Sodium Pool Fires Research Articles

Numerical analysis of liquid sodium spreading on sloped floor surface under pool fire scenario in SFR systems

The accidental leaks of coolant from the heat transport circuit of Sodium-cooled Fast Reactor (SFR) can create fire events due to the reactivity of sodium with atmospheric oxygen, which can pose threat to safe operation of the reactor. The leaked sodium eventually reaches the cell floor, spreads gradually, and forms a burning pool. The thermal consequences of pool fire depend on its surface area, which is determined by the spreading process of sodium on the floor surface. In this context, numerical investigation has been carried out on the spreading behavior of burning sodium pool on the plane/sloped cell floor by developing a 3-D numerical model based on the viscous-gravity current equation of the shallow layer model. The governing equations of the model have been solved numerically by implementing the Finite Volume Method. The model developed has been validated using the experimental results published on liquid pool spreading under burning and non-burning conditions on plane and sloped floors for different leak rates. Using this model, numerical simulations have been carried out on sodium pool spreading under various leak scenarios envisaged in SFR cells based on the specific burning characteristics of sodium pool fire. The time-varying pool size and shape, pool height profile, and maximum pool area have been estimated for a wide range of sodium leak rates and typical floor slopes of interest to nuclear power plants. These predictions contribute towards the numerical evaluation of sodium pool fire scenarios pertaining to the safety of SFR.

Experimental investigations of extinguishing sodium pool fires using modified expandable graphite powders

A new type of composite fire-extinguishing powder was prepared with expandable graphite (EG) and sodium carbonate as the main materials. The morphology, structure, and thermal stability of the extinguishing powder were characterized. A fire-extinguishing experimental system was subsequently constructed to investigate the powder's fire-extinguishing performance for sodium fires. The results indicated that, compared with the unmodified EG, the modified EG had a 62.53% lower particle size and exhibited higher thermal stability. The fire control mechanism was also analyzed; the results revealed that sodium carbonate and EG exert a synergistic effect on flame suppression, and sodium carbonate improves fire-extinguishing efficiency.

Analysis of liquid sodium spills spreading on floor surface pertaining to sodium pool fire events in SFR cells

Analysis of postulated sodium leak events and resultant pool fire scenarios is essential in the safety evaluation of Sodium-cooled Fast Reactors (SFR). In this connection, numerical studies have been carried out to analyze the spreading characteristics of accidental liquid sodium spills on the cell floor surface under pool fire conditions in SFR systems. The dynamics of sodium pool spreading has been investigated by implementing the integral model, which is commonly employed for liquefied hydrocarbon fuel spills spreading on land or water surface by using the specific characteristics of sodium pool fire. The governing equations have been solved numerically, and the model predictions have been validated with the liquid sodium and water spreading experimental results available in the literature. The present analysis results have shown that the 1-D integral model could fairly predict the overall behaviour of sodium pool spreading, under burning conditions. The validated model has been used to evaluate liquid sodium spreading and pool formation for different leak scenarios under postulated accidental conditions in SFR. The model is useful to simulate the spreading behaviour of sodium spills with the numerical codes developed to evaluate the sodium fire safety.

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.

The role of powder physicochemical properties on the extinction performance of an extinguishing powder for sodium fires

The French Atomic and alternative Energy Commission (CEA) aims to reuse its sodium fire (carbonate base) extinguishing powder after long term storage (stock from the dismantlement of its old sodium facilities). As the composition of the powder appears to change during the storage, the efficiency on the extinction as a function of the physicochemical properties was questioned. Small sodium fire extinction experiments were carried out with powders of different compositions. The results demonstrated a dominant role of water of crystallization on the extinction. Two steps are proposed for the extinction mechanism that includes: (1) the formation of liquid sodium hydroxide and (2) the melting of carbonate mixture at eutectic composition. The sodium hydroxide behaves as a protective layer and insulates the sodium surface from prolonged contact with oxygen. Consequently, it provides rapid decrease of temperature, unlike the slow melting of carbonates eutectic and its porous layer formed due to its higher viscosity. The presence of trona (aging product) does not alter the extinction capacity of the powder. To extrapolate the results to large fires, 35 g of water of crystallization are necessary to extinguish 1 m2 of sodium pool fire. Finally, the particle size appears to be a non-significant parameter to the quality of extinction except for the spreading performance.

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.

New developments in sodium pool fire and SFR containment aerosol modeling with ASTEC-Na

Being able to model sodium pool fires, containment pressurization, and aerosol transport is an essential part of sodium fast reactor safety studies and source term evaluation. This paper describes recent developments to incorporate these capabilities into ASTEC-Na, a sodium-specific severe accident simulation tool currently under development by IRSN. In addition, this paper presents a comprehensive set of historical small, medium, and large-scale sodium fire experiments with which to compare experimental results. The gas temperature, gas pressure, and aerosol concentration predictions produced by the code are compared against this set of 1980’s-era sodium pool fire experiments. The code and experiments tend to agree fairly well for temperatures and pressures. There are some important discrepancies for the aerosol concentration, but these tend to be more during the cool down phase of an experiment after the fire had been extinguished. That being said, important areas for improvement have been identified that can be implemented in the ASTEC-Na in the future, e.g., new models the aerosol generation rates, are discussed.

Aerosol generation from sodium pool fires: Learning from the 1980s-era EMIS experiments and modelling

A sodium fire during a major accident in a sodium fast reactor could have serious consequences. The fire could damage plant components and safety devices, and release highly toxic combustion product aerosols into the air. It is particularly important to characterize aerosol generation, since when the fires involve contaminated sodium from the primary coolant system, these aerosols are the main vector by which radionuclides are transported throughout containment and ultimately out into the surrounding environment. As such, being able to quantify the aerosolization of material from a sodium fire is essential in evaluating potential releases from the plant during an accident. This paper presents a model for sodium combustion product aerosolization, and exploits a series of 1980s-Era sodium fire experiments in order to develop and refine the model.

Radionuclides emissions from sodium pool fires: Learning from the 1970s-era FANAL experiments

The emission of radionuclides from sodium fires is a very important concern when considering the potential radiological consequences of an accident at a sodium fast reactor. Radionuclides can be released when contaminated sodium from, for example, the primary coolant system, catches fire. Not all radionuclide species will behave the same way, however, and some will have a higher tendency to become aerosolized than others. This paper uses historical experimental results from the 1970s-era FANAL program in order to come up with estimates for the combustion partition coefficients for radioiodine, radiocesium, and several other radionuclides from pool fires, and uses information from the program to characterize the chemical interaction between radioiodine vapor and sodium combustion product aerosols.

In-containment source term predictability of ASTEC-Na: Major insights from data-predictions benchmarking

Modeling the containment response to a sodium pool fire is to be one of the key aspects of any comprehensive safety evaluation of the new generation of sodium cooled fast reactors. Through a peer review of earlier experimental investigations some useful data can be collected and then used for assessing the current analytical capabilities to model severe accidents or some of their specific aspects. This paper provides major insights into the in-containment aerosol behavior predictability of ASTEC-Na (CPA∗ module) during Na-pool fires. By comparing against tests from the ABCOVE (AB1 and AB2) and FAUNA (F2) programs, it has been shown that experimental trends can be roughly reproduced with a single-cell approach whenever natural convection is effective in making the vessel atmosphere uniform both thermally and in composition. Nonetheless, the present heavy parametrization of ASTEC-Na models should be avoided or strongly supported by further experimentation that allows setting sound default values, concerning both combustion energy distribution and aerosol formation and distribution. Anyway, the peer data review has highlighted that a meaningful comparison to predictions is not always feasible due to large data uncertainties, particularly at the beginning of Na burning. As for the particle ageing, the comparisons set seems to indicate that transformation from oxides to hydroxides is predicted to be too slow; nevertheless, a more extensive benchmarking should be conducted to confirm it.

Evaluation of sodium pool fire and thermal consequence in two-cell configuration

Evaluation of sodium pool fire and thermal consequence in two-cell configuration

Theoretical assessment of particle generation from sodium pool fires

Potential sodium discharge in the containment during postulated Beyond Design Basis Accidents (BDBAs) in Sodium-cooled Fast Reactors (SFRs) would have major consequences for accident development in terms of energetics and source term. In the containment, sodium vaporization and subsequent oxidation would result in supersaturated oxide vapours that would undergo rapid nucleation creating toxic aerosols. Therefore, modelling this vapour nucleation is essential to proper source term assessment in SFRs. In the frame of the EU-JASMIN project, a particle generation model to calculate the particle generation rate and their primary size during an in-containment sodium pool fire has been developed. Based on a suite of individual models for sodium vaporization, oxygen natural circulation (3D modelling), sodium-oxygen chemical reactions, sodium-oxides-vapour nucleation and condensation, its consistency has been partially validated by comparing with available experimental data. As an outcome, large temperature and vapour concentration gradients set over the sodium pool have been found which result in large particle concentrations in the close vicinity of the pool.

Numerical modeling of sodium fire – Part II: Pool combustion and combined spray and pool combustion

The risk of sodium-air reaction has received considerable attention after the sodium-fire accident in Monju reactor. The fires resulting from the sodium-air reaction can be detrimental to the safety of a sodium fast reactor. Therefore, predicting the consequences of a sodium fire is important from a safety point of view. A computational method based on CFD is proposed here to simulate sodium pool fire and understand its characteristics. The method solves the Favre-averaged Navier-Stokes equation and uses a non-premixed mixture fraction based combustion model. The mass transfer of sodium vapor from the pool surface to the flame is obtained using a sodium evaporation model. The proposed method is then validated against well-known sodium pool experiments of Newman and Payne. The flame temperature and location predicted by the model are in good agreement with experiments. Furthermore, the trends of the mean burning rate with initial pool temperature and oxygen concentration are captured well. Additionally, parametric studies have been performed to understand the effects of pool diameter and initial air temperature on the mean burning rate.Furthermore, the sodium spray and sodium pool combustion models are combined to simulate simultaneous spray and pool combustion. Simulations were performed to demonstrate that the combined code could be applied to simulate this. Once sufficiently validated, the present code can be used for safety evaluation of a sodium fast reactor.

Suppression of sodium fires with liquid nitrogen

Sodium has unusual fire hazards, including autoignition when heated in air or exposed to liquid water. Owing to limitations of existing suppression agents for sodium pool fires, suppression using liquid nitrogen (LN2) is examined here. Sodium pools of 5–80g were heated in stainless steel beakers. At about 290°C, pool surface autoignition occurred and caused a rapid pool temperature increase. Vapor phase combustion occurred when the pools reached 320–450°C, ultimately leading to pool temperatures up to 700°C. For suppression tests, LN2 delivery (at 2.7g/s) began when the fires became fully-developed, near a pool temperature of 600°C. Liquid nitrogen was found to be an effective suppression agent. The minimum amount of LN2 required to suppress a fully-developed sodium pool fire was found to be about three times the initial sodium pool mass.

Sodium pool combustion phenomena under natural convection airflow

Sodium pool fire is a design basis accident of sodium-cooled fast reactor. In this study, a numerical method for multi-dimensional modeling of sodium pool fire has been developed. It considers coupling of thermal–hydraulics, chemical reaction and aerosol dynamics equations. From the present multi-dimensional computation, phenomena of sodium pool fire are understood such as flow and temperature fields and aerosol mass distribution of various sizes. It has been found that the burning rate varies along the radial direction and the mass and heat transfer around the pool peripheral is maximum and most influential. The thermal–hydraulic phenomena in the near-surface region are very important to determine the sodium pool fire consequence such as the burning rate and aerosol emission. The area-averaged burning rate and aerosol release fraction calculated by the present numerical method are in agreement with experimental data.

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 Simulation of Sodium Pool Fire

A numerical simulation thermal-hydraulics code called SPOOL based on computational fluid dynamics considering sodium reaction and aerosol transport is developed. Sodium pool fires are simulated using the SPOOL code, and periodic oscillation of the flame is observed with frequency similar to that observed for small-scale pool fire experiments with industrial fuels. The calculated mass-burning rate differs slightly from experimental results, yet it increases with pool temperature in agreement with experimental trends. The mass flux of aerosol driven by thermophoresis is calculated to be about 100 times larger than that by gravity, and the aerosols become concentrated at the edge of the pool. The release fraction, obtained by dividing the total mass of aerosol released into the atmosphere by that produced, increases with pool temperature in qualitative agreement with experiments.

Prediction of Release Rate of Burnt Sodium as Aerosol

Evaluation of the consequences of sodium pool fire requires a prediction model that can calculate the fractions of the aerosols of Na2O and Na2O2 being transported into the atmosphere and onto the sodium pool surface from the reaction zone. In the present model, we applied the laminar diffusion flame theory in a thin layer in the neighborhood of the pool surface. The combustion was modeled assuming the instantaneous chemical equilibrium because the chemistry is fast compared with the mixing. The aerosol transport was assumed to be due to a sum of the drag force, the thermal force and the gravity. The calculated results are in good agreement with experimental data obtained by other investigators for the burning rates and the aerosol release fractions of sodium pool fire. This analysis forms the basis of prediction of aerosol release fraction in the sodium pool fire.

A fundamental study of sodium pool combustion.

A theoretical study has been conducted to investigate the burning rate of sodium pool fires, which may be caused by the spillage of liquid sodium in fast-breeder nuclear reactors. In the present model, a thin layer (approximately l mm in thickness) of vapor is assumed to exist upon the surface of the liquid pool. The sodium vapor diffuses toward the flame front at which a chemical reaction between sodium and oxygen occurs. The calculated results are in good agreement with experimental data obtained by other investigators for the burning rates of sodium pool fires. The earlier conventional model, however, tends to underestimate the burning rates since the vapor layer is neglected and the flame temperature is assumed to be equal to the pool surface temperature.

Investigation of Burning Rate of Sodium Pool Fires

KEYWORDS: sodium pool firesburning rateanalysisvapor layerflame temperaturesteady-state conditionsfree convectionheat transfermass transfer