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

Military theories regarding surprise has a conceptual way of being described which may influence the effect that surprise actually has on a specific command level. Without sufficient explanations there might be unclear reasons for the conversion of these theories into practice. The existing research shows an increased possibility of surprise within the strategical command level and how to achieve it. It also shows the absence of research regarding what causes surprise on tactical command level, even if some theories describe surprise as being more effective at that level. Special operations can be included within the tactical command level and may offer an explanation. This case study examines which causes may lead to surprise on a tactical command level by analyzing descriptions of special operations within the conduct of regular warfare. With theoretical and empirical references, surprise was achieved in both of the selected cases. The results of the study can be divided in to two categories: the direct and the indirect causes. The categorization gives a further explanation in how to better understand surprise within a specific command level. The study contributes to the existing research by revealing a better understanding for commanders within a tactical command level in how to achieve surprise.

In many ore deposits throughout the world, brecciation often accompanies or occurs in association with mineralization (Sillitoe, 1985). Such is the case in the Tintic Mining District (Ag-Pb-Zn) of north-central Utah, where unique breccia features called pebble dikes occur alongside significant mineralization. Pebble dikes are tabular bodies of breccia, which consist of angular to rounded clasts of quartzite, shale, carbonate, and minor igneous rock cemented in a fine-grained clastic matrix. All clasts now lie above or adjacent to corresponding source rocks. Dikes are thin, typically less than 0.3 m wide to as much as 1 m, and can exceed 100 m in length. The average of the largest clast sizes is less than 3 cm but correlates positively with pebble dike width. Contacts are sharp and an envelope of fine breccia surrounds roughly half of the dikes. Pebble dikes are mostly hosted in an Eocene rhyolite lava flow, which displays argillic to silicic alteration when in contact with a pebble dike, but are also hosted in an assortment of folded Paleozoic sedimentary rocks. The dikes show a strong northeast trend in orientation, following a regional fabric of northeast-trending strike-slip and oblique-slip faults.The formation of pebble dikes has been historically attributed to the intrusion of the Silver City Stock, the Tintic District's main productive intrusion (Morris and Lovering, 1979; Hildreth and Hannah, 1996; Kim, 1997; Krahulec and Briggs, 2006). However, pebble dikes are spatially associated with a previously unrecognized porphyritic unit, informally named the porphyry of North Lily, which is texturally, mineralogically, and chemically distinct from the Silver City Stock, and like pebble dikes, is emplaced in northeast-trending plugs and dikes. Pebble dikes show a strong spatial correlation to outcrops of the porphyry of North Lily. Additionally, clasts of the porphyry of North Lily have been found in pebble dikes, while pebble dike quartzite clasts have been found as xenoliths in the porphyry of North Lily. These similarities and interactions suggest simultaneous formation. Low-grade alteration associated with pebble dikes indicates that they formed at elevated temperatures (<150°C). Stable isotope characteristics of rhyolite altered during the emplacement of pebble dikes suggests that the dikes formed in the presence of heated groundwater, with little to no magmatic water association. The overall physical, spatial, and chemical characteristics of pebble dikes of the Tintic Mining District suggest that they formed by the mobilization of breccia in the explosive escape of groundwater that had been heated by the porphyry of North Lily. This escape occurred along pre-existing northeast-trending faults and fractures. Pebble dikes then became pathways for later ore fluids, easing the creation of the district's abundant mineral resources.

<p> Detailed mapping was carried out to provide reliable data concerning the geologic structure which is relevant to the original pebble fabrics. </p> <p> Mathematical determination of an ellipsoidal shape from any two cross-sections and its application to the study of deformed ellipsoidal particles are developed and discussed. </p> <p> Final pebble fabric resulting from special coaxial superpositions of the tectonic strain ellipsoid upon originally ellipsoidal pebbles, is employed to determine the strain ratio and the original pebble orientation and shape. </p> <p> Some general techniques are outlined for different cases to determine the strain ellipsoid and the original shape of deformed pebbles. Weighted vector mean is introduced in the orientation analysis of deformed pebbles. The concept of average axial ratio in the case of unextractable pebbles is examined. </p> <p> Pebble size, pebble ductility ratio, shortening strain, and strain rate are all calculated and discussed. Finally, selective veins developing in the less-deformed pebbles are subjected to detailed orientation and petrofabric analyses. </p> / Thesis / Doctor of Philosophy (PhD)

No description available.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Regulators and designers of nuclear reactors regard knowledge of the pebble fuel temperature as important, due to the role that it plays in maintaining structural integrity and the production of neutrons. By using special fuel assemblies fitted with measuring equipment it is possible to measure the fuel temperature in stationary fuel reactors. This, however, is not possible in the pebble bed modular reactor due to its dynamic core. Designers of the pebble bed modular reactor have reserved special inspection channel borings inside the center reflector for fuel temperature measurement. By means of optical fibers and interferometry, the temperature can be measured inside such a channel. Currently the only way to control the fuel surface and core temperature is by measuring the gas inlet and outlet temperatures. This thesis attempts to determine the pebble temperature by measuring the temperature in a reflector channel. This is done by constructing an electrically heated pebble bed experimental setup simulating a cutout section of a pebble bed modular reactor core. An additional computational fluid dynamics simulation of the experimental setup was also performed. This thesis also attempts to determine if there is a measureable temperature peak that can indicate where a pebble was in contact with the reflector surface. This could then be used in future studies to determine the pebble fuel velocity as it moves down the reactor core. The computational fluid dynamics results were validated by experimental measurements. In the computational fluid dynamics model and experimental setup, it was found that there was indeed a measureable temperature difference on the temperature gradient along the reflector wall. The heat being conducted away from the pebble through the contact area can explain this. These differences were only observed when the channel was moved closer to the pebbles and it is therefore advised that some redesigning of the channel should be done if the in-core temperature is to be accurately interpreted by the designers at PBMR (Pty) Ltd. / AFRIKAANSE OPSOMMING: Reguleerders en ontwerpers van kern reaktore beskou die kennis van die korrel brandstof temperatuur as belangrik. Dit is weens die rol wat die brandstof temperatuur speel met die behoud van strukturele integriteit en die produksie van neutrone binne-in die reaktor. Met behulp van spesiale brandstof montasies toegerus met die meetings instrumentasie, is dit moontlik om die brandstof temperatuur in stilstaande brandstof reaktore te meet. Dit is egter nie moontlik in die korrel bed modulêre reaktor nie, as gevolg van sy dinamiese kern. Ontwerpers van die korrel bed modulêre reaktor het spesiale kanale in die binnekant van die middel reflektor vir brandstof temperatuur meeting gereseveer. Deur middel van optiese vesel en interferometrie, kan die temperatuur binne so 'n kanaal gemeet word. Tans is die enigste manier om die brandstof-oppervlak temperatuur te berekern, net moontlik deur gebruik te maak van die gemete gas inlaat-en uitlaat temperature van die reaktor. Hierdie tesis poog om vas te stel of die korrel brandstof temperatuur deur die meet van die oppervlak temperatuur in 'n reflektor-kanaal bepaal kan word. Dit word gedoen deur 'n elektriese verhitte korrel bed eksperimentele opstelling te bou wat 'n gedeelte van 'n korrel bed modulêre reaktor simuleer. 'n Bykomende numeriese simulasie van die eksperimentele opstelling was ook uitgevoer. Hierdie werk het ook probeer om vas te stel of daar 'n meetbare temperatuur piek op die temperatuur profiel aandui kan word waar 'n korrel in kontak is met die reflektor se oppervlak. Dit kan dan in toekomstige studies gebruik word om te bepaal wat die korrel brandstof spoed was soos dit in die reaktor beweeg. Die numerise simulasie uitslae was deur eksperimentele metings bevestig. In die numerise simulasie model en die eksperimentele opstelling, is daar gevind dat daar inderdaad 'n meetbare temperatuur verskil op die temperatuurgradiënt teen die reflektor oppervlak is. Dit kan verduidelik word as gevolg van die hitte wat weg van die korrel gelei word deur middel van die kontak area. Hierdie verskille was slegs waargeneem wanneer die kanaal nader aan die korrels geskuif is en dit word as n aanbeveling aan PBMR (Pty) Ltd gemaak om sommige herontwerpe aan die kanaal te doen indien die in-kerntemperatuur gemeet wil word en akkuraat geinterpreteer wil word.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The Pebble Bed Modular Reactor (PBMR) is a Generation IV graphite-moderated helium cooled nuclear reactor which is being developed in South Africa. The PBMR design is based on the German Arbeitsgemeinschaft Versuchreaktor (AVR). The AVR was decommissioned in December 1988 due to operational and safety problems. The PBMR project has put a lot of emphasis on safety and therefore all safety issues relating to the AVR have to be addressed before this technology can be implemented. After the decommissioning of the AVR plant, technicians found radioactive isotopes of cesium 55Cs137, 55Cs134, silver 44Ag110 and strontium 38Sr90 as well as graphite dust in the primary coolant loop of the reactor. These isotopes as well as the graphite dust have to be removed from the helium coolant stream because it can be potentially harmful to equipment, personnel and the general public. The main objective of this thesis is therefore to investigate a separation method for removing the graphite dust (and with it the radioactive isotopes) from the helium coolant stream and also test this method under different operating conditions and geometrical configurations to determine its dust separation efficacy. The device chosen to investigate is the Ranque-Hilsch vortex tube. The Ranque-Hilsch vortex tube (RHVT) is a simple device having no moving parts that produces a hot and cold air stream simultaneously at its two ends from a compressed air source. The vortex generated by the vortex generator located at the inlet of the RHVT causes strongly rotating flows similar in speed to that of a gas centrifuge. The gas centrifuge is used for isotope separation. The RHVT, in theory, can therefore be implemented to separate the graphite/silver isotopes from the helium coolant with the added benefit of either cooling or heating the coolant and was thus selected as the separation technique to be tested experimentally. The dust separation efficiency of the RHVT was tested experimentally using different grades of graphite dust, different fluids, various inlet volumetric flow rates and volume fractions and different RHVT geometries. The experimental results showed that the RHVT has a dust separation efficiency of more than 85 %. A regression analysis was also done with the experimental data to obtain a correlation between the different operating conditions (such as volumetric flow rate) and the dust separation efficiency that can be used to predict the dust efficiency under different operating and geometric conditions (such as the PBMR environment). An analytical model is also presented to describe the ‘temperature separation’ phenomenon in the RHVT, using basic thermo-physical principals to gain a better understanding of how the RHVT works. A CFD analysis was also attempted to supplement the analytical analysis but the solution did not converge and therefore only the preliminary results of the analysis are discussed. / AFRIKAANSE OPSOMMING: Die “Pebble Bed Modular Reactor” (PBMR) is `n vierde generasie grafiet gemodereede en helium verkoelde reaktor wat in Suid-Afrika ontwikkel word. Die PBMR ontwerp is gebaseer op the Duitse Arbeitsgemeinschaft Versuchreaktor (AVR) wat buite werking gestel is in Desember 1988 as gevolg van operasionele en veiligheidsprobleme. Die PBMR projek lê baie klem op veiligheid en daarom moet alle veiligheidskwessies van die AVR eers aangespreek word voor die tegnologie geimplementeer kan word. Nadat die AVR buite werking gestel is, het AVR tegnisie radioaktiewe isotope van cesium 55Cs137, 55Cs134, silwer 44Ag110 en strontium 38Sr90 asook grafiet stof in die primêre stroomkring van die reaktor gevind. Hierdie isotope sowel as die grafiet stof moet uit die helium verkoelingsmiddel in die primere stroomkring van die reaktor verwyder word aangesien dit dalk skadelik kan wees vir toerusting, personeel en die publiek. Die hoofdoelwit van hierdie tesis is dus om `n skeidingstekniek te ondersoek wat die stof (en dus ook die radioaktiewe isotope) uit die helium verkoelingsmiddel kan verwyder. Hierdie tegniek moet dan getoets word onder verskillende operasionele en geometriese toestande om die skeidingsbenuttingsgraad te bepaal. Die toestel wat gekies is om ondersoek te word is die “Ranque-Hilsch Vortex Tube”. Die “Ranque-Hisch Vortex Tube” (RHVT) is a eenvoudige uitvindsel wat geen bewegende parte bevat nie en wat warm en koue lug gelyktydig produseer vanaf `n saamgepersde lugbron. ‘n Baie sterk roteerende vloei word gegenereer in die RHVT wat dieselfde snelhede bereik as die lug in `n gas-sentrifugeerder. Die gas- sentrifugeerder word gebruik as `n isotoopskeidingsapparaat. In teorie kan die RHVT dus ook gebruik word om partikels te skei as gevolg van die sterk roteerende vloei, met die voordeel dat dit ook die lug kan verhit en verkoel. As gevolg van hierde redes is die RHVT gekies as die skeidingstegniek om te ondersoek en dus experimenteel te toets. Die benuttingsgraad van die RHVT se vermoë om die grafiet stof van die lug te skei was gevolglik eksperimenteel getoets deur gebruik te maak van verskillende gehaltes grafiet stof, verskillende vloeistowwe (lug of helium), verskillende inlaat volumevloeitempos en volume fraksies en RHVT geometrieë. Die experimentele resultate het getoon dat die RHVT `n benuttingsgraad van meer as 85 % het. `n Regressie analise was ook gedoen met die eksperimentele data om `n korrelasie tussen die verskillende opersionele toestande (soos volumevloeitempo) en die stof skeiding benuttingsgraad te kry. Hierdie korrelasie kan dan gebruik word om die stofskeidingsbenuttingsgraad onder ander operasionele en geometriese omstandighede, soos die PBMR omgewing, te voorspel. `n Analitiese model word ook voorgestel om die “temperatuur-skeidings” meganisme in die RHVT te verduidelik, met die hulp van basiese termo-fisiese beginsels, om beter te verstaan hoe dit werk. Daar was ook gepoog om `n CFD analise te doen wat die analitiese model kon aanvul, maar die numeriese oplossing het nie gekonvergeer nie en dus word net die voorlopige resultate van dié analise bespreek.

ropositional proof complexity is the study of the resources that are needed to prove formulas in propositional logic. In this thesis we are concerned with the size and space of proofs, and in particular with the latter. Different approaches to reasoning are captured by corresponding proof systems. The simplest and most well studied proof system is resolution, and we try to get our understanding of other proof systems closer to that of resolution. In resolution we can prove a space lower bound just by showing that any proof must have a large clause. We prove a similar relation between resolution width and polynomial calculus space that lets us derive space lower bounds, and we use it to separate degree and space. For cutting planes we show length-space trade-offs. This is, there are formulas that have a proof in small space and a proof in small length, but there is no proof that can optimize both measures at the same time. We introduce a new measure of space, cumulative space, that accounts for the space used throughout a proof rather than only its maximum. This is exploratory work, but we can also prove new results for the usual space measure. We define a new proof system that aims to capture the power of current SAT solvers, and we show a landscape of length-space trade-offs comparable to those in resolution. To prove these results we build and use tools from other areas of computational complexity. One area is pebble games, very simple computational models that are useful for modelling space. In addition to results with applications to proof complexity, we show that pebble game cost is PSPACE-hard to approximate. Another area is communication complexity, the study of the amount of communication that is needed to solve a problem when its description is shared by multiple parties. We prove a simulation theorem that relates the query complexity of a function with the communication complexity of a composed function. / <p>QC 20170509</p>

The premise of the work presented here is to use a common analytical tool, Computational Fluid dynamics (CFD), along with a difference turbulence models. Eddy viscosity models as well as state-of-the-art Large Eddy Simulation (LES) were used to study the flow past bluff bodies. A suitable CFD code (CFX5.6b) was selected and implemented. Simulation of turbulent transport for the gas through the gaps of the randomly distributed spherical fuel elements (pebbles) was performed. Although there are a number of numerical studies () on flows around spherical bodies, none of them use the necessary turbulence models that are required to simulate flow where strong separation exists. With the development of high performance computers built for applications that require high CPU time and memory; numerical simulation becomes one of the more effective approaches for such investigations and LES type of turbulence models can be used more effectively. Since there are objects that are touching each other in the present study, a special approach was applied at the stage of building computational domain. This is supposed to be a considerable improvement for CFD applications. Zero thickness was achieved between the pebbles in which fission reaction takes place. Since there is a strong pressure gradient as a result of high Reynolds Number on the computational domain, which strongly affects the boundary layer behavior, heat transfer in both laminar and turbulent flows varies noticeably. Therefore, noncircular curved flows as in the pebble-bed situatio n, in detailed local sense, is interesting to be investigated. Since a compromise is needed between accuracy of results and time/cost of effort in acquiring the results numerically, selection of turbulence model should be done carefully. Resolving all the scales of a turbulent flow is too costly, while employing highly empirical turbulence models to complex problems could give inaccurate simulation results. The Large Eddy Simulation (LES) method would achieve the requirements to obtain a reasonable result. In LES, the large scales in the flow are solved and the small scales are modeled. Eddy viscosity and Reynolds stress models were also be used to investigate the applicability of these models for this kind of flow past bluff bodies at high Re numbers.

Irradiation experiments for the pebble bed modular reactor PBMR fuel (coated fuel particles and pebble fuel) are planned at the South African First Atomic Reactor Installation (SAFARI-1). The experiments are conducted to investigate the behavior of the fuel under normal operating and accelerated/accident simulating conditions because the safe operation of the reactor relies on the integrity of the fuel for retention of radioactivity. For fuel irradiation experiments, the accurate knowledge and analysis of the neutron spectrum of the irradiation facility is required. In addition to knowledge of the neutron spectrum in the irradiation facility, power distributions and knowledge of nuclear heating values has to be acquired. The SAFARI-1 reactor boosts operating fluid temperatures of about 300 K. On the contrary, the PBMR can reach temperatures in up to about 1370 K under normal operating conditions. This calls for design of high temperature irradiation rigs for irradiation of the PBMR fuel in the SAFARI-1 reactor. The design of this instrument (rig) should be such that to create an isolated high temperature environment in the SAFARI-1 reactor, to achieve the requirements of the PBMR fuel irradiation program. The design of the irradiation rig is planned such that the rig should fit in the existing irradiation channels of the SAFARI-1 reactor, a time and cost saving from the licensing perspective. This study aims to establish the know-how of coated particle and pebble modeling in using the Monte Carlo N-Particle code (MCNP5). The study also aims to establish the know-how of rig design. In this study, the Necsa in-house code Overall System for the Calculation of Reactors (OSCAR-3), a software known as OScar 3-Mcnp INTerface (OSMINT) linking OSCAR-3 and MCNP5, also developed at Necsa, as well as MCNP5 code developed and maintained by the Los Alamos team, are used to calculate neutronic and power distribution parameters that are important for fuel irradiations and for rig design. This study presents results and data that can be used to make improvements in the design of the rig or to confirm if the required operational conditions can be met with the current preliminary rig design. Result of the neutronic analysis are presented for the SAFARI-1 core, core irradiation channel B6 (where the PBMR fuel irradiation rig is loaded for the purpose of this study), the rig structure and the pebble fuel are presented. Furthermore results of the power distribution and nuclear heating values in the reactor core, the irradiation channel B6, the rig structures and the pebble fuel is also presented. The loading of the PBMR fuel irradiation rig in core position B6 reduces the core reactivity due to the fact that the loading of the rig displaces the water moderator in channel B6 introducing vast amounts of helium. This impacts on the keff value because there will be less neutron thermalization and reproduction due to the decreased population of thermal neutrons. The rig is found to introduce a negative reactivity insertion of 46 pcm. The loading of this rig in the core leads to no significant perturbations on the core power distribution. The core hottest channel is still localized in core channel C6 both with RIG IN and RIG OUT cases. A power tilt is observed, with the south side of the core experiencing reduced assembly averaged fission power, with correspondingly small compensations from the assemblies on the north side of the core. The perturbations on the core assembly averaged fluxes are more pronounced in the eight assemblies surrounding B6. Core position B6 suffers an 18% neutron flux depression with the loading of the rig. The fluxes in core positions A5, A6, A7, B5, B7 and C7 are increased when the rig is loading. The largest increases are noted as 12% in A7, 9% in A6 and 6% in A5 and B7. All the eight core positions surrounding B6 experience reduced photon fluxes with the loading of the rig. Core position B6 shows a flux depression of up to 20%, with 10% reduction in core position A6. The remainder seven positions surrounding B6 shows flux depressions of no more than 5%. Further on, due to decreased moderation effects, the axial neutron flux in core position B6 is reduced by 20% when the rig is loaded. The energy dependent neutron flux in B6 decreases by 50% in the thermal energy range with corresponding increases of up to 50% in the resonance and fast energy regions. The axial and the energy dependent photon flux in core position B6 decreases by up to 20% when the rig is loaded. The magnitude of the neutron and photon fluxes is found to have a direct proportion on the neutron and photon heating values. While the amount of neutron heating in core position B6 increases by one order of magnitude, when the rig is loaded, the photon heating values increases by up to 60% in the region spanning ±10cm about the core centerline. The amount of photon heating in the rig structural materials dominates neutron heating, except in the helium regions of the rig, where neutron heating dominates photon heating. In the fuel region of the pebble, fission heating (3803W) largely dominates photon heating (119W). / Thesis (M.Sc. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2009