Equilibrium considerations in aerosol formation during sodium combustion

Combustion of liquid sodium is of interest in the safety assessment of liquid metal cooled fast breeder reactor systems. In the present study, a detailed thermodynamic analysis of sodium-air system has been carried out for equivalence ratios in the range of 0.1–1.9 and for flame temperatures ranging from 1100 to 1950 K. In addition to this, decomposition calculations presented for product aerosols such as sodium oxide (Na₂O), sodium dioxide (Na₂O₂) and sodium hydroxide (NaOH) in normal oxygen and oxygen-deficient conditions (which are some form of phase diagrams of these aerosols) are used rigorously to find out the predominant aerosol that should be present in and outside the burn pan for a pool fire of liquid sodium. The conditions of occurrence of various sodium oxides under two different fire conditions namely pool and jet fires have been worked out. It is established that heterogeneous reactions involving sodium oxide are responsible for the formation of sodium dioxide and sodium hydroxide. It is necessary to take account of the rates of these heterogeneous reactions as well as the equivalence ratio-dependent decomposition calculations to correctly estimate the aerosol product mix in practical situations.     

Origin and Components of Radioactive Aerosols on the Cover Site at the Chernobyl Nuclear Power Plant

It is shown that essentially three sources determine the ¹³⁷Cs concentration in aerosols at the site of the object Cover at the Chernobyl nuclear power plant: a natural source (global transport of air masses and local wind uplift of dust) and two technogenic sources (work performed at the industrial site and operation of the object). Continual aerosol sampling for two weeks at the site in 2000 using FPP-15-1,5 filters shows that the ¹³⁷Cs/²⁴¹Am ratio remained in the range 50–70. This shows that during that period the artificial radioactive aerosols resulted from work performed at the site and wind uplift. The effect of work performed inside the object Cover on the aerosol conditions around the object is studied for the fire which occurred on January 14, 1993 in room 805/3.     

Response Surface Modeling of Aerosol Release Fraction in Sodium Pool Combustion

A response surface model has been proposed to evaluate an aerosol release fraction during sodium pool fire in a liquid metal fast reactor (LMFR). Air containing aerosols are radiative and they influence the allocation of combustion heat from the flame to atmospheric gas or sodium pool. Hence, the aerosol release fraction needs to be quantified based on the behavior of the aerosols and physics of mass and heat transfer. However, the aerosol release fraction is one of user-specified parameters of computer codes for the sodium fire safety analysis of the LMFR. In the present study, a response surface model of the aerosol release fraction has been developed based on numerical experiments of aerosol dynamics. For developing the model, aerosol dynamic equation has been solved coupled with thermal-hydraulics and chemical reaction. The authors obtained good agreement of the aerosol release fraction between the numerical experiments and the past experiments. Therefore, the aerosol behavior model has been validated with regard to the pool combustion phenomena and is reasonably applicable to the numerical experiment. Three influential variables on the release fraction are identified as pool temperature, gas temperature and oxygen molar fraction in the air. The proposed response surface model is a quadratic expression of the influential variables and can be easily employed in the sodium fire analysis code.     

AHMED experiments on hygroscopic and inert aerosol behaviour in LWR containment conditions: experimental results

Hygroscopic NaOH, CsI, CsOH and inert Ag aerosol behaviour at different temperatures and relative humidities (RH) has been studied in a well instrumented and controlled vessel of 1.81 m³ total free volume. Homogeneous thermal-hydraulic conditions for aerosol measurement in the vessel were achieved. The aerosol number and mass concentration were measured continuously during the experiments using a Condensation Nucleus Counter and a Tapered Element Oscillating Microbalance. The particle size distribution and chemical composition in the test conditions were measured by Berner low pressure impactors. In the case of NaOH the half life of the aerosol mass concentration was more than four times longer at low RH (22%) as compared to high RH (96%). The half lives of the CsOH and CsI aerosols were only twice as long at low RH as compared to high RH. Thus at high RH (96–97%) the half lives of CsOH and CsI were twice as long as the half life for the NaOH aerosol. The faster decay of the NaOH aerosol is due to the smaller density decrease of NaOH during water condensation. CsOH particles grew rapidly to their equilibrium size at all humidities. The measured equilibrium size for CsOH aerosol agree well with the calculated particle size at different RHs. Experimental results were also compared with calculations obtained by severe accident computer codes. These calculated results will be presented in a later paper.     

Observation of Radioactive Aerosol Emissions from the Sarcophagus at the Chernobyl Nuclear Power Plant

It is shown that high wind velocity outdoors results in a higher concentration of radioactive aerosols in the ventillation exhaust from the sarcophagus. Aerosol samples from manholes in the roof of the sarcophagus have been collected, for the first time ever, in January–December 2002 on trilayer filter packets. The ¹³⁷Cs concentration in outgoing flows is 0.7–2.3 Bq/m³. The activity median aerodynamic diameter of the aerosol carrier particles is 0.7–1.8 m. At the same time, the ¹³⁷Cs concentration in the atomspheric layer at the ground dear the sarcophagus was 1000 times lower and the carrier-particle sizes 2–4 times larger.     

Behavior of Sodium Oxide Aerosol in a Closed Chamber

The behavior of sodium oxide aerosol in a closed chamber was studied for the safety analysis of a Na-cooled fast reactor. The experimental apparatus and techniques are first described. The aerosol was released during a short time by blowing air onto heated Na in a 1 m³ chamber. The maximum mass concentration of the aerosol in the form of Na₂O ranged of 0.05 ～ 10g/m³. The particle size distribution, the aerosol mass concentration and the mass deposition rates were measured as a function of time.

It was found that the mass median diameter of the aerosol was related to the maximum mass concentration. To determine the character of the behavior of sodium oxide aerosol in the chamber, the density of the aerosol material and the thickness of the boundary layer through which the particles deposit on the chamber wall were observed. The initial half-time of the aerosol mass concentration was compared with the values numerically calculated under certain assumptions.     

Experiments and computational models for aerosol behaviour in the containment

Aerosols generated by condensation of volatile fission products during nuclear reactor core meltdown accidents represent a major fraction of the accidental airborne radioactivity. A comprehensive experimental research programme was performed at Battelle to investigate the transport and deposition behaviour of aerosols in the containment, in order to support the development of computer models which estimate the fission product behaviour in the containment and the source term for potential radionuclide releases to the environment. Important steps in the investigations were: (1) DEMONA experiments. The first large scale aerosol test series performed in the Battelle model containment (BMC) (total volume 640 m³), using an open (quasi one-room) geometry and condensation aerosols from a plasma torch generator. (2) VANAM experiments. Advanced aerosol tests in the BMC, using a multi-room geometry, mixed hygroscopic/non hygroscopic condensation aerosols, a double injection period, and varying thermohydraulic conditions. One of the experiments was subject of the International Standard Problem ISP 37. (3) KAEVER experiments. A systematic investigation of aerosol materials and mixtures and the related deposition behaviour, using a simplified one-room test vessel (10 m³ volume) and advanced instrumentation. Important computer codes developed and/or validated in connection with the experiments are FIPLOC and NAUA; aerosol codes CONTAIN, MELCOR and GOTHIC-MAEROS were also applied. Some important results from the investigations and code developments are: (1) significant local aerosol concentration differences can occur in a multi-room geometry; (2) concentration differences can be caused by atmospheric stratification; and (3) deposition is strongly affected by material hygroscopicity and atmospheric humidity. (4) Satisfactory prediction requires a consistent treatment of multi-room thermal hydraulics, aerosol transport and steam condensation on particles. (5) Prediction results can be affected by numerical stability and nodalization (user experience). This paper presents a number of results of the experimental investigations and the present state of code modelling, with special reference to the findings of ISP37.     

Sodium Fires and Aerosols

Release of ¹³¹I associated with burning of contaminated Na, decay of airborne concentration of the released ¹³¹I and the size distribution of aerosols referred to radioiodine are investigated in experimental runs conducted on laboratory scale. These investigations are carried out in conjunction with similar investigations for the Na matrix. An experimental chamber (5.4 m³) is used for burning a small pool of Na (–50 g) containing spiked ¹³¹I (40–500 μCi). Values of the specific activity ratio, viz. the ratio of the concentration of ¹³¹I in the aerosol Na to that in pool Na lay in the range of 0.9×10^?2—5.7×10^?2 at pool temperatures of about 300°C. The concentration decay half-time and the aerosol size distribution characteristics referred to ¹³¹I remain similar to those applicable to the Na content of the aerosols. Surface concentrations of ¹³¹I in the residues examined differ from the bulk concentrations (calculated) in the pool and are lower by a factor of ?20. The concentrations of ¹³¹I in the aerosols released are further lower than the surface concentrations in the residues by a factor of ?2.     

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.     

旋风组钠气溶胶去除系统性能验证

钠气溶胶是钠冷快堆发生钠火时产生的特征性产物,需通过除尘净化系统对钠气溶胶进行去除净化,以减少排放到环境的放射性剂量和钠气溶胶浓度。采用干式气固分离路线,设计了一套旋风组钠气溶胶去除系统,通过开展真实钠火环境下的钠气溶胶去除试验,考察了金属钠燃烧过程中钠气溶胶浓度变化、粒径分布、压降、去除效率等主要参数,最终认为该系统对钠气溶胶具有很高的去除效率。     

钠冷快堆中池式钠火的计算分析

文章论述了根据池式钠火的特点建立了理论模型 ,编制了SPOOL程序。该程序模拟钠燃烧过程中钠和氧气的化学反应 ,钠燃烧热在各种介质中不同方式的传递 ,钠气溶胶的产生、沉积 ,以及在各种通风条件下多种介质的质量和能量交换等瞬态过程 ,描述了钠燃烧过程中各种特征参数随时间的变化。其主要的计算参数包括房间内气体的压力和温度、房间建筑结构的温度、钠气溶胶质量浓度等等。用俄罗斯别洛雅尔斯克核电站实验和法国卡桑德拉 3号实验的数据 ,对SPOOL程序进行验证的结果表明 ,该程序的计算结果可信。该程序为国内钠冷快堆中池式钠火事故的安全分析提供了分析方法     

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.     

Clogging of industrial pleated high efficiency particulate air (HEPA) filters in the event of fire

The IRSN and AREVA NC are currently conducting a common interest fire research programme with the aim, among other things, of improving knowledge of clogging of high efficiency particulate air (HEPA) filters and developing an empirical model for clogging of such filters by combustion aerosols. This model must - insofar as possible - be independent of the nature of the fuel and be able to be integrated in a calculation code covering the interaction between the ventilation and the fire. This paper discusses the influence of various “direct” factors such as the filtration velocity, the mass of deposited aerosol per filter area, the diameter and morphology of the combustion particles, the condensate content of the aerosols, and “indirect” factors such as the air flow feeding the fire and its oxygen content, which influence the evolution of the aeraulic resistance of a clogged filter.     

Detecting small leaks in pipes in nuclear power plants by measuring aerosol parameters

It is found experimentally that when through defects appear in water pipes the concentration and dispersion composition of the aerosols in the room air change. A water or steam leak in a pipe at an early stage of the development of a defect, when the leak still has no effect on the standard operation of the equipment, can be detected by monitoring the aerosol component of the air. The stage where a leak can be detected by aerosol monitoring depends on the background aerosol concentration in the room air, the temperature and pressure in the pipe, and the arrangement of samplers in the monitoring system. The leak-detection method proposed is much more sensitive than moisture content measurements.Translated from Atomnaya Énergiya, Vol. 97, No. 3, pp. 189–195, September, 2004.     

Sodium boiling experiments in a seven-pin bundle: Flow patterns and two-phase pressure drop

Two series of quasi-steady state sodium boiling experiments have been carried out in an electrically heated seven-pin bundle. The power levels (130–170 and 30–40 W/cm²) and other test conditions were selected to correspond to the core and radial breeder zones of a typical LMFBR. The test procedure involved the gradual reduction of mass flow rate through the bundle in a simulation of the consequences of a slowly growing blockage in the lower part of a reactor subassembly. By this means it was possible to study the development of quasi-steady state boiling up to the onset of permanent dryout. The results obtained provide information on flow regimes in the two-phase region, vapour qualities and flow rates at which cooling of the bundle can be effectively maintained, and the ultimate incidence of dryout. A relation for the two-phase pressure drop multiplier obtained from adiabatic pressure drop measurements in this geometry is given and compared with earlier results from single-channel geometry tests.     

Analytical evaluation method of J-integral in creep-fatigue fracture for type 304 stainless steel

Creep-fatigue crack growth at the operating temperature of LMFBR can be characterized by ΔJ_F and J′ (same as C^*). Type 304 stainless steel, the main structural material of the Japanese LMFBR, shows notable cyclic hardening at elevated temperatures. Evaluation of these J-integrals with the finite-element method is strongly affected by the reference strain range when the cyclic hysteresis' is used as the stress-strain relation.In this paper, an evaluation method for ΔJ_F and J′ with a cyclic stress-strain curve (Δσ – Δ relation) is proposed and verified by experimental results. The evaluation method proposed here does not require cyclic calculations but is monotonic and the effect of the reference strain range is relatively small.     

Development of micro-beam NRA for 3D-mapping of hydrogen distribution in solids: Application of tapered glass capillary to 6 MeV N ion

A micro-beam NRA system, by means of a resonant nuclear reaction ¹H(¹⁵N, αγ)¹²C, has been developed for the purpose of the 3D mapping of the hydrogen distribution in solids. To obtain the tens μm size of the beam spot, the combination of the newly proposed tapered glass capillary and a conventional quadrupole magnetic lens is employed. An Y patterned film on a substrate is prepared as an application of the developed system. The 6 MeV ¹⁵N beam focused by glass capillaries down to 50 μm successfully shows the hydrogen distribution. The in-plane NRA profile implies that the beam emitted from the glass capillary outlet is parallel, although the original beam has a considerable divergence. The NRA measurements in the 10³ Pa N₂ atmosphere due to the low gas conductance of the glass capillary is also demonstrated.     

Characterization of carbonaceous materials in PM2.5 and PM10 size fractions in Morogoro, Tanzania, during 2006 wet season campaign

Atmospheric aerosol samples in PM10 and PM2.5 size fractions were collected in parallel at a rural site in Morogoro during wet season in March and April 2006. All samples were analysed for the particulate matter mass, for organic, elemental, and total carbon (OC, EC, and TC), and for water-soluble OC (WSOC). The average PM10 and PM2.5 mass concentrations and associated standard deviations were 14 ± 13 μg/m³ and 7.3 ± 4 μg/m³ respectively. On average, TC accounted for 33% of the PM10 mass and 44% of the PM2.5 mass for the campaign. The average OC/PM percentage ratios were 27% and 33% in PM10 and PM2.5 size fractions respectively and a larger fraction of the OC was water-soluble. The observed low EC/TC mean percentage ratios of 10-14% respectively for PM10 and PM2.5 fractions indicate that the carbonaceous aerosol originates mainly from biogenic aerosols and/or biomass burning. A simple source apportionment approach was used to apportion the OC to biofuel and charcoal burning. On average, 93% of the PM10 OC was attributed to biofuel and 7% to charcoal burning in the 2006 wet season campaign. However, it is suggested that a contribution to the OC at Morogoro could also come from other natural biogenic matter, and/or biomass burning aerosols. The results for the sources of OC at Morogoro should therefore be considered with great caution.     

Evaluation of plutonium oxide aerosol release from an LMFBR in a hypothetical accident

This work was undertaken to prepare a computer code for the hazard evaluation of plutonium oxide aerosol released to the atmosphere in the event of a hypothetical accident in a 50 MW(th) scale LMFBR. The reactor building structure consists of semi-double containments as follows: the primary containment has a large volume in comparison with the secondary annular containment in which a part is connected to the atmosphere through an emergency filter system. Sodium oxide aerosol containing PuO₂---UO₂ fuel, fission products and structural steel agglomerates quickly by coagulation due to its high concentration. Simultaneously, the aerosol concentration decreases due to settling, plating and thermophoresis. Using the present code, the amount of PuO₂ aerosol leakage to the atmosphere was evaluated.     

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.