Iodine retention, filters

The retention systems which are installed in the Canadian Candu plants show a similar design (McArthur and Salaff, 1988). They consist of a combination of an aerosol filter with different pore sizes and an impregnated charcoal filter with an iodine retention efficiency of better than 99.9%. 

Chaney13 observed a correlation between the retentivity of a carbon and the adsorptive power for iodine, which he termed the activity. The activity is obtained by determining the amount of iodine which 1 gram of dried 200-mesh carbon will adsorb from SO milliliters of 0.2 N iodine-potassium iodide solution. The mixture is agitated for 3 minutes, filtered, and the first portions of the filtrate are discarded. The iodine concentration of an aliquot portion of the filtrate is determined by titration. Highly active carbons for gas-adsorption will remove approximately 90 % of the iodine. 

The most hazardous volatile constituents are the iodine and rudienium fission products. Though more than 95 % of the iodine is volatilized in the dissolver (as I2, HI and HIO mainly) most of it is caught in the off-gas scrubber and most of what remains is removed by the filters. With these techniques the retention of iodine in the plant is >99.5%. 

Systems designed to vent the containment atmosphere and to filter molecular iodine from the vented flow are in service and a body of research results characterizing the effectiveness of these systems also exists. Application of the research data to other systems of different design is possible in principle. Retention capability of wet scrubbers for organic iodides are not well established. 

Following a loss-of-coolant accident, a fraction of the radionuclides released from the primary circuit remains airborne in the containment atmosphere, as was discussed in the preceding sections. In order to reduce the airborne activity concentrations, standby filters are installed in the annulus of the German PWR plants, consisting of aerosol filters and iodine filters. In the German RSK Guidelines a retention efficiency of 99.9% for particulate iodine, of >99.99% for h and of >99% for organoiodides is required under accident conditions. In certain other countries, the specified iodine species as well as the required retention efficiencies differ somewhat from these values. 

Finally, the standby filters of the annulus air extraction system will act as an additional barrier, preventing the release of radionuclides from the annuli to the environment. The comparatively high humidity of the off-gas under accident conditions does not adversely affect the function of the aerosol filters, but potentially reduces the efficiency of the iodine filters. Nonetheless, a retention efficiency of >99% for I2 and of >90% for CH3I can be assumed, even under such unfavorable conditions. 

In the Swedish BWR plant Barseback, a so-called vented containment was employed for the first time, consisting of a 10,000 m gravel bed filter to which the gas-steam mixture from the containment would be directed in the event of a severe accident. The steam will condense on the initially cold pebble surfaces, resulting in an effective retention of aerosols and iodine. However, for various reasons this solution proved not to be optimal. 

When compared to the solid state filters, liquid-phase retention systems prove to have several advantages. The heat introduced by the venting flow as well as by the decay of the absorbed radionuclides is passively removed by boiling of the water phase, the load capacity for aerosols is virtually unlimited, and the retention of volatile iodine compounds can be guaranteed by appropriate chemical conditioning of the water phase. An important prerequisite for high retention efficiency, besides the thermal and radiation stability of the chemicals applied, is a very intimate contact between the gas-steam flow and the liquid phase of the retention system, in order to obtain a fast and effective exchange of matter. 

The steel fiber filter and the Venturi scrubber were tested under similar conditions (Dickinson et al., 1990 McCormack et al., 1990). The decontamination factors for manganese and cesiiun with the steel fiber filter were on the order of 10 for particle iodide, a retention on the same order of magnitude was measured, whereas the decontamination factor for total iodine was only on the order of 10 , probably due to a partial decomposition of Csl or to an incomplete consumption of HI during the generation of the Csl aerosol. The tests performed with the Venturi scrubber showed aerosol decontamination factors of more than 10 here also, the iodine decontamination factor was lower by a factor of about 10. More than 99% of the retained matter remained in the scrubber solution, less than 1% in the steel fiber filter, which is installed downstream. In contrast with the solid-state filters, in the Venturi scrubber no problems resulted from the aerosol load of the retention system. 

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