Abstract
In the decommissioning of damaged Fukushima Daiichi reactors, the melted and re-solidified fuel debris in the bottom of the reactor pressure vessel and primary containment vessel need to be cut into small pieces before removing them from reactor buildings. During the cutting operations, submicron radioactive aerosol particles are expected to be generated and dispersed into the atmosphere of the primary containment vessel. Those suspended particles must be removed from the air atmosphere inside the containment before escaping to the environment. The water spray system in the upper part of the primary containment vessel is an effective and applicable method to remove airborne radioactive aerosol particles. Computational Fluid Dynamics simulation of aerosol scavenging by spray droplets is complicated but necessary to investigate the aerosol removal process inside the vessel. In this paper, a numerical model was developed and implemented into an open-source computational fluid dynamic code OpenFOAM to simulate the aerosol removal by water spray droplets with considering the collection mechanisms of inertial impaction, interception, and Brownian diffusion. In this model, the dispersed spray droplets were described using the Lagrangian particle tracking method, the continuous particle-laden gas was described using the Eulerian method, and a two-way interaction between dispersed and continuous phases was considered. The polydisperse aerosol particles at different diameters from 0.2 – 1 μm were treated as different gas species of the continuous phase. Continuity equations of each gas specie were solved using a passive scalar transport equation. The numerical model was validated by comparing the simulation results with the experimental data obtained from UTARTS facility. Simulation results agreed well with the experimental results. The simulation results provided more insights to better understand the aerosol removal process, including the time evolution of aerosol mass fraction and flow field of the gas phase.
Highlights
In the decommissioning of Fukushima Daiichi reactors, the resolidified fuel debris in the bottom of the reactor pressure vessel (RPV) and primary containment vessel (PCV) need to be cut into small pieces before removing them from reactor buildings (Porcheron et al, 2018)
A numerical model of aerosol scavenging by water spray droplets with considering collection mechanisms of inertial impaction, interception, and Brownian diffusion was developed and implemented into OpenFOAM
Nine size groups of aerosol particles with diameters varying from 0.2 to 1 μm were treated as nine gas species in the continuous phase and their movements were solved using passive scalar transport equations
Summary
In the decommissioning of Fukushima Daiichi reactors, the resolidified fuel debris in the bottom of the reactor pressure vessel (RPV) and primary containment vessel (PCV) need to be cut into small pieces before removing them from reactor buildings (Porcheron et al, 2018). The aerosol clouds will be generated in the cutting process and the small particles attached on the surface of internal structures may resuspend These aerosol particles will disperse in the air atmosphere of PCV and must be suppressed inside the reactor buildings. The water spray system in the upper part of PCV is originally designed to depressurize the containment vessel and mix the stratified atmosphere during a hypothetical severe accident. It is an effective and applicable mitigation method to remove the suspended aerosol particles in the PCV atmosphere. Within the size range of 0.1–1 μm ( called Greenfield gap), neither mechanism works efficiently, and the total collection efficiency is much lower than that of other size ranges (Greenfield, 1957)
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