SOLID PARTICLE EFFECTS on HEAT TRANSFER in MULTILAYERED MOLTEN POOLS with GAS INJECTION

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1 OECD Workshop on Ex-Vessel Debris Coolability Karlsruhe, Germany, November 1999 Organised in collaboration with Forschungszentrum Karlsruhe (FZK) GmbH ABSTRACT SOLID PARTICLE EFFECTS on HEAT TRANSFER in MULTILAYERED MOLTEN POOLS with GAS INJECTION Rosa Marina Bilbao y León and Michael L. Corradini Nuclear Safety Research Center Department of Engineering Physics University of Wisconsin - Madison 1500 Engineering Drive Madison, WI Phone: ; FAX: sam@loca.neep.wisc.edu corradini@engr.wisc.edu In the very unlikely event of a severe reactor accident involving core melt and pressure vessel failure, it is important to identify the circumstances that would allow the molten core material to cool down and resolidify, bringing core debris to a coolable state. To achieve this, it has been proposed to flood the cavity with water from above forming a layered structure where upward heat loss from the molten pool to the water will cause the core material to quench and solidify. In this situation the molten pool becomes a three phase mixture: e.g., a solid and liquid slurry formed by the molten pool as it cools to a temperature below the temperature of liquidus, agitated by the gases formed in the concrete ablation process. The present work quantifies the partition of the heat losses upward and downward in this multi-layered configuration, considering the influence of the viscosity and the solid fraction in the pool, from data obtained from an intermediate scale experimental facility that has been developed at the University of Wisconsin - Madison. Recent experimental results showing the heat transfer behavior for multilayered pools with various viscosities and solid fractions are presented. 1. INTRODUCTION In the design of the new generation of nuclear reactors and in the safety assessment of currently operating nuclear power plants, it is necessary to evaluate the probability of experiencing a severe accident and to identify the strategies to follow in order to mitigate the possible consequences. For example, in the unlikely event of a severe reactor accident involving core melt and pressure vessel failure, it is important to identify the circumstances that would allow the molten core material to cool down and resolidify, bringing core debris to a safe and stable state. In this type of postulated accident, the molten material which escapes from the reactor pressure vessel is expected (1)

2 to accumulate as a molten pool in the reactor cavity below. This material, usually called corium, is formed mainly by urania, zirconia, zirconium and stainless steel. The molten corium can thermally attack the concrete underneath and decompose it, producing gases which agitate the pool, enhancing heat losses to the boundaries as fission product decay heat and chemical reactions continue to add energy to the process. To achieve coolability of the corium in this configuration it has been proposed to flood the cavity with water from above forming a layered structure where upward heat loss from the molten pool to the water will cause the core material to cool and solidify. The effectiveness of this procedure depends largely on the rate of upward heat loss as well as on the formation and stability of an upper crust. In this situation the molten pool can become a three phase mixture: the solid and liquid slurry formed by the molten pool cooled to a temperature below the temperature of liquidus, agitated by the gases formed in the concrete ablation process. Because this process occurs at large scales and with materials whose physical properties are not well determined, the phenomenology involved is not completely understood. In addition, many of the currently most widely used models were not specifically obtained to simulate this phenomenology and do not always predict the experimental observations. Various attempts have been made to reproduce the problem experimentally by using either prototypic or simulant materials. Some of these are integral experiments that try to reproduce the entire scenario to pinpoint all the processes involved (Farmer et al., 1992) (Thompson et al., 1992), while others are separate effect studies focused on the more detailed analysis of very specific phenomena (Greene, 1982, 1988a, 1988b, 1991) (Felde, 1980). However, these approaches have always ignored the effects of solids within the molten pool and their impact on the apparent viscosity of the pool, as well as their effect on heat transfer. The objective of this work is to quantify the heat losses upward and downward in this multi-layered configuration, as well as the corresponding heat transfer coefficients, considering viscosity effects, the solid fraction within the lower heated pool and the presence of an overlying solid layer. To complete this task, an intermediate scale experiment has been designed, in which simulant materials are used to model a molten pool with another liquid layer above. The design includes volumetric heating, gas injection from the bottom and solids within the pool. 2. THE EXPERIMENTAL TEST SECTION AND PROCEDURE A diagram of the apparatus used in this study is displayed in Figure 1. It consists of a rectangularly shaped test section 6.5 in wide (16.51 cm), 8 in long (20.32 cm) and 16 in tall (40.64 cm), whose walls were made of 0.5 in (1.27 cm) thick Lexan. The fluid cell can be subdivided in two parts by insertion of a stainless steel wire mesh. The lower half of the pool holds the heating assembly, which consisted of two stainless steel electrodes, connected through a grid of high resistivity nickel-chromium heating wires wrapped into a spiral shape and connected to a DC Power Supply. Air was injected into the steel (2)

3 pressure box on the bottom and entered the pool through the bronze porous plate, which ensured a uniform bubble distribution in the pool. Figure 1. Schematic Diagram of the Test Section On the top and bottom of the fluid cell there are two cooling assemblies, formed by two aluminum plates each. Machined into one of the plates was the cooling water channel, which had a double spiral water flow pattern designed to alternate the hot and cold legs of the flow, resulting in nearly isothermal cooling plates. In the bottom cooling assembly, these two plates were mounted together with the porous plate. Holes of 1/8 in (3.175 mm.) diameter were drilled in all the aluminum plates to allow for the injected air to go through. The inlet and outlet cooling water temperature and the surface temperature in the cooling assemblies, the temperature of the pool at several heights and the inlet and outlet air temperature were measured with redundant E-type thermocouples. Flowmeters were used to measure the air and cooling water flow rates. The electrical power input by the (3)

4 DC power supply unit was measured by a digital multimeter and a DC current probe. The relative humidity of the inlet and outlet air was measured with a humidity and temperature transmitter. A DAS connected to a 486 PC was used to collect the voltage signals from all the thermocouples. Each data point is the result of recording 300 samples for each thermocouple at a sampling rate of 1 Hz. The simulant fluids used for this investigation were water, glycerin and white mineral oil. The solid particles were polystyrene beads, with an average size of about in (0.5 mm), and a specific gravity of In order to include the effect of solids in a density stratified multi-layer configuration, it was necessary to devise a way of keeping the solids within the appropriate fluid layer. For that purpose, a stainless steel wire screen with mesh size small enough to stop the solid polystyrene beads was placed at the interface of the fluid layers. The light fluid in these tests was always low viscosity mineral oil. For the lower layer, water, a mixture of water and 10% in volume of polystyrene beads, 96% pure glycerine and a mixture of glycerine and 10% in volume of polystyrene beads were used. Only the lower layer was heated. In addition, it was of interest to simulate the effect of a porous solid crust in between the fluids. Due to the adequate density ratio among the two fluids and the polystyrene beads, these tended to agglomerate at the interface of the fluids, and the desired configuration was achieved. 3. ANALYSIS OF THE EXPERIMENTAL RESULTS Previous reports analyzed in detail the results of the other two series of experiments, the tests with one fluid layer (Bilbao y León et al., 1999b) and two fluid layers (Bilbao y León, 1999a). In this paper we focus on the tests involving three layers: two fluid layers with a solid interface in between. Figure 2 shows the power split for these tests, where we observe a quite different trend from what was observed in two previous series of tests: the power split up/down has changed from 80/15 to about 60/35. This result is not surprising because of the effect of inserting the wire mesh, which collects the solid particles as a crustal layer which obstructs the process of heat and mass transfer at the interface by severely limiting the surface renewal and entrainment processes. Therefore, the increase in thermal resistance for the heat transfer upward results in a proportional increase in the fraction of the heat transferred downward. Applying this observation to the MCCI phenomenology, one may deduce that even if a full size solid crust does not form at the interface between the molten pool and the coolant, the existence of a very porous layer would substantially hinder the upward heat transfer. As for the influence of viscosity and solid fraction on the power split, we observe that the fraction of power being transferred downward is slightly larger for glycerine and its suspensions (pink symbols) than for water. This trend makes sense because as the fluid gets more viscous the screen creates a more significant obstruction to the flow and the heat transfer. (4)

5 Figure 2. Power Split for all the three layer tests 1. When analyzing the heat transfer coefficients (HTC), the same trends as in single layer tests were observed: overall, the HTC increases with the superficial gas velocity of the injected air and decreases with the viscosity of the fluid mixture in the lower part of the pool. No first order effect on the HTC has been detected as a direct consequence of the presence of solids in the pool, only the effect resulting from the increase in the effective viscosity due to the solids in the pool. However, if we look at the results more closely, one observes that as the viscosity of the pool increases, the heat transfer coefficients decreases. In addition, the heat transfer coefficient decreases as the fluid in the pool changes to a different fluid of higher viscosity, but it increases as the viscosity of the pool increases for a given fluid within the pool. We explain this behavior by coupling together all the phenomena happening in the pool simultaneously: as the superficial gas velocity increases, the temperature of the pool decreases due to the improvement in the heat transfer by the agitation of the pool. At the same time, the overall viscosity of the pool increases because of the descent in temperatures, which in turn induces a decrease in the heat transfer coefficient. Furthermore, the void fraction in the pool increases with the superficial gas velocity, inducing a descent in the effective three phase viscosity of the suspension. This means that the viscosity and the superficial gas velocity induce opposite effects on the heat transfer. 1 The results for the two layers tests have also been plotted for comparison purposes (5)

6 Several attempts were made in order to quantify this phenomenon. First, the effective three phase viscosity of the pool, i.e. including the effect of the solids and the void fraction was estimated. Since no model was found in the literature to estimate the viscosity of this three-component three-phase mixture, a classical approach was taken. The suspension formed by the fluid with the solid particles was assumed to be a homogeneous fluid whose viscosity was given by Thomas equation (Seiler et al., 1996) and the overall effective viscosity of the pool was estimated as ( α) Φ µ 3Φ = α µ air + 1 µ 2 (1) where α is the average void fraction in the pool, which can be calculated using the well known drift flux model. However, this approach failed to agree with the trends found experimentally. A more successful approach was to find a way of expressing the heat transfer coefficient as a dimensionless combination of the parameters that we considered most significant to describe the phenomenology: h = f ( µ 2Φ, jsup) (2) This HTC should be directly proportional to the superficial gas velocity, because we expect an increase of the HTC when j sup increases. At the same time, the HTC will be inversely proportional to the viscosity of the pool, since the HTC will become smaller as the viscosity increases. Using the theorem Π for dimensionalization, we obtain the parameter: 1 µ 2Φ Ψ = = Re Fr ρ 2Φ g 3 j This in turn will produce correlations of the form: Nu = C ) sup b b (Re Fr = C Ψ (4) Figure 3 shows the appeareance of our experimental data for the upward and downward heat transfer coefficients when using this approach. We observe that all the data, for both fluids, with and without solids in the pool collapse very well, and behave as describe by our hypothesis. As the dimensionsless parameter Ψ increases, i.e. as the effective viscosity in the pool increases and the superficial gas velocity decreases, the heat transfer coefficient does decrease. The interfacial heat transfer coefficients show they same trends than the upward and downward ones but they are substantially larger (Figure 4). However, these values are smaller than the ones found in the two layer series of tests due to the presence of the intermediate layer that limits the entrainment and makes the heat and mass transfer a lot less effective. Greene et al. (1982; 1988a; 1988b; 1991) and Werle (1978) performed (3) (6)

7 several experiments in similar conditions, using different fluid pairs with different density ratios, which produced different levels of entrainment. In their experiments, however, there was not intermediate layer between the two fluids. The comparison between these previous data and ours is shown in Figure 5. Bilbao's data for both water and glycerine show the same trends as Greene's data for mercury but they are substantially lower. This behavior is more accentuated than expected because water and mercury have similar viscosities, but the density ratio water/mercury or oil/mercury is a lot larger than water/oil or glycerin which would translate into smaller rates of entrainment. With these premises, for a fluid pair such as water/oil we would expect to obtain larger HTCs than for mercury/oil. Therefore, it seems that the interlayer screen does have a significant effect on the HTCs. Also in Figure 5, we have plotted the correlations suggested by Greene and Szekely (1963) to represent the behavior of the interfacial heat transfer coefficient between to fluid layers. It is observed, that both numerical correlations show trends similar to our data. They predict relatively well our experimental results for glycerin while they seem to underestimate the HTC for water. Ψ Ψ Ψ Figure 3. Upward and downward Nusselt numbers versus dimensionless number Ψ (7)

8 4. CONCLUSIONS The analysis of the experimental data obtained in the present work, and the comparison with previous models and correlations, allow us to draw some important observations: First, we concluded that the viscosity of the fluid in the pool has a dominant effect on the obtained heat transfer coefficients to the boundaries. The heat transfer coefficient decreases as the viscosity of the pool increases. On the other hand, the solid fraction in the pool does not have a first order effect on the heat transfer coefficients. The solids in the pool increase the effective viscosity of the fluid, and in this sense the solids do have an impact on the heat transfer coefficient, but this impact is not different than the impact obtained by increasing the viscosity of the pool without adding solids. In addition, a very imbalanced power split was found for the one and two layer tests, with 80% of the power being transferred upward and only 15% downward. The power split measured in the three layer tests is very different, with an up/down ratio of 60/35 approximately, due to the presence of a non-entraining interface between the fluid layers. Ψ Ψ Figure 4. Interfacial Nusselt number versus dimensionless parameter Ψ For the configurations studied here, where there is gas injection into the pool, we concluded that it is necessary to consider the overall effect. As the superficial gas (8)

9 velocity increases, the temperature of the pool decreases due to the improved heat transfer produced by the gas agitation. But, as the temperature of the pool decreases, the viscosity of the pool increases too, producing the opposite effect. It is necessary to account for both competing effects in order to predict successfully the behavior of the system. Furthermore, for the study of the heat transfer between density stratified fluids, the determination of the rate of entrainment between layers is of paramount importance in order to model the behavior of the system adequately. Figure 5. Comparison of Bilbao y León s data with previous experimental and numerical studies 2. Finally, we can say that the existing models and correlations are not able to predict the new data for the entire range of viscosities and superficial gas velocities. Empirical correlations have been provided for our data (Figures 3 and 4), but these need to be used cautiously within the range of appropriate conditions. 2 The data from the two layer tests as well as Greene and Werle data with high entrainment have also been plotted in Figure 4 for comparison purposes. (9)

10 NOMENCLATURE A Laplace constant [m] g Gravity [9.8 m/s 2 ] j sup Superficial gas velocity [m/s] k Thermal conductivity [W/m 2 K] Fr Froude number, ρj 2 / ρgd Nu Nusselt number, ha/k Re Reynolds number, ρjd/µ α void fraction µ dynamic viscosity [N s / m 2 ] Φ solid fraction ρ density [kg/m 3 ] Subscripts air air l liquid s solid 2Φ two phase mixture or suspension 3Φ three phase mixture or suspension REFERENCES R. M. Bilbao y León, 1999a. Interfacial Heat Transfer in Multiphase Molten Pools with Gas Injection, Ph.D. Thesis, Department of Nuclear Engineering and Engineering Physics, University of Wisconsin Madison. R. M. Bilbao y León, M. L. Corradini, 1999b. Solid Particle Effects On Heat Transfer In A Molten Pool With Gas Injection, Ninth International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-9), San Francisco, California, October 3-8, M. T. Farmer, B. W. Spencer, D. R. Amstrong, D. J. Kilsdonk, R. A. Aeschlimann and M. Fisher, Results of MACE Test M0 and M1, Proceedings of the second OECD (NEA) CSNI Specialist's Meeting on Core Debris-Concrete Interactions, Karlsruhe, Germany. D. K. Felde, H. S. Kim and S. I. Abdel-Khalik, Convective Heat Transfer Correlations for Molten Core Debris Pools Growing in Concrete, Nuclear Engineering Design, v. 58, pp G. A. Greene et al., Heat Transfer betweeen Immiscible Liquids Enhanced by Gas Bubbling, International Meeting on Thermal Nuclear Reactor Safety, Chicago IL. G. A. Greene and T. F. Irvine Jr., 1988a. Heat Transfer Between Stratified Immiscible Liquid Layers Driven by Gas Bubbling Across the Interface, Proceedings of 1988 National Heat Transfer Conference, Houston TX. (10)

11 G. A. Greene, C. Finfrock and S. B. Burson, 1988b. Phenomenological Studies on Molten Core-Concrete Interactions, Nuclear Engineering Design, v. 108, p G. A. Greene, Heat, Mass and Momentum Transfer in a Multifluid Bubbling Pool, Advances in Heat Transfer, v. 21, p C. S. Miner, N. N. Dalton, et al., Glycerol, Reinhold Publishing Corp., New York. J. M. Seiler and J. Ganzhorn, September Viscosities of Corium-Concrete Mixtures, 4th European Conference Thermophysical Properties INSA, Lyon, France. J. Szekely, Mathematical Model for Heat or Mass Transfer at the Bubble Stirred Interface of Two Inmiscible Liquids, International Journal of Heat and Mass Trasfer, v. 6, p D. H. Thompson, J. K. Fink, B. W. Spencer, D. R. Amstrong, B. R. Sehgal, Thermal-Hydraulic Aspects of the Large-Scale Integral MCCI Tests in the ACE Program, presented at the 2 nd OECD (NEA) CSNI Specialists MTG. On Core Debris Concrete Interactions, Karlsruhe, Germany. H. Werle, Experimental Investigation of Heat Transfer between Two Horizontal Liquid Layers with Gas Injection, Proc. Fourth PAHR Info. Exch. Mtg., Ispra Italy, v. 1, p.164. (11)