Combination of ozone feed and wet electrostatic precipitator: Experimental study of an innovative system to filter gaseous iodine and iodine containing particles

Efficient mitigation systems capable of reducing as much as possible the radioactive discharge to the environment in a severe Nuclear Power Plant (NPP) accident are a necessity. Nuclear Power Plants are equipped with filtration systems which have been characterized for aerosol retention in a short term, but to a far lesser extent for the retention of volatile iodine or for the long-term behaviour of the system. After the Fukushima accident, one of the main concerns of the nuclear industry has been to verify the ability of filtration systems to mitigate the possible source term to the environment. Radiotoxic iodine has a significant contribution to a possible source term in a severe NPP accident. An innovative system has been developed at VTT Technical Research Centre of Finland Ltd. to filter gaseous iodine and iodine containing particles (e.g. IxOy). The system consists of a combination of an ozone feed and a modern wet electrostatic precipitator (WESP). The electrostatic precipitation (ESP) technique is widely used in the industry to filter out impurities (i.e. particles) in gases. The advantage of a WESP is that the impurities are removed from the system with a solution. They can thus directly be transported to a water container, such as the sump in a nuclear power plant. The addition of ozone feed at the inlet of the WESP allows the oxidation of gaseous iodine into particles. The efficiency of the system for the filtration of gaseous molecular iodine and methyl iodine has been tested and the subsequent observations are further discussed in this paper. The results showed that the combination of WESP together with ozone feed results in a good filtering efficiency against molecular iodine under air atmosphere and air/steam atmosphere.