Steam droplet transport

Because of its potential to form volatile species, the behavior of fission product iodine is of particular significance in this context. In Section 4.3.3. it was pointed out that both in the primary coolant and in the steam generator secondary-side water iodine is present as non-volatile iodide the measured carry-over rates to the main steam are identical with those of fission product cesium, indicating that carryover is exclusively effected by droplet transport (entrainment). 

A possibility is to saturate at different temperatures the reactants before they enter into the stack [33]. This approach can be accomplished by several procedures based on external dewpoint, external evaporation, steam injection with downstream condensers, or flash evaporation. High temperature values allow to absorb significant water amount in gas streams and then transport it inside the stack compensating the water losses due to internal fast evaporation. However, the main problem with external humidification is that the gas cools after the humidifier device, the excess of water could condense and enter the fuel cell in droplet form, which floods the electrodes near the inlet, thereby preventing the flow of reactants. On the other hand, internal liquid injection method appears preferable for example with respect to the steam injection approach because of the need of large energy requirement to generate the steam. 

Fission product noble gases entering the water-steam circuit in the event of a tube leak are completely volatilized and transported with the steam to the main condenser where they are extracted and released via the off-gas stack. This release is monitored by a continuously operating detector device located in the condenser off-gas line. Non-volatile fission and activation products which are transported over the leak to the water-steam circuit remain completely in the water phase of the steam generator by the action of the blowdown purification system their activity concentration is kept at a level which is controlled by the injection rate on the one hand and by the purification rate on the other. Because of the very low vapor pressures of these elements and their chemical compounds (dissolved ions or insoluble oxides/hydroxides), their transport to the steam under the prevailing conditions (270 °C, 7 MPa) is only possible by droplet entrainment. This means that partitioning between liquid and steam phases is proportional to the steam moisture content, which is usually well below 0.1%. 

The carry-over of corrosion product radionuclides with the main steam in the direction of the turbine is effected, on the one hand, by droplet entrainment with the residual moisture content of the steam and, on the other, by steam volatility. Usually, droplet carry-over is the most significant transport mechanism however, the oxides of the primary system metals show a measurable solubility in steam even at BWR operating conditions. At different plants, concentrations of dissolved cobalt on the order of 60 ng/kg were measured in condensed samples of main steam, i. e. significantly higher than could be explained by droplet entrainment (e. g. Hepp et al., 1986). These observations are consistent with the fundamental results on steam volatility of weakly dissociated compounds under BWR operating conditions which were reported by Styrikovich and Martynova (1963). Since only non-dissociated substances are volatile with steam, it has to be assumed that a fraction of the cobalt present as dissolved ions in the reactor water at ambient temperature is converted to non-dissociated oxide, hydroxide or ferrite at the plant operating temperature. 

According to experiments performed under appropriate conditions, about 1% of the fission product iodine present in the flashed primary coolant volume is transported with the steam phase, about half of it in form of aerosols (Aim and Dreyer, 1980). This figure agrees well with that reported by Morell et al. (1985) and Hellmann et al. (1991), obtained in flashing experiments from small-diameter pipes (see Section 6.2.2.). In the experiments of Aim and Dreyer (1980), which were carried out in an uncoated steel vessel, only particulate iodide and elemental h were detected in the atmosphere of the vessel. The rate of plate-out of iodine from the atmosphere was found to depend on the specific geometric conditions of the experimental setup. In a comparatively small vessel, deposition halftimes of 8.5 hours for h and of 0.9 hours for aerosol iodide were measured within the first two hours after blowdown these values increased to 9.9 and 5.7 hours, respectively, during the following 22 hours. From other experiments markedly different values have been reported (e. g. CSE tests, see Section 7.3.S.3.8.). The reasons for these differences are not only due to the dimensions and true surface areas present in the respective experimental facility, but also to various other parameters, such as initial concentrations, turbulences in the atmosphere, rate of water droplet plate-out and of steam condensation. 

In a flash evaporation the liquid is atomized, forming very fine droplets with diameters of less than 20 pm which will be transported in part by the steam flow these droplets are the carriers of non-volatile radionuclides. In order to determine the extent of entrainment, Li was used as a non-volatile tracer in these experiments. The results of the experiments performed over periods of more than 30 minutes each (in order to obtain information on the initial discharge phase as well as on the steady-state phase) showed an entrainment of liquid with the steam to the condenser tank on the order of 0.2% (related to the total amount of tracer used), independent of the geometrical position of the orifice (in a horizontal or a vertical direction the arrows shown in Fig. 6.7. indicate schematically the direction... 

Non-volatile radionuclides transported as aerosols out of the containment are partly washed down during steam condensation in the annuli, as was already mentioned above. The temperatures of the sumps here are well below the boiling point, so that a noticeable resuspension of radionuclides by droplet carry-over is not to be assumed. However, a small resuspension of non-volatile radionuclides cannot be ruled out under these conditions, with the mechanism for this being the formation of small droplets by the action of the air flow. As was discussed in Section 6.2.2., experimental studies have yielded a very small carry-over of non-volatiles to the air flow. Due to the differences in the conditions, the results of these investigations cannot be directly applied to the release from sumps in the annuli or in the auxiliary building, but it can be assumed that the experimental values obtained in... 

The entrainment of liquid from the film is governed by the relative velocity and film Reynolds number. The minimum relative velocity required for the onset of the entrainment is given by V . in Table I. The steam velocity in the cavity of 192 m/s far exceeds this onset of entrainment velocity of 89 m/s at the assumed cavity pressure of 0.2 MPa. Thus significant entrainment of the film into droplets is expected. The calculated entrainment rate is 12.6 g/cm sec. At this rate, the characteristic time constant is 5.3 seconds. This value should be compared with the film residence time of 3.6 seconds. The two characteristic time constants indicate that the film transport and entrainment mechanisms are in the same order of magnitude. A little less than one half of the molten corium is expected to be entrained by the streaming gas and the remaining mass discharged from the cavity to the lower compartment as a liquid film. 

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