Sodium Oxide Aerosols Research Articles

Experimental investigation on sodium leak behaviour through a pinhole

Sodium is most preferred coolant in Fast Breeder Reactors (FBR) due to its favourable neutronic and excellent heat transfer properties. However, when exposed to atmosphere, sodium reacts with oxygen and moisture resulting in sodium fire leading to rise in ambient temperature and release of harmful sodium oxide aerosols. Owing to this, prevention and early detection of sodium leak is of highest priority in the design of sodium systems. In all sodium systems, wire type leak detectors are provided for early detection of sodium leak. However, sodium leak from small defects such as hairline crack and pinholes may lead to oxidation of sodium and deposition of reaction products plugging the leak path. Such leaks may go undetected, cause local corrosion of structural material and may lead to sudden failure. Hence, investigation on sodium leaks from small openings is vital for sodium safety. Series of experiments were carried out to understand such sodium leak scenario through a small aperture simulating a pinhole defect. Sodium plugging phenomenon was studied at various oxygen concentrations from 0.5% to 3% in the leak zone, as a function of driving pressure. Conditions for unplugging were also established by varying sodium temperature and driving pressure, for plugged cases.

Development of a New Method for Simulating Gas-to-Particle Conversion During Sodium Pool Fires in SFR Containment

Sodium combustion oxide aerosols are the main carriers of radioactive materials in a sodium-cooled fast reactor (SFR) during sodium fire accidents. Therefore, it is of great significance to simulate aerosol behavior in sodium pool fires to evaluate radioactive source terms in the containment or environment. In this work, a numerical method has been developed to simulate sodium oxide aerosol behavior during sodium pool fires. The Classical Nucleation Theory has been taken into account to simulate gas-to-particle conversion (GPC). The model has been evaluated theoretically in 280 cases with three main parameters: sodium pool temperature, pool diameter, and oxygen concentration. The correlation established by fitting data points is associated with the sodium evaporation rate. The SFA code has been developed based on advanced sodium pool combustion and aerosol models coupled with GPC correlations. In comparison with the experimental data, the code-calculated average atmospheric temperature, airborne aerosol concentration, and particle size are in good agreement with the data, which indicate that the method is reliable and can be applied in code development in the future.

Three-dimensional computational fluid dynamics modelling of sodium oxide aerosol atmospheric dispersion from indoor sodium fire

In order to estimate the atmospheric impact of the release of sodium oxide aerosols from an indoor sodium fire, two chemical models describing the evolution of sodium oxide aerosols have been implemented in the atmospheric version of the computational fluid dynamic model Code_Saturne. The first chemical model is a shrinking core model. It describes the hydration and transformation of sodium oxide particles to carbonate by assuming that particles are made of layers of different compositions (sodium oxide, sodium hydroxide and carbonate). In the second model, a reactive absorption model describes more accurately the carbonate formation from sodium hydroxide. Both model implementations favorably compare to the published results produced by the model authors, as well as to experimental data resulting from experiments in laboratory. They differ in the speed of carbonate formation, and notably in the atmospheric humidity dependence of this formation. The second model may be more adapted to relative humidity larger than 50% where the core of particles is liquid. The atmospheric dispersion of sodium aerosols is modelled with Code_Saturne by assuming a release particle size distribution and by taking into account the sedimentation and deposition processes. The differences between the shrinking core model and the reactive absorption model largely influence the composition of sodium particles during their dispersion in the atmosphere. Further comparisons to experimental measurements of sodium oxide in the real atmosphere are required to evaluate the accuracy of these models at determining atmospheric sodium carbonate concentrations occurring from the transformation of sodium oxides.

Modeling of atmospheric dispersion of sodium fire aerosols for environmental impact analysis during accidental leaks

In this study a chemical kinetic model which converts sodium oxide aerosols to sodium carbonate aerosols as a function of CO2 and humidity is implemented in the FLEXPART atmospheric dispersion model for impact assessment during sodium fire accidents in sodium cooled fast reactors. The model is validated with open environment aerosol dispersion experiments conducted at Kalpakkam coastal site. Simulated total aerosol concentrations and deposition agree with experimental data. Simulations indicate rapid conversion of NaOH to Na2CO3 during atmospheric transport under high humidity levels (57–80%) at Kalpakkam. Parametric tests with different particle size indicated that a particle diameter of 5 μm produced realistic aerosol deposition closely comparing with observations. A series of simulations were conducted for various meteorological conditions to assess the chemical impact in a range of 25 km during a postulated accident. Simulations for extended 8-h release scenarios in different seasonal flow conditions indicate the concentrations beyond 1.5 km fall to very low values (≤0.1 mg/m3 for NaOH and ≤4 mg/m3 for Na2CO3) indicating no chemical impact. Short-duration (1 h) releases during worst environmental conditions (sea breeze, low winds) indicate that the aerosol concentrations during sea breeze are about 4–5 times high compared to winter time calm winds due to wide diffusion of aerosols in winter. The concentrations in these two cases are found to be higher (∼0.2 mg/m3 for NaOH and 6 mg/m3 for Na2CO3) in the site distance range but thereafter rapidly fall, thus indicating no inhalation hazard during sodium fire scenarios at the coastal site.

A lumped parameter modelling of particle generation from Na-pool fires in SFR containments

Modelling of sodium-evaporation and formation of sodium-oxide aerosols from a sodium-pool fire is of fundamental importance for the assessing of the radiological consequences in Sodium-cooled Fast Reactors severe accidents. This paper summarizes the derivation of a simple model to estimate the amount and size of particles being generated from Na-pool fires and its performance assessment, once implemented in an integral severe accident tool (ASTEC-Na), against available large-scale separate effect tests. The model has been transposed in analytical correlations which implementation in lumped-parameter severe accident codes is straightforward. According to the comparisons to data set, the correlations do not adversely impact the code estimates with respect to other more empirical alternative approaches and, in addition, the correlations remove any need of user-defined ad-hoc parameters in the input deck concerning Na-based particles behaviour, as other alternates do. Regarding code behaviour, the model predictions yield the same order of magnitude both in terms of suspended aerosol concentration and diameter as data and capture the reliable measured data trends.

Severe accident source terms for a sodium-cooled fast reactor

In order to support the demonstration of a risk-informed approach to the design optimization of a sodium-cooled fast reactor (SFR), it was necessary to make realistic estimates of the consequences of severe accident scenarios. This paper describes the database, models, and assumptions used to estimate the offsite consequences of characteristic severe accident scenarios. As required for comparison with the NRC’s technology neutral framework limit curve, the offsite dose at one mile from the plant boundary is calculated using conservative meteorology.The reference plant design is a 1000MWt pool-type design with metallic fuel. Because an integrated analysis tool comparable to MELCOR does not exist for SFR accident scenario analysis, it was necessary to write a computer code that would assess release of radionuclides from the fuel and transport within the reactor primary system and to link those analyses with results from existing computer codes that assess the dynamic response of the reactor, containment thermal–hydraulics, and radionuclide transport processes within the containment. The analyses indicate that the offsite source terms for SFR severe accident scenarios tend to be small because of the low melting temperature of the fuel, likelihood of significant retention of fission products within the sodium pool, augmentation of containment deposition processes by interaction with sodium oxide aerosols, and small driving force for release from the containment to the environment. A number of major sources of modeling uncertainty are identified as requiring further development effort. An integrated modeling capability, similar to the MELCOR code, is required to direct further research and ultimately to support the licensing of an SFR.

Initial Size Distribution of Sodium Combustion Aerosol

The behavior of sodium hydroxide aerosol is one of the important features in the safety evaluation of a sodium-cooled fast reactor. The vapor phase combustion of the sodium results in the generation of sodium oxide aerosols in the flame zone. The physical and chemical changes of sodium oxide aerosol after leaving the flame zone are very important to understand the sodium oxide aerosol process. The particle size distribution of sodium hydroxide aerosol after leaving the flame zone is measured using both optical and impaction techniques in the Aerosol Test Facility. The mass median diameter is found to be ~1.0 μm for the particles older than 20 s. The size distribution is essentially attributed to sodium hydroxide aerosol. The initial size distribution of particles does not vary with ignition temperatures over the range of 250 to 550°C for a confined pool, whereas the size increases with increase in relative humidity.

A numerical study of radiation heat transfer in sodium pool combustion and response surface modeling of luminous flame emissivity

A response surface model of the luminous flame emissivity of sodium pool fire has been proposed for use in safety analysis computer codes of a liquid metal fast reactor. The liquid sodium burns in air resulting in not only heat generation but also release of sodium oxide aerosols of sub-micron diameters. Aerosols levitating in air are radiative and they influence the allocation of combustion heat from the flame to atmospheric gas or sodium pool. The emissivity of the flame needs to be quantified, as it is one of user-specified parameters of the computer codes for the sodium fire analysis. The response surface model of the flame emissivity is developed based on numerical experiments on the physics of mass and heat transfer and behavior of the aerosol. Thermal-hydraulic equations have been solved coupled with aerosol dynamics and chemical reaction. Three influential variables on the emissivity are identified as pool temperature, gas temperature and oxygen molar fraction in the air. It has been found that the emissivity is calculated reasonably as a function of the three variables. The proposed response surface model can be easily employed in the sodium fire analysis codes because it is a simple quadratic expression. For the safety evaluation of the sodium fire, combined use is recommended of the proposed model and the lumped-mass zone model code.

Numerical investigation of multi-dimensional characteristics in sodium combustion

Multi-dimensional sodium combustion behavior has been numerically investigated in the present paper. A new computer code AQUA-SF has been developed for this purpose. The code includes two sodium combustion models (so called ‘spray combustion’ and ‘pool combustion’), a mass and heat transfer model considering a six-flux gas radiation and a coagulation and sedimentation model of sodium oxide and hydroxide aerosols. The sodium spray combustion rate is evaluated by a summation of the combustion rate of each sodium droplet with an individual diameter. A flame sheet model is applied to situations where sodium spreads out on the floor and a pool combustion takes place. The model assumes an infinitely thin flame above the pool surface and is based on a mass and energy balance in the flame. As the results of numerical analyses of a sodium spray combustion test, a location of high-temperature core region and a maximum temperature agrees with the experiment. Good agreements of an overall transient behavior are obtained in a large-scale sodium combustion test analysis. The numerical analyses also demonstrate that the distributions of temperature and chemical species concentration vary with sodium combustion modes. If sodium scatters and the spray combustion is dominant, the distributions vary in space. When a large amount of sodium exists on a floor and the pool area is enlarged, the distributions are more uniform in space.

F134 ナトリウム燃焼で発生したエアロゾル雰囲気中での 3 次元透過解析

A method to estimate optical properties of sodium oxide aerosol is developed. They are used to solve the radiative heat transfer problems in a system with aerosol generated by sodium fire. For the purpose, three-dimensional radiative transfer in an absorbing-scattering media is solved by the Monte Carlo method. The Mie's scattering theory is used to handle particle scattering during the analysis. By comparing the experimental results of the intensity and the angular distribution of the transmitted energy with the analyzed results obtained by assuming the complex index of refraction, it is shown that the value if the complex refractive index can be estimated.

Sodium Fire Code SFIRE1C for Pool Fire Characteristics

Sodium pool fire code, SOFIRE II, written for the constant value of stoichiometric combustion ratio and heat of reaction is used to compute the buildup of pressure and temperature in a containment. In the SOFIRE II model, for the formation of a mixture of Na2O and Na2O2 in the sodium pool, the input stoichiometric combustion ratio and heat of formation values need to be varied to corresponding values admissible for the mixture. In the present work, the SOFIRE II one-cell model is revised and the present version SFIRE1C (Sodium FIRE 1 Cell model) accounts for the formation of Na2O in an early stage of the fire and shifts to the formation of Na2O2 at a later stage. Thus SFIRE1C computes in a more realistic manner the reaction products which are formed in the pool. The model for sodium oxide aerosol release is also modified in this version, by incorporating a more appropriate aerosol release rate equation. The calculated values using the SFIRE1C one-cell model are compared with sodium pool fire experimental results.

Sodium Fire Code SFIRE1C for Pool Fire Characteristics.

Sodium pool fire code, SOFIRE II, written for the constant value of stoichiometric combustion ratio and heat of reaction is used to compute the buildup of pressure and temperature in a containment. In the SOFIRE II model, for the formation of a mixture of Na2O and Na2O2 in the sodium pool, the input stoichiometric combustion ratio and heat of formation values need to be varied to corresponding values admissible for the mixture. In the present work, the SOFIRE II one-cell model is revised and the present version SFIRE1C (Sodium FIRE 1 Cell model) accounts for the formation of Na2O in an early stage of the fire and shifts to the formation of Na2O2 at a later stage. Thus SFIRE1C computes in a more realistic manner the reaction products which are formed in the pool. The model for sodium oxide aerosol release is also modified in this version, by incorporating a more appropriate aerosol release rate equation. The calculated values using the SFIRE1C one-cell model are compared with sodium pool fire experimental ...

Effect of Relative Humidity on Growth of Sodium Oxide Aerosols

Effect of Relative Humidity on Growth of Sodium Oxide Aerosols

Effect of relative humidity on growth of sodium oxide aerosols.

Abstract Behavior of aerosol resulting from sodium fires in a closed vessel is investigated and the changes in the particle size distribution of the aerosol due to coagulation and humidity have been studied. The initial mass concentration is in the range of 80~500mg/m3 and the relative humidity is varied between 50 to 98%?. The initial size of the released aerosol is found to be 0.9 μm. Equilibrium diameters of particles growing in humid air have been computed for various humidity levels using water activity of sodium hydroxide. Both theoretical and experimental results have yielded growth ratios of about 3 at about 95%? relative humidity. It is recommended that the computer codes dealing with aerosol coagulation behavior in reactor containment should include an appropriate humidity-growth function.

Experiments on sodium fires and their aerosols

A number of new sodium fire and aerosol experiments were undertaken to provide data for LMFBR safety analyses: (1) Experiments on the burning of single drops of liquid sodium falling in air have been performed to aid in model development for sodium spray fire codes. (2) The leakage of sodium oxide aerosols through a straight smooth capillary tube, representative of the maximum size of a hypothetical gas leak in the wall of the secondary containment of an LMFBR, has been studied. Even in those cases in which the capillary did not plug, <11% of the entering mass was of a respirable size as it emerged from the capillary. In addition, there were a number of conditions under which the capillary plugged. (3) Experiments on the behavior of high temperature, high concentration aerosols have shown a rapid depletion of the aerosol concentration in the first 6 sec following injection of ∼800 g/m 3 aerosols at ∼1000°C into a closed vessel. This depletion has been correlated with the early formation of 100 to 200 μm agglomerates which fall out promptly.

Behavior of Sodium Oxide Aerosol in Closed Chamber under Thermal Convection Flow

The present work was undertaken to obtain the effect of thermophoretic adherence of nuclear aerosol on the reactor containments in the event of a hypothetical accident of LMFBR. In the accident, sodium oxide aerosol containing PuO2-UO2 fuel and fission products may be suspended in the containment and tends to deposit on the cold wall of the containment, so that the aerosol concentration decreases by thermophoresis in addition of settling and diffusional plating. Experiments were made to determine the degree of thermophoretic adherence for sodium oxide aerosol in a chamber during sodium burning. The results obtained by the experiments were compared with those of the calculations and the agreement was satisfactory.

Behavior of Sodium Oxide Aerosol in Closed Chamber under Thermal Convection Flow

The present work was undertaken to obtain the effect of thermophoretic adherence of nuclear aerosol on the reactor containments in the event of a hypothetical accident of LMFBR. In the accident, sodium oxide aerosol containing PuO2-UO2 fuel and fission products may be suspended in the containment and tends to deposit on the cold wall of the containment, so that the aerosol concentration decreases by thermophoresis in addition of settling and diffusional plating. Experiments were made to determine the degree of thermophoretic adherence for sodium oxide aerosol in a chamber during sodium burning. The results obtained by the experiments were compared with those of the calculations and the agreement was satisfactory.

Description of Aerosol Release Associated with Sodium Burning

Incipient temperatures of sodium oxidation, sodium oxide aerosol release and sodium ignition were observed in relation with oxygen concentration in supplying gas by a thermobalance method. On this results, a burning rate and an aerosol release fraction were determined by burning about 0.5 g of sodium samples on a crucible in a burn tube under conditions ranging 400 to 600°C in sodium temperature, 0.1 to 1.0 Nl/min in the supplying gas flow rate and 2.5 to 20v/0 in the oxygen concentration. Particle sizes of sodium aerosols released from the samples made a log-normal distribution, of which mass median diameter ranged from 0.6 to 2.0 μm. The burning rate increased with the increases of the sodium temperature, of the gas flow rate and of the oxygen concentration, and the aerosol release fraction also increased in proportion to the burning rate at fixed oxygen concentrations. It was found that the aerosol release rate defined as a product of the burning rate and the aerosol release fraction describes fairly well not only the present experimental data but also reported ones.

On the filtration of sodium oxide aerosols by multilayer sand bed filters

Multilayer sand bed filters are developed and the clogging and penetration behaviour of sodium oxide aerosols in these filters with flow rates up to 5 cm/sec and Reynolds numbers between 1 and 10 are investigated. Sand bed filters were optimized to take up high amounts (500 g/m 2) of chemically aggressive aerosols at reasonable pressure differences with efficiencies as good as Hepa-filters. Besides experimental results a mechanism explaining the clogging and penetration of sand bed filters is discussed.

Formation of Volatile Radioiodine Compounds in Sodium Pool Burning

In sodium pool burning occurring in the case of an LMFBR accident, some radio-iodine in the sodium coolant may be transported into the gas phase and act in common with sodium oxide aerosol. If some iodine is converted to volatile compounds, the radioactivity may remain for many hours in the gas phase of the reactor containment. The present work was carried out in an attempt to throw more light on these circumstances. Reactor-grade sodium with sodium iodide tagged with 131I in an amount of about 1 ppm was burned by heating in a closed vessel containing air. Most of the iodine released into the gas phase took the form of aerosol, but some amount remained in vaporous state. It was determined by Maypacks and radio-gaschromatography that the volatile radio-iodine compounds were of organic form. The proportion of organic iodide referred to total airborne iodine in the present experiments falls within the experimental data on what will be formed in a loss of coolant accident involving a light water reactor. It is concluded that volatile iodine formation would present similar aspects under accidental conditions affecting both LWR and FBR, though the mechanisms governing the two cases would be different.