Particle deflection and plate-out dynamics in a helium stream

Steyn, Hermanus Johannes (2009-12)

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009.

Thesis

ENGLISH ABSTRACT: In the primary circuit components of high temperature reactors, various unwanted particles 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 253 days. The presence of radioactive particles in the primary circuit components constitutes an unwanted maintenance problem from a radiation hazard point of view. The development of a method to remove these particles from the helium stream is therefore needed. This thesis consideres two possible methods of removing silver from the helium stream, namely laser ablation of microparticles and electrostatic precipitation. These methods require the generation of silver ions or charged particles, the deflection of these particles in a helium gas stream passing through an electric field and the subsequent plate-out of these particles onto deflection electrodes. To find a suitable method of generating ions, various methods to generate the silver ions were investigated and evaluated. These methods include existing ion sources, thermionic, field and photoelectric emission and laser ablation of microparticles. No existing ion sources could be found which could be utilised in helium at high pressure. From calculations it was concluded that thermionic, field and photoelectric emission could also not be used to raise the energy of the emitted electrons sufficiently to ionize silver in a helium flight path. These methods were found not to be feasible ion sources in helium at high pressures. However, laser ablation of microparticles was found to constitute a feasible technology. Laser ablation was successfully utilised by Nichols et al. (2000) to deflect silver nanoparticles in an electric field across a two bar helium stream. An apparatus, similar to the one developed by Nichols et al. (2000), was designed and built. The apparatus included a silver insertion mechanism and tests with this apparatus were called the microparticle tests. To determine the efficacy of the silver insertion mechanism, the microparticle tests were done without the use of a laser. It was found that a laser was not necessary as microparticles collected on both the deflection electrodes. Dielectrophoresis was 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 the deflection of atoms, ions and polarized particles by using probability theory. From this model it was found that the Brownian motion force is far larger that the force created by the polarizibility of the atom due to an electric field. The deterministic deflection model describes the deflection of larger particles in a continuum. From this model it was found that a silver microparticle with a radius of 3 mm in a helium stream with bulk velocity of 0.0198 m/s would deflect 4.6 mm per helium flight path length of 140 mm. From these calculation it was found that the apparatus which had been built was not long enough to deflect and plate-out all the silver microparticles. The dielectrophoresis force on nanoparticles cannot be calculated, as the theory of dielectrophoresis is only valid for particles with diameter larger than 1 mm. Changes were therefore made to the apparatus to generate nanoparticles as their mobility is larger than that of microparticles. The nanoparticles were created by means of an arc discharge in helium; therefore tests with this modified apparatus were called the arc discharge test. The nanoparticles so created, deflected and deposited on both deflection electrodes. With the use of an atomic force microscope some of the particles could be classified as microparticles. According to the deterministic deflection model they should 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 and diffusion charging calculations, to support this hypothesis. A reasonable correlation between the theoretical model and this experimental results was obtained. Based on the arc discharge test, electrostatic precipitation was proposed as the indicated means of scrubbing silver and other particles such as graphite from a helium stream. It is recommended that a new apparatus be built and that the deterministic deflection model be used to predict the deflection of the particles. With this apparatus the uncertainties of breakdown voltage, the effect of thermionic emission and the size of the particles, all of which have been identified as being important, can then also be determined.

AFRIKAANSE OPSOMMING: In die primêre komponente van die kringloop van hoë temperatuurreaktors, was verskillende ongewenste partikels soos grafiet en silwer-110 (110mAg) teenwoordg. Silwer- 110 (110mAg) is radioaktief met ’n halfleeftyd van 253 dae. Vanuit ’n radiasie-risiko oogpunt word daar onderhoudsimplikasies geskep deur die teenwoordigheid van radioaktiewe partikels in die primêre komponente. Die ontwikkeling van ’n metode om hierdie partikels uit die heliumstroom the verwyder was dus nodig. Hierdie tesis ondersoek twee moontlike metodes van verwydering van silwer uit die heliumstroom, naamlik laser-ablasie van mikropartikels en elektrostatiese presipitasie. Hierdie metodes benodig die generasie van silwer ione of gelaaide partikels, die defleksie daarvan in ’n heliumstroom wat deur ’n elektriese veld vloei en die platering van die partikels op defleksie elektrodes. Om ’n geskikte metode the vind wat ione genereer was, verskillende metodes om die silwer ione te verkry, ondersoek en geëvalueer. Hierdie metodes sluit in bestaande ioonbronne, termioniese, veld en fotoëlektriese emissie en laser-ablasie van mikropartikels. Geen ioonbronne was gevind wat gebruik kan word in helium by hoë druk nie. Die gevolgtrekking is gemaak vanaf berekeninge dat termioniese, veld en fotoëlektriese emissie ook nie gebruik kan word om die energie van die voortgebronge elektrone genoeg te verhoog om silwer in ’n heliumstroom te ioniseer nie. Daar was gevind dat hierdie metodes nie geskik is as ioonbronne in helium by hoë druk nie. Daarenteen was laser-ablasie van mikropartikels gevind om ’n geskikte tegnologie voor te stel. Laser-ablasie van mikropartikels was suksesvol deur Nichols et al. (2000) gebruik om silwer nanopartikels te deflekteer in ’n elektriese veld oor helium van twee bar. ’n Apparaat soortgelyk aan Nichols et al. (2000) se eksperiment, was dus ontwerp en gebou. Die apparaat het ’n silwer insitmeganisme bevat en toetse met hierdie apparaat was die mikropartikel toetse genoem. Om die effektiwiteit van die insitmeganisme te bepaal, was toetse gedoen sonder opstelling van die laser. Daar was eksperimenteel gevind dat die laser nie nodig was nie, omdat mikropartikels op beide defleksie elektrodes geplateer het. Dielektroforese was voorgestel as ’n moontlike verduideliking vir deflektering en platering vir die silwer mikropartikels. Om die defleksie van silwer partikels teoreties te moduleer was twee modelle voorgestel, naamlik deterministiese en stogastiese defleksiemodelle. Laasgenoemde beskryf die defleksie van atome, ione en gepolariseerde partikels deur gebruik te maak van waarskynlikheidsteorie. Die stogastiese defleksiemodel dui aan dat die Brownian bewegingskrag veel groter is as die krag wat geskep word deur die polarisasie van ’n atoom as gevolg van ’n elektriese veld. Die deterministiese defleksiemodel beskryf die defleksie van groter partikels in ’n kontiuum. Met hierdie model was gevind dat silwer mikropartikels met ’n radius van 3 mmin ’n heliumstroom van snelheid van 0.0198 m/s, 4.6 mm sal deflekteer per 140 mm van heliumstroom lengte. Dit bewys dat die apparaat wat gebou was, se lengte onvoldoende was om al die silwer mikropartikels te deflekteer en te laat neerslaan. Die dielektroforese krag van nanopartikels kan nie uitgewerk word nie, omdat die dielektroforese model slegs geldig is vir partikels groter as 1 mm. Veranderings was dus aan die apparaat gemaak om nanopartikels te genereer omdat hul mobiliteit hoër is as die van mikropartikels. Die nanopartikels was geskep deur gebruik van ’n boogontlading in helium; daarom was toetse met hierdie gemodifiseerde apparaat die boogontladingstoets genoem. Die nanopartikels wat so geskep was, het gedeflekteer en het op beide elektrodes neergeslaan. Met die gebruik van ’n atomiese krag mikroskoop was dit gevind dat sommige van hierdie partikels mikropartikels was. Volgens die deterministiese defleksiemodel moes hul nie gedeflekteer het nie. Gekombineerd met die feit dat daar, weens oksidasie van die elektrodes, suurstof in die plasma was, was ’n hipotese van bipolêre lading voorgestel. Die deterministiese defleksiemodel is saam met die veld- en diffusielading gebruik om hierdie hipotese te staaf. ’n Redelike korrelasie tussen die teoretiese en eksperimentele data was gevind. Gebaseer op die boogontladingstoets, was elektrostatiese presipitasie voorgestel as ’n metode om silwer en ander partikels soos grafiet uit ’n heliumstroom te verwyder. Daar word voorgestel dat ’n nuwe apparaat gebou word en dat die deterministiese defleksiemodel gebruik word vir die bepaling van defleksie van die partikels. Deur die nuwe apparaat te gebruik kan die onsekerhede van deurslagspanning, effek van termioniese emissie en grootte van die partikels wat geidentifiseer is as belangrik, ook bepaal word.

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