6th European Review Meeting on Severe Accident Research (ERMSAR-2013) Avignon (France), Palais des Papes, 2-4 October, 2013

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1 Analytical support to experiment QUENCH-17 and first post-test calculations with ATHLET-CD C Bals, H Austregesilo, T Hollands Gesellschaft für Anlagen- und Reaktorsicherheit (GRS), Garching (GE) ABSTRACT The experiment QUENCH-17 was performed by KIT on 3-31 January 213 in the frame of the European SARNET2 network The primary test objective was to investigate the formation and cooling of a debris bed after complete oxidation of the bundle within a height of about 5 m The course of the experiment should be driven in a way that no melt formation occurs Therefore, the 12 peripheral heated rods had hafnium claddings and also the shroud tube was made of hafnium, which has a higher melting temperature and a lower oxidation rate compared to zirconium, while the inner 9 unheated rods with Zry-4 claddings filled with pre-fragmented zirconia pellets should collapse after the preoxidation period and build the debris bed To support the experimenters in the choice of appropriate test conditions, pre-test analytical support with ATHLET-CD was performed with a program version where the oxidation kinetics of Hafnium and the material properties of Hf and HfO 2 were considered for the shroud and the outer ring of heated rods The pre-test simulations with a flow rate variation between 3 and 8 g/s for steam and argon showed that in order to get an optimized oxidation the bundle temperature should be kept between 175 and 1 K to avoid a temperature escalation and to reach the aim of cladding oxidation in a time as short as possible, where higher flow rates resulted in a shorter test time With low flow rates of 3 g/s, similar as used in the test (2 g/s), the predicted time for complete oxidation of Zry-4 claddings between 65 and 115 mm was 65 s, compared to ~ 72 s as indicated in the test The preliminary post-test analyses with boundary conditions as adopted in the experiment resulted in a good agreement of temperature behaviour and hydrogen generation The maximum temperature of ~ 1 K within the pre-oxidation period was slightly underpredicted in comparison with experimental data; also the cooling of the debris after start of injection showed that a temperature escalation above melting temperature was avoided as demonstrated in the test The good agreement in hydrogen production besides an underprediction of ~ 2 % in the last phase of pre-oxidation proves that the overall amount of oxidation could be simulated in the right extent 1 INTRODUCTION The accident in TMI-2 had shown that the behavior of the debris bed plays an important role in cooling down the degraded hot core after a severe accident with heavy core damage in a LWR Particulate debris beds may be formed during a severe accident mainly due to thermal stresses followed by the mechanical failure and fragmentation of preoxidized fuel rods during reflooding Therefore, different experimental programs have been launched to investigate whether a particulate debris bed configuration is coolable These experiments concentrate on the question whether an initially hot dry bed can be Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 1

2 quenched before further heat-up and melting due to decay heat and possibly further oxidation leading to a finally uncoolable melt pool configuration At the French Institut de Radioprotection et de Sureté Nucléaire (IRSN) there are the PEARL and PRELUDE experiments, which were performed in the frame of the European Severe Accident Research Network of Excellence (SARNET2), both with the objective to improve the knowledge of the thermal-hydraulics in a debris bed during reflooding The data of these experimental programs should be used for the validation of numerical codes to improve the models for prediction of reflood behavior of a severe damaged reactor core with a debris bed configuration While the PRELUDE experiments investigated test sections with a diameter from 1 to 3 mm the design of the PEARL facility allows debris bed sizes of 54 mm diameter and up to 5 mm height where multidimensional effects could be studied The results of the lower dimensioned PRELUDE tests influenced the design of the bigger PEARL facility The test matrix comprises the variation of system pressure, inlet water flow rate and water temperature, heating power, initial temperature and particle size of the debris bed [1] While the IRSN investigated debris beds with uniform shape of particles, the current work programme of the DEBRIS experiments at the Institute of Nuclear Technology and Energy Systems (IKE), University of Stuttgart, includes the investigation of poly-disperse particle beds (different diameters) and particle beds with irregularly shaped particles In a second configuration of the facility 2D-flow conditions are simulated with a perforated downcomer (bottom and lateral flow) [2] The results of the tests focus on the validation of the MEWA model, developed by IKE and coupled to the German system code ATHLET-CD for the simulation of in-vessel behavior during late phase core degradation The QUENCH-Debris experiment, performed also in the frame of the European SARNET2 network at KIT, was the 17 th experiment at the QUENCH test facility The primary test objective was to investigate the formation and coolability of a debris bed in the core under more prototypical boundary conditions, i e at much higher temperatures starting in a bundle configuration 2 CONDUCT AND FIRST RESULTS OF THE QUENCH-17 EXPERIMENT The QUENCH-17 experiment was conducted successfully on 3 31 January 213 at KIT after an intensive planning and preparation phase, mainly based upon the experiences of the previous QUENCH experiments [3] and the experience of IRSN involved in processing of debris tests [4] The primary aims of the experiment QUENCH-17 on debris formation were to examine the formation of a debris bed inside the completely oxidised region of the bundle without melt formation, and to investigate the coolability phenomena during the reflood of the damaged bundle The course of the experiment showed clearly that the objectives were met [5] The test bundle with a length of about 2 m consisted of the shroud tube made of hafnium, twelve heated peripheral rods, all with hafnium cladding, and nine unheated internal rods (Fig 1) Additionally, four hafnium corner rods with integrated inner thermocouples were installed at the bundle periphery Hafnium was chosen because of its higher melting temperature and the about one order of magnitude lower oxidation rate compared to zirconium, ie the heated rods, the shroud and the corner rods should survive the hard test conditions The claddings of the inner nine rods were made of Zry-4 They were filled with pre-fragmented zirconia pellets The debris would be produced by collapse of the rods with oxidised Zircaloy claddings Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 2

3 Figure 1: Cross section of the test bundle [6] In common with the previous QUENCH experiments, the bundle was heated by a series of stepwise increases of electrical power from room temperature to a maximum of 9 K in an atmosphere of flowing argon (2 g/s) and superheated steam (2 g/s) The bundle was stabilised at this temperature, the electrical power being 4 kw During this time the operation of the various systems was checked To initiate the oxidation the bundle was first heated to 17 K during s by power increase to 1 kw (Fig 2) The power was further increased stepwise to about 12 kw after s to reach a maximum temperature of 1 K This power level was maintained constant during the most of the oxidation phase to reach the target extent of oxidation During this phase the peak temperature slightly decreased to 175 K due to 1) diminishing chemical energy release, and 2) additional gas flow via leakage of unheated rods and shroud To increase the oxidation rate the power was increased to 14 kw at 67 s and kept at high level until the end of oxidation phase at 7777 s The hydrogen release was continuously monitored and showed the behaviour expected according the planning calculation with total release of about 1 g At this time the indicating devices showed that complete oxidation was achieved at bundle elevation of 65 mm, as required by the test specification No clear indication of debris bed formation was so far observed The possibly surviving inner Zry-4 claddings were destroyed by the application of an axial mechanical force resulting in material relocation and formation of a heterogeneous debris bed consisting of the pre-segmented pellets and larger cladding tube fragments at the two middle grid spacers The debris bed formation was confirmed by the response of fluid thermocouples installed at those locations Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 3

4 Figure 2: Test conduct Preparation for reflood was initiated at s by simultaneously turning off the steam flow, reducing the power to 4 kw, and switching the argon injection to the top of the bundle Water was then injected at 1 g/s The sequence meant there was a delay of 7 s between stopping the steam flow and starting the water injection Power was then kept constant at 4 kw during the reflood A temperature increase of about 15 K was observed from fluid thermocouples during the delay period, indicating heat transfer from debris to the nearly stagnant steam and argon Following the start of injection a temporary reduction was observed lasting about 6 s This may have been due to rapid steam generation for a short period at the start of refilling A second temperature increase of up to 12 K then followed, lasting about 25 s while refilling of the lower volume continued, with the result that a maximum temperature of 125 K inside the debris bed was observed at 782 s This coincided with the first indication of rod quenching at the bundle bottom, after which the debris temperatures progressively reduced At the same time the quench front advanced from the bottom of the bundle to the debris region, at which point the temperatures thereat dropped sharply to saturation indicating quench of the debris The quench progression correlated with a steady increase in the evaporation rate until the start of debris quenching, when the rate of increase remained approximately constant for about 15 s, before accelerating again This suggests a possible hindering of the quench process inside the debris itself No significant hydrogen release was indicated during the quenching of debris bed Generally the quench progression indicated from thermocouples corresponded to the refilling as indicated by the measured collapsed water level However, the water level stagnated at 7872 s when the elevation of 85 mm was reached, indicating the position of shroud rupture A sharp increase in hydrogen release at this time indicated water penetration through the breach and consequent strong reaction with outer metallic side of shroud First inspections of the bundle top using endoscope shows formation of a heterogeneous debris bed consisting of segmented pellets and large cladding fragments between two upper grid spacers (Fig 3) Both spacers were heavily oxidised but remained intact Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 4

5 Figure 3: Debris collected between two upper grid spacers 3 PRE-TEST CALCULATIONS OF THE QUENCH-17 EXPERIMENT WITH ATHLET-CD The boundary conditions of the test were determined on the basis of pre-test calculations by 6 SARNET partners (PSI, IRSN, NRI, IBRAE, RUB and GRS) [6], where GRS used the code ATHLET-CD (22C) for the simulation One of the main objectives of the test was to avoid the formation of melt to allow debris bed formation consisting of solid fragments To produce an adequate debris bed at the time of reflood initiation the bundle should be significantly oxidised Therefore, the pretest calculations had the goal to find the right conditions for a controlled oxidation of the Zry claddings, where total oxidation should be reached over a height of at least 5 m without temperature escalation within a time as short as possible to avoid shift operation of experimenters In this first step of test schedule the pre-test calculations comprised the pre-oxidation period of the experiment, where an optimal set of the main test parameters as power and inlet flow rates of steam and argon should be found to get the aimed conditions required for the formation of the debris bed 31 Analytical model used for pre-test calculations The basis for the pre-test simulations with ATHLET-CD was the standard dataset used for several previous QUENCH experiments in the frame of code validation In Fig 4 the used nodalization is shown, where 3 different rod types in the thermo-fluid object BUNDLE represent the central inner rod (ROD1), the inner ring (ROD2) and the outer ring of rods (ROD3) Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 5

6 Figure 4: Nodalization used for ATHLET-CD pre-test simulations As given by KIT [7] material data for Hf and data for Hf oxidation kinetics were programmed in a special ATHLET-CD version on the basis of the current code version 22C The oxidation reaction rate of Hf, calculated as K = K e E/(RT), with K = 76 kg/m 2 s 1/2 and E= J/mol, is much lower compared to that of Zry (Fig 5) The exothermal heat for Hf oxidation is given with Δh Hf = kj/mol at K T 1 K, where a constant value of Δh Hf = 63 kj/mol = J/kg Hf is used for ATHLET-CD Furthermore, thermal properties of HfO 2 according to [8] are adopted for the oxidized Hf material These Hf data are applied for the outer ring of rods (ROD3) and for the shroud which is simulated with the conventional heat slab (HECU) model As planned for the test, the central rod and the inner ring of rods (ROD1, ROD2) remain unheated while the 12 Hf rods of the outer ring (ROD3) are heated by a tungsten heater with the same geometry as used for previous QUENCH experiments Rate Constante 1e+2 * XP ((g/cm**2)**2 / s) e 5 5e 6 1e 6 5e 7 ATHLET CD: Reaction Rates for Steam Oxidation of Zr / Hf IOXMOD 15: Cathcart Prater/Courtright (refcase) 19: Leistikov Prater/Courtright 25: Hf: Steinbrück/Stuckert 1e Reziprocal Temperature /T (1/K) Figure 5: Zry- steam oxidation correlations used for QUENCH-17 simulations Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 6

7 32 Results of pre-test calculations As recommended by KIT the same initial and boundary conditions were used as they existed for the QUENCH-16 test, where the system pressure was 21 bar and the initial gas temperature (steam, argon) was K In the performed simulations of the pre-oxidation period the influence of the following parameters was investigated: - inlet flow rate of steam and argon (3 5 8 g/s); - power ramp dependent on inlet flow rate to get a maximum temperature of ~ 175 K; - external electrical resistance ( mω/rod); - options for Zry-steam oxidation model: selected correlation (Cathcart-Prater/Courtright; Leistikov-Prater/Courtright) (Fig 5); oxide layer thickness for transition from parabolic to linear rate (1 2 3 µm, no transition); - variation of shroud insulation thickness to get flat temperature profile; - position of grid 3 (shift from 55 mm to 45 mm) The calculations showed that a maximum temperature higher than 1 K resulted in an escalation due to quick oxidation, where melting could not be avoided; temperatures lower than 17 K showed a too slow oxidation of levels below 75 and above 95 mm bundle height Therefore, to keep the main objectives, which were to avoid a temperature escalation and to reach the aim of cladding oxidation over a height of at least 5 m in a time as short as possible, the maximum temperature has to be kept between 175 and 1 K The power ramp was defined in a way, where a first steep increase resulted in a quick oxidation of the hottest position (95 mm) In a second more flat increase the maximum value of power is not reached until the hottest position is fully oxidized; then the power is kept constant at a value where the temperature of ~ 175 K could be kept (Fig 6) 3 Quench Debris: Pre Test Calculation with ATHLET CD 22c 25 Electric Power (kw) power input, G(st,ar)=8g/s bundle power, G(st,ar)=8g/s power input, G(st,ar)=5g/s bundle power, G(st,ar)=5g/s power input, G(st,ar)=3g/s bundle power, G(st,ar)=3g/s Power (WHRES=45mOhm/rod) Figure 6: Selected power ramp for different inlet flow rates as boundary condition for pre-test calculations Dependent on inlet flow rates with increasing steam and argon flow due to increasing heat conduction higher power values can be chosen The parameter variation proved that higher flow rates resulted in a shorter test time Therefore, the calculation with steam and argon flow rates of 8 g/s and a maximum power of 29 kw was taken as a reference case, in which complete oxidation of Zry claddings between 65 and 115 mm elevation was reached after 35 s [6,9], as Fig 7 shows Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 7

8 Considering a simulation with low flow rates of 3 g/s, similar as later used in the experiment (2 g/s), with oxidation options as used for the benchmark calculations of QUENCH-1 and QUENCH-16 (Cathcart-Prater/Courtright correlation, no transition from parabolic to linear rate is made), the predicted time for total oxidation of Zry claddings in a height of 5 mm was 65 s, compared to 72 s as indicated in the test [1] (Fig 8) Quench Debris: Pre Test Calculation with ATHLET CD 22c ROD2, 45 mm ROD2, 55 mm ROD2, 65 mm ROD2, 75 mm ROD2, 85 mm ROD2, 15 mm ROD2, 115 mm ROD2, 125 mm ROD3, 15 mm Oxide layer thickness (mm) Quench Debris: Pre Test Calculation with ATHLET CD 22c ROD2, 45 mm ROD2, 55 mm ROD2, 65 mm ROD2, 75 mm ROD2, 85 mm ROD2, 15 mm ROD2, 115 mm ROD2, 125 mm ROD3, 15 mm Rod cladding temperatures (Rod2: Zr / Rod3: Hf) Oxide layer thickness (Rod2: Zr / Rod3: Hf) Figure 7: Results of reference calculation (flow rate 8 g/s) Quench Debris: Pre Test Calculation with ATHLET CD 22c ROD2, 45 mm ROD2, 55 mm ROD2, 65 mm ROD2, 75 mm ROD2, 85 mm ROD2, 15 mm ROD2, 115 mm ROD2, 125 mm ROD3, 15 mm Oxide layer thickness (mm) Quench Debris: Pre Test Calculation with ATHLET CD 22c ROD2, 45 mm ROD2, 55 mm ROD2, 65 mm ROD2, 75 mm ROD2, 85 mm ROD2, 15 mm ROD2, 115 mm ROD2, 125 mm ROD3, 15 mm Rod cladding temperatures (Rod2: Zr / Rod3: Hf) Figure 8: Results of pre-test calculation for flow rate 3 g/s Due to uncertainties mainly in the oxidation model (selection of correlation, transfer from parabolic to linear rate), where the variation in the model parameters resulted in a band width of ΔT ~ 1 C, it was recommended to keep the cladding temperatures below 175 K to be sure that no escalation of temperatures up to melting occurs In a further step of simulations [1] it was shown that the consideration of grid oxidation had little influence on the overall behavior of rod temperatures and that a shift of grid 3 to a lower position results in slightly increased temperatures ( reduced time for total oxidation) at the former grid position at 55 mm and above due to lower flow velocities and therefore lower heat transfer to the fluid, if the cross sectional area is not restricted by the grid Oxide layer thickness (Rod2: Zr / Rod3: Hf) Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 8

9 In the ATHLET-CD pre-test simulations an amount of ~ 8 g hydrogen was produced within the time period which was needed for a total oxidation between the bundle elevations 65 and 115 mm, which is similar to the result of the experiment up to the time when the power was increased from 122 to 143 kw after ~ 67 s [6,11] After this time a further steeper increase of hydrogen generation is indicated in the test which is not simulated by ATHLET-CD due to constant power (Fig 9) 12 Quench Debris: Pre Test Calculation with ATHLET CD 1 intlh2 exp acc H2 out calc Integral mass of H2 release (g) Total Hydrogen Release Figure 9: Comparison of measured hydrogen generation with pre-test calculation (flow rate 3 g/s) 4 FIRST POST-TEST CALCULATIONS OF TEST QUENCH-17 WITH ATHLET-CD After receiving the real boundary conditions of the test, where a lower inlet flow rate for steam and argon (2 g/s) was adopted compared to the chosen pre-test conditions, the first post-test calculations started with ATHLET-CD For these simulations the same program version 22C with the above described modifications for the Hf components was used as applied for the pre-test parameter study 41 Analytical model used for post-test calculations 411 Nodalization of the test facility As the 2D MEWA model was intended to be applied it was necessary to model the bundle in radial direction in more detail compared to the one-bundle nodalization used for the pretest calculations and previous simulations of QUENCH experiments In this nodalization the central rod (ROD1), the inner ring of rods (ROD2) and the outer heated ring (ROD3) each are placed in an own fluid channel with cross connections between the central and middle channel, the middle and outer channel and an additional connection between the outer channel and the bypass (Fig 1) Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 9

10 Figure 1: Nodalization of the QUENCH-17 rod bundle used for the ATHLET-CD post-test simulation with the MEWA model 412 Simulation of the debris bed with MEWA With the MEWA model, developed by IKE and coupled to ATHLET-CD, the processes in a strongly degraded core or a particulate debris bed in the lower head or in the reactor cavity are described in a quasi-continuum approach using a two-dimensional description [12] Solid, melt and two-phase fluid (gas/steam) are modelled as separate phases with thermal non-equilibrium between all phases Details considering the conservation equations and the constitutive laws can be found in [12] and [13] Validation calculations for quenching of superheated, initially dry debris beds were performed by IKE for different experiments, addressing various aspects of quenching as different flooding conditions, quenching of non-uniform, irregular shaped particles, effects of stratified bed configurations and effects of particle size It was shown that the inclusion of capillary effects becomes important to explain the behavior in cases with small particle sizes (~ 1 mm), especially with non-homogeneities and stratification For further clarification and support of modelling, MEWA was checked by means of new additional experiments conducted in the frame of the SARNET2 programme (DEBRIS, PEARL) Finally, examples for the quenching of debris under reactor severe accident conditions highlighted the importance of multi-dimensional effects allowing water ingression along the boundaries of the bed or through bypasses, which provides a mechanism to facilitate quenching, together with the cooling effects of steam flows through the hot dry zone [12] The module MEWA was started with a time dependent criterion after an ATHLET-CD simulation with an intact core region The time defined for the activation was 775 s, Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper 212 1

11 when the experiment indicated debris relocation by thermocouples installed at the top of grid spacer 2 [11] respectively 77 s, when quenching was activated in the later simulation The size of the debris bed was defined by input data horizontally over the region of ROD1 (central rod) and ROD2 (inner ring of rods) and vertically in an elevation between 392 m (upper elevation of grid 2) and 77 m, corresponding to the first estimations considering the test results The selected debris height includes three thermofluid volumes in the chosen nodalization With the activation of the MEWA model all differential equations describing the rod behavior (ECORE model) as also the thermo fluid behavior in the MEWA region are switched off in the integration module FEBE The boundary conditions for the still remaining ATHLET-CD equations now are defined by special coupling routines of the MEWA module The selected particle diameter was taken as 35 mm corresponding to the height of the pellet segments and also 5 mm in a further parameter study due to the fact that the debris collected after the test showed several large parts of cladding (Fig 3) which seem to allow an improved permeability for the quench water Additional important model parameters for MEWA are taken as default as listed below: - correlation for heat conductivity in the debris bed: Imura/Vortmeyer model; - controller for calculation of capillary forces for fluid: model not used; - flag for bed permeability: model of Ergun is used; - flag for two-phase friction model: Reed model is used 42 First main results of post-test calculations From the comparison of hydrogen generation it can be shown that the heat up and preoxidation period could be simulated with good agreement until ~ 67 s when the power increase occurred in the test (Fig 11) The observed increase of hydrogen generation rate in the time region after the power increase and before start of cool down (67 77 s) was not simulated sufficiently with ATHLET-CD; the resulting H 2 mass of 89,2 g in the simulation after s underestimates the measured mass of 111 g by ~ 2 % 12 1 Post Test Calculation of QUENCH 17 with ATHLET CD intlh2 exp acc H2 out calc Post Test Calculation of QUENCH 17 with ATHLET CD ATHLET power input prelim ATHLET bundle power E55, P exp (corr) Integral mass of H2 release (g) Power (kw) Total Hydrogen Release Electrical Bundle Power Figure 11: Comparison of hydrogen generation of post-test calculation bundle power Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper

12 From Fig 12, which shows the oxide layer thicknesses calculated for the Zry claddings of the inner (ROD2: Zry) and outer ring of rods (ROD3: Hf) it can be seen that at the elevation of 65 mm the Zry claddings are oxidized totally after ~5 s while the resistance measurement shows complete oxidation at this height later after 72 s On the other hand the intended complete oxidation over a height of 5 m (65 to 115 mm) is reached in the simulation after ~ 7 s which coincides with the time when the quenching starts Oxide layer thickness (mm) Quench17: First Post test Calculation with ATHLET CD 22c ROD2, 45 mm ROD2, 55 mm ROD2, 65 mm ROD2, 75 mm ROD2, 85 mm ROD2, 15 mm ROD2, 115 mm ROD2, 125 mm ROD3, 15 mm Oxide layer thickness (mm) Quench17: First Post test Calculation with ATHLET CD 22c ROD2 outer, 65 mm ROD2, 75 mm ROD2, 85 mm ROD2, 15 mm ROD2, 115 mm ROD2, 125 mm ROD2, 135 mm ROD2 inner, 65 mm ROD2, 75 mm ROD2, 85 mm ROD2, 15 mm ROD2, 115 mm ROD2, 125 mm ROD2, 135 mm Oxide layer thickness (Rod1/Rod2: Zr, Rod3: Hf) Oxide layer thickness (Rod1/2: Zr, Rod3: Hf) Figure 12: Calculated oxide layer thicknesses for rods As aimed for the test with the use of Hf as cladding material for the outer heated ring of rods the simulation shows that the maximum oxide layer thickness reached for hafnium is ~ µm (ROD3, 85 / 95 mm) and therefore low compared to that of Zry The first evaluation of the test had shown noticeable internal oxidation at upper bundle elevations with about 2 µm internal oxide layer thickness at 1328 mm bundle elevation [11] which is not calculated by the code (Fig 12, right diagram) In the simulation the highest values of internal oxide layer are ~ 1 µm at the positions of maximum temperature (85/95 mm); at the position of 135 mm the activation temperature for the beginning of the internal reaction (T = 1 K) was not reached As shown in [11], grid spacer 4 was completely oxidized after the experiment which is in agreement with the simulation, where grid 3 and grid 4 are totally oxidized after ~ 5 s (Fig 13) Therefore, it is assumed that the underestimation of the internal oxidation at upper positions is the reason for a lower hydrogen generation in the later time period of the simulation compared to the test data An additional reason for the lower hydrogen production in the calculation is the fact that the oxidation of the shroud outer surface is not considered In Fig 14 the rod temperatures are shown over the whole period of the experiment The cladding temperatures of the outer heated Hf rods are calculated slightly higher (ROD3) compared to the temperatures of the inner unheated ring of rods (ROD2) which form the debris in the quench period of the test In the test only corner rod and shroud temperatures are measured over a longer period at upper elevations; the shown corner rod temperature at maximum temperature level (95 mm) was corrected after the test by about 1 K towards higher values In comparison to the simulation the corrected values of TIT A/13 are about 1 K higher than the calculated data, the measured and also corrected shroud temperature is lower as expected and in good agreement with the simulation Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper

13 Oxide layer thickness (µm) GRID2 B1, 35mm GRID2 B2, 35mm GRID2 B3, 35mm GRID3 B1, 15mm GRID3 B2, 15mm GRID4 B1, 135mm GRID4 B2, 135mm Oxide layer thickness of grids Figure 13: Oxide layer thickness of grids While the calculated temperatures at lower elevations (35 75 mm), where the debris bed is built after the pre-oxidation period are generally in good agreement with data, the temperatures at levels above mm are overestimated in the simulation In spite of this the internal oxidation at this upper elevations above 15 mm is underestimated (see Fig 12, right) due to the fact that an activation temperature of T > 1 K is adopted for the start of internal reaction as recommended in the user manual and vertical steam flow inside of the rod is not possible in the simulation ROD3, 35 mm ROD3, 45 mm ROD3, 55 mm ROD3, 65 mm ROD2, 65 mm ROD2, 75 mm TFS 16/7, 35mm, exp TSH 8/9, 45mm, exp TSH 8/27, 45mm, exp TSH 9/, 55mm, exp TSH 9/18, 55mm, exp TSH 1/27,65mm, exp TIT C/11,75mm, exp TSH 11/18,75mm, exp TSH 11/,75mm, exp ROD2, 85 mm ROD3, 15 mm ROD3, 115 mm ROD3, 125 mm ROD3, 135 mm TIT D/12,85mm, exp TFS 12/12,85mm, exp TIT A/13,95mm, exp TFS 13/13, 95mm, exp TSH 13/9, 95mm, exp TFS 14/14, 15mm, exp TSH 14/9, 15mm, exp TSH 14/27, 15mm, exp TFS 15/15, 115mm, exp TSH 15/18, 115mm, exp TFS 16/16, 125mm, exp TSH 16/18, 125mm, exp TSH 16/, 125mm, exp TFS 17/17, 135mm, exp Rod cladding temperatures (lower bundle region) Rod cladding temperatures (upper bundle region) Figure 14: Rod temperatures The MEWA model was started together with the initiation of the quench water injection at 77 s (reference calculation) Fig 15 (left) is a representation of the MEWA region in the bundle and shows the particle temperatures in the debris bed at the time directly after the activation of MEWA The given height is the length of the bundle and corresponds to the effective height given for the experiment as height = length 475 m The state of the ECORE region at t = 77 s is demonstrated at the right side of Fig 15, where the axial temperature distribution of the rods and the ballooning of the cladding can be seen Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper

14 Figure 15: Graphic demonstration of temperatures in MEWA region (left) and ECORE region (right) In a preliminary simulation where the debris bed was not simulated and the rods were assumed as undestroyed the temperature decrease after start of quenching is too fast compared to measured data at the elevations 65 to 95 mm as the water level increase is too fast while the temperatures are calculated in good agreement in the region below (Fig 16, left) In the reference simulation, where the MEWA model was used with a particle diameter of 35 mm, the cool down of the positions above the simulated debris bed (75 95 mm) occurs at the right time so that after 78 s the bundle is quenched in agreement with the experiment At positions inside of the debris bed (35 65 mm) after a period of good agreement with test data until ~ 78 s the rewetting happens at too high temperatures and therefore the final temperature decrease occurs too early (Fig 16, right) This behavior is similar to the simulation where the debris bed model was not used ROD3, 35 mm ROD3, 45 mm ROD3, 55 mm ROD3, 65 mm TFS 16/7, 35mm, exp TSH 8/9, 45mm, exp TSH 8/27, 45mm, exp TSH 9/, 55mm, exp TSH 9/18, 55mm, exp TSH 1/27,65mm, exp TIT C/11,75mm, exp TSH 11/18,75mm, exp TIT D/12,85mm, exp TIT A/13,95mm, exp 1 ROD3, 35 mm ROD3, 45 mm ROD3, 55 mm ROD3, 65 mm TFS 16/7, 35mm, exp TSH 8/9, 45mm, exp TSH 8/27, 45mm, exp TSH 9/, 55mm, exp TSH 9/18, 55mm, exp TSH 1/27,65mm, exp TIT C/11,75mm, exp TSH 11/18,75mm, exp TIT D/12,85mm, exp TIT A/13,95mm, exp Figure 16: Rod temperatures during quenching without (left) / with (right) MEWA simulation In an additional parameter study the particle diameter was changed to 5 mm due to the fact that the debris bed in the test contained larger parts of the broken cladding As shown in Fig 17 (left) the larger particle diameter with the increased permeability for the quench water accelerates the cool-down at the 85 and 95 mm positions while the temperatures within the debris bed remain nearly unchanged Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper

15 As a further test the shroud leak was considered in the simulation at a height of 85 mm where after a time of 7872 s 9 % of the injected water was lost through the leak This assumption is derived from the first test results as there is no more increase of the water level after this time point [11] For this variation ATHLET-CD calculates significantly delayed quenching times for the elevations 85 and 95 mm (Fig 17, right) which is not in agreement with the measured data ROD3, 35 mm ROD3, 45 mm ROD3, 55 mm ROD3, 65 mm TFS 16/7, 35mm, exp TSH 8/9, 45mm, exp TSH 8/27, 45mm, exp TSH 9/, 55mm, exp TSH 9/18, 55mm, exp TSH 1/27,65mm, exp TIT C/11,75mm, exp TSH 11/18,75mm, exp TIT D/12,85mm, exp TIT A/13,95mm, exp 1 ROD3, 35 mm ROD3, 45 mm ROD3, 55 mm ROD3, 65 mm TFS 16/7, 35mm, exp TSH 8/9, 45mm, exp TSH 8/27, 45mm, exp TSH 9/, 55mm, exp TSH 9/18, 55mm, exp TSH 1/27,65mm, exp TIT C/11,75mm, exp TSH 11/18,75mm, exp TIT D/12,85mm, exp TIT A/13,95mm, exp Figure 17: Rod temperatures with MEWA simulation during quenching with increased particle diameter (left) / with simulation of the shroud leak (right) The application of the MEWA debris bed model shows that the cooling of the debris bed could be simulated with satisfactory results for the temperature decrease It works well in the two phase region but shows heavy oscillating mass flows and therefore small time steps due to an unfavorable coupling of the MEWA region at the boundary to the ECORE model in the case that the MEWA cells are totally filled with liquid Therefore, the transition to a single phase incompressible medium should be optimized The comparison of grid temperatures shows a wider band of data for the test compared to the simulation (Fig 18) as most of the temperatures are measured at the top of grid spacer 2 ( mm) where the measuring sensor could be in contact with the debris bed or not The measured grid temperatures at the 35 mm level agree well with the calculated temperatures of GRID2 which is an average temperature of the heat conductor volumes at a level of 35 mm, while the higher measured temperatures of grid 2 coincide with the calculated rod temperatures at the levels 35 and 45 mm As shown for the quench period of the experiment (Fig 18, right) the cool down of grid 2 is generally calculated in good agreement with data besides a slightly too fast rewetting time as evident also for the rods at lower positions Due to lower temperature of the grid compared to the rod claddings at 35 mm the grid is cooled slightly earlier in comparison to the rod while this is not the case for the test In agreement with the test, the upper spacer grids 3 and 4 both are fully oxidized at the end of the experiment [11] In the simulation full oxidation is achieved at about 15 to 1 K after ~ s for GRID3 and at ~ 13 K after 5 s for GRID4 (Fig 13, Fig 18) Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper

16 GRID2 B1, 35mm GRID2 B2, 35mm GRID2 B3, 35mm GRID3 B2, 15mm GRID4 B2, 135mm ROD3, 35 mm ROD3, 45 mm TGS 5i, 35mm, exp TGS 7i, mm, exp TGS 5, mm, exp TGS 3, mm, exp TGS 3i, mm, exp TGS 9i, mm, exp TFS 16/7i, 35mm, exp GRID2 B1, 35mm GRID2 B2, 35mm GRID2 B3, 35mm ROD3, 35 mm ROD3, 45 mm TGS 5i, 35mm, exp TGS 7i, mm, exp TGS 5, mm, exp TGS 3, mm, exp TGS 3i, mm, exp TGS 9i, mm, exp TFS 16/7i, 35mm, exp Figure 18: Comparison of grid temperatures 5 CONCLUSIONS After a successful performance of experiment QUENCH-17 it was evident that the results of the pre-test calculations of ATHLET-CD 22C, done with a program version where the oxidation kinetics of Hf and properties of Hf and HfO 2 were considered for the shroud and the outer ring of heated rods, were confirmed up to the time when the outer oxidation in the intended amount was reached The predicted time for complete oxidation of Zry-4 claddings between the levels 65 and 115 mm with low flow rates of 3 g/s, similar as used in the test (2 g/s) was 65 s compared to 72 s as indicated in the experiment Also the predicted hydrogen generation of ~ 8 g during this time period was in good agreement with the later test results The first post-test calculations, performed with an extended nodalization for the application of the debris bed model MEWA, also shows good agreement of temperature and oxidation behavior which can be derived from the measured hydrogen mass In the last phase of pre-oxidation the increase of power leads to an increase of oxidation rate which is probably attributed to considerable inner oxidation at upper positions which is not calculated by the code Therefore, the model adopted for the activation of inner side oxidation has to be improved to enable inner oxidation also for temperatures below the present activation criterion if a cladding failure at lower elevations was detected In spite of this underestimation there is a general good agreement of oxidation behavior which results in an underprediction of ~ 2 % at the end of the test The application of the MEWA debris bed model shows that the cooling of the debris bed could be simulated with satisfactory results for the temperature decrease in the reference calculation (particle diameter 35 mm) A temperature escalation above melting temperature was avoided as demonstrated in the test The MEWA model works well in the two-phase region but shows some instability in case of transition from two-phase to single phase liquid flow Therefore, the coupling of MEWA to ECORE for this special condition should be optimized The parameter variation with higher particle diameter of 5 mm for the debris bed compared to 35 mm in the reference calculation as it seems reasonable considering the configuration of the debris as detected after the test shows a too early quenching An additional simulation with consideration of the shroud leak did not show the expected concordance with test data regarding the temperature decrease at upper levels Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper

17 In spite of remaining problems for the application of MEWA in the late phase of refilling it can be concluded that this first post-test calculation of the test QUENCH-17 can be considered as successful for the verification of MEWA within ATHLET-CD regarding the low experience with simulations of debris bed cooling in bundle experiments ACKNOWLEDGEMENT The development and the assessment of ATHLET-CD are sponsored by the German Federal Ministry of Economics and Technology REFERENCES [1] Repetto, G, Garcin, T, Foubert, F, et al, Experimental program on debris reflooding (PEARL); Status of the PRELUDE experiments, 16 th International QUENCH Workshop, November 16-18, 21, Karlsruhe, Germany; [2] Buck, M, Bürger, M, Rashid, M, Kulenovic, R, Status of IKE experiments and modelling activities, SARNET2 WP% COOL Topical Meeting on QUENCH-Debris and related experiments, KIT, November 18, 21, Karlsruhe, Germany; [3] Stuckert, J, Steinbrück, M, Große, M, QUENCH-Debris: Bundle Tests on Debris Formation and Coolability, SARNET-2 WP 51 proposal, KIT, November 18, 21, Karlsruhe, Germany; [4] Chikhi, N, Repetto, G, SARNET2 Meeting: QUENCH-Debris test preparation, QUENCH Workshop, KIT, November 18, 21, Karlsruhe, Germany; [5] Stuckert, J, Conduct and first results of the QUENCH-17 experiment, KIT, February 213, Karlsruhe, Germany; (received via ) [6] Stuckert, J: Status of the preparation of the QUENCH-Debris experiment, 18 th International QUENCH Workshop, November 2, 212, Karlsruhe, Germany; [7] Stuckert, J: QUENCH-Debris Bundle Tests on Debris Formation and Coolability, SARNET- 2 WP51 proposal, Topical Meeting on QUENCH-Debris, KIT, ; [8] Barin, I, Thermochemical Data of Pure Substances Part I Ag-Kr VCH, Verlagsgesellschaft mbh, Weinheim, ISBN , 1993; [9] Bals, C, Austregesilo, H, Analytical Support for QUENCH-Debris Test: First calculations with ATHLET-CD, ATHLET-CD specification calculations for QUENCH-Debris, GRS, February 212; [1] Bals, C, Austregesilo, H, Analytical Support for QUENCH-Debris Test: Further calculations with ATHLET-CD, ATHLET-CD specification calculations for QUENCH-Debris, GRS, April 212; [11] Stuckert, J: First results of the QUENCH-17 bundle test on debris formation, SARNET2 WP5-COOL Review Meeting, February 11-14, 213, Fuerteventura; [12] Buck, M, Bürger, M, Rahmann, S, Pohlner G: Validation of the MEWA Model for Quenching of a Severely Damaged Reactor Core, Joint OECD/NEA-EC/SARNET2 Workshop on In-Vessel Coolability, Paris, October 12-14, 29; [13] Buck, M, Pohlner G, Rahmann, S,: Documentation of the MEWA code, IKE, Universität Stuttgart, March 212; Session 2: IN-VESSEL CORIUM AND DEBRIS COOLABILITY, Paper