Study of level swell for a Filtered Containment Venting System at the VEFITA facility

A Filtered Containment Venting System (FCVS) is a safety system incorporated in Nuclear Power Plants (NPPs) to filter the steam and noncondensable gases released from the containment during a severe accident scenario to avoid overpressure in the containment structures while isolating most radioactive products inside. This paper presents the experimental and numerical simulation results from confirmatory tests performed in the VEFITA (Venting Filter Assessment) test facility at the Paul Scherrer Institut (PSI) in Switzerland. This facility is designed to evaluate the performance of an FCVS by assessing key phenomena and parameters such as internal recirculation patterns, changes in inventory, level swell, and the Decontamination Factor (DF) of aerosol and iodine species in simulated venting processes. The current study focuses on the level swell (gas holdup) phenomenon. The level swell during gas injection is particularly of interest since it is associated with the heights of gases and liquids in the venting vessel and thus affects the design and operation of an FCVS. The experiment is performed by injecting the noncondensable gas and steam at various mass flow rates through a sparger nozzle to the vessel, which is partially filled with washing fluid as a bubble column. The level swell and other physical variables are recorded and analyzed. System codes (RELAP5 and TRACE) input decks are developed for this facility, and experimental transients are simulated. The code simulation results, as well as previous physical gas holdup models based on homogeneous-heterogeneous regime transition, are compared with current experimental data to study the level swell in this FCVS.