Experimental evaluation of re-entrainment from wet scrubber filtered containment venting systems

Nuclear Engineering and Design 369 (2020) 110837

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Experimental evaluation of re-entrainment from wet scrubber filtered containment venting systems

T

L.S. Lebel , E. Lessard, K. Batten, T. Clouthier ⁎

Canadian Nuclear Laboratories, Chalk River, Ontario, Canada

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Pool scrubbing experiments Aerosol re-entrainment Phase Doppler anemometry Filtered containment venting systems

Wet scrubber-based filtered containment venting systems (FCVSs) are commonly deployed at nuclear power plants, owing to the high degree of aerosol, iodine, and organic iodine retention that is possible through the pool scrubbing action that takes place during their operation. However, after a FCVS has been operating during a severe accident, the pool water will become extremely contaminated with previously-captured radionuclides, and the bubbling action of gases passing through the pool, could result in the re-entrainment of contaminated liquid droplets into the gas stream. This would produce a secondary source term during the late phases of an accident. As such, a set of experiments have been conducted in order to evaluate the aerosol re-entrainment rates and size distribution from a system prototypical of a pool-type FCVS. Entrainment rates were measured by tracking the carry-over of a NaCl tracer into a set of liquid traps, while the size of the re-entrained droplets was measured with a phase Doppler anemometer. The experiment employed a prototypical venturi nozzle and gas injection conditions to examine the influence of gas flow rate, liquid level, and pool water temperature. Entrainment factors between 5× 10−5 and 1× 10−3 were measured, and the water droplet count median diameters were between 3 and 13 µm, and fairly wide particle size distributions, with mass mean diameters were on the order of 40–140 µm. These experimental results help the assessment of the potential magnitude of the late phase source term hazards posed by the fission products retained in pool-type FCVS (secondary release), and the potential loading on the secondary dry filter stage (metal fiber filters) employed in FCVSs after the primary pool scrubbing stage. The secondary metal fiber filters should have a high efficiency in capturing water droplets produced through this mechanism, given the droplet’s relatively large size.

1. Introduction Filtered containment venting systems (FCVS) offer a considerable degree of protection against radiological releases, and are an important part of severe accident management strategies in many jurisdictions

⁎

around the world (Jacquemain et al., 2014). Wet scrubber-based systems are a common type deployed at nuclear power plants, owing to the high degree of aerosol, iodine, and organic iodine retention that is possible through the pool scrubbing action that takes place during their operation. A considerable amount of research has been conducted

Corresponding author. E-mail address: [email protected] (L.S. Lebel).

https://doi.org/10.1016/j.nucengdes.2020.110837 Received 14 April 2020; Received in revised form 9 June 2020; Accepted 31 August 2020 Available online 18 September 2020 0029-5493/ Crown Copyright © 2020 Published by Elsevier B.V. All rights reserved.

Nuclear Engineering and Design 369 (2020) 110837

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comprehensive model. Other studies have analytically examined the fluid dynamics of the droplet formation phenomena (Duchemin et al., 2002; Lu and Xie, 2017; Reinke et al., 2001), and a number of experimental studies have been conducted that examined entrainment phenomena under different contexts (Cosandey et al., 2003; Günther et al., 2003; Zhang et al., 2017; Kim and No, 2005). An overall review of entrainment phenomena was also presented by Bagul et al. (2013). After bubbling out of the liquid pool, the gas will carry a certain quantity of liquid droplets along with it, and the ratio between the mass flow rate of liquid droplets, ml , and the mass flow rate of gas out of the pool, mg , is the entrainment factor, E , according to Eq. (1).

E=

ml mg

(1)

The main droplet formation mechanism for small droplets is the breakup of the film of bubbles at the surface of the pool (film droplets) (Cosandey et al., 2003; Kataoka and Ishii, 1984; Newitt et al., 1954), and it has been found that entrainment rates for these droplets increase with the superficial velocity of the gas, jg , through the system. The quantity of liquid that is entrained by the gas decreases quite strongly as a function of height above the liquid pool. Close to the interface between the pool and the gas headspace, liquid entrainment is very high because of the large amount of splashing and jet droplets that are produced. A lot of these larger droplets will reach a maximum height before falling back down into the pool. The upper limit of these maximum heights is the boundary of the “momentum controlled region”. Above this, the only droplets still present are those carried along in the gas flow (droplet terminal settling velocity less than the superficial velocity of the gas, jg ). This region is called the “deposition controlled region”. Further de-entrainment in this region occurs due to deposition on the walls of the vessel. The momentum controlled region has less practical interest to FCVSs because they are designed to be tall enough that the pool surface and droplet separators are sufficiently spaced that the larger droplets do not reach the separators. Thus, the finer film droplets are the only ones that are of concern to pass through the secondary droplet separators. Assuming Stokes flow around the droplets and a uniform gas velocity (Hinds, 1999), the critical droplet diameter, dcrit , where the particle settling velocity equals the superficial velocity is given by Eq. (2). This is an upper limit to the particle size in the deposition controlled region, as larger particles would eventually re-settle into the pool due to gravity.

Fig. 1. Typical wet scrubber FCVS, including pool scrubber stage and droplet separation stage, from Jacquemain et al. (2014).

assessing the decontamination factors for wet-scrubber FCVSs (Dong and Yang, 2019), and the technology continues to be an important topic internationally (Gupta et al., 2017). However, the bubbling action through the liquid pool also results in entrainment of water droplets as the gas bubbles break at the surface. After the FCVS has been operating during a severe accident, the pool water will be extremely contaminated with previously-captured radionuclides (Lebel et al., 2017), and so the re-entrainment of contaminated liquid droplets represents a significant potential secondary source term. This would be the case for both intermittent venting, where the system opens and closes after prescribed pressures, as well as continuous venting, where the system remains open for the duration of the accident. Events can occur late into the accident, such as the melt through of corium through the reactor pressure vessel/calandria vessel into a wet cavity, which can result in sudden increases in the steam generation rate that require added containment venting. This is the main reason that wet scrubber-type FCVSs have a secondary droplet separation and micro aerosol filtration stage, as shown in Fig. 1. The droplet separators capture re-entrained droplets and return the liquid to the pool. Their effectiveness depends on the quantity of re-entrained droplets, as well as the particle size distribution, as the separators will be less effective against very fine water droplet particulates. Assessing re-entrainment rates and droplet particle size distributions in FCVS-type systems is the primary topic of this study. Liquid drop entrainment has been an important consideration in the chemical industry and geophysical sciences for many decades, but it has also been important in the nuclear industry because of the role it plays as a mechanism for radiological particle release from water pools during accident conditions (Cosandey et al., 2003; Kataoka and Ishii, 1984). The fundamental mechanisms of droplet formation were described by Newitt et al. (1954), who observed two main sources of droplets. Film droplets, as shown in Fig. 2 a) to e), are generated when the thin membrane of water above the bubble bursts, forming very small droplets that are on the order of microns and tens of micron in diameter. Following this, jet droplets, Fig. 2 f) to h), are formed after the bubble bursts, water flows into the cavity left behind by the gas, rushes towards the centre and piles up in a jet. As the jet extends upward, part of the liquid near the apex will detach, forming droplets on the order of 100 to 1000 µm, substantially larger than the film droplets. Jet droplets, are much less of a concern in the context of FCVSs, because they are more likely to either splash back down into the liquid pool, or be easily captured in the droplet separator if they manage to be transported there by the gas flow. Film droplets, on the other hand, can be carried along with the gas much more easily due to their small size, making them of greater concern in FCVSs. Mechanistic descriptions of pool entrainment phenomena have been developed in the past, and Kataoka and Ishii (1984) present a

dcrit =

18jg µg g

(2)

l

Here, µg is the gas viscosity, g is the acceleration due to gravity, and is the liquid drop density. Kataoka and Ishii (1984), proposed correlations for the momentum controlled and deposition controlled regions. They express it in terms of dimensionless versions of the superficial gas velocity, jg , and height above the pool surface, h , where: l

jg =

h =

jg 4

g

/

2 g

(3)

h /g

(4)

In these expressions, is the liquid surface tension, g is gas density, and is the difference between the liquid and gas density. In the Kataoka and Ishii (1984) correlations, entrainment in the momentum controlled region is proportional to the ratio jg / h , and is also expressed in terms of a dimensionless viscosity, Nµg = µg / mensionless hydraulic diameter, Dh = Dh / 2

/g

.

g

/(g

) , di-

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Fig. 2. Mechanism of the burst of a bubble on the surface of the water, adapted from Newitt et al. (1954).

1.5 E = 2.21Nµg Dh 1.25

0.31

g

jg h

We =

(5)

l

g

1

jg 3 exp

0.205

h Dh

(6)

Entrainment varies continuously between these two regions, and the transition between the momentum and deposition controlled regions occurs at the dimensionless transition height, hcrit , according to:

hcrit

0.33 0.42 1.97 × 103N µg Dh

g

(8)

Here, Dinj is the diameter of the injection nozzle, and l is the surface tension of the liquid. The purpose of this study is to experimentally evaluate liquid reentrainment from gas injection into water pools with a prototypical venturi nozzle. The influence of the gas flow rate will be examined in order to characterize how re-entrainment increases with the average superficial gas velocity under these conditions. The effect of pool temperature will also be characterized to explore any potential influences when it is higher or lower than the gas injection temperature. The influence of liquid level will also be explored because it is anticipated that the superficial gas velocity will be less uniform near the pool surface, and more influenced by the hydrodynamics of the gas injection, when the water level is lower. The experiments are designed such that the gas injections are carried out with injection Weber numbers above and below the transition to the jet injection regime at We = 105.

This correlation holds, at least, in the “low” gas flux regime, where jg / h < 6.39 × 10 4 . In the deposition controlled regime, Kataoka and Ishii (1984) proposed the following correlation for entrainment, which is proportional to the cube of superficial velocity, but decays exponentially with respect to the height above the pool. 0.5 E = 7.13 × 10 4N µg

2 l Dinj vinj

0.23

(7)

2. Experiment details

The previous work discussed has considered bubbly flows where the gas flow rate is uniform across the surface of the pool, which is not the case in FCVSs. One of the unique considerations with FCVSs are that special types of nozzles are used to inject the gases into the pool at high speeds. Commercial FCVSs often use a proprietary nozzle design (usually variations on a venturi nozzle) to enhance contact between the liquid and gas phases, therefore increasing the possible decontamination factor of the pool (Albiol et al., 2018; Goel et al., 2018). The hydrodynamics and re-circulation in the pool are driven by the high velocity gas injection, and these will have a strong influence on liquid reentrainment because the flow patterns will influence the local superficial gas velocity and bubble size at the top of the pool. Near the injection site, there are two characteristic regimes that depend on the injection velocity, vinj : the globule regime and the jet regime. With the former, the bubbles that form at the nozzle periodically detach as they rise up, while in the latter, the gas penetrates as a jet much farther into the liquid before breaking up into smaller bubbles (Albiol et al., 2018; Herranz et al., 1997, 2018). The transition between the globule regime and the jet regime occurs when the injection Weber number is on the order of 105, where the Weber number, We , is defined as:

The experiments conducted in this study employ an injection of a steam/air mixture through an upward-facing venturi nozzle into a water pool. A generic venturi nozzle design, similar to the design presented in Goel et al. (2018), was used in the experiments. While the current experiments only employ a single venturi nozzle, commercial FCVSs will typically have banks of nozzles in order to increase the overall throughput. The apparatus was also constrained to be shorter than a commercial FCVS, which are often several meters high. The injection flow conditions were designed such that the Weber numbers span between about 104 to about 106, covering both the globule and jet injection regimes (transition from globule to jet at We = 105). This allows a range of hydrodynamic conditions within the pool to be studied. Droplet entrainment depends on the superficial gas velocity at the surface, so a range of superficial gas velocities (assuming uniform gas flow) from about 0.033 to 0.37 m/s were used in these experiments. The aerosol carry-over was quantified by measuring the salinity rise in a pair of liquid traps. The particle size distribution of the released droplets in the headspace above the pool was measured with a phase 3

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Fig. 3. Schematics of the experimental apparatus.

Doppler anemometer (PDA).

125 cm above the tip of the venturi nozzle. The liquid traps where the aerosol carry-over was captured each contained about 9 L of clean water (initial salinity less than ~5 mg/L). Gases from the second liquid trap were subsequently vented to the atmosphere. Omega model PX309 pressure transmitters were located at the top and bottom of each vessel to measure the water level. The volume of water added to the main vessel from steam condensation was tracked this way, which was an important metric for keeping track of the dilution of the initial concentration of NaCl over time from the initial concentration of 50 g/L. Thermocouples were used to measure the temperature of the liquid inside the main vessel and recirculation circuit. Pressure and temperature are also monitored just upstream of the venturi nozzle in order to record the thermal-hydraulic conditions of the gas injection. Videos of the bubble hydrodynamics in the main vessel were recorded with a high resolution camera.

2.1. Apparatus A schematic of the overall test apparatus is shown in Fig. 3(a). The aerosol resuspension occurs in the main pool scrubber vessel, which is a 19 cm diameter, 149 cm high glass tube. It contains between 15 and 26 L (depending on the water level tested) of a saline solution with an initial concentration of 50 g/L of NaCl. Flows of air and steam are combined through a mixing tee before being introduced into the main vessel through the Pease-Anthony type submerged venturi scrubber nozzle shown in Fig. 3(b). The tip of the venturi nozzle was 17 cm from bottom of the vessel. References to liquid height will typically be with respect the tip of the venturi nozzle in this paper, and not bottom of the vessel, unless otherwise noted. The steam generator was a Sussman MBA12 electric boiler, Swagelok VAF-M4 variable area flowmeters were used to measure the steam flow rate, and a Cole-Parmer EW32908 differential pressure mass flowmeter was used to measure the air flow rate. The main vessel was connected to a liquid recirculation circuit, composed of a pump, a chiller, and an in-line heater, to maintain a constant water temperature. Through the bubbling action between the liquid and gas, a mixture of air, steam, and suspended particles was carried through the top of the main vessel towards two in-series liquid traps, built out of 11-cm diameter, 94-cm high polycarbonate tubes. A tee was installed at the outlet of the main vessel to create two horizontal outlets. This allowed gases to carry the fine droplets (produced through film breakup mechanism) out, while preventing the larger splash droplets from transferring to the liquid traps, mimicking the conditions of an FCVS. The inlets to the tee had internal diameters of 34 mm, and their centerline was located

2.2. Aerosol carry-over measurements Re-entrainment rates were assessed by quantifying the flow rate of liquid droplet aerosols exiting the main vessel. The water in the main vessel contained between 30 and 50 g/L of NaCl in solution, and the transfer of aerosols could be quantified by tracking the transfer of the NaCl into the two downstream liquid traps. The NaCl concentration in the liquid traps can be recorded in real time, which allows for an online measurement of the aerosol carry-over. The droplets contain the same NaCl concentration as the water in the main vessel, and so the NaCl acts as an aerosol tracer to quantify the mass flow rate of liquid drops exiting the main vessel. NaCl is also useful as a tracer because it should behave analogously to soluble fission products (e.g., CsI, CsOH). The NaCl concentration in the liquid traps was monitored with 4

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Jenway 4510 conductivity meters. The values of interest include the initial salinity level in each vessel at the onset of the experiment (i.e., before injecting the gas mixture), and the rate of increase in the salinity in the liquid traps (and decrease in the main vessel) during the experiment. The rate of increase in the salinity (i.e., the slope of the absolute salinity plotted over time) was linear during the steady-state period of a given test condition. These rates, dcti /dt , in the two liquid traps are used to derive the aerosol carry-over rate, ml , as defined in Eq. (9).

ml =

l

cv

· Vt1

dct1 dct 2 + Vt 2 dt dt

(9)

Here, Vt1 and Vt2 are the volumes of water in the first and second liquid traps, respectively. The NaCl concentration in the main vessel, c v , was known by adding a known quantity of NaCl to water to start with a solution of about 50 g/L. The concentration would become diluted over the course of a test series by the steam condensate that accumulated in the main vessel, but this was tracked by performing a volume balance on the amount of water present. In the majority of tests, the second liquid trap did not experience a statistically-significant salinity increase, in which case the second term in the brackets in Eq. (8) could be dropped. The use of two liquid traps allows the capture efficiency of this method to be known, in order to reduce experimental uncertainty. A photo of the liquid traps in operation is shown in Fig. 4. Another important point of discussion is the sampling bias introduced by the tee at the outlet to the main vessel. As the goal of the experiments are to characterize entrainment of smaller particles produced through the film breakup mechanism, a system that could avoid the carry-over of large droplets was desired. The tee was oriented horizontally, by design, to prevent larger droplets from splashing directly into the outlet. All droplets that are adjacent to the tee openings can become sampled, but as it is anisokinetic, there is a degree of bias as a function of particle size. The sampling concentration ratio, C /Co , can be estimated according to Eq. (4) (Hinds, 1999), where us is the velocity entering the tees, is the particle relaxation time, and Ds is the (34 mm) diameter of tee inlet. This was computed for a range of vessel superficial velocities, and presented in Fig. 5. For large particles, there is a larger degree of sampling bias at higher values of jg , but there is limited sampling bias for particles below ~ 40 µm at all vessel superficial velocities.

C 1 = Co 1 + 0.62us / Ds

Fig. 5. Approximate particle sampling bias of the outlet tee as a function of particle size (assuming stationary particles next to the tee).

particle size distribution of the re-entrained water droplets in the head space above the liquid in the main vessel. It has the capability to measure particles in the 0.02 µm to 200 µm range, approximately. During the pool temperature study and pool height study (listed in Table 1), the PDA measurement location was 113 cm above the tip of the venturi nozzle, along the centre of the vessel. However, due to the larger degree of water swell and increased optical density in the headspace during the high gas velocity study, the measurement location had to be moved higher and closer to the vessel wall. For these, the measurements were done at a height of 121 cm above the tip of the venturi nozzle, and 2 cm from the vessel wall. The PDA measurements were triggered for one-minute durations when stable test conditions had been reached, and three such sets of measurements were taken at five minute intervals during the steady-state period of a given test condition. It should be noted that these measurements also included larger droplets produced through splashing. Many of these larger droplets would not have passed through the tee-junction at the outlet of the main vessel to be carried over to the first liquid trap and, as such, there is likely some degree of bias between the PDA and salinity measurement techniques that needs to be taken into consideration. A photo of the PDA setup is provided in Fig. 6.

(10)

2.3. Phase Doppler anemometer measurements

2.4. Test conditions

A Dantec Dynamics PDA system was employed to measure the

The parameters being evaluated in this study are i) the influence of the injection Weber number, Weinj , ii) the liquid height above a venturi nozzle, hl , iii) the pool temperature, Tl , and iv) the superficial gas velocity through the system, jg . A review of the literature on entrainment phenomena established that there is a strong dependence on the superficial gas velocity for liquid re-entrainment mechanisms, for a uniform gas flow rate across the surface of the pool (Kataoka and Ishii, 1984). However, in FCVSs, where the high velocity gas injections induce strong re-circulation patterns in the liquid, it is unclear whether there are regions of the pool surface that will have different local superficial gas velocities and, therefore, different re-entrainment rates. It is possible to study these factors by conducting a set of separate effects experiments, which vary the gas injection flow rate, Finj , the pool liquid height, hl , and the pool temperature, Tl . The injection Weber number depends on the venturi nozzle outlet diameter and the gas velocity, according to Eq. (8). Both the steam and air flow rates are recorded during the experiments, but it is unclear whether the steam was partially condensed after being mixed with the air. As such, the total volumetric flow rate of the gas injection, Finj , was calculated using

Fig. 4. Liquid traps in operation in the experiments. 5

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Table 1 Test conditions under evaluation in the three sub-studies.

Pool liquid temperature, °C Pool liquid level*, cm Gas inlet temperature, °C Gas injection rate, NL/min Injection Weber number Superficial gas velocity, m/s

Pool temperature study

Pool liquid level study

High gas velocity study

60–85 60 80 40–265 1.8× 104–2.4× 105 0.033–0.14

70 30–70 80 40–225 1.8× 104–1.7× 105 0.036–0.12

70 60 80 375–600 5.7× 105–1.2× 106 0.24–0.37

* Note: the pool liquid level is the collapsed liquid level, excluding swell from the bubble injection, and is with respect to the height above the tip of the venturi nozzle.

number is higher. Thus, separate effects tests exploring the effects of pool water level and flow velocity (affecting the Weber number) were performed in this study to provide information on the effects of nonuniformity in the superficial gas velocity. The experimental conditions that have been evaluated in this study are listed in Table 1. The experimental program was divided into three subsets in order to perform separate effects experiments of different parameters (pool temperature, pool liquid level, and gas velocity). The pool temperature study explores the influence of the thermal conditions in the pool on the aerosol re-entrainment. Pool temperatures that were between 20 °C colder and 5 °C hotter than the incoming gas flow were studied, as this changes the net amount of steam condensation experienced by the incoming gas. A range of different gas injection rates was employed, but all the tests were performed with a 60 cm pool liquid level and an 80 °C gas inlet temperature. The pool liquid level study explores the influence of the amount of water in the vessel on re-entrainment. Collapsed liquid levels (excluding the swell from the bubble injections) above the venturi nozzle between 30 and 70 cm were studied. It is during these tests, in particular, that influence of non-uniformity in the superficial gas velocity can be examined in more detail, as when the liquid level is lower, the influence of the high-velocity gas injection will be more pronounced. Higher liquid levels, on the other hand, give the bubble swarm more space for the internal flows to become more uniformly distributed across the cross-section. A range of different gas injection rates was employed, but all the tests were performed with a 70 °C pool temperature and an 80 °C gas inlet temperature. The high gas velocity study explores the influence of gas injections in the jet injection regime (We ≫105). This separate effects study is important because the highest superficial gas velocities and, therefore, highest rates of aerosol re-entrainment, are anticipated for these conditions. The influence of the high speed gas injection (i.e., high We ) is also anticipated to be quite significant, because of the much higher velocities involved. It should be noted, however, that these higher gas injection rates were also much more demanding on the apparatus because of the larger degree of pool swell, higher demands on the temperature regulation systems, and more limited capture efficiency in the liquid traps. As such, only a limited number of tests were carried out under these conditions. All the tests were performed with a 70 °C pool temperature, a 60 cm liquid level, and an 80 °C gas inlet temperature. The test conditions for all the separate effects studies were explored by running the experimental apparatus at steady state, with the gas flow rates and systems temperatures controlled at the desired operating points. The pool temperature was controlled by adjusting the degree of heating or cooling on the water recirculation loop from the main vessel, while the gas injection rate and temperature were controlled by adjusting the steam and air flows into the system. The water levels in the main vessel and the liquid traps were not actively controlled during the steady-state period, but were filled to near the desired conditions beforehand, and then monitored using the pressure difference between the bottom and top of the liquid pools. Salinities were also monitored, and the rate of the increase in salinity in each liquid trap was used to derive the aerosol carry-over rates using Eq. (9).

Fig. 6. View of PDA measurement location during the experiments.

the water saturation pressure at the temperature of the gas, P sat (Tg ) , the measured gas pressure, Pg , and the volumetric flow rate of air, Fair , as per Eq. (5). The gas injection velocity, vinj , could then be calculated from the cross-sectional area of the outlet of the venturi nozzle as per Eq. (6).

Finj =

1

Fair (P sat (Tg )/ Pg )

(11)

Finj

vinj =

2 /4· Dinj

(12)

The superficial gas velocity nominally depends on Finj but, more precisely, depends on the gas flow rate after it has come to thermal equilibrium with the pool water, Feq . In the experiments, an air/steam mixture is injected, but some of the steam in the gas condenses as it cools down (or some of the water evaporates as the gases heat up) to the temperature of the surrounding water, and it can be assumed that this process occurs fairly rapidly. The gas breaking the surface will then have an equilibrium flow rate, Feq , that depends on the volumetric flow rate of air and the pool temperature, according to Eq. (13). In this case, P sat (Tl ) is the vapour pressure of water at the temperature of the liquid pool, and Pv is the pressure in the main vessel.

Feq =

1

Fair (P sat (Tl )/ Pv )

(13)

A “nominal” superficial gas velocity can then be calculated from the cross-sectional area of the main vessel. However, the inherent assumption in this value is that the gas flow is uniform across the diameter of the vessel. This would only be true, though, if the plume of bubbles had enough space to spread out evenly, and if there were no influence on the flow conditions within the pool imparted by the high velocity gas injection from the venturi nozzle. It is not anticipated that this should be true at all in this system. The non-uniformity in superficial gas velocity is expected to be more prominent, though, when the influence of the injection nozzle is higher, such as when the Weber 6

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qualitatively described from the videos that were taken. The bubble swarms were not stationary, but moved around the vessel in a dynamic way owing to the high degree of turbulence in the system. At higher injection velocities, the dynamics within the system were stronger, and the liquid appeared to recirculate at higher velocities with a larger degree of turbulence. These observations qualitatively support the hypothesis that local superficial gas velocities in this situation can be different from the nominal “average” superficial gas velocity. The point injection of the gas at high speed induces fast-rising bubble swarms and recirculation patterns, and these would not be present to the same degree in other cases with a more uniform gas injection or boiling surface. Quantifying the bubble hydrodynamics within the pool was out of the scope of this study, but the higher aerosol entrainment rates that could result was directly measured. There was also a much larger degree of swell at the higher gas flow rates. The initial water level and the volume of water was the same in the two examples shown in Fig. 8, but there was about 10 cm of swell in Fig. 8(a) versus about 35 cm of swell in Fig. 8(b) with the higher Weber number. The swelled water level, hsw , could be visually estimated from videos taken of the bubble hydrodynamics during the gas injection. These are shown as the fractional increase over the collapsed water level, ho , in Fig. 9. Note that hsw and ho are both with respect to the bottom of the vessel in this ratio. The error bars are fairly large because the surface of the pool was quite turbulent, which made the visual estimation of its average position quite challenging. Another important factor to be considered is the distance between the surface of the pool and the outlet at the top of the vessel. Droplets above the critical diameter, dcrit , defined in Eq. (2), cannot be entrained in the gas indefinitely, but be entrained into the gas up to a certain height, due to the momentum that they have when they are ejected from the surface. This critical height was described, according to Kataoka and Ishii (1984), in Eq. (7). For the current experimental setup, the hcrit value is about 72 cm above the pool surface. Given the visual observations of the liquid surface height, the pool surface-outlet distances ranged from 21 cm to 92 cm, but the majority test conditions were conducted with the outlet in the momentum controlled region. However, as the swelled water height was not directly controlled in these experiments, there is considerable experimental uncertainty associated with the pool surface-outlet distances.

Different test conditions were often performed one after the other, in sequence over a single day, and the 35 different test conditions reported herein were carried out over seven days of testing. The system was held at a steady state for approximately 10–15 min during each test condition, which was long enough to observe a clear positive trend in the salinity measurements in the liquid traps, and acquire a sufficiently large amount of data on the exact operating points of the other system conditions. Three, one-minute long acquisitions of the PDA measurements were conducted at each test condition as well, one near the beginning, one near the middle, and one near the end of the 10–15 min steady-state period. 3. Results The following sections present the experimental results. The flow conditions that were considered in the experiments are discussed first, including the range of superficial velocities/injection Weber numbers that were studied, and a qualitative description of the observed bubble hydrodynamics. The entrainment factors that were observed as a result of the NaCl carry-over measurements are presented next. Finally, the observations on the droplet particle sizes in the headspace above the pool, as measured with the PDA, are given. 3.1. Flow conditions As shown in Fig. 7, the flow conditions that were studied in these experiments covered a wide range of superficial velocities and injection Weber numbers. Both of these factors depend on the gas injection rates, but the relationship also changes based on the pool water temperature. At thermal equilibrium, the gas exiting the pool will have a smaller water vapour content when the pool temperature is lower and, therefore, the equilibrium flow rate and superficial velocity will be smaller, even though the gas injection flow rate and Weber number might be the same. The error bars on the data in Fig. 7 were estimated from the uncertainties associated with the air flow rate measurement, pressure measurements, and temperature measurements used to derive these values in Eqs. (11) and (13). Fig. 8 shows photos of the bubble hydrodynamic conditions within the liquid pool in the main vessel at two different injection Weber numbers, with the same initial water level. In both cases, the flow within the liquid pool appeared quite turbulent, with swarms of bubbles rising to the surface quite quickly, and counter-current flows of water recirculating down to the bottom of the vessel. It is not possible to quantify any of the internal velocities, but the general behaviour can be

3.2. Aerosol entrainment rates Aerosol carry-over rates from the main vessel to the liquid traps was quantified by monitoring the salinity increases in the liquid traps. The value of ml , according to Eq. (9), can be calculated from the slope of the salinity increase over time, along with the known liquid volume in the liquid traps and salinity level in the main vessel. The mass flow rate of steam and air, mg , leaving the water pool is obtained from the estimated equilibrium volumetric flow rate, Feq , (Eq. (13)) and the density of the air/steam mixture at that temperature. The ratio of these two mass flow rates give the entrainment factor, as per Eq. (1). The aerosol entrainment results from the pool temperature, pool liquid level, and high gas velocity sub-studies are shown as a function of the nominal superficial velocity in Fig. 10. The error bars on the entrainment factor are a function of the estimated uncertainty of the salinity increase measurements in the liquid traps, in addition to the uncertainties of the main vessel salinity, the liquid trap water volumes, and the equilibrium gas flow rate out of the pool. Fig. 10(a) shows the results from the pool temperature sub-study, where all the tests were carried out with the same pool liquid levels and gas injection temperatures (~60 cm and ~ 80 °C, respectively), but where a number of different pool temperatures and superficial velocities were explored. Overall, there was a clear increase in the aerosol carry-over rate as the superficial gas velocity increased. It is more difficult, however, to make any clear conclusions on the effect of the pool

Fig. 7. The nominal superficial velocities and injection Weber numbers for the flow conditions studied in the experiments. 7

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Fig. 8. Bubble hydrodynamics in the main vessel induced by the gas injection.

Fig. 9. Fractional water swell, city.

(

hsw ho

height is less clear, however, because the results that were obtained at similar flow conditions often varied within the bounds of the experimental uncertainties (i.e., overlapping error bars), thus no general conclusions were possible. This is contrary to what would be expected from the Kataoka and Ishii (1984) correlations in Eqs. (5) and (6). Changing the initial water level also changes the distance between the pool surface and the outlet tee, and reducing the initial water level should also lead to lower aerosol concentrations at the height of the outlet. Lower water levels also mean that the pool surface is closer to the jet of gas exiting the venturi nozzle, and that the superficial velocities at the pool surface would be less uniform, with more substantial spikes in the local velocity. It appears from the data that these two influences counterbalance each other. Fig. 10(c) shows the results from the sub-study on higher gas injection velocities. These were much more challenging conditions for the apparatus and, therefore, only a limited number of test were carried out. Some modifications had to be made to the apparatus, including the installation of flowmeters and pressure transmitters with higher measurement ranges. Due to the larger degree of swell in the vessel, the PDA measurements had to be carried out at a higher location in the headspace of the main vessel. The larger degree of swell in the liquid traps also required that a lower volume of water (initial water level) be used, and these were the only tests where salinity increases were observed in the secondary liquid trap, in addition to the primary liquid trap. All the tests were conducted with the same volume of water in the main vessel (~60 cm collapsed level), pool temperature (~70 °C), and gas injection temperature (~80 °C). A very clear increase was observed in the entrainment factor for higher superficial velocities, with a maximum value of about 10−3 being obtained in the experimental program. There are other experimental studies available in the literature that

)

1 , as a function of superficial gas velo-

temperature, as any potential trends are not much greater than the experimental uncertainty on the entrainment factor measurements. Fig. 10(b) shows the results from the pool liquid level sub-study, where all the tests were carried out with the same pool liquid and gas injection temperatures (~70 °C and ~ 80 °C, respectively), but where a number of different pool liquid levels and superficial gas velocities were explored. Once again, there was an increase in the entrainment factor for higher nominal superficial velocities. The influence of the pool 8

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Fig. 10. Entrainment factors as a function of superficial gas velocity.

can be compared to the tests conducted in this study. Cosandey et al. (2003) compared their experimental results with a number of other previous studies on droplet entrainment from boiler water pools and air/steam mixtures (Garner et al., 1954; Bunz et al., 1992; Kudo et al., 1994; Muller, 1997). The Cosandey et al. (2003) study considered cases where the superficial gas velocity was below 0.1 m/s, as did most of the other studies, and they measured entrainment factors between 4×10−6 and 3×10−4. Garner et al. (1954) considered much higher superficial velocities, and also obtained higher entrainment factors, on the order of 6×10−5 to about 1×10−3 from their experiments that bubbled steam through a water pool in an evaporator apparatus. The studies by Bunz et al. (1992), Kudo et al. (1994), and Muller (1997) considered re-entrainment from boiling water pools (either through direct heating, or depressurization-induced flashing), which is why the superficial velocities were smaller. Cosandey et al. (2003) also considered a boiling pool, but bubbled air into the pool as well in some of their tests. These literature results, which involve entrainment factors obtained from experiments with more uniform bubbly flows, are compared to the results from this study in Fig. 11. At the lower superficial gas velocities that were considered in this study, the results overlap with those from previous literature on boiling or flashing water pools, where the nominal superficial velocities were similar. There is a lot of scatter in the literature data in this regime,

Fig. 11. Comparison of the entrainment factor results from this study, as a function of the nominal superficial gas velocity, with literature data from Cosandey et al. (2003), Garner et al. (1954), Bunz et al. (1992), Kudo et al. (1994), and Muller (1997). 9

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however. Cosandey et al. (2003) attributed this to differences in the experimental setup and diameter of the pool, relative to the diameter of the vessel. It is also possible that there was some bias from how the aerosol sampling measurements were carried out. However, at higher superficial velocities the entrainment factors were much larger than the general trend from other studies, and are about an order of magnitude higher than the Garner et al. (1954) results where the superficial velocities overlapped. The Garner et al. (1954) study employed a 0.3 m diameter vessel, and conducted measurements between 0.5 m and 1.0 m above the pool surface, from a horizontal outlet, and employed a condenser to capture the salt-laden water droplets. The hcrit transition height between the momentum and deposition controlled regions was approximately 0.8 m for their tests. The experimental setup was fairly comparable with the one in this paper, except that they injected a pure steam flow into a downcomer, which would have resulted in a much more uniform bubble rise and superficial gas velocity across the crosssection of the vessel. The experimental study herein allows for nonuniform gas flow rates across the surface of the pool more so than the previous studies from literature. This lends credence to the hypothesis that the strong injection velocities result in flow patterns and local bubble velocities that exceed the nominal superficial gas velocity, resulting in higher overall entrainment factors. A follow up study that examines the pool and gas space hydrodynamics should be performed in order to conclude this more definitively.

two statistics were calculated from the measured particle sizes based on Eqs. (14) and (15) (Hinds, 1999). In both cases, the error bars on these values was determined from the standard deviation of the three repeat measurements that were taken during each test condition. The plots also compare the size distribution statistics to the critical droplet diameter, dcrit , of the deposition controlled region (Fig. 14).

dg = exp

dmm =

lndi N

mi di = mi

(14)

di4 di3

(15)

It is difficult to discern any systematic trends in the data with respect to nominal superficial gas velocity, water temperature, or liquid level. At the higher flow conditions, owing to the droplet entrainment factors, the total particle count rates were much higher in the PDA observation, meaning that there is a larger degree of statistical significance in the reported count median diameter values. Comparing to literature, the observed particle size distributions and values of the count median diameter from this study are consistent with the droplet sizes reported in Cosandey et al. (2003). The reason that there is such a large difference between the count median and mass mean diameters is because the particle size distributions were very broad. The geometric standard deviations are not shown, but were typically between 2 and 4. The measured mass mean diameters were often greater than the critical diameter, but this is as expected given that the PDA measurements were taken from within the momentum controlled zone. The particle size distributions are broad because there was a mix of film droplets and jet droplets. Given estimated sampling bias of the outlet tee (Fig. 5), it is likely that there would have been a bias toward small particles in the aerosol entrainment rate measurements reported in the previous section. However, the sampling bias mostly affects particles that are larger than the critical diameter. Conducting future experiments in a taller vessel would not drastically affect the conclusions about the entrainment of the smaller film mechanism droplets.

3.3. Particle size Particle size measurements were carried out using the PDA system, with the measurement volume located in the headspace of the main vessel above the liquid pool. It was located high enough that it was not affected by the water swell, and was outside of the range of most of the splashing. Because the PDA measurements were at a fixed location in the vessel, the distance between the pool interface and the measurement varied from as little as 17 cm to as much as 80 cm. Therefore, the majority of measurements would have been conducted in the momentum controlled regime above the pool. The measurements recorded the diameter of each individual droplet passing through the measurement volume. As stated before, the data acquisition with the PDA was activated for one minute at a time, and these acquisition periods were repeated three times per test condition. Typical particle size distributions are given in Fig. 12. The count median diameters were on the order of 3–13 µm, as shown in Fig. 13, while the mass mean diameters were on the order of 40–140 µm. These

4. Discussion The main reason for conducting this study was to assess the potential for the re-entrainment of radionuclides in a pool scrubber-type FCVS. Droplet re-entrainment is a known phenomenon, and many of these types of FCVS are fitted with secondary droplet separation stages

Fig. 12. Typical particle size distribution histograms. 10

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Fig. 13. Particle count median diameters as a function of superficial gas velocity.

for precisely this reason. Normally, these are metal fiber filters capable of capturing larger droplets. The first outstanding question that this study aimed to answer was whether the high velocity injection from a venturi nozzle would have an impact on the liquid drop entrainment rates, as compared to previous studies that considered boiling pools or other configurations where the bubble flows were more uniform. This study has shown conclusively that point-source gas injections do lead to much higher entrainment factors. Entrainment factors observed in this study were as much as an order of magnitude higher than those in the literature (e.g., Garner et al. (1954), where gas was bubbling up fairly uniformly across the cross-section of the pool). Assessing the bubble hydrodynamics in the pool, as well as the gas velocity profiles in the headspace above the pool, is a topic of future study as it would provide further evidence to verify the general conclusions made in this study. The second outstanding question was whether radionuclide re-entrainment might be a significant late-phase source term when employed in a FCVS during an accident, and whether the size of the droplets would be small enough that they would not be effectively captured by the secondary droplet separators. A FCVS that is used during an accident would capture a large fraction of what would otherwise be released to the environment and, as such, might contain on the order of 1018 Bq of radioiodine species, or 1016 Bq of radiocesium species, along with other aerosols, as in the study by Lebel et al. (2017) that analyzed

the operation of a FCVS during an unmitigated station blackout at a CANDU-type nuclear power plant. That study showed that, even late into the accident, containment would have to be periodically vented. Each of these venting periods would require gas flow rates on the order of 10 kg/s, for durations of about an hour at a time. For an entrainment factor of 10−3, the droplet loading on the secondary filters could be around 0.01 kg/s. This would be in line with releases of about 0.5% of the radionuclide inventory that is captured in the pool, per hour-long venting period (depending on the total volume of water in the in FCVS pool, and whether the water is periodically refreshed). However, given the relatively large size of the film droplets that are generated through this re-entrainment mechanism, it is expected that the majority of this would be captured in the secondary filters. Observed count median diameters were > 3 µm, fairly wide particle size distributions were observed, and mass median diameters were > 40 µm. The aerosols would of course shrink if 100% humidity in the FCVS were not maintained, due to evaporation of the water. This is not anticipated to be a problem in a wet scrubber-based FCVS, however. The efficiency of the secondary filters as a function of particle size is design-specific, but high capture efficiencies for particles in this size range would be anticipated. Droplets that would pass through the secondary droplet separators would shrink substantially once released to the environment as they come into equilibrium with the outside humidity. The majority of the water would evaporate, leaving the residual dissolved material. 11

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Fig. 14. Particle mass mean diameters as a function of superficial gas velocity.

The results of this study can be applied to the re-entrainment soluble fission products (e.g., CsI, CsOH) from the pool of a wet scrubber-type FCVS, as these would likely behave as passive tracers in the water droplets, as NaCl did in this study. Re-entrainment of I2 and organic iodine species would be an entirely different question, as it is subject to interfacial mass transfer process and water chemistry in the pool. With that being said, the dissolved components of the iodine species would still be subject to the same droplet entrainment processes as soluble fission products. Insoluble particles were not explored in this study, but they are important in FCVS re-entrainment, as important fission product species, like TeO2, are insoluble (Dickson and Glowa, 2019). Insoluble particles suspended in the pool can affect the bubble breakup at the pool surface and sedimentation processes in the gas space, and these are usually affected by the size of the suspended particles. Re-entrainment of insoluble particles was explored in Cosandey et al. (2003).

action as gases pass through the pool could act to re-entrain contaminated liquid droplets into the gas stream and, therefore, operating a FCVS late in an accident could potentially cause a secondary release of previously captured radionuclides. The magnitude of this secondary release, however, would depend on the magnitude of the liquid droplet entrainment factors from the bubble burst mechanisms at the surface of the pool, as well as the ability of the secondary filters that are installed in FCVSs to handle the droplet load. The experimental study herein was conducted in order to quantify the droplet entrainment factors that would be expected in a prototypical FCVS, and also to quantify the sizes of the water droplets that would be released as a result. Entrainment factors between 5×10−5 and 1×10−3 were observed in this study, which is higher than what was observed in similar experiments that studied either boiling water or more uniformly distributed gas injections. The high velocity gas injection from the venturi nozzle resulted in strong recirculation patterns within the liquid pool, and it can be inferred that this led to much higher local superficial gas velocities. The more non-uniform gas flow rates across the surface of the pool seen in this study, in turn, led to higher overall entrainment factors. Even though the entrainment factors were higher than anticipated, the size of the droplets that were produced suggests that they can be effectively captured in the secondary metal fiber filters in FCVSs. The entrained droplets were found to have count median diameters between

5. Conclusion Filtered containment venting systems serve an important role in mitigating the potential release of radionuclides into the environment during a severe accident. Wet scrubber-based systems are employed at many nuclear power plants throughout the world. They accumulate radionuclides in a liquid pool and, as a result, this pool water would be extremely contaminated after several cycles of operation. The bubbling 12

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3 and 13 µm, and fairly wide particle size distributions, with mass mean diameters were on the order of 40–140 µm. The efficiency of the secondary metal fiber filters should be high with respect to these larger particles, which should serve to reduce potential late phase secondary release of radionuclides from wet scrubber-based FCVSs. An important future topic of study that could compliment this work would be a more detailed experimental study of the bubble hydrodynamics and flow conditions in the gas headspace. Non-uniformity in the flow is the major hypothesis for the higher than expected entrainment factors. Studies on the re-entrainment of iodine species and insoluble particles would be other topics that can be considered in the future.

Reactor Safety Program, Report EUR 14635 EN, 1992. Cosandey, J.O., Günther, A., von Rohr, P.R., 2003. Transport of salts and micron-sized particles entrained from a boiling water pool. Exp. Therm Fluid Sci. 27 (8), 877–889. Dickson, R.S., Glowa, G., 2019. Tellurium behaviour in the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 204, 49–65. Dong, S., Yang, J., 2019. Overview of the experimental studies and numerical simulations on the filtered containment venting systems with wet scrubbers. Ann. Nucl. Energy 132, 461–485. Duchemin, L., Popinet, S., Josserand, C., Zaleski, S., 2002. Jet formation in bubbles bursting at a free surface. Phys. Fluids 14 (9), 3000–3008. Garner, F.H., Ellis, S.R., Lacey, J.A., 1954. The size distribution and entrainment of droplets. Trans. Inst. Chem. Eng. 32, 222–235. Goel, P., Moharana, A., Nayak, A.K., 2018. Measurement of scrubbing behaviour of simulated radionuclide in a submerged venturi scrubber. Nucl. Eng. Des. 327, 92–99. Günther, A., Wälchli, S., von Rohr, P.R., 2003. Droplet production from disintegrating bubbles at water surfaces: Single vs. multiple bubbles. Int. J. Multiph. Flow 29 (5), 795–811. Gupta, S., Herranz, L.E., Van Dorsselaere, J.P., 2017. Integration of Pool scrubbing Research to Enhance Source-term Calculations (IPRESCA). 8th European Review Meeting on Severe Accident Research. Herranz, L.E., Peyrés, V., Polo, J., Escudero, M.J., Espigares, M.M., López-Jiménez, J., 1997. Experimental and analytical study on pool scrubbing under jet injection regime. Nucl. Technol. 120 (2), 95–109. Herranz, L.E., Iglesias, R., Fontanet, J., 2018. Mitigation of source term in suppression pools: Large uncertainties in predictability. Ann. Nucl. Energy 120, 509–515. W.C. Hinds, Aerosol Technology, Wiley-Interscience, 2nd ed., 1999. D. Jacquemain, S. Guentay, S. Basu, M. Sonnenkalb, L. Lebel, J. Ball, H-J. Allelein, M. Liebana, B. Eckardt, N. Losch, and L. Ammirabile, “Status report on filtered containment venting”, Organisation for Economic Co-Operation and Development report NEA-CSNI-R-2014-7, 2014. Kataoka, I., Ishii, M., 1984. Mechanistic modeling of pool entrainment phenomenon. Int. J. Heat Mass Transf. 27 (11), 1999–2014. Kim, C.H., No, H.C., 2005. Liquid entrainment and off-take through the break at the top of a vessel. Nucl. Eng. Des. 235 (16), 1675–1685. Kudo, T., Yamano, N., Moriyama, K., Maruyama, Y., Sugimoto, J., 1994. Experimental study of aerosol re-entrainment from flashing pool in ALPHA program. Proceedings of the 3rd International Conference on Vessel Design and Operation. Lebel, L.S., Morreale, A.C., Korolevych, V., Brown, M.J., Gyepi-Garbrah, S., 2017. Severe accident consequence mitigation by filtered containment venting at Canadian nuclear power plants. Ann. Nucl. Energy 102, 297–308. Lu, M., Xie, H., 2017. An investigation of pool entrainment based on the method of Volume of Fluid. Nucl. Eng. Des. 318, 72–84. M. Muller, “Re-entrainment von Aerosolen wahrend der gefilterten Druckentlastung nach einem schweren Kernschmelzunfall”, Ph.D. thesis ETH No. 12138, Zurich, Switzerland, 1997. Newitt, D.M., Dombrowski, N., Knelman, F.H., 1954. Liquid entrainment: the mechanism of drop formation from gas vapour bubbles. Trans. Inst. Chem. Eng. 32, 244–261. Reinke, N., Vossnacke, A., Schütz, W., Koch, M.K., Unger, H., 2001. Aerosol generation by bubble collapse at ocean surfaces. Water Air Soil Pollut. Focus 1, 333–340. Zhang, P., Li, W., Di, Z., Hu, X., Chen, L., Chang, H., Chen, P., 2017. An experimental study of pool entrainment with side exit. Ann. Nucl. Energy 110, 406–411.

CRediT authorship contribution statement L.S. Lebel: Conceptualization, Methodology, Investigation, Visualization, Supervision. E. Lessard: Investigation, Visualization. K. Batten: Resources, Investigation. T. Clouthier: Resources, Investigation. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Funding for this work was provided by the Atomic Energy of Canada Limited under the auspices of their Federal Nuclear Science and Technology Program. References Albiol, T., Herranz, L.E., Riera, E., Dalibart, C., Lind, T., Del Corno, A., Kärkelä, T., Losch, N., Azambre, B., Mun, C., Cantrel, L., 2018. Main results of the European PASSAM project on severe accident source term mitigation. Ann. Nucl. Energy 116, 42–56. Bagul, R.K., Pilkhwal, D.S., Vijayan, P.K., Joshi, J.B., 2013. Entrainment phenomenon in gas–liquid two-phase flow: a review. Sadhana 38 (6), 1173–1217. H. Bunz, M. Koyro, B. Propheter, W. Schock, and M. Wagner-Ambs, “Resuspension of fission products from sump water”, Commission of the European Communities

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