Low-volatility substances

The transport and the deposition of the condensation aerosols in the reactor coolant system depend on a number of parameters such as heat and mass transfer, chemical reactions and aerosol kinetics. With respect to their transport behavior, all the substances volatilized from the reactor core do not directly enter the primary circuit. First, they have to pass through the upper plenum of the reactor pressure vessel their residence time here depends on the specific accident sequence. The masses deposited here on the walls and structures will differ and, consequently, the fraction which, after having passed through the upper plenum, enters the actual primary circuit will depend on the details of the accident. 

The aerosols of the low-volatility substances which enter the primary system of the reactor consist mainly of refractory oxides of the less volatile fission products, as well as of those of the core structural and component materials metals such as silver and cadmium from the PWR control rod materials are also present. In addi- 

In the primary system, physical mechanisms such as diffusion, gravitational settling, inertial impact and thermophoresis will effect a deposition of the aerosol 

Once an aerosol particle is deposited on a structural surface it is subjected, on the one hand, to gravitational and electrostatic (Van der Waals) adhesive forces, which tend to hold the particle to the surface and, on the other hand, to forces that tend to effect resuspension of the particle. Resuspension in the reactor primary system may reduce the retention effectiveness and, consequently, enhance the source term to the containment. According to Benson and Bowsher (1988), resu-pension of deposited aerosols can be caused by one of three processes  

In a steam-containing gas flow, the presence of a significant fraction of soluble substances (e. g. CsOH) on the surfaces of the deposited aerosols will result in a drastic decrease in the degree of resuspension, even at very high gas velocities. In the Lace experiments, the measured resuspension fluxes were two to three orders of magnitude lower with mixed aersosols (MnO and CsOH) than those obtained for pure insoluble deposits (Rahn, 1988). Similarly, it was observed in these experiments that chemical effects could not be ignored. 

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Low cost is one of the main advantages of the vapor pressure methods, as compared with calorimetric techniques. An apparatus to measure the vapor pressures of low boiling temperature liquids can be built easily in an undergraduate chemistry laboratory. However, the same is not quite true if we want to measure the vapor pressures of low-volatility substances, such as most solids. In these cases, Knudsen cells are usually the method of choice, but they require more expensive high-vacuum equipment [36]. 

When in a system the components are essentially immiscible, then the vapor pressures of the individual components add up, and both components distill in the ratio of their vapor pressures. Most common is the use of water as main component therefore, this particular process is addressed as steam distillation. Steam distillation is a common method for the purification of low volatile substances. Of course, the components should not react in the distillation process. The vapor pressures of some components are shown in Fig. 6.14. 

The method uses speeial effusion cameras with holes of a definite form, maintaining high vacuum in the system. The method is widely applied to the measurements of a vapor pressure of low volatile substances. 

Entrainers, modifiers, and co-solvents are basically mixed solvent systems and provide another dimension to supercritical fluid extraction. The entrainers enhance the solubility of the low volatile substance in the solvent, provide selective solubility in multi-solute instances, and enhance the sensitivity of the solubility and selectivity to temperature, pressure, and composition. The entrainers may be reactive and are also useful as slurrying media. Table 21.1.4 shows representative data on the effect of entrainers on vapor liquid systems that has been systematically studied by Brunner s group. Kumik and Reid as well as Johnston s group present data for dense gas-solid systems. 

The most interesting characteristics of SCFs, on which are based all the SCFs processes, is related to their tunability with pressure and temperature, especially the tunability of their solvent power [8], The dissolving effect of a SCF is dependent on its density value. Solubility increases with increasing density (ie, with increasing pressure). The relationship with temperature is a little more complicated. Fig. 12.3 shows the solubility of a substance of low volatility in a sub- and supercritical fluid. The solubility in the SCF increases at constant pressure up to temperatures slightly below the of the solvent. A further increase in temperature leads at low pressures to a decrease of the dissolved amount of the low-volatility substance in the subcritical liquid solvent and at high pressures still to an increase. High and low pressures refer to a medium pressure level. 

FIGURE 12.3 Variations in the solubility of a low-volatility substance (liquid or soBd) in a subcritical (temperature < critical temperature, T ) or supercritical fluid (temperatnre> T ) as a function of process temperature, process pressure (segmented lines correspond to equal pressure or isobaric conditions), and solvent density (dotted lines correspond to equal density conditions). (Reproduced fmm Ref. [4] with permission of Elsevier.)... 

At low pressures, solubility of the low-volatility substance in the supercritical solvent decreases with temperature since density of the supercritical solvent decreases rapidly with increasing temperatnre at near critical pressures. At high pressures, density changes with temperature are far more moderate, so that the increase of vapor pressure is the dominating factor, while at low pressures loss in solvent power induced by lower density prevails. Analogous solubility behavior can be found in systems of an SCF with a solid substance or a low-volatility liquid [4]. 

Only on-column Injection allows the quantitative and reproducible sample injection on to the column without losses. Low-volatile substances, in particular, are transferred totally without discrimination. 

Testing for migration of plastics additives into foods or food simulants in many cases will be costly and difficult, particularly with fatty food and the fatty food simulants owing to analjUiical difficulties in isolating and determining lipophilic, low volatility substances at low levels in the fat matrix. Moreover, it is recognised that, in the main. 

Here, /f is equal to /r if T < Tc and it is equal 0.25 fr for T > T. Kraska et al. [21 ] have applied this equation of state for the modeling of the p VT behavior of pure CCIF3 which is used as a supercritical solvent. The parameters are listed in Table 1.10 and those for CO2 are given in [21,37], This equation of state has been employed by Kraska et al. [21] for the calculation of the solubility of several low-volatile substances in supercritical fluids with the fugacity approach. 

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