Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The primary circuit components of very high temperature reactors (VHTRs) experience various unwanted fission products such as Kr, Xe, I, Cs, Sr, and Ag. These particle are generated during normal operation of the reactor from abaration, cracks and/or deffects are transported by the helium coolant. The main candidate that has been identified as a cause for concern and the focus of research to minimizing radioactive contamination of the reactor coolant circuit is silver. This is because the design of the coated particles limits the release of fission products into the coolant except for silver(Ag110m). Ag110m is a long lived metallic fission product formed inside the nuclear reactor core and is the only known element released out of the coated particles into the coolant at any temperature above 1150 ◦C when the reactor starts to heat up. The release occurs on intact coated particles, failed particles and also from defective particles. The amount of released silver is initially small and occurs as the pebble heats up and this is strongly dependent on the temperature of the core. It is therefore able to reach the surface of the reactor core and enter into the Helium coolant flowing throughout the reactor. Thus Ag110m will be circulated through the reactor circuit until it reaches the cooler sides of the main power system (MPS) where it will start to plate out. The presence of this radioactive silver in the primary circuit components may result in unwanted maintenance problems from a radiation hazard point of view. The development of a method to remove particles from the helium stream is therefore needed. In this work, two theoretical deflection models used to deflect the silver particles are proposed, namely the stochastic and the deterministic deflection models. The latter describes the deflection of microparticles in a helium medium. It uses the dielectrophoresis (DEP) technique to investigate the deflection of a silver µm moving in a helium medium with the bulk velocity of 0.021 ms−1 and subjected to a dielectrophoretic force only deflect an amount of 0.52039 nm and 4.49882 nm in the x - and z -directions on average. The former (stochastic deflection model) describes the deflection of ions and polarized particles by using probability theory, namely kinetic theory of gases. This model showed that the mean free time that the particle spends while deflected by a uniform electric field is short so much that there is not enough time for a silver particle to be appreciably deflected between collisions. For example, when an electric field of 100kV/m was applied on a single silver ion for a time of 0.1 µs, the deflection distance obtained was 33.38 mm for a free time of 0.189285 ns and under pressure and temperature conditions of 1 bar and 20◦ C. The Brownian motion was then compared to the effects of a nonuniform electric field in polarizing and deflecting an atom. This is done by comparing the Brownian motion and the polarizibility of an atom using nonuniform electric fields. It is found that the silver speed produced from Brownian motion (79.563 ms−1) is far larger than that produced from the polarizibility of an atom (4.69455×106 nms−1). The deterministic and stochastic deflection models using nonuniform electric fields proved that the dielectrophoresis technique is negligibly small in deflecting particles and cannot be used to deflect silver particles as required in a VHTR. / AFRIKAANSE OPSOMMING: Die primˆere siklus komponente van baie ho temperatuur reaktore (VHTRs) word bloodgestel aan verskeie ongewenste fisie produkte soos Kr, Xe, I, Cs, Sr, en Ag. Hierdie deeltjies word gegenereer gedurende normale werking van die reaktor van abarasie, krake en / of defekte word vervoer deur die helium verkoelingsmiddel. Die belangrikste kandidaat wat gedentifiseer is as ’n rede vir kommer en die fokus van navorsing op die minimalisering van radioaktiewe besoedeling van die reaktor verkoelingsmiddel siklus is silwer. Die rede hiervoor is die ontwerp van die omhulsel wat die vrylating van die fisie produkte in die koelmiddel behalwe vir silwer (Ag110m) beperk. Ag110m is ’n metaal fisie-produk met ’n lang leeftyd wat gevorm word binne-in die kern van die reaktor en is sover bekend die enigste element wat vrygestel word deur die bedekte deeltjies in die verkoelingsmiddel by enige temperatuur bo 1150 ◦C wanneer die reaktor begin verhit. Die vrystelling kom voor by ongeskonde brandstofomhulse, nie funksionele deeltjies en ook van gebrekkige deeltjies. Die bedrag van vrygestel silwer is aanvanklik klein en kom voor as die brandstofelemente verhit en heirdie vrystelling is sterk afhanklik van die temperatuur van die kern. Dit is dus in staat om die oppervlak van die reaktor kern te bereik en betree die Helium verkoelingsmiddel vloeistelsel en beweeg regdeur die reaktor. Dus sirkuleer die Ag110m deur die reaktor kring totdat dit die koeler kante van die MPS bereik waar dit sal begin uitplatteer. Die teenwoordigheid van hierdie radioaktiewe silwer in die primˆere stroombaan komponente kan lei tot ongewenste onderhoud probleme van ’n straling gevaar oogpunt. Die ontwikkeling van ’n metode om deeltjies te verwyder uit die helium stroom is dus nodig. In hierdie werk word van twee teoretiese defleksie modelle gebruik gemaak om die silwer partikels se defleksie te beskryf, naamlik die stogastiese en die deterministiese defleksie modelle. Laasgenoemde beskryf die defleksie van mikro grootte partikel in ’n helium medium. Dit maak gebruik van die dielektroflorosensie (DEP) tegniek om ondersoek in te stel na die defleksie van ’n silwer deeltjie met ’n radius van 3 μm. Dit is vanaf hierdie model waargeneemdat ’n silwer mikrodeeltjie met ’n radius van 3 m in ’n helium medium beweeg met die snelheid van 0,021 ms−1 en onderworpe is aan ’n dielektroforetiese krag dit net met ’n gemiddelde van 0,52039 nm en 4,49882 nm in die x - en z -rigtings deflekteer. Die voormalige (stogastiese defleksie model) beskryf die defleksie van ione en gepolariseerde partikels deur gebruik te maak van waarskynlikheidsteorie, naamlik die kinetiese teorie van gasse. Hierdie model toon dat die gemiddelde vrye tyd wat die deeltjie spandeer terwyl dit gedeflekteer word deur ’n uniforme elektriese veld sovel korter is dat daar nie genoeg tyd is vir ’n silwer deeltjie is om aansienlik tussen botsings gedeflekteer kan word nie. Byvoorbeeld, wanneer ’n elektriese veld van 100kV/m toegepas word op ’n enkele silwer ioon vir ’n tyd van 0.1 μs, die defleksie afstand van 33,38 mm verkry word vir ’n vrye tyd van 0.189285 ns en onder druk en temperatuur voorwaardes van 1 bar en 20 ◦C. Die Brown-beweging was dan vergeleke met die uitkoms van n univorme elektriese veld wat n polariserende atoom deflekteer. Dit word gedoen deur die vergelykings van die Brown-beweging en die polariseerbaarheid van ’n atoom met behulp van nie-uniform elektriese veld te gebruik. Daar word gevind dat as die silwer spoed van Brown se beweging (79,563 ms−1) veel groter is as di van die polariseerbare atoom (4,69455 × 10−6 nms−1). Die deterministiese en stogastiese defleksie modelle deur gebruik te maak van nonuniform elektriese velde bewys dat dielectrophoresis tegniek is weglaatbaar klein in defleksie van deeltjies en kan dus nie gebruik word om silwer partikels te buig soos wat in ’n VHTR vereis word nie.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / The PBMR has called for research into the possibility of distributed in-core temperature measurement. In this thesis, several methods for distributed temperature measurement in high-pressure, -radiation and -temperature environments have been investigated by means of a literature study. The literature study has revealed FBG temperature sensors as the most feasible solution to the temperature measurement challenge. Various parameters affecting the propagation of light in optical fibres and consequently the FBG reflection profile was researched. The differential equations describing FBG structures were solved and implemented in Matlab in order to simulate WDM of a distributed FBG sensing system. Distributed sensing with apodized FBGs written in sapphire optical fibre show the most promise of becoming a solution to the measurement challenge. However, practical testing of sapphire FBGs exposed to the environment in the PBMR core is required. With this long-term goal in mind, a general test platform for FBG temperature sensors was assembled. A heater controller was built for a specialized fibre heating element capable of controlling the temperature of a single FBG up to 1600 C. Temperature measurement using wavelength division multiplexing of apodized FBGs written in silica optical fibre were demonstrated in the test platform with great success. The measured results corresponded very well with the theory. Finally, the implementation of FBGs in the PBMR is discussed and recommendations are made for future work

This study introduces an evaluation of the downstream utilization of oxygen produced by the hybrid sulfur process (HYS). Both technical and economic aspects were considered in the production of primarily synthesis gas and hydrogen. Both products could increase the economic potential of the hybrid sulfur process. Based on an assumed 500MWt pebble bed modular nuclear reactor, the volume of hydrogen and oxygen produced by the scaled down HYS was found to be 121 and 959 ton per day respectively. The partial oxidation plant (POX) could produce approximately 1840 ton synthesis gas per day based on the oxygen obtained from the HYS. The capital cost of the POX plant is in the order of $104 million (US dollars, Base year 2008). Compared to the capital cost of the HYS, this seems to be a relatively small additional investment. The production cost varied from a best case scenario $9.21 to a worst case scenario of $19.36 per GJ synthesis gas. The profitability analysis conducted showed favourable results, indicating that under the assumed conditions, and with 20 years of operation, a NPV of $87 mil. and an IRR of 19.5% could be obtained, for the assumed base case. The economic sensitivity analysis conducted, provided insight into the upper and lower limitations of favourable operation. The second product that could be produced was hydrogen. With the addition of a water gas shift and a pressure swing adsorption process to the POX, it was found that an additional 221 ton of hydrogen per day could be produced. The hydrogen could be produced in the best case at $2.34/kg and in the worst case at $3.76/kg. The investment required would be in the order of $50 million. The profitability analysis for the base case analysis predicts an NPV of $206 million and a high IRR of 23.0% under the assumed conditions. On financial grounds it therefore seemed that the hydrogen production process was favourable. The thermal efficiency of the synthesis gas production section was calculated and was in good agreement with that obtained from literature. The hydrogen production section’s thermal efficiency was compared to that of steam methane reforming of natural gas (SMR) and it was found that the efficiencies were comparable but the SMR process was superior. The hydrogen production capacity of the HYS process was increased by a factor of 1.83. This implied that for every 1 kg of hydrogen produced by the HYS an additional 1.83 kg was produced by the proposed process addition. This lowers the cost of hydrogen produced by the HYS from $6.83 to the range of approximately $3.93 - $4.85/kg. In the event of a global hydrogen economy, traditional production methods could very well be supplemented with new and innovative methods. The integration of the wellknown methods incorporated with the new nuclear based methods of hydrogen production and chemical synthesis could facilitate the smooth transition from fossil fuel based to environmentally friendly methods. This study presents one possible integration method of nuclear based hydrogen production and conventional processing methods. This process is technically possible, efficient and economically feasible. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2009.

It is a great challenge in the design of the PBMR to accurately predict gas flow and heat transfer in the reactor. Understanding the heat transfer at the core-reflector interface in particular is a very important aspect as the reactivity of the control rods housed in the reflectors is highly temperature dependent. It is also very important because the core-reflector interface is on the critical path for heat removal during accident conditions. PBMR has developed an OECD/NEA coupled neutronic/thermal-hydraulic benchmark to aid in the understanding of the different modelling approaches currently employed at PBMR. A comparison of THERMIX-KONVEK and DIREKT results showed large temperature differences at the core-reflector interfaces. Further investigation showed that these differences are as a result of the numerical methods used i.e. Cell-Centred (CC) vs. Vertex-Centered (VC). The present study extended this comparison to Star-CD (CC) and Flownex (VC) which are also used to simulate the reactor at PBMR. An ID MATLAB program that mimics the CC and VC numerical methods was verified against Star-CD and Flownex. This program was then used to model an ID version of the OECD/NEA benchmark. Results were compared with DIREKT and THERMIX-KONVEK. Although the results compared well, there were significant errors at the core-reflector interfaces. The findings of this study were that different numerical methods will predict different temperatures, heat fluxes and (temperature-dependent) sink terms. It was also shown that in addition to the differences resulting from numerical methods, differences were seen between Star-CD and DIREKT and Flownex and THERMIX-KONVEK in the region of the core-reflector boundary. In general, for complicated simulations like that of the pebble bed, the numerical basis of software used to simulate the problem needs to be understood for the problem to be correctly modelled. / Thesis (M.Sc. Engineering Sciences (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010.

Due to the constantly growing environmental concerns about global warming, there is immense pressure on the coal-to-liquids (CTL) industry to lower carbon dioxide emissions. This study evaluates the cogeneration of electricity and process steam, using coal and nuclear heat obtained from a High Temperature Gas Cooled Reactor (HTGR) such as a Pebble Bed Modular Reactor (PBMR), for the use in a CTL plant. Three different cogeneration processes were investigated to resolve what influence nuclear cogenerated electricity and process steam would have on the carbon dioxide emissions and the unit production cost of electricity and process steam. The first process investigated utilises coal as combustion medium and an extraction/condensing steam turbine, together with the thermodynamic Rankine cycle, for the cogeneration of electricity and process steam. This process was used as a basis of comparison for the nuclearbased cogeneration processes. The second process investigated utilises nuclear heat generated by a HTGR and the same power conversion system as the coal-based cogeneration system. Utilising a HTGR as a heat source can decrease the carbon dioxide emissions to approximately zero, with a 91.6% increase in electricity production cost. The last process investigated is the nuclear-based closed cycle gas turbine system where a gas turbine and Brayton cycle is coupled with a HTGR for the cogeneration of electricity and process steam. It was found on technical grounds that this process would not be viable for the cogeneration of electricity and process steam. The unit production cost of electricity and process steam generated by each process were determined through an economic analysis performed on each process. Overall it was found that the CTL industry could benefit a great deal from utilising nuclear heat as a heat source. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.

This study introduces an evaluation of the downstream utilization of oxygen produced by the hybrid sulfur process (HYS). Both technical and economic aspects were considered in the production of primarily synthesis gas and hydrogen. Both products could increase the economic potential of the hybrid sulfur process. Based on an assumed 500MWt pebble bed modular nuclear reactor, the volume of hydrogen and oxygen produced by the scaled down HYS was found to be 121 and 959 ton per day respectively. The partial oxidation plant (POX) could produce approximately 1840 ton synthesis gas per day based on the oxygen obtained from the HYS. The capital cost of the POX plant is in the order of $104 million (US dollars, Base year 2008). Compared to the capital cost of the HYS, this seems to be a relatively small additional investment. The production cost varied from a best case scenario $9.21 to a worst case scenario of $19.36 per GJ synthesis gas. The profitability analysis conducted showed favourable results, indicating that under the assumed conditions, and with 20 years of operation, a NPV of $87 mil. and an IRR of 19.5% could be obtained, for the assumed base case. The economic sensitivity analysis conducted, provided insight into the upper and lower limitations of favourable operation. The second product that could be produced was hydrogen. With the addition of a water gas shift and a pressure swing adsorption process to the POX, it was found that an additional 221 ton of hydrogen per day could be produced. The hydrogen could be produced in the best case at $2.34/kg and in the worst case at $3.76/kg. The investment required would be in the order of $50 million. The profitability analysis for the base case analysis predicts an NPV of $206 million and a high IRR of 23.0% under the assumed conditions. On financial grounds it therefore seemed that the hydrogen production process was favourable. The thermal efficiency of the synthesis gas production section was calculated and was in good agreement with that obtained from literature. The hydrogen production section’s thermal efficiency was compared to that of steam methane reforming of natural gas (SMR) and it was found that the efficiencies were comparable but the SMR process was superior. The hydrogen production capacity of the HYS process was increased by a factor of 1.83. This implied that for every 1 kg of hydrogen produced by the HYS an additional 1.83 kg was produced by the proposed process addition. This lowers the cost of hydrogen produced by the HYS from $6.83 to the range of approximately $3.93 - $4.85/kg. In the event of a global hydrogen economy, traditional production methods could very well be supplemented with new and innovative methods. The integration of the wellknown methods incorporated with the new nuclear based methods of hydrogen production and chemical synthesis could facilitate the smooth transition from fossil fuel based to environmentally friendly methods. This study presents one possible integration method of nuclear based hydrogen production and conventional processing methods. This process is technically possible, efficient and economically feasible. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2009.

It is a great challenge in the design of the PBMR to accurately predict gas flow and heat transfer in the reactor. Understanding the heat transfer at the core-reflector interface in particular is a very important aspect as the reactivity of the control rods housed in the reflectors is highly temperature dependent. It is also very important because the core-reflector interface is on the critical path for heat removal during accident conditions. PBMR has developed an OECD/NEA coupled neutronic/thermal-hydraulic benchmark to aid in the understanding of the different modelling approaches currently employed at PBMR. A comparison of THERMIX-KONVEK and DIREKT results showed large temperature differences at the core-reflector interfaces. Further investigation showed that these differences are as a result of the numerical methods used i.e. Cell-Centred (CC) vs. Vertex-Centered (VC). The present study extended this comparison to Star-CD (CC) and Flownex (VC) which are also used to simulate the reactor at PBMR. An ID MATLAB program that mimics the CC and VC numerical methods was verified against Star-CD and Flownex. This program was then used to model an ID version of the OECD/NEA benchmark. Results were compared with DIREKT and THERMIX-KONVEK. Although the results compared well, there were significant errors at the core-reflector interfaces. The findings of this study were that different numerical methods will predict different temperatures, heat fluxes and (temperature-dependent) sink terms. It was also shown that in addition to the differences resulting from numerical methods, differences were seen between Star-CD and DIREKT and Flownex and THERMIX-KONVEK in the region of the core-reflector boundary. In general, for complicated simulations like that of the pebble bed, the numerical basis of software used to simulate the problem needs to be understood for the problem to be correctly modelled. / Thesis (M.Sc. Engineering Sciences (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010.

Due to the constantly growing environmental concerns about global warming, there is immense pressure on the coal-to-liquids (CTL) industry to lower carbon dioxide emissions. This study evaluates the cogeneration of electricity and process steam, using coal and nuclear heat obtained from a High Temperature Gas Cooled Reactor (HTGR) such as a Pebble Bed Modular Reactor (PBMR), for the use in a CTL plant. Three different cogeneration processes were investigated to resolve what influence nuclear cogenerated electricity and process steam would have on the carbon dioxide emissions and the unit production cost of electricity and process steam. The first process investigated utilises coal as combustion medium and an extraction/condensing steam turbine, together with the thermodynamic Rankine cycle, for the cogeneration of electricity and process steam. This process was used as a basis of comparison for the nuclearbased cogeneration processes. The second process investigated utilises nuclear heat generated by a HTGR and the same power conversion system as the coal-based cogeneration system. Utilising a HTGR as a heat source can decrease the carbon dioxide emissions to approximately zero, with a 91.6% increase in electricity production cost. The last process investigated is the nuclear-based closed cycle gas turbine system where a gas turbine and Brayton cycle is coupled with a HTGR for the cogeneration of electricity and process steam. It was found on technical grounds that this process would not be viable for the cogeneration of electricity and process steam. The unit production cost of electricity and process steam generated by each process were determined through an economic analysis performed on each process. Overall it was found that the CTL industry could benefit a great deal from utilising nuclear heat as a heat source. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.

Philosophiae Doctor - PhD / The framework agreement in the year 2000 by the international community to launch Generation IV program with 10 nations, to develop safe and reliable nuclear reactors gave rise to the increased interest in the studies of SiC and the effect of different irradiations on solids. Silicon carbide is a preferred candidate used in harsh environments due to its excellent properties such as high chemical stability and strong mechanical strength. The PBMR technology promises to be the safest of all nuclear technology that have been developed before. SiC has been considered one candidate material being used in the fabrication of pebble bed fuel cell. Its outstanding physical and chemical properties even at high temperatures render it a material of choice for the future nuclear industry as whole and PBMR in particular. Due to the hostile environment created during the normal reactor operation, some of these excellent properties are compromised. In order to use this material in such conditions, it should have at least a near perfect crystal lattice to prevent defects that could compromise its strength and performance. A proper knowledge of the behavior of radiation-induced defects in SiC is vital. During irradiation, a disordered crystal lattice occurs, resulting in the production of defects in the lattice. These defects lead to the degradation of these excellent properties of a particular material. This thesis investigates the effects of various radiation effects to 6H-SiC. We have investigated the effects of radiation induced damages to SiC, with a description of the beds and the importance of the stability of the SiC-C interface upon the effects of radiations (y-rays, hot neutrons). The irradiated samples of 6H-SiC have been studied with various spectroscopic and structural characterization methods. The surface sensitive techniques such as Raman spectroscopy, UV-Vis, Photoluminescence and Atomic Force Microscopy will be employed in several complimentary ways to probe the effect of irradiation on SiC. The obtained results are discussed in details.

The Pebble Bed Modular Reactor (PBMR) is a high temperature graphite-moderated nuclear reactor that uses helium as a coolant. The triple coated (TRISO) particles contain enriched uranium oxide fuel which is coated with layers of various forms of pyrolytic carbon and silicon carbide. The TRISO particles are further embedded in the matrix of spherical graphite pebbles. The graphite matrix is a composite moulded from a compound containing natural flake graphite (64 wt.%), synthetic graphite (16 wt.%) and a phenolic resin binder (20 wt.%) heated to 1800 °C in inert atmosphere. The graphitic composite provides structural integrity, encasement and act as a moderator material. In this work, low density model graphite composites similar to those used in nuclear applications as encasement material in fuel pebbles were made by uniaxial cold compression moulding. The graphitic composites contained various ratios of natural flake graphite and synthetic graphite at fixed phenolic novolac resin binder content of 20 wt.% (green state). The fabrication process employed entails mixing the graphite powders, followed by addition of methanol phenolic resin solution to the graphite powder mix, drying, grinding, milling and sieving; and finally compression moulding in a stainless steel die at 13 MPa using a hydraulic press. The green moulded disc specimens were then carbonized at 900 °C in nitrogen atmosphere to remove volatiles followed by annealing at 1800 °C in helium atmosphere. The annealing step diminishes structural defects and result in densification of the composites. The microstructure of fabricated graphitic composites was characterized using various techniques. Particle Size Distributions determined using Laser diffraction showed that the inclusion of the binder leads to agglomeration. The composite powders had larger mean particle sizes than the raw graphite powders showing the binding effect of the novolac phenolic resin. X-ray diffraction studies showed that the graphitic composites had a hexagonal crystal structure after annealing. Raman spectroscopy revealed the presence of the structurally disordered phase derived from the resin carbon (indicated by the pronounced D-band in the Raman spectra). XRD and Raman observations were consistent with literature and gave results supporting existing knowledge base. Optical microscopy revealed a flake-like microstructure for composites containing natural graphite and needle-coke like particles for composites containing mainly synthetic graphite. Optical microscopy confirmed that the effect of the manufacturing route employed here was to align the particles in the direction perpendicular to the compression moulding direction. As a result, the graphitic composites exhibited anisotropic property behavior. The bulk density of the composites increased with the increase in the natural graphite content due to compactability of natural flakes in the manufacturing route. Thermogravimetric analysis studies on the composites showed that they were stable in air to 650 °C. Composites containing mainly synthetic graphite were thermally more stable in air compared to their natural graphite counterparts. The linear coefficients of thermal expansion of the composites were measured using thermomechanical analysis (20-600 °C). In the moulding direction, the average CTE (αP) values were in the range (5-9) × 10-6 K-1 and increased with increment in the natural graphite content in the composite. In the direction perpendicular to moulding direction, the average CTE (αN) values were in the range (1.7-2.1) × 10-6 K-1 showing that the expansion was similar or constant in this direction. Therefore an anisotropic expansion ratio, i.e. αP:αN, of about 3 was observed in the composites. This anisotropy is attributable to the alignment of the filler particles in the manufacturing route. The thermal conductivity of the annealed composites were measured in the pressing direction from 100 to 1000 °C and the values ranged from 19 to 30 W m-1 K-1. Anisotropy was also observed as far as strength was concerned. A composite containing 64:16:20 wt.% ratio had the best mechanical properties, high thermal conductivity and slightly high expansion coefficient. This work demonstrates the complimentary properties of the graphite fillers in the composites. It also reports for the first time, data on the effect of variation of the filler graphites on microstructure and properties of model low density compression moulded graphitic composites. / Thesis (PhD)--University of Pretoria, 2013. / gm2014 / Chemistry / unrestricted