Filtered Containment Venting System (FCVS) for removal of elemental and organic iodine during severe nuclear power plant accidents

Structural integrity may be threatened due to over pressurization by accumulation of steam and noncondensable gases in a scenario of `beyond design basis accident' leading to a situation of potential fission product release to the environment. Filtered Containment Venting System (FCVS) has been considered for depressurization while keeping off-site doses low or within prescribed limits. Various types of FCVS have been installed on many nuclear power plants world-wide. In case of a nuclear meltdown accident followed by an over-pressurization that may require venting, the mixture of steam and non-condensable gases containing iodine is passed through the FCVS. Radioactive iodine in various organic and inorganic forms is scrubbed in a specific scrubbing solution while the cleaned steam-gas mixture can then be discharged into the environment. Among the spectrum of fission nuclides generated as a result of fission, iodine with its nine oxidation stages is perhaps the most reactive fission product generated and released into the primary coolant system during the event of severe NPP accidents. Because of fuel degradation and melting, a huge inventory of iodine is volatilized. Understanding of iodine behavior in environment similar to that of containment is necessary prerequisite for planning effective mitigation system for efficient retention of iodine within containment boundary. The prevailing conditions i.e. iodine concentration, pH, temperature and dose rate will govern the iodine behavior in the sump water as well as in the containment. Development of an efficient, fast and reliable mechanism to retain volatile iodine species in a containment of a nuclear reactor during a severe accident has been an active area of research in the recent years. The need for such research is greatly enhanced in Post Fukushima nuclear era. This paper reviews the available FCVS options and specifically outlines the related research and development activities being carried out at Pakistan Institute of Engineering & Applied Sciences (PIEAS).