Filtration of iodine with a wet electrostatic precipitator

Potential radioactive release to the environment in case of a postulated severe accident emphasizes the importance of preserving the containment integrity of Nuclear Power Plants (NPPs) and of relying on efficient mitigation systems capable of reducing as much as possible the radioactive discharge to the environment. After the stress tests that followed the Fukushima-1 accident, the interest in Filtered Containment Venting Systems and even better mitigation systems has been renewed. The electrostatic precipitation (ESP) technique is widely used in the industry to filter out impurities in gases. It has been found to be a reliable and effective filtration method in a variety of applications. The filtration efficiency of a modern wet electrostatic precipitator (WESP) can lead to a decontamination factor higher than 1000. Another advantage of the WESP is that the impurities are removed from the system with water. They can thus directly be transported to a water container, such as a sump in a nuclear power plant (NPP). Radiotoxic iodine has a significant contribution to a possible source term in a severe NPP accident. Therefore, the applicability of the WESP technique on the filtration of fission products, especially gaseous and particulate iodine, is investigated in this study. The ESP technique can also be used to filter gaseous pollutants when the gas flow is pre-treated before the filtration unit. The water droplets fed into the system adsorb gaseous impurities. The gaseous compounds can also be oxidized to form solid particles which are filtered with the ESP technique. In this study on iodine filtration with a WESP, both proposed methods are used for the decontamination of gaseous species in containment conditions. Gaseous iodine is oxidized with additional ozone and water droplets are fed to the gas flow just before the filtration unit of WESP. The preliminary experiments were carried out with TiO2 aerosol. The applied electric voltage between the electrodes, the residence time of particles inside the ESP chamber and the injection of water droplets before the ESP chamber were varied in the first experiments. The injection of water droplets significantly increased the trapping efficiency of TiO2 particles for applied electric voltage less than 15 kV (negative). In the iodine experiments, the gas flow containing elemental iodine (2.2 l/min, 16 ppm of I2 in gas phase) was mixed with ozone (3 l/min, > 1000 ppm) in order to oxidize all gaseous iodine to iodine oxide particles. The number size distribution and the number concentration of the formed iodine oxide particles were measured online with ELPI and SMPS devices. A result of the first filtration experiments with iodine oxides, more than 98 % of IOx particles was filtered with the WESP when the applied electric voltage was in a range from -10 kV to -25 kV.