Research on iodine mitigation systems: Experimental study on the combination of ozone feed and wet electrostatic precipitator

An innovative system has been developed by VTT Technical Research Centre of Finland Ltd. to filter gaseous iodine and iodine containing particles (e.g. IxOy). It consists of a combination of an ozone feed and a modern wet electrostatic precipitator (WESP). An electrostatic precipitator (ESP) removes aerosols from gas flow due to forces induced by strong electric fields. The addition of a water solution mist to the ESP, the combination called as Wet ESP (WESP), enhances the particle growth in diameter and the charging of particles. Hence, the retained impurities can directly be transported to a container, such as a sump in a nuclear power plant (NPP), via the flow of water solution on the WESP collection electrode surfaces. The WESP has been found to be a reliable and effective filtration method in a variety of applications to clean air from particles but the technique needs some improvements to have an effect on the gaseous impurities. The innovation in the system developed by VTT Technical Research Centre of Finland Ltd. comes from the implementation of an ozone feed upstream the inlet of WESP, in order to oxidize gaseous iodine into particles. The applicability of the combination of these two techniques on the filtration of fission products, especially gaseous and particulate iodine, was investigated in this study. The system consists of a steam generator, a gaseous iodine feeding system, an ozone generator, a reaction chamber, a droplet spray chamber, the electrical filter WESP and two sampling furnaces (Figure 1). Figure 1: Schematic figure of the experimental facility. Gaseous and particulate iodine samples were collected at the inlet and outlet of the WESP (spray chamber + WESP chamber). The scrubber solutions and leachants from filters were analysed with Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Aerosol number size distributions were measured online with Electric Low Pressure Impactors (ELPI) both at the inlet and outlet of the WESP. Two TSI (series 3775) Condensation Particle Counters (CPC) were also used to measure the aerosol number concentration both at the inlet and outlet of the WESP. The results are presented as a comparison between the ELPI, CPC and ICP-MS measurements at the inlet and at the outlet of the WESP. From these results the filtration efficiency of the WESP was calculated. First, the best parameters related to the WESP performance were defined with tests performed in dry conditions (i.e. without steam content) and at room temperature, varying parameters such as applied voltage, number of corona needles, pH of the sprayed droplet solution, use of wall flushing solution in the WESP, for three flow rates (56, 86 and 132 l/min). The WESP showed its best filtration efficiency in dry atmosphere with the following conditions in the studied set-up: - Ozone generator max. output [O3]i ~ 9000 ppm; - NaOH wall flushing: yes; - Droplet feed (~80 g/m3, pH~12): yes; - Maximum applied voltage: -25 kV. Next, the aim was to assess the behaviour of WESP in more realistic conditions, at higher temperature, steam content, varying molecular iodine (I2) concentration and testing the WESP with methyl iodide (CH3I), which is another form of iodine present in significant concentration in the containment building in case of severe accident. For these experiments, the facility was heated up to 120 °C and the temperature inside the WESP chamber was 65 °C. Overall, the filtration efficiency calculated for mass and particle number was good. In case of the molecular iodine filtration tests (~12 or ~100 ppm initial concentration of I2 before the dilutions, downstream the iodine generator), the filtration efficiency was > 95 % for particle number (Figure 2) and > 97.6 % for mass of gaseous iodine and particles.Figure 2: The filtration efficiency of WESP for particle number at -25 kV, calculated from ELPI measurements according to particle aerodynamic diameter for the experiments performed with an initial I2 concentration of about 12 ppm. Concerning the performance with methyl iodide (~6 ppm initial concentration of CH3I before the dilutions, downstream the iodine generator), the filtration was not satisfactory. This was because only 5 % of iodine measured at the inlet sampling was in the form of particles. It seemed that the formed particles decomposed due to contact with steam before the particles were retained. The filtration efficiency (for total iodine mass) decreased when the steam content of carrier gas was increased (and when temperature was increased). However, the difference between the different steam contents (11, 34 and 45 l/min) was not obvious. Further studies are required, especially on changing the location of the iodine oxidation step to avoid particle decomposition before the WESP. Overall, this study has shown that the use of "ozone feed and WESP" together enables the filtration of both gaseous molecular iodine and iodine containing particles (e.g. IxOy). The results obtained for several conditions showed efficient trapping of iodine when iodine was oxidized into iodine oxide particles.