Particle deflection and plate-out dynamics in a helium stream

[摘要] ENGLISH ABSTRACT: In the primary circuit components of high temperature reactors, various unwantedparticles have been found. These particles include, for example, graphite and silver-110 (110mAg). The silver-110 (110mAg) particles are radioactive, with a half-life of 253days. The presence of radioactive particles in the primary circuit components constitutesan unwanted maintenance problem from a radiation hazard point of view. Thedevelopment of a method to remove these particles from the helium stream is thereforeneeded. This thesis consideres two possible methods of removing silver from thehelium stream, namely laser ablation of microparticles and electrostatic precipitation.These methods require the generation of silver ions or charged particles, the deflectionof these particles in a helium gas stream passing through an electric field and thesubsequent plate-out of these particles onto deflection electrodes.To find a suitable method of generating ions, various methods to generate the silverions were investigated and evaluated. These methods include existing ion sources,thermionic, field and photoelectric emission and laser ablation of microparticles. Noexisting ion sources could be found which could be utilised in helium at high pressure.From calculations it was concluded that thermionic, field and photoelectric emissioncould also not be used to raise the energy of the emitted electrons sufficiently to ionizesilver in a helium flight path. These methods were found not to be feasible ion sourcesin helium at high pressures. However, laser ablation of microparticles was found toconstitute a feasible technology.Laser ablation was successfully utilised by Nichols et al. (2000) to deflect silver nanoparticlesin an electric field across a two bar helium stream. An apparatus, similar to theone developed by Nichols et al. (2000), was designed and built. The apparatus includeda silver insertion mechanism and tests with this apparatus were called the microparticletests. To determine the efficacy of the silver insertion mechanism, the microparticletests were done without the use of a laser. It was found that a laser was not necessaryas microparticles collected on both the deflection electrodes. Dielectrophoresiswas proposed as a possible explanation for the deflection and the plate-out of the mcroparticles.To theoretically model the deflection of the silver particles, two models were proposed,namely the deterministic and the stochastic deflection models. The latter describes thedeflection of atoms, ions and polarized particles by using probability theory. From thismodel it was found that the Brownian motion force is far larger that the force createdby the polarizibility of the atom due to an electric field. The deterministic deflectionmodel describes the deflection of larger particles in a continuum. From this model itwas found that a silver microparticle with a radius of 3 mm in a helium stream withbulk velocity of 0.0198 m/s would deflect 4.6 mm per helium flight path length of 140mm. From these calculation it was found that the apparatus which had been built wasnot long enough to deflect and plate-out all the silver microparticles.The dielectrophoresis force on nanoparticles cannot be calculated, as the theory of dielectrophoresisis only valid for particles with diameter larger than 1 mm. Changeswere therefore made to the apparatus to generate nanoparticles as their mobility islarger than that of microparticles. The nanoparticles were created by means of an arcdischarge in helium; therefore tests with this modified apparatus were called the arcdischarge test. The nanoparticles so created, deflected and deposited on both deflectionelectrodes. With the use of an atomic force microscope some of the particles couldbe classified as microparticles. According to the deterministic deflection model theyshould not have deflected. Combined with the fact that oxygen was in the plasma,due to the oxidation of the electrodes, a hypothesis of bipolar charging was thus proposed.The deterministic deflection model was used and supplemented with field anddiffusion charging calculations, to support this hypothesis. A reasonable correlationbetween the theoretical model and this experimental results was obtained.Based on the arc discharge test, electrostatic precipitation was proposed as the indicatedmeans of scrubbing silver and other particles such as graphite from a heliumstream. It is recommended that a new apparatus be built and that the deterministicdeflection model be used to predict the deflection of the particles. With this apparatusthe uncertainties of breakdown voltage, the effect of thermionic emission and the sizeof the particles, all of which have been identified as being important, can then also bedetermined.

[发布机构] Stellenbosch University