SCALING OF WATER SPRAY IN LARGE ENCLOSURES APPLICATION TO NUCLEAR REACTOR SPRAYING SYSTEMS. J. Malet, E. Porcheron, P. Cornet, J.

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1 The 10 th International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-10) Seoul, Korea, October 5-9, SCALING OF WATER SPRAY IN LARGE ENCLOSURES APPLICATION TO NUCLEAR REACTOR SPRAYING SYSTEMS J. Malet, E. Porcheron, P. Cornet, J. Vendel Institut de Radioprotection et de Sûreté Nucléaire (IRSN) DPEA/SERAC, Laboratoire de Physique et de Métrologie des Aérosols et du Confinement. BP 68, Gif-sur-Yvette cedex, France K. Fischer Becker Technologies GmbH Kölner Strasse 6, D-65760, Germany KEYWORDS Spray, large enclosures, reactor containment, scaling, nuclear reactor, droplets ABSTRACT During the course of a hypothetical severe Pressurized Water Reactor accident, pressurization of the containment occurs and hydrogen can be produced by the reactor core oxidation. Inner spray systems are used in order to reduce the pressure, mix the gaseous mixture and collect the fission products on the droplets. Complex phenomena are involved during the spray injection, making difficult to set up reliable models (using lumped-parameter codes as well as CFD codes) and experiments or experimental facilities to scale without distortion. The French Institute for Radioprotection and Nuclear Safety (IRSN) is conducting experimental and numerical separate-effect studies to understand the different phenomena involved during the course of an accident (Cornet et al. 2002). For this purpose, the TOSQAN facility has been designed to reproduce typical thermal-hydraulic conditions. Spray interaction with the atmosphere is studied in the TOSQAN vessel, neglecting water interaction with walls and compartments (separate-effect test). A scaling study has been performed in the frame of a European Thematic Network (SCACEX, Fischer et al. 2002). The objective of this paper is to present scaling rules for spray studies in large enclosures such as TOSQAN. One problem of spray scaling is related to the description of the real containment conditions (droplet size distribution, turbulent flow induced by the spray, thermodynamic changes). The second problem is related to the scaling of two-phase flows as spray systems. 1. INTRODUCTION During the course of an hypothetical severe accident in a Pressure Water Reactor (PWR), hydrogen can be produced by the reactor core oxidation and distributed into the reactor containment according to convection flows and water steam wall condensation. In order to assess the risk of detonation generated

2 by a high local hydrogen concentration, hydrogen distribution in the containment has to be known. The TOSQAN experimental program has been created to simulate typical accidental thermal hydraulic flow conditions in the reactor containment and to study different phenomena such as water steam wall condensation in the presence of non-condensable gases. A second part of TOSQAN program is devoted to study the water spraying effect used as mitigation mean (Malet et al., 2003). In some particular hypothetical nuclear accidents, water steam could be released in the containment leading to an increase of the total pressure. In some cases, fission products could also be released into the containment. Inner spray systems are used in French NPPs in order to reduce the total pressure by condensing the water steam on the spray droplets and cooling down the containment walls. A second objective of these spray systems is to remove the fission products from the gas containment: fission products (aerosols of micrometric size) can be captured by the large spray droplets (200 to 1000 µm), and fall down into the bottom of the containment (the sump). However, the use of spray systems in the case of hypothetical nuclear accidents can generate new questions concerning a particular phase of one type of accident. During the course of a hypothetical severe accident in a Pressurised Water Reactor (PWR), hydrogen can be produced by the reactor core oxidation and can be distributed into the reactor containment according to convection flows and wall condensation (so called hydrogen risk ). In order to assess the risk of detonation generated by a high local hydrogen concentration, the hydrogen distribution in the containment has to be known. The use of this spraying system can lead to several counter-phenomena concerning the gas distribution and can be summarised by these two questions:! Can the hydrogen risk be reduced by a high mixing level of the gas mixture when the spray droplets are injected?! Can the hydrogen risk be enhanced due to the local reduction of steam, leading to higher local concentration of hydrogen due to condensation of steam on the spray droplets? 2. SPRAY SYSTEMS IN REACTOR APPLICATIONS 2.1. Real-scale characteristics The approximated volume of French PWR containment is ~50000 m 3 (900 MWe PWR) or ~75000 m 3 (1300 MWe PWR P4 and P 4). The dome volume is estimated at ~47000 m 3 for the 1300 MWe PWR (OECD 1999). The steam is generally at saturation conditions; the hydrogen concentration in case of an accident can be between 7 % and 16 %. The spray system is automatically activated when the total pressure reaches 2.6 bar. The French containments have generally two series of 253 nozzles placed in circular rows (see Figure 1), with a total water mass-flow-rate of about 560 kg.s -1. More precisely, for the 900 MWe PWR, there are exactly 4 rings of nozzles: two rings with diameters 10 and 14 m at respectively m and m from the bottom, and 2 other rings with diameters 25 and 27 m, at respectively m and m from the bottom (Evrard 2002). So it can be assumed that there is approximately one ring at height 54.5 m of a radius of 6m and another ring at height 51 m of 13 m radius.. The type of nozzles used in French containment is a SPRACO 1713A where an injection pressure of 3.5 to 5.25 bar is applied. The temperature of the injected water is 20 C or C, depending on the kind of process (the C process is the so-called recirculation mode for which the water of the spray is collected in the sump and recirculated up into the nozzles). The nozzle orifice diameter is 9.5 and 11.1 mm (French 900 Mwe PWR and 1300 MWe PWR, respectively).

3 Spray in the containment: 2 rings of nozzles Side view containment Slice view dome 1 st ring of nozzles (R ring1 ~6m, H ring1 ~54.5m) ~ 60 m ~ 35 m Sprays compartements ~ 40 m 2 nd ring of nozzles (R ring2 ~13m, H ring2 ~51.5m) 506 nozzles for all rings 1 st ring: 1 nozzle each ~15cm 2 nd ring: 1 nozzle each ~32cm Valid for French 900MW PWR Figure 1: Spray in PWR (not at scale). The variation of the flow-rate with the pressure supply has been measured under standard pressure and temperature conditions (1 bar, 20 C) (Ducret et al. 1993). From these measurements, it can be verified that the flow-number FN of the real-scale nozzle is constant for different pressure levels, and see section 4.5 for the definition of the FN). This is an important result since it means that the SPRACO nozzle has the same hydrodynamic behaviour as many nozzles used for general spray studies of the open literature (Buchlin 1995). As a result, common correlations (Lefevre 1989) give the mean Sauter diameter of a spray as a function of the flow number and other fluid characteristics could be applied to these SPRACO nozzles. The spray obtained from a single nozzle is a full-cone spray.the half-angle is found to be of about 33. An important parameter of a spray is its mean droplet size. Concerning the size-distribution, a study has been performed leading to droplet diameters between 25 and 1000 µm (Ducret et al.1993). Care has to be taken before drawing conclusions from these results, since the aim of this study was not to characterise precisely the spray, but to have an estimation of the size-distribution. Measurements at large size can be distorted if particles are not spherical (for large sizes) or if measurements are performed in too dense zones of the spray. Furthermore, these size distributions have been measured at atmospheric pressure and ambient temperature. Even if the pressure drop through the nozzle is the same as in the real case, the size distribution may not be the same if different pressure or temperature is considered for the atmosphere. Using adequate correlations for plain-orifice atomizers (Tanasawa and Toyoda 1955 in Lefevre 1989), the corresponding droplet diameters can be determined for thermodynamic conditions corresponding to the one of a hypothetical accidents. Such correlations can lead to a diameter reduction, in the considered case, of a factor 0.5 (at higher mean gas mixture densities) to 0.9. It has to be emphasised that several relations on this subject exist (Lefevre 1989) and that they are empirical, generally obtained for small droplets and for higher pressures and temperatures than the ones considered for containment accident. The last point to be considered is the kind of mean diameter that has to be used in a model. For spray modelling, the Sauter diameter is generally used: it is defined as the diameter of a drop having the same volume/surface ratio as the entire spray (D 32 =D 30 3 /D 20 2 ). From the measured size distributions presented above, the Sauter diameter has been calculated and is found to be 474 µm and 687 µm for the two pressure cases, respectively. Considering the uncertainty of the measurements, especially at large size,

4 some doubts exist concerning the particles with diameters larger than 1000 µm at a pressure drop of 3.9 bar. If this class of particle is not considered in the calculation, the mean Sauter diameter of the spray is then 607 µm instead of 687 µm. Considering the reduction of the diameter at higher pressure and temperature, it is found: for a total pressure of 2.5bar, a mean temperature of 120 C and and relative humidity of 75%, the reduction factor is found to be For a range of various possible conditions, this reduction factor is found to be between 0.74 and 0.90, leading to Sauter diameters between 448µm and 544µm. As a result, the Sauter diameter of the real-scale spray that has to be considered in the modelling is 500 µm. This can be considered as a real semi-empirical evaluation of the Sauter diameter in the real conditions. In this scaling study, no size distribution is considered. However, if a size-distribution is used, at present the most widely used expression for drop size distribution is the one of Rosin-Rammler. The latter is quite simple and permits data to be extrapolated into the range of very fine drops. Relations can be found in (Lefevre 1989) Spray systems in large-scale facilities In this section, special care will be given to the scaling of sprays systems in various large-scale facilities (LSF) in order to determine whether the existing experiments are suitable for the scaling analysis in this report. Outer spray tests like HDR/E11.2 or E11.2 are not considered here (OECD 1999). The NUPEC-Tadsotsu facility (OECD 1994) is a vessel simulating a PWR used for the study of thermal-hydraulics. Several tests have been done in NUPEC, and the test with spray is called M7-1 (corresponding to ISP-35). Test M8-2 also considers spray injection (OECD 1999). The CSE facility (Kmetyk 1994) is a containment vessel used to evaluate the performance of sprays for the removal of aerosols and iodine vapour. Determination of the containment response to postulated accident conditions was the main purpose of the thermal-hydraulic tests performed in CVTR (Schmitt et al. 1970). The main geometrical parameters of the three considered LSF are given in Table 1. The different spray tests and initial gas mixture characteristics are given in Table 2. Table 1: Main geometrical characteristics of LSF used for internal spray tests. Facility NUPEC-Tadsotsu CSE CVTR VESSEL CHARACTERISTICS Vessel volume (m 3 ) Vessel height (m) (26+9 for dome) Vessel diameter (internal) (m) m Vessel material metallic Stainless steel Nb. compartments 25 6 MEASUREMENTS Concentration measurements 29 (helium) 14 Temperature measurements 182 (wall, gas, sump) 5 49 (+23 in wall liner) Other measurements 10 flow measurements between compartments 7 pressure transducer

5 Table 2 : Spray tests characteristics in LSF. Facility NUPEC-Ta dsotsu CSE CVTR SPRAY TESTS CHARACTERISTICS Test number M7-1 A3 A4 A6 A7 A8 A Total spray-flow-rate (kg/s) Injection temperature ( C) Duration of spray injection 30min 600 s, 1800 s, 3600 s, intermittently spray or 12min operating continuously, with recirculation from the sump after the fresh water has been used Type of sequence Steam and - producing the initial mixing (# steady-state) No steam injection during spraying helium - injecting the aerosols; No spray recirculation injection - injecting the spray during spraying Injection height 16 to 19m ~17m 26m SPRAY CHARACTERISTICS Nozzle reference 7G3 7G3 7G3 7G3 A20 A50 7G3 7G3 F35 F35 F18 F18 Number of nozzles Shape of the spray (HC: Hollow-cone FC: full-cone) HC FC FC FC FC HC HC (half-angl ~33 ) Nozzle orifice diameter (mm) Droplet diameter (µm) Average : GAS MIXTURE CHARACTERISTICS BEFORE SPRAY INJECTION Gas mixture (Air, Steam, Helium) A-S A A A-S A-S A-S A-S A-S Mean gas temperature ( C) C to 113 C Absolute pressure Initial relative humidity (%) dry near saturation

6 Conclusion on scaling of LSF From the experiments described above, the respect of the spray surface coverage seems to be the only scaling rule considered. Gas mixture conditions are taken as in the real case (scaling 1:1 for the gas characteristics), and spray nozzles seem to be similar to the real one (scaling 1:1 for the spray characteristics). No special care on the particle size distribution or on droplet concentration is taken in the described experiments. It should be noted that some studies, not presented here, have considered these effects. Droplet concentration effects on pressure and temperature before and after combustion have been considered in (Berman and Cummings 1984, from Paillère 1996). Small droplet sizes (fog spray rather than droplet spray) have been studied in order to evaluate the influence of a spray having a higher total surface available for heat and mass transfer (Torok et al.1983, from Paillère 1996). This work seems to conclude that turbulence generated by this fog spray leads to faster depressurisation Scaling of sprays in the open literature Only a few studies can be found in the open literature concerning the scaling of sprays. Two kind of studies are generally investigated for the spray characteristics: - correlations to determine the droplet size distribution under different temperature, pressure and gas mixture conditions from measurements performed at atmospheric pressure, ambient temperature, generally in air; several correlations exist and are summarised in (Lefevre 1989); - correlations to determine the Sauter diameter, the gaseous entrainment flow-rate, the droplets velocities and gas velocities as a function of the nozzle characteristics, the injection pressure and flow-rate, and the liquid type; these correlations are nozzle and spray dependant ; (Moreau 1994) has performed an experimental study for full-cone nozzles. In (Malet 2000), information can be found on scaling of sprays used for mitigation of industrial hazards, where the same problem exists (large-scale spray versus smaller spray used in experimental facility). Concerning the gaseous entrainment by sprays in closed enclosures, only very few studies have been found for thermal hydraulic conditions representative of nuclear applications. Many studies exist on sprays in motor injectors, but are of much smaller sizes, under high pressure conditions (up to 40 bar), high temperatures (>600 C) and for other gas mixture conditions. Since these studies are generally experimental, they seem to be too far away from nuclear applications to be considered. Experimental and numerical studies on gas entrainment and mixing by spray or row of sprays have been performed under conditions more representative of nuclear applications (Malet 1999, Pretrel and Buchlin 1997a, Pretrel and Buchlin 1997b, St-Georges and Buchlin 1995, St-Georges 1993). These studies concern generally the mitigation of gas accidental release in industrial (chermical) applications by sprays (Buchlin 1994). They are not performed in a completely closed vessel (wind tunnel), but some of the considerations presented in this report are based on information learned from these studies. 3. PRESENTATION OF THE TOSQAN SPRAY PROGRAM The TOSQAN experimental programmes have been established to simulate accidental thermal hydraulic flow conditions in the reactor containment and to study different phenomena such as wall condensation in the presence of non-condensable gases (Cornet et al. 2002, Vendel et al. 2002, Malet et al. 2001), sump-atmosphere interaction, spray effects on gas distributions or on aerosol wash-out. The TOSQAN facility is thus devoted for separate-effect tests and code validation Description of the facility and instrumentation The TOSQAN facility (see Figure 2) consists of a closed vessel (7 m 3 volume, 4 m high, 1.5 m inner diameter) into which steam-air-helium mixtures can be established (Brun et al. 2002). The walls are thermostatically controlled (temperature from 30 to 160 C). The spray is injected on the vertical axis, at

7 a distance between 20 and 90 cm from the top. Optical windows allow laser measurements (LDV, PIV, Raman spectroscopy) at four different levels (Porcheron et al. 2002). The vessel contains also intrusive measurement techniques, such as 54 sampling points for mass spectrometry and 110 thermocouples. The mixture can thus be characterized by its velocity, turbulence intensity, steam-air-helium concentrations and temperature. TOSQAN EXPERIMENTAL FACILITY FOR SPRAY TESTS Thermostatically controlled 2R=1.5m wall temperature 2*R = 1. 5m Thermostatically control wall temperature Water Spray Total Height 4m Figure 2 : TOSQAN experimental facility for spray tests Spray tests scenario The proposed TOSQAN test sequence consists of an injection of water-spray at a flow-rate Q inj for a duration t inj in an air-steam mixture (helium added for some tests) at a given pressure P g and temperature T g of the mixture. The walls are heated at a constant temperature. The atmosphere is initially produced in the TOSQAN vessel and is at rest. No injection of steam or helium is performed during the spray phase. This kind of scenario has the advantage of being simple for modelling and scaling analysis. The disavantage of such a scenario is that it is a transient test, making measurements more difficult to obtain, especially if spatial measurements are required. A test will have to be performed several times and its repeatability has to be checked carefully. The spray water falling into the sump will be removed as fast as possible to avoid water accumulation and evaporation. The test matrix will be composed of three reference tests, for which all local measurements, including laser diagnostics, will be performed, and of about 10 sensitivity tests, for which fewer measurements will be performed Scaling problematic for TOSQAN spray tests The objective of this chapter is to use a scaling method for spray systems for the kind of scenario proposed for TOSQAN tests in the former section. The scaled test has to be representative of the main physical phenomena that occur in the real case (spray in a PWR). In other words, since the geometry of the vessel is fixed, the only way to generate flows representative of the real case is to change the spray and/or the gas mixture characteristics for the TOSQAN facility. The aim of this scaling study is to determine those characteristics. The approach used here is proposed for the definition of spray tests in model facilities, but it does not pretend to be a scaling method of all phenomena occurring in the real containment.

8 There are two reasons that explain why the real-scale nozzle cannot be used in a facility like TOSQAN. Since the flow-number of the real-scale nozzle is very high, indicating that high power system has to be used to produce such a flow. In practice, injection flow from firemen water supply has to be used to produce a spray with this nozzle, leading to technical problem for the use of this nozzle in TOSQAN. Furthermore, the resulting depressurisation will be too fast when using a real-scale nozzle. It can be shown that the depressurisation rate is proportional to the condensed flow-rate, which is, as a first approximation, proportional to the injection mass flow-rate and depends upon the gas properties The slope of depressurisation in TOSQAN (index t, 1 nozzle) is then 28 times steeper than the one in the PWR (index r, 253 nozzles). This effect would lead to very short test during which no measurements could be performed. If the flow-rate is reduced, no spray is obtained with such a large nozzle diameter, so that the use of the real scale nozzle in TOSQAN is not possible. 4. DEVELOPMENT OF THE SCALING METHOD FOR SPRAY SCALING In the frame of a European Concerted Action, SCACEX (Fischer et al. 2002, Fischer et al. 2002b), conducted during 2002, it has been proposed to use the H2TS method to determine the most relevant phenomena for adequately scaling containment spray systems. The H2TS method (Hierarchical-Two-Tiered Scaling method (Zuber et al.1998)) is based on the analysis of individual transport mechanisms that are characterised by time and length scales. The use of such a time-preserving approach is also a recommendation from the SOAR on Containment Thermalhydraulics and Hydrogen distribution (OECD 1999). It is emphasize that this European Concerted Action on the scaling of spray systems has been performed in the frame of the TOSQAN project, i.e. considering French NPPs and taking into account the restrictions of the TOSQAN facility. According to the H2TS method, a complex system is divided into sub-systems, modules, constituents, and phases. For spray in large enclosures, the following assumptions are made: - Only large individual enclosures are considered, and specifically only the dome of the containment will be considered; the water falling inside the compartments and flowing on the walls is not considered; - The water collected in the sumps is not considered here, only the gas space is taken into account; - No aerosol and no collection of aerosols on the spray droplets is considered; - Liquid film on walls, due to spray impingement on the side walls or the compartment walls, or liquid films due to wall condensation is not considered; Natural or forced convection due to other sources (steam jet/plume, wall condensation) is not considered here. Concerning the geometry, the boundary conditions are the volume of the vessel, the height of the vessel, the height of spray injection, the number of nozzles and position on the containment radius. The initial and boundary conditions for the spray characteristics are the nozzle type and spray shape, the spray half-angle, the spray flow-rate, the droplet initial size distribution, the droplet initial velocity (near the nozzle exit), the droplet initial temperature. The initial and boundary conditions for the gas mixture are the initial gas temperature, the total pressure, the saturation rate, the steam volume fraction, the helium volume fraction and stratification. The driving forces and dominant physical phenomena are divided into two parts, the thermodynamic forces mainly governed by condensation on droplets, and the mechanical forces mainly governed by the entrainment of gas by the spray. The detailed thermodynamic forces are as follows: - For the gas mixture: - Steam condensation on droplets, leading to the reduction of steam partial pressure and total

9 pressure (primary importance), - Convection in the gas, leading to a mixing of the gas and a reduction of its temperature (secondary importance), - Steam condensation in the gas leading to fog formation (to be neglected in this study), - For the spray: - Heat and mass transfer exchanges (convection and condensation) makes the temperature and size of the droplets higher (primary importance), - Coalescence of the droplets induces higher diameters of the drops (to be neglected in this study), - The possible evaporation of the droplets could counter-balance the latter phenomenon (to be neglected in this study). The detailed mechanical forces are: - A source of momentum, by entrainment of the gas mixture by the spray (primary importance), - The source of buoyancy forces, leading to thermal and mass stratification (secondary importance for steam-air cases, primary importance for hydrogen-steam-air cases) - The dissipation or the production of turbulence by the particle-turbulence interactions, that can lead to a higher or lower mixing (secondary importance), The primary forces resulting from heat and mass transfer (condensation) and the ones resulting from momentum transfer and buoyancy forces are shown schematically in Figure 3. Condensation zone Depressurisation rate Idealized Helium layer Mixing: Momentum exchanges (gas entrainement induced by the spray Simplified recirculation loop Spray Gaseous mixture Buoyancy forces Interaction with the structure neglected here TOSQAN vessel Figure 3 : Primary forces for spray tests. The relevant variables to characterise the resulting two-phase flows during the spray injection, considering the above-mentioned primary phenomena are: - The ones concerning the goals of the scaling analysis: final total pressure (depressurisation) and final vertical mass stratification (mixing and buoyancy forces); - The others concerning local measurements for more detailed phenomena analysis: droplet sizes and temperatures at different locations, steam-helium distribution in the whole enclosure, gas temperature (thermal stratification). The H2TS method has been used to define the three transfer processes that are the most important and for which scaling rules will be derived in this section. The scaling analysis will use either time or frequency variables (or ratio of times or frequencies), as proposed in the H2TS method. However, a rigorous and global H2TS method is not proposed here. Furthermore, before deriving the scaling rules, some criteria that are not concerned with the processes but with the gas mixture or the geometrical

10 conditions will be stipulated. The notation in the following sections will be as follow: - (Ci): i criteria - (Si): i scaling rule 4.1. Scaling of gas mixture conditions For the scaling analysis, the thermal-hydraulic conditions of the gas mixture are taken to be prototypical, as performed in previous studies in LSF: (C1): gas conditions prototypical (scaled 1:1) The gas mixture characteristics will be: - Initial mean gas temperature T g, taken between 100 C and 160 C, which corresponds to the range of temperature in PWRs under severe accident conditions; - Initial containment total pressure P, which will be taken between 1.4 bar and 5 bar, 2.5 bar being close to the nominal total pressure for sprays in French PWR (2.6 bar); the lower limit for the pressure (1.4 bar) is the one used in some of the large scale facilities, the higher limit (5 bar) is the maximum total pressure in a PWR; - Saturation conditions: initial conditions will be considered mainly superheated, in order to make modelling (especially by computer codes) easier; - Gas mixture: air-steam mixture are considered as well as air-helium-steam mixtures; a distortion in scaling is here the use of helium instead of hydrogen; - Mass distribution: in case of the presence of helium, it will be assumed that the helium initially forms a concentrated layer at the top of the vessel, since this is the extreme case which is probably the most difficult to mix (except bubbles of helium situated in some compartments, but this geometrical part of the containment is not considered here); a higher helium volume fraction will be used in TOSQAN compared to the ones obtained in case of a hypothetical accident for hydrogen in order to enhance the light gas effects General considerations for the spray scaling Concerning the spray parameters, the following will not be considered for the scaling study: - The spray nozzle will be taken as simple as possible (circular orifice nozzle), since the real scale nozzle is also very simple; furthermore, the kind of nozzle will have a significant impact on the droplet atomisation process and thus on the kind of size distribution obtained; this is however an inlet parameter, the resulting one is the droplet diameter, that will be the considered parameter; - Spray type (full-cone, flat-fan, hollow-cone, etc.) is not considered here, and a full-cone spray will be used; however, if we consider that in the real case, the nozzles placed in a ring form one unique large spray, a better modelling of this kind of spray could be a hollow-cone nozzle. That is probably the reason why in some large scale facilities, hollow-cone nozzle have been used; - Concerning the number of nozzles: for design and simplification reasons, one single nozzle will be used in this theoretical study. This means that the sprays in the real case are supposed to form one large spray with approximately the ring diameter, but that the air recirculation between this ring of spray and the walls is considered (the sprays near the walls are neglected here, since they are considered as liquid films); - The droplet size distribution will not be considered here: a single diameter is considered for the particles, even if the size distribution of the real scale nozzle is very wide Geometrical scaling considerations Several criteria on geometrical parameters have to be considered. These criteria are not notified as scaling rules since they are not related to a transfer process: there are based on basic (and quite obvious) considerations of similarity between the small-scale vessel and the real case. Since they are not based on conservative equations, it is proposed to respect these criteria in an approximate way. In other words,

11 deviations from these criteria have to be very large in order to be considered as important. The first one is the criterion respecting the horizontal occupied surface of the spray in comparison to the total horizontal surface of the containment cross-section. It can be easily understood that if the spray is occupying the whole section of the vessel, the induced flows by the spray entrainment will not be the same as if it is not. The criterion is thus: (C2): S spray S tot real S = S spray tot model In the real case, the sprays are arranged in 2 series of two rings. The first ones can be considered to be at a radius between 5 and 7m. The second one are near the wall, and since the spray are directed toward the wall, it can be considered that only the half surface of the spray near the walls is to take into account in the calculation of S spray. The maximum radius of the spray can be deduced from photos that have been taken during nozzle tests, and is approximately between 1 and 1.5 m. The second geometrical criterion concerns the total height of the scaled vessel. It can be easily understood that a minimum height has to be considered. This height will be taken arbitrarily as a function of the stopping distance D stop : (C3): D stop H real D = H stop model The stopping distance is the length of the droplet path which it travels before reaching the terminal settling velocity (see section 4.5 for the derivation of the equations). The last geometrical consideration concerns the height of the spray injection. It is easy to understand that the gas space above the spray nozzle has an effect upon the recirculation flow patterns, especially in the region next to the nozzle. If the nozzle is located on the roof, air entrainment will occur only from the side of the sprays. If the nozzle is placed too far away from the roof, entrainment induced by the spray could come mainly from the top. Furthermore, for helium tests, in the frame of hydrogen bubble simulation, the spray has to be inside this bubble. The scaling criteria proposed here is given by the injection height relative to the vessel height: (C4): H H inj real = H H inj model 4.4. Scaling of heat and mass transfer: depressurisation The transfer process can be characterised by the rate of depressurisation, the length scale by the containment volume, and the system response by the duration of the injection. The proposed scaling rule S1 is to keep the depressurisation rate constant. dp dt real = dp dt model (S1) Since the gas conditions are prototypical, i.e. the total pressure is approximately the same in the real case and in the model, this scaling rule S1 can be considered as a frequency. As a first approximation, the condensation flow-rate can be expressed in the following form:

12 Q = Q Cp cond inj ini ( Tsat Tinj ) L cond This expression does not include any specification of the droplets characteristics. Other expressions are available in (Plumecocq 1997). If the temperature T g is considered constant, and if steam is considered as a perfect gas. The following equations can be assumed to be valid: s a [( ) ( )] RpgTg P = + M m sv M m av Since the mass of air m a remains constant: Q = dm dt s cond = dp = R dt T pg g M sv Q R M s T V pg cond g dp dt 4.5. Scaling of momentum transfer: mixing by air entrainment The momentum transfer process can be characterised by the rate of gaseous entrainment by the spray, the length scale by the containment volume, and the system response by the time for one flow recirculation. The proposed scaling rule S2 is to keep constant the entrainment flow-rate to the containment volume ratio: Q V entr real Q = V entr model (S2) This scaling rule S2 is a frequency. (Mc Quaid 1975) found the following simple empirical correlations for gas entrainment flow-rate of a full-cone spray: ρ w Qentr = 18.7 Qinj FN 2 4R e and ρ for 4R w 2 e FN 0.1 ρ w ρ w Qentr = 1.33Qinj FN 2 for FN 1 4R 2 > e 4R e The validity of these correlations for a single spray nozzle for application in real-scale reactor should be investigated. The radius of the spray envelope R e will be defined at the height where the spray is not developing anymore, i.e. at the height where the droplets reach their terminal settling velocity: the stopping distance D stop (see Figure 4). In order to calculate this radius R e, the relaxation time τ d of the particle has to be calculated. For these calculations, thermodynamic changes of the droplet are neglected. The latter point should not lead to severe distortion for the scaling analysis (the changes in diameters by mass transfer are too small to produce a significant variation of the spray envelope radius R e ). The droplets are also assumed to be spherical. No droplets interaction is considered. No dispersion of the droplets by turbulence is assumed.

13 Maximum radius of a spray Entrained gaseous mixture Induced by the spray Spray Entrained gaseous mixture Dstop Droplets are deccelerating Gas mixture velocity Spray (1 nozzle) 2R e Droplets at constant velocity Gas mixture velocity TOSQAN vessel Spray will not spread infinitly # reaches a max. diameter Figure 4 : Gas mixture entrainment induced by the spray and assumed maximum radius of the spray. The considered equations are as follows: U set = 8m ( ρ ρ ) d w g w g g πd² ρ ρ CD where the drag coefficient CD is defined by: CD ( Re_p) if 1 < Re_p < 1000 Re_p 24 Re_p if Re_p if Re_p 1000 and the Reynolds number of the particle is Rep = Uset Uset The relaxation time is therefore: τ d = g The stopping distance will be assumed to be based on the injection velocity: D =τ U stop d inj This is a distortion from the real case, since as it has already been specified before, the place where the droplets are formed is not at the injection nozzle, since a liquid sheet is present at the outlet nozzle. The initial velocity of the droplet is therefore smaller than this calculated velocity. The spray envelope at that height is R = D tan( α) e stop Using the definition of the flow-number: Qinj FN = ρ w P the entrainment flow-rate can then be calculated. D ν g

14 Single spray (TOSQAN case) Interacting sprays (real case) View from the top Spray Containment View from the side Entrained gas Gas volume neglected Droplet growth neglected View from the top Entrainment flow rate Q entr Entrainment flow rate Q entr reduced Figure 5 : Effect of interacting spray on the entrainment flow-rate (simplified drawings not at scale). In order to obtain the exact ratio S2, it is necessary to inverse the equations given above, and several solutions can exist. Fulfillment of these conditions depends upon the availability of nozzles having the given parameters and leading to the given range of mean droplet diameter Scaling of buoyancy forces: mixing of a light gas In this part, we consider a simplified case where pure helium is located at the top of the vessel, over a height Z h. Turbulent diffusion is neglected. The transfer processes can be characterized by a convection time and a buoyancy time, the length scale by the radius of the spray envelope at a given height and the system response by a Froude number. The proposed scaling rule S3 is to keep constant the Froude number: Fr buoy ) Fr) real t t = = = model tconv t real buoy conv model This scaling rule S3 is a non-dimensional number based on a ratio of characteristic times. The characteristic length is defined as the radius R c of the spray envelope. If the height Z h is smaller than the stopping distance, R c is taken as the spray envelope radius at height Z h. In the opposite case, it is taken as the R e radius defined in the previous part (see Figure 4). The characteristic variable for the convection, i.e. for the flow induced by the spray, will be a velocity U conv defined as: U Q = π R entr conv 2 c (S3) where the entrainment flow-rate is taken from the former section. The characteristic time for convection t conv is then: t conv 2R = U c conv For buoyancy, the classical definition of the time scale, based on the same length scale R e, is:

15 ρ t = 2R g bouy c g ( ρg ρh) For the latter time scale, the assumption is made that buoyancy forces are induced between pure helium and an air-steam gas mixture. Idealized Helium layer Z h 2R h D stop 2R e Spray Gaseous mixture TOSQAN vessel Figure 6 : Spray envelope radii R h and R e considered for the characteristic length in Froude number expression. 5. SCALING RESULTS 5.1. TOSQAN resulting spray test matrix From these scaling rules, the TOSQAN vessel is found to be adequate for the spray study if the following parameters are considered: spray injection flow-rate of 30g/s, 200µm droplet diameters, injection velocity of 7m/s, and spray half-angle of 30. The deduced test matrix is given in Table 3. However, some geometrical scaling criteria have to be relaxed. Further work will concern the quantification of these distortions using computational fluid dynamics calculations, in order to precise the coduitions of the proposed test matrix.

16 Table 3 : Resulting spray tests matrix for TOSQAN Initial gas mixture characteristics Spray characteristics Mixtur e T g ( ) P (bar) X s RH( %) X h T sat ( C) T g -T sat ( C) Q inj (g/s) T sat T inj ( C) T inj ( C) D (µm) Ref.101 A-S Ref. 102 A-S-H A-S A-S A-S A-S A-S A-S A-S-H A-S-H A-S A-S Scaling ratios of various experiments From the deduced scaling rules and scaling criteria, evaluation of the large scale facilities spray experiments regarding to these rules can be performed. Results are given in Table 4. Table 4 : Evaluation of the scaling ratios for several experiments. Real case TOSQAN CVTR (test 5) CSE (test A6) NUPEC (C1) gas conditions scaled 1:1 Temperature conditions colder (C2) 0.16 to (given) 1 (deducted) (C3) (C4) to (S1) 0.5 to 1.8 (from 0.6 to to to (bar/h) calculated sequences) (calculated) (Experimental) (Experimental) (Experimental) (S2) (s -1 ) to Considering the assumptions made to develop all the scaling rules and criteria, evaluation of the scaling cannot be done in order to respect the scaling ratios exactly, but to have the same order of magnitude. It can be seen that NUPEC and CSE experiments had the objective of covering fast all the horizontal surface of containment, whereas, CVTR tests seem to be more realistic (C2). For this point of view, these experiments are quite different from the other ones and from the real case. Concerning the total height of the vessel (C3), the CSE experiment seems to be not high enough regarding to the droplet size that was used. The injection height criteria (C4) is well respected in all experiments except for the CVTR case where it seems to be too low. Concerning the depressurisation rate (S1), the one of CVTR is quite high compared to the other ones. The entrainment scaling rule (S2) seems to be not respected in CSE (too low mixing, but this was probably better for aerosol capture experiments) and in NUPEC (too high mixing, observation also made by the experimental team).

17 As a conclusion, it can be seen that no experiment respect all scaling criteria proposed here. Furthermore, since a relatively low level of instrumentation is proposed in those facilities, experiments scaled with these scaling rules and performed in TOSQAN and in an other larger facility well instrumented would be of great importance. 6. CONCLUSION The H2TS method has been used in order to determine the conditions of the scaling analysis for containment sprays. Three relevant phenomena have been selected: depressurisation of the containment, gas mixing by entrainment, and light gas mixing. They have been described by simple equations leading to three scaling rules. Other scaling criteria, not associated to a transfer process, are also given. Assumptions made during this scaling analysis and distortions from the scaling have been presented. The approach used here is proposed for the definition of spray tests in model facilities (not only for TOSQAN experiments), but it does not pretend to be a scaling method of all phenomena occurring in the real containment during spray injection. Using the limited information available on real-spray containment systems, some characteristics of these sprays have been estimated/calculated, especially concerning the mean droplet size under real conditions and spray size and shape. Combined with the proposed scaling-rules, they lead to the determination of the TOSQAN experimental conditions for spray tests. Comparison of the scaling ratios obtained for the large-scale facilities was performed and show that the latter do not respect all the proposed scaling criteria. The applicability of the scaling rules and the validity of the associated approximations will be examined once the first spray test results from the TOSQAN facility will become available. Some of the scaling rules or scaling criteria can be evaluated in TOSQAN by changing some boundary conditions (especially for the spray characteristics); however, it may turn out that experiments in a test facility larger than TOSQAN are desirable to remove unwanted effects from scaling distortions.

18 7. NOMENCLATURE (Ai) : assumption/distortion (i=1 to 20) FN : flow-number of a nozzle (Ci) : scaling criteria (i=1 to 4) CD : drag coefficient Cp ini : initial specific heat Do : nozzle orifice diameter D32 : Sauter diameter D30 : volume diameter D20 : surface diameter D : droplet mean diameter D stop : particle stopping distance Fr : Froude number g : gravity h : height in a vessel H : total height of a vessel H inj : spray injection height H r : PWR considered height (dome) H ring : Nozzles ring height in a PWR H t : TOSQAN height L cond : latent heat of condensation M s : molar mass of water steam M a : molar mass of air m d : mass of a droplet (assumed to be spherical) m s : mass of steam m a : mass of air P : initial total pressure (before spray injection) P_fin : final total pressure (after spray injection) N nozzle : number of nozzle in a row of spray in a PWR Q inj : spray injection flow-rate Q cond : condensation flow-rate Q entr : gas entrainment flow-rate : radius of the spray envelope at height R h R pg Z h : perfect gas constant (8.31SI) Rs : perfect gas constant related to steam (Rpg/Ms) R d : ratios of the depressurisation rate dp/dt between model and prototype R e : spray envelope radius R max : maximum spray envelope radius R ring : inner spray ring radius in the real reactor Rep : Particle/droplet Reynolds number RH : relative humidity S : saturation, relative humidity (Si) : scaling rule (i=1 to 3) S spray : maximum horizontal surface of the spray S tot : total horizontal surface of the cylindrical part of the containment T g : intial gas mixture temperature (before spray injection) Tg_fin : final gas mixture temperature (after spray injection) T inj : water spray injection temperature Tsat : saturation temperature t : time t buoy : characteristic time for buoyancy t conv : characteristic time for forced/induced by the spray convection t inj : duration of the spray injection t res : particle residence time U conv : forced/induced by the spray convection velocity U inj : injection velocity at the nozzle exit U set : droplet terminal settling velocity V : total volume of a vessel V r : PWR total volume V t : TOSQAN total volume X s : steam molar fraction X h : helium molar fraction We inj : particle Weber number at the injection We inj_cr : critical particle Weber number at the injection Z h : height of a pure helium layer located on the top of the vessel α : spray half-angle P : pressure drop of a nozzle orifice ν g : gas cinematic viscosity µ w : water dynamic viscosity ρ g : gas mixture density ρ h : helium density ρ w : water density τ d : droplet relaxation time : water tension surface σ w

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