and 19. One can see the local oxygen starvation point (when parameter becomes equal to unity). The study of these graphs brings to the conclusion that the duration of oxygen starvation is determining the quantity of Zr nitride phase formed in a cladding. The longer the starvation phase is, the more massive nitride zones are formed in dioxide layer (compare Figure 1 and Figure 26). The calculated dynamics of layers ZrO 2 and -Zr(O) is presented in Figure 19. One can see the same laws in inter-phase fronts behaviour as in test problem (see Figure 7). Figures 21 to 23 show the oxygen (which is a constituent part of the air) mass flow rates at the inlet and at the outlet part of the fuel assembly. One can see from these Figures that the oxygen consumption grows as long as the cladding temperature becomes higher. Finally, the situation arises when all the oxygen entering the fuel assembly is consumed for oxidation of zirconium claddings in medium bundle elevations. This state is called as total oxygen starvation. The total oxygen starvation was observed in all three bundle experiments on air ingress QUENCH-16 [9] (Figure 23), QUENCH-1 [7] (Figure 21) and PARAMETER-SF4 [8] (Figure 22). SOCRAT underpredicts the beginning time of total oxygen starvation in QUENCH-16. This may be due to some overestimation of temperatures at medium levels in calculations. Figure 18: Calculated local oxygen starvation in tests QUENCH-1 and QUENCH-16 Session Severe accident scenarios and codes 18/26

19 Figure 19: Calculated local oxygen starvation in test PARAMETER-SF4 5 ZrO2 thickness level 75 mm ZrO thickness level 75 mm Time, s Figure 2: Calculated disappearance of ZrO 2 layer and growth of -Zr(O) layer in Q-16 Session Severe accident scenarios and codes 19/26

20 .4.3 SOCRAT O2 entry SOCRAT O2 exit Experiment O2 exit Mass flow rate, g/s Time, s Figure 21: QUENCH-1 oxygen mass flow rate Mass flow rate, kg/s E-5 4E-5 SOCRAT O2 entry SOCRAT O2 exit Experiment SF4 exit O 2 starvation Time, s Figure 22: PARAMETER-SF4 oxygen mass flow rate Session Severe accident scenarios and codes 2/26

21 Figure 23: QUENCH-16 oxygen mass flow rate Figures 24, 25, 28 show hydrogen mass flow rates at the working section exit. SOCRAT underestimates in some extent H 2 mass flow rate at initial phases and at final quench phase. At this phase a lot of H 2 was generated in the experiment due to: a) melt oxidation, b) nitrides oxidation, c) continued oxidation of claddings by steam through very porous nitride layer. The first and third above factors seem to be more influent compared to second one. The integral hydrogen productions are presented in Figures 27, 29. The analysis of hydrogen production graphs shows that the change of oxygen diffusion coefficients only by a factor of two (K diff =2) in cladding region corresponding to dioxide layer and neighbouring -Zr(O) layer with ZrN zones resulted in a drastic calculation spike of hydrogen generation as it was observed in QUENCH-16 and PARAMETER-SF4 tests. The quantity K diff =2 was chosen in the calculations as it corresponds roughly to the volume fraction of ZrN in dioxide layer obtained in the experiment (see Figure 1). For QUENCH-1 K diff =1 is a good approximation due to respectively small volume fraction of ZrN in dioxide layer as it is possible to see from Figure 26. Once again, Figure 26 clearly demonstrates that the volume fraction of ZrN formed in ZrO 2 layer for QUENCH-1 is very small compared to QUENCH-16. This is why final hydrogen spike in QUENCH-1 is negligible in comparison with QUENCH-16. Session Severe accident scenarios and codes 21/26

22 Figure 24: QUENCH-1 calculated and experimental hydrogen release rate Figure 25: QUENCH-1 calculated and experimental hydrogen release rate at reflood Session Severe accident scenarios and codes 22/26

23 6th European Review meeting on Severe Accident Research (ERMSAR-213) Figure 26: QUENCH-1: only local presence of nitride in zirconium dioxide layer at axial level 85 mm at the end of air ingress phase [5] SOCRAT new version Kdiff=1 SOCRAT new version Kdiff=2 Experiment Integral hydrogen production, g Time, s Figure 27: PARAMETER-SF4 calculated and experimental integral hydrogen release Session Severe accident scenarios and codes 23/26

24 Hydrogen mass flow rate, kg/s 6th European Review meeting on Severe Accident Research (ERMSAR-213) Figure 28: QUENCH-16 calculated and experimental hydrogen release rate (with Kdiff=2) SOCRAT new version Kdiff=1 SOCRAT new version Kdiff=2 SOCRAT new version Kdiff=3 SOCRAT new version Kdiff=5 Integral hydrogen production, g Time, s Figure 29: QUENCH-16 calculated and experimental integral hydrogen release Session Severe accident scenarios and codes 24/26

25 6th European Review meeting on Severe Accident Research (ERMSAR-213) 4 CONCLUSIONS The air ingress phase was an important feature of the PARAMETER-SF4 and QUENCH-16 tests, drastically influencing the test behavior. An analytical model is developed to consider oxygen diffusion in a Zr-based cladding under air ingress conditions. As a result of application of transparent ZrO2 layer model, the sharp hydrogen generation rise is calculated, as it was observed in tests. The transparent ZrO2 layer occurrence is connected with formation of nitrides in -Zr layer in the oxygen starvation phase. The transparency is modeled by enhancement of oxygen diffusion coefficient. The quantity Kdiff=2 (factor of diffusion coefficient enhancement) seemed to be a reasonable value for QUENCH-16 and PARAMETER-SF4 experiments. This quantity roughly corresponds to the ZrN volume part in dioxide layer in QUENCH-16 which was observed in post-test bundle investigations. SOCRAT underpredicts in some extent the beginning time of oxygen starvation which may be connected with overestimation of oxidation by air at fuel assembly medium levels in calculations due to slightly overpredicted temperatures. On the whole, the calculated and experimental thermal-hydraulic and chemical data are in a reasonable agreement, which shows the adequacy of modeling the complicated thermohydraulic behavior including the air ingress phase and the bottom reflood in the QUENCH1, PARAMETER-SF4 and QUENCH-16 tests. 5 ACKNOWLEDGMENTS The work has been performed in the frame of the cooperation agreement between IBRAE and KIT in the field of nuclear energy research. The author would like to thank Dr. M. Veshchunov from IBRAE, Moscow, for their valuable suggestions and the help at interpretation of the calculated results. The author also greatly acknowledges Dr. Stuckert and Dr. Steinbrueck from KIT, Germany, for numerous discussions about air ingress phenomena revealed in tests. References [1] J.M. Broughton, P. Kuan and D.A. Petti, A Scenario of the Three Mile Island Unit 2 Accident, Nuclear Technology, vol. 87, p. 34 (1989). [2] V.P. Vasilevskii, A.I. Ionov, L.N. Podlazov et.al. Analysis of First Phase of Accident Sequence at Unit 4 of Chernobyl NPP, Atomnaya Energiya, V. 64, 1, p (1988). [3] M. Fukasawa, Overview of Fukushima-Accident Analysis, Proc. 212 SARNET International Meeting (SARNET 212), Cologne, Germany, March 21-23, 212. [4] M. Steinbrueck, U. Stegmaier, T. Ziegler, Prototypical Experiments on Air Oxidation of Zircaloy-4 at High Temperature, Report FZKA 7257, Forschungszentrum Karlsruhe, Germany, January 27. [5] J. Stuckert, A. Miassoedov, G. Schanz, L. Sepold, U. Stegmaier, L. Steinbock, M. Steinbrueck, Air Ingress Results from the QUENCH Facility, SARNET 3rd Annual Meeting, GRS Garching, January 3 February 2, 27. [6] T. Hollands, M.K. Koch, Modelling of the Nitrogen Impact during Air-Ingress, Proceedings of the 15th International QUENCH Workshop, KIT, Karlsruhe, Germany, November 3-5, 29. Session Severe accident scenarios and codes 25/26

26 6th European Review meeting on Severe Accident Research (ERMSAR-213) [7] G. Schanz, M. Heck, Z. Hozer, L. Matus, I. Nagy, L. Sepold, U. Stegmaier, M. Steinbrück, H. Steiner, J. Stuckert, P. Windberg, Results of the QUENCH-1 Experiment on Air Ingress, Forschungszentrum Karlsruhe, FZKA 787, SAM-LACOMERAD9, May [8] A.E. Kisselev, V.F. Strizhov, A.D. Vasiliev, Application of Thermal Hydraulic and Severe Accident Code SOCRAT/V2 to Bottom Water Reflood Experiment PARAMETER-SF4, Nuclear Engineering and Design, 246, pp (212). [9] J. Stuckert, M. Große, Z. Hozer, M. Steinbrück, 213, Results of the QUENCH-16 Bundle Experiment on Air Ingress, Scientific Report KIT-SR 7634, Karlsruhe Institute of Technology. [1] J. Birchley and L. Fernandez-Moguel, Simulation of air oxidation during a reactor accident sequence: Part 1 Phenomenology and model development, Ann. Nucl. Energy, 4, pp (212). [11] L. Fernandez-Moguel, C. Bals, E. Beuzet, C. Bratfisch, O. Coindreau, Z. Hozer, J. Stuckert, A. Vasiliev, P. Vryashkova, SARNET-2 WP5 Benchmark on Air Ingress Experiments QUENCH-1, -16, Proc. of 6th SARNET International Meeting (SARNET 213), Avignon, France, October 2-4, 213. (In print). [12] L. Bolshov and V. Strizhov, SOCRAT The system of codes for realistic analysis of severe accidents, Proc. ICAPP 6, Reno, NV, USA, 26, Paper [13] A.V. Berdyshev, L.V. Matveev, M.S. Veshchunov, Development of Database for Kinetic Steam Oxidation Model of Zircaloy-4 at High Temperatures ( C), Preprint IBRAE-975, Moscow, Nuclear Safety Institute of Russian Academy of Sciences, [14] M. Steinbrueck, M. Jung, May 2-5, 211, High-Temperature Reaction of -Zr(O) with Nitrogen, Proc. ICAPP 211 Conf., Nice, France. Paper [15] J. Stuckert, M.S. Veshchunov, Behaviour of Oxide Layer of Zirconium-Based Fuel Rod Cladding under Steam Starvation Conditions, Report FZKA 7373, April 28. [16] J. Stuckert, M. Steinbrueck, Experimental Results of the QUENCH-16 Bundle Test on Air Ingress, Proc. ICAPP 212 Conf., Chicago, US, June 24-28, 212. Paper [17] S. Leistikow, G. Schanz, H.v. Berg, March 1978, Kinetics and morfology of isothermal steam oxidation of Zrcaloy-4 at 7-13 C, KfK [18] J.P. Abriata, J. Garces, R.Versaci, Bull. Alloy Phase Diagrams, 7, p.116 (1986). [19] M.Veshchunov, P.Hofmann, J.Nuc. Mat., 21, p.11 (1994). [2] J.-C. Brachet, M. Le Saux, V. Vandenberghe, L. Portier, D. Gilbon, J.-P. Mardon, A. Cabrera (EDF/Septen) et al., Overview of past and on-going CEA Sudies of Nuclear Fuel Clad Materials Behavior upon LOCA, Proceedings of the 18th International QUENCH Workshop, KIT, Karlsruhe, Germany, November 2-22, 212. [21] A.A. Samarskii, A.V. Gulin. Numerical Methods of Mathematical Physics. Nauchny Mir, 316 pp. (23). (In Russian). Session Severe accident scenarios and codes 26/26