核反应堆中气溶胶颗粒再悬浮行为研究进展

Progress on Particle Resuspension in Nuclear Reactors

  • 摘要: 在核反应堆中,微细的粉尘颗粒通常是放射性物质的重要载体,严重事故下沉积在壁面上的颗粒在瞬态气流作用下发生的再悬浮行为是反应堆放射性源项分析的重要内容。发生再悬浮的颗粒通常呈现单层或多层的颗粒沉积结构,从物理过程来说,单层颗粒的再悬浮过程取决于气流对颗粒的气动力以及壁面对颗粒的黏附力,而多层颗粒再悬浮过程还与沉积颗粒层的结构、颗粒与颗粒之间的内聚力等密切相关,此外,在核反应堆中,冷凝液滴带来的毛细力以及高温烧结导致的烧结力等作用使得颗粒的再悬浮过程更加复杂。本文首先介绍了颗粒再悬浮的几种运动模式以及影响因素,再从颗粒受力分析的角度介绍了再悬浮过程中主要的作用力和作用特点,然后介绍了目前研究颗粒再悬浮的方法以及发展出来的各种计算模型,最后总结了目前压水堆、高温气冷堆以及聚变堆中颗粒再悬浮研究的进展。

     

    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).

     

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