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The droplet size distributions in an industrial self-priming Venturi scrubber with liquid film injection have been investigated for varying gas flow rates and different submergence levels. For this purpose, imaging method analysis, commonly applied in spray measurements, was adopted and validated by means of synthetic data. The droplet size distributions characterized by probability density functions show a unimodal shape and can be satisfactorily represented by Nukiyama-Tanasawa distribution functions. For an increase in the gas flow rate, the cumulative distribution function shifts consistently to smaller equivalent droplet diameters. On the other hand, no statistically significant difference in the droplet size distribution was found for a change in the submergence level. The present results are compared with two droplet size correlations based on spray injection and commonly applied to Venturi scrubbers. Both correlations significantly overpredict the experimental results by a wide margin. In contrast, droplet size correlations developed for annular flows in straight tubes show a superior consensus in magnitude with the experimental data. The pronounced decreasing trend in droplet size with increasing gas superficial velocity, however, is weaker in our results, possibly due to the geometry and the self-priming characteristics of the Venturi scrubber., International Journal of Multiphase Flow, 142, ISSN:0301-9322, ISSN:1879-3533

The primary aims of the QUENCH-18 bundle test were to examine the oxidation of M5® claddings in air/steam mixture following a limited pre-oxidation in steam, and to achieve a long period of oxygen and steam starvations to promote interaction with the nitrogen. Additionally, the QUENCH-18 experiment investigated the effects of the presence of two Ag-In-Cd control rods, and two pressurized unheated rod simulators (6 MPa, He). The twenty low-pressurized heater rods (0.23 MPa, similar to the system pressure) were Kr-filled. In a first transient, the bundle was heated in an atmosphere of flowing argon and superheated steam by electrical power increase to the peak cladding temperature of 1400 K. During this heat-up, claddings of the two pressurized rods were burst at temperature of 1045 K. The attainment of 1400 K marked the start of the pre-oxidation stage to achieve a maximum cladding oxide layer thickness of about 80 µm. In the air ingress stage, the steam and argon flows were reduced, and air was injected. The first Ag-In-Cd aerosol release was registered at 1350 K and was dominated by Cd bearing aerosols. Later in the transient, a significant release of Ag was observed. A strong temperature escalation started in the middle of the air ingress stage. During the air ingress stage, a period of oxygen starvation occurred, which was followed by almost complete steam consumption and partial consumption of the nitrogen indicating formation of zirconium nitrides under oxygen starvation conditions. The temperatures continued to increase and stabilized at the melting temperature of Zr bearing materials until water injection. Almost immediately after the start of reflood there was a temperature excursion, leading to maximum measured temperatures of about 2430 K. Final quench was achieved after about 800 s. A significant quantity of hydrogen was generated during the reflood (238 g). Nitrogen release (>54 g) due to re-oxidation of nitrides was also registered. Residual zirconium nitrides were observed in the bundle middle. The metallographic investigations of the bundle show strong cladding oxidation and Zr melt formation. The Zr melt relocated downwards to the lower bundle part was strongly oxidized. Partially oxidized Zr-bearing melt was found down to elevation 160 mm; this elevation was the lowest with evidence of relocated pellet material. At the bundle bottom, only frozen metallic melt containing Zr, Ag, In and Cd was observed between several rods. The experiment exhibited a multiplicity of phenomena for which the data will be invaluable for code assessment and for indicating the direction of model improvements. Example of code application with SCDAPSim is given at the end of this paper.

The international EU-SGTR and ARTIST projects investigated the transport of fission products in the form of aerosols during SGTR severe accidents. The major finding of the two projects was that there was significant retention of aerosols in the steam generator secondary side, and that the retention was increased by more than an order of magnitude if the secondary side of the steam generator was flooded with water. Furthermore, the experiments with the flooded secondary side showed that the aerosol particle retention was significantly increased due to the presence of submerged structures, i.e., the tube bundle, as compared to an empty pool. The increased aerosol retention was attributed to the interactions of the high velocity gas jet discharged from the tube break with the dense bundle of the steam generator tubes. Under these conditions, the two-phase flow is very complex due to the high gas velocities and complicated geometry of the steam generator secondary side. To determine the effect of the tube bundle on aerosol retention, hydrodynamic characteristics of an empty pool and a flooded steam generator secondary side were measured in a facility equipped with wire-mesh sensors. The facility was equipped with either a tube bundle consisting of 221 steam generator tubes, or with a single tube in the center of the facility. The flow development could be followed by making the measurements at different distances between the gas injection and the measurement point, and using different flow rates. The facility was operated at close to ambient conditions. Void fraction, bubble size distributions, gas phase velocity as well as interfacial area concentration were determined based on the wire-mesh sensor data. In addition, the penetration depth of the initial large gas bubble into the channel was studied for the closest break-sensor distances. The investigations show distinct differences between the flow characteristics in the single tube geometry and in the tube bundle. As compared to the single tube, the flow was more confined in the tube bundle due to the interactions of the flow with the tubes. The interfacial area between the liquid and gas phase is larger with the tube bundle than with the single tube and also the bubble size distributions show distinct differences between the two geometries., Nuclear Engineering and Design, 366, ISSN:0029-5493, ISSN:1872-759X

In order to develop an iodine scrubbing model for nuclear severe accidents, experimental data on filtration efficiency of a wet scrubber for gaseous molecular iodine are collected to improve understanding of the basic mechanisms involved in iodine chemical scrubbing. A bubble column reactor 1.5 m high and 0.2 m in diameter equipped with an injection nozzle and a bubble breaker is used to investigate retention under relevant flow regimes. Scrubber is loaded with a scrubbing solution containing sodium hydroxide and sodium thiosulphate, no irradiation is provided. Retention efficiency is found to be strongly dependent on flow regime and on residence time as models predict. It was found that jet-like injection has a strong effect on filtration efficiency introducing a significant dependence on iodine initial concentration in gas phase believed to be due to nozzle hydrodynamics.

Much is still not known about the end-state of core materials in each of the units at Fukushima Daiichi Nuclear Power Station (Daiichi) that were operating on March 11, 2011. The Nuclear Energy Agency of the Organization for Economic Development has launched the Preparatory Study on Analysis of Fuel Debris (PreADES) project as a first step to reduce some of these uncertainties. As part of the PreADES Task 1, relevant information was reviewed to confirm the accuracy of graphical depictions of the debris endstates at the damaged Daiichi units, which provides a basis for suggesting future debris examinations. Two activities have been completed within the PreADES Task 1. First, relevant knowledge from severe accidents at the Three Mile Island Unit 2 and the Chernobyl Nuclear Power Plant Unit 4 was reviewed, along with results from prototypic tests and hot cell examinations, to glean insights that may inform future decommissioning activities at Daiichi. Second, the current debris endstate diagrams for the damaged reactors at Daiichi were reviewed to confirm that they incorporate relevant knowledge from plant observations and from severe accident code analyses of the BSAF (Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Station) 1 and 2 projects. This paper highlights Task 1 insights, which have the potential to not only inform future Decontamination and Decommissioning activities at Daiichi but also provide important perspectives for severe accident analyses and management, particularly regarding the long-term management of a damaged nuclear site following a severe accident.

Integral source-term analyses are performed using MELCOR for a PWR Station Blackout (SBO) sequence leading to induced steam generator tube rupture (SGTR). In the absence of any mitigation measures, such a sequence can result in a containment bypass where the radioactive materials can be released directly to the environment. In some SGTR scenarios flooding of the faulted SG secondary side with water can mitigate the accident escalation and also the release of aerosol-borne and volatile radioactive materials. Data on the efficiency of aerosol scrubbing in an SG tube bundle were obtained in the international ARTIST project. In this paper ARTIST data are used directly with parametric MELCOR analyses of a mitigated SGTR sequence to provide more realistic estimates of the releases to environment in such a type of scenario or similar. Comparison is made with predictions using the default scrubbing model in MELCOR, as a representative of the aerosol scrubbing models in current integral codes. Specifically, simulations are performed for an unmitigated sequence and 2 cases where the SG secondary was refilled at different times after the tube rupture. The results, reflecting the experimental observations from ARTIST, demonstrate enhanced aerosol retention in the highly turbulent two-phase flow conditions caused by the complex geometry of the SG secondary side. This effect is not captured by any of the models currently available. The underlying physics remains only partly understood, indicating need for further studies to support a more mechanistic treatment of the retention process.

Large Eddy Simulations (LES) have been conducted to investigate turbulent air flow and particulate depletion inside a cubical cavity of length 0.7 m and temperature difference of 39 K between the hot and cold opposing vertical walls, thus resulting in a Rayleigh number of 109. Initial confrontation of the idealized flow field against experimental data ( Kalilainen, Rantanen, Lind, Auvinen, & Dehbi, 2016 ) revealed that wall to wall radiation needs to be taken into account in order to reproduce the enhanced turbulence levels and reduced thermal stratification observed in the experiment. We introduce thereafter radiation effects indirectly by specifying measured temperatures rather than zero convective heat flux on the adiabatic walls. The LES predictions with realistic boundary conditions are in very good overall agreement with the measured velocity fields and temperature profiles. This accurate flow field is used to perform Lagrangian tracking simulations for spherical SiO2 particles with aerodynamic diameters of 1.4 μm and 3.5 μm. Here too, the computed particle depletion rates are in excellent agreement with the experimental data. Further Euler/Lagrange simulations are conducted for particles with hypothetical diameters in the range of 0.5 μm to 10 μm. Particles with diameters larger than 3 μm are removed at rates comparable to those predicted by the simple “stirred settling” model. However, as particle diameters decrease, the deposition rates are increasingly faster than predicted by stirred settling, and the decay constants tend towards an asymptotic value that is independent of particle size. Additionally, sensitivity computations show that thermophoresis has little effect on the removal rates of particles, but the inclusion of the thermophoretic force modifies the deposition pattern of sub-micron aerosols. The strong turbulent diffusion is thus the overriding cause for the significant deposition rates of smaller particles.

In experimental prototypes of pebble-bed reactors, significant quantities of graphite dust have been observed due to rubbing between pebbles as they flow through the core. At the typical operating conditions in these reactors, which feature high temperatures, pressures, and a helium atmosphere, limited data is available on the frictional properties of the pebble surfaces, and as a result, a conceptual design of a scaled-down version of a pebble-bed reactor has been proposed to investigate this issue in detail. However, this raises general questions about how the granular flow in a scaled facility will emulate that in a full-size reactor. To address this, simulations of granular flow in pebble-bed reactors using the discrete-element method (DEM) have been carried out in a full-size geometry (using 440,000 pebbles) and compared to those in geometries scaled down by factors of 3:1 and 6:1. Differences in velocity profiles, pebble ordering, pebble wear, and stresses are examined, and the effect of friction is discussed. The results show complex behavior due to discrete pebble packing effects, although several simple scaling rules can be derived.

The depletion dynamics of 1 µm and 2.5 µm SiO2 aerosol particles inside a differentially heated cavity was investigated experimentally using a cubical DIANA cavity with two opposing isothermal vertical walls and adiabatic top, bottom, front and back walls. The top, front and back walls are made of glass to allow optical access for different laser devices. The cavity atmosphere consisted of air and the isothermal wall temperatures were set to approximately 330.6 K and 291.3 K. The Rayleigh number of the flow was approximately 109, indicating turbulent conditions. The particle deposition rates were investigated by measuring the intensity of the reflected light from the particles and by using tapered element oscillating microbalance to measure the change in airborne particle mass concentration. The flow field in the mid-plane joining the isothermal walls was investigated using particle image velocimetry. Gas temperature measurements were collected using K-type thermocouple. The flow field and temperature measurements described turbulent flow near the isothermal and horizontal walls encircling the cavity stagnant core region with a stratified temperature distribution. Measurements indicated that the particle concentration at any time was approximately uniform throughout the cavity atmosphere. The measured depletion rate were compared to the theoretical “stirred settling” model predictions. While the decay rate of 2.5 μm particles was close to that predicted by the theoretical “stirred settling” model, it was found that 1 μm particles deposited two times faster than the theory predicted.

A steam generator tube rupture in a pressurized water reactor may cause accidental release of radioactive particles into the environment. Its specific significance is in its potential to bypass the containment thereby providing a direct pathway of the radioactivity from the primary circuit to the environment. Under certain severe accident scenarios, the steam generator bundle may be flooded with water. In addition, some severe accident management procedures are designed to minimize the release of radioactivity into the environment by flooding the defective steam generator secondary side with water when the steam generator has dried out. To extend our understanding of the particle retention phenomena in the flooded steam generator bundle, tests were conducted in the ARTIST and ARTIST II programs to determine the effect of different parameters on particle retention. The effects of particle type (spherical or agglomerate), particle size, gas mass flow rate, and the break submergence on particle retention were investigated. Results can be summarized as follows: increasing particle inertia was found to increase retention in the flooded bundle. Particle shape, i.e., agglomerate or spherical structure, did not affect retention significantly. Even with a very low submergence, 0.3 m above the tube break, significant aerosol retention took place underlining the importance of the jet–bundle interactions close to the tube break. Droplets were entrained from the water surface with high gas flow rates carrying aerosol particles with them. However, compared to particle retention in the water close to the tube break, the effect of droplet entrainment on particle transport was small.

A steam generator tube rupture (SGTR) in a pressurized water reactor (PWR) might be a major source of accidental release of radioactive aerosols into the environment during severe accident due to its potential to by-pass the reactor containment. In the ARTIST program, tests were carried out at flow conditions typical to SGTR events to determine the retention of dry aerosol particles inside a steam generator tube. The experiments with TiO2 agglomerates showed that for high velocities in the range of 100–350 m/s, the average particle size at the outlet of the tube was significantly smaller than at the inlet due to particle de-agglomeration. Earlier, particle de-agglomeration has not been considered significant in nuclear reactor severe accidents. However, the tests in ARTIST program have shown that there is a possibility that TiO2 aerosol particles de-agglomerate inside a tube and in the expansion zone after the tube exit under SGTR conditions. In this investigation, we measured TiO2 aerosol de-agglomeration in the tube with very high flow velocities with two different TiO2 aerosols. The de-agglomeration was determined by measuring the size of the agglomerates at the inlet and outlet of the test section. The test section was composed of tubes with three different lengths, 0.20, 2.0 and 4.0 m, followed by an expansion zone. The main results were: (i) the de-agglomerate process was relatively insensitive to the initial particle size distribution, (ii) the agglomerates were observed to de-agglomerate in all the tubes, and the resulting particle size distributions were similar for both TiO2 aerosols, (iii) at high flow rates, increasing the gas mass flow rate did not produce further de-agglomeration, and (iv) the agglomerates did not de-agglomerate to primary particles. Instead, after de-agglomeration the particles had a median outer diameter Dc = 1.1 μm. Based on analysis using computational fluid dynamics (CFDs), the de-agglomeration was caused by the turbulent shear stresses due to the fluid velocity difference across the agglomerates in the viscous subrange of turbulence. It has to be noted that the particles used in this investigation were TiO2 agglomerates, and not prototypical nuclear aerosols with significantly different characteristics. Therefore, the results of this investigation cannot be directly used to determine whether the nuclear aerosol particles may de-agglomerate in SGTR sequences. However, this investigation highlights the possibility of particle de-agglomeration under SGTR conditions, and identifies the mechanism of the de-agglomeration inside the broken tube and when the aerosol is discharged to an open space.

In order to determine penetration curve of the electrostatic precipitator (ESP) as a function of aerosol particle diameter in the range of 10–1000 nm measurement series were carried out in real scale power plant conditions. Differential mobility particle sizing (DMPS) system was used to measure the particle mobility distributions before and after ESP. MICRON -algorithm (constrained regularization) was used to invert mobility distribution to the corresponding number distributions. Penetration curve was calculated from the measured number distributions.

Aerosol formation in pulverized coal combustion have been studied experimentally at the real scale power plant. Combustion aerosol mass and number size distributions have been determined, when burning bituminous coal from Poland. Mass size distributions have been measured by low pressure impactor and number distributions by differential electrical mobility (DMA) method.

A steam generator tube rupture (SGTR) event with a stuck-open safety relief valve constitutes one of the most serious accident sequences in pressurized water reactors (PWRs) because it may create an open path for radioactive aerosol release into the environment. The release may be mitigated by the deposition of fission product particles on a steam generator's (SG's) dry tubes and structures or by scrubbing in the secondary coolant. However, the absence of empirical data, the complexity of the geometry, and the controlling processes have, until recently, made any quantification of retention difficult to justify. As a result, past risk assessment studies typically took little or no credit for aerosol retention in SGTR sequences. To provide these missing data, the Paul Scherrer Institute (PSI) initiated the Aerosol Trapping In Steam GeneraTor (ARTIST) Project, which aimed to thoroughly investigate various aspects of aerosol removal in the secondary side of a breached steam generator. Between 2003 and 2011, the PSI has led the ARTIST Project, which involved intense collaboration between nearly 20 international partners. This summary paper presents key findings of experimental and analytical work conducted at the PSI within the ARTIST program.

A steam-generator-tube rupture (SGTR) in a pressurized-water reactor is a design-basis accident with which plants have been designed to cope. Its specific significance is in its potential to bypass the containment thereby providing a pathway for radioactivity release to the environment. In combination with significant core damage, uncontrolled SGTR may lead to accidental release of radioactivity in the form of aerosol particles and gaseous compounds. In this paper, we describe the phenomena identification and ranking tables (PIRT) prepared as an international collaboration for uncontrolled SGTR in connection with core damage. This PIRT is limited to fission-product transport and retention in the steam generator and release to the auxiliary building, i.e., the source term. The phenomena affecting the fission-product retention in the steam generator and release to the environment were divided into those relevant in a dry steam-generator secondary side, and those relevant in a flooded secondary side. Rankings were given to 17 phenomena in the dry and 16 phenomena in the flooded secondary side regarding their: i) impact on the source term, ii) the availability of experimental data applicable to SGTR severe-accident conditions, and iii) the availability of codes/models to describe the phenomenon under the conditions of SGTR severe accidents. The SGTR boundary conditions and the considered phenomena are briefly described. The results are given in the form of ranking tables separately for the dry and flooded steam generator secondary side. The most important phenomena and their interactions with other phenomena are discussed with references to existing data and models. The most important uncertain phenomena in the dry steam generator scenario are particle bounce and fragmentation; whereas inertial impaction and retention by droplets in the jet region are the most uncertain ones in the flooded configuration. The need for improved knowledge is, though, tempered by overall high aerosol retention in the latter scenario. The applicability of the existing data and models for SGTR accident analysis is considered in the discussion.