[en] In a severe accident scenario, the inadequate heat removal in a nuclear reactor can lead to over pressurization of the containment thus challenging its integrity. If not controlled, this can lead to release of radionuclides and high pressure steam in the environment. To ensure that the containment building remains intact and the reactor depressurizes, the vent line from the reactor is directed to a scrubber tank consisting of multiple venturi scrubbers, metal fiber filter and demister pad (known as Filtered Containment Venting System (FCVS)). This is a passive safety measure suggested for installation in advanced and existing nuclear reactors post Fukushima event. The venturi scrubber plays an important role in depressurizing the containment and filtering the contaminated gas. A numerical model is developed and validated with experimental results to understand the hydrodynamics and scrubbing of iodine vapors in such passive mode of operation. (author)

[en] Highlights: • A computational model to predict the efficiency of venturi scrubber is developed. • Efficiency increased with increase in gas velocity and liquid injection velocity. • Dimensionless Performance Parameter is found to increase with increasing efficiency. • Improved efficiency for multi-stage and variable injection diameter system is presented. - Abstract: To understand the performance of a venturi scrubber operating in self-priming mode, the present study develops a computational model to simulate the interacting three phase flow occurring in the venturi scrubber. The computational model is based on an Euler- Lagrangian framework where parcels of droplets and aerosols are tracked throughout the flow domain. The model incorporates the effect of droplet collision and increased momentum source in the regions of high liquid concentration. Additionally, the droplet inertial effects on the shear induced multi-jet breakup is included to predict the evolution of the droplet size distribution. The predicted local liquid loading, pressure drop and the collection efficiency were found to be in good match with the cited experimental values. The jet penetration and the droplet mean diameter is combined to form a unique dimensionless number, called the Performance Parameter, to quantify the multi-jet characteristics in the throat section of the scrubber. Higher values of Performance Parameter corresponded to higher efficiencies of the scrubber, hence serves as an analytical tool to assess the performance of the scrubber. Based on the parametric study for different throat gas velocities, liquid injection velocities, changing throat length and changing orifice diameter, the applicability of such a scrubber to self-priming mode is discussed. Further, for transients incurring lower gas velocities and lower operating height, the advantages of multi-stage injection and variable injection diameter are presented.

[en] Highlights: • Pressure profile in submerged venturi scrubber has been predicted. • Injection characteristics of submerged venturi scrubber are discussed. • Iodine retention in venturi scrubber is evaluated. - Abstract: Post Fukushima, many utilities are planning to install the Filtered Containment Venting System (FCVS) to protect the nuclear reactor containment under severe accident scenarios. FCVS aims at depressurizing the reactor containment and filtering the radioactive gas leaving the containment boundary under such scenarios. A key component of FCVS is the venturi scrubber. The hydrodynamics of the fluid flow inside the FCVS is complex involving multi-phase and multi-component systems with droplets formed by entrainment of high velocity gas stream with liquid jet. The size of droplets and their trajectories are critical with regard to decontamination of iodine which combines with the droplets inside the venturi. Thus, understanding the injection characteristics, hydrodynamics and absorption of iodine vapour with the droplets in the venturi scrubber is very important. There are very limited studies on this aspect. In view of this, a CFD simulation study has been performed in this work for understanding of venturi scrubber operation and performance in self-priming mode. The interaction of the continuous gas phase and droplets is assumed to occur through drag force. The gas flow rate is varied from 1700 to 3400 lpm for two submergence heights of 3 m and 4 m. The liquid loading is estimated from pressure drop at the nozzles observed in the experiments. The retention of iodine in venturi scrubber is calculated assuming instantaneous mass transfer at gas droplet interface. The mass transfer coefficient is evaluated from Steinberg and Treybal correlation. Our results indicate that the liquid loading to the scrubber increases at lower flow rates of 1700 lpm gas flow and further decreases with the increase in gas flow. The two phase pressure profile predicted is close to measured values for each case. The pressure decreases in the converging section; decreases sharply in the throat section due to liquid injection as droplets and shows negligible recovery in the diverging section of the venturi scrubber. The retention of iodine in the venturi scrubber increases with the increase in gas flow rate in the venturi scrubber. This indicates that the participation of venturi scrubber in the iodine retention increases as the air flow rate increases.

[en] Highlights: • Submerged Pease-Anthony type venturi scrubber is experimentally investigated. • Axial variation of pressure drop across the venturi scrubber is measured. • Scrubbing performance of iodine vapors is simulated. • The effect of submergence height on scrubbing performance has been studied. • The decontamination factor for iodine vapors has been presented. - Abstract: Post Fukushima accident, the societal impact of radiological leakage to the environment necessitated further exploration and robust strategy to safeguard the nuclear reactor containment. In view of this, design of Filtered Containment Venting System (FCVS) plays an important role in depressurizing the reactor and preventing release of radionuclides to the environment. In this context, a venturi scrubber submerged in alkaline solution along with demister pad housed in a scrubber tank is investigated experimentally at prototypic conditions for retention of iodine vapors in the scrubber system. The hydrodynamics and the scrubbing performance of the submerged venturi scrubber is simulated at air flow rates of 1500–3500 lpm at the submergence height of 4 m. The iodine concentration retained in the scrubber tank and its bulk decontamination is measured by ICP-OES and ICP-MS analysis respectively. It is observed that the overall pressure drop in the venturi scrubber increases with an increase in flow rate. The two phase axial pressure drop characteristics reveal an increase in the pressure drop in the converging section with increase in flow rate. The pressure drop also increases after the scrubbing liquid from the hydrostatic pool is suctioned and no recovery is found in the diverging section of the venturi scrubber. The iodine retention in the scrubber tank increases sharply with an air flow rates up to 2200 lpm after that the increase is gradual with the maximum of 95% retention at 3400 lpm. It is seen that in all the experiments the amount of iodine leaving the facility is negligible and the bulk decontamination is high. Further, effect of submergence height on the retention of iodine vapors in the scrubber tank is compared at 3 m and 4 m. It is seen that an increase in submergence height significantly increased the percentage of iodine arrested in the scrubber tank at 1700 lpm air flow but further increase in air flow rate has lesser impact on the iodine retention in the scrubber tank despite the increase in submergence height.