Decontamination process of elemental iodine with Filtered Containment Venting System

In the event of a nuclear power plant accident pressure within the containment can increase. A Filtered Containment Venting System (FCVS) allows for over-pressure release through multi-scrubbers (Venturi-scrubber, bubbling-scrubber, metal fiber filter and molecular sieve) and reduces the radioactive release. However, FCVS performance changes depending on operational conditions, e.g. steam flow rate, pressure, operating time and so on. The Central Research Institute of Electric Power Industry (CRIEPI) full-height FCVS test facility is constructed to measure FCVS performance under several conditions and can evaluate the decontamination factor (DF) of three major targets (aerosol, elemental iodine (I₂) and organic iodine (CH₃I)). This project is intended to acquire a systematic database of FCVS performance and optimize the FCVS operation procedure. The CRIEPI test vessel is about 8 m high, with an internal diameter of 0.5 m. FCVS performance tests were conducted under the following conditions: maximum pressure and temperature of 0.8 MPa and 170°C, inlet gas flow of steam (∼1600 kg/h) and air (∼300 kg/h) and containing aerosol/iodine/organic iodine. If fission product iodine gas is released into the environment during a severe accident, it will have a major impact on public health. This paper addresses the iodine decontamination performance by the bubbling effect. Iodine is effectively soluble in an alkaline solution. Accordingly, 0.5 wt% sodium hydroxide (NaOH) or a mixture of 0.2 wt% sodium thiosulfate (Na₂S₂O₃) and 0.5 wt% NaOH is used as an iodine filter (absorber) and during the experiment, an alkaline solution with composition equivalent to the actual equipment is used. The concentration of elemental iodine is quantified with an Inductively-Coupled Plasma with Mass Spectrometry (ICP-MS), while iodine DF is defined by the concentration ratio at the inlet and outlet. Iodine DF shows low dependence on flow dynamics, but dependence on solution property. Where iodine concentration is low, DF is high (between 10⁴ and 10⁵) and vice versa when the iodine concentration (saturate) increases.