Progress on Particle Resuspension in Nuclear Reactors

Abstract

In nuclear reactors, fine aerosol particles are one of the important carriers of radioactive substances. In the case of a serious accident in a nuclear reactor, the resuspension behavior of particles under the effect of transient airflow may cause a radioactive leak, which is an important content of the radioactive source term analysis. According to the deposition patterns of particles on the wall, particle resuspension can be divided into monolayer resuspension and multilayer resuspension. For the monolayer resuspension, the particle resuspension motion can be divided into lift-off motion determined by lift and adhesion, slide motion determined by drag and friction, and roll motion determined by aerodynamic torque and adhesion torque. For the multilayer resuspension, particle deposition structures affect particle resuspension and produce additional effects including cohesion, shielding effects, coverage effects, restructuration and saltation, which complicates the resuspension process. The resuspension fraction, resuspension rate, and resuspension rate constant are used to describe the resuspension process quantitatively. In terms of physical force, the resuspension process usually depends on the forces from flow including drag, lift, Brownian force, and thermophoresis force, as well as the forces from walls or other particles including cohesion, van der Waals adhesion and friction. For a nuclear reactor system, wall forces acting on particles during the resuspension are more complicated due to the potential capillary force of condensate droplets (pressurized water reactor (PWR)) and the potential sintering force under the high-temperature condition (high-temperature gas-cooled reactor (HTGR)). To predict the resuspension process, scholars proposed various prediction methods including empirical formula, mechanism model (force/torque equilibrium model and energy accumulation model), and numerical simulation method. The empirical formula is only applicable to limited working conditions, the mechanism model may lack key parameters and result in uncertain accuracy, and numerical methods including computational fluid dynamics and the Monte Carlo method can consider more realistic physical processes, but the computational cost in engineering applications is enormous. Based on above principles and methods, the particle resuspension study in the field of nuclear reactors was reviewed in this paper. For PWRs, aerosol resuspension can be induced by continuous fluid flow under convection, transient fluid flow from hydrogen explosion, and bubbling of bubbles in the liquid phase. The resuspension caused by continuous fluid flow possesses the characteristics of long duration and wide occurrence region, which accounts for the main proportion of aerosol resuspension in PWR. For HTGRs, micron-scale graphite dust is the main radioactive carrier. Available reactor experimental data are only obtained from the AVR reactor. Existing research usually uses a combination of resuspension mechanism models and modeling tests to study its resuspension fraction and evaluate dust leakage under accidents. For fusion reactors, dust is mainly generated by the fragmentation of the co-deposited layers formed by plasma and walls. The experiment is usually conducted through a simplified device of the vacuum chamber. Existing experimental research mainly focuses on the influence of geometric parameters of the breach under LOVA (loss of vacuum accidents).

Keywords

• nuclear reactor
• aerosol particle
• resuspension