High Temperature Gas-cooled Reactor (HTGR) is a Generation IV reactor under consideration by Department of Energy and in the nuclear industry. There are two categories of HTGRs, namely, Pebble Bed Modular Reactor (PBMR) and Prismatic reactor. Pebble Bed Modular Reactor is a HTGR with enriched uranium dioxide fuel inside graphite shells (moderator). The uranium fuel in PBMR is enclosed in spherical shells that are approximately the size of a tennis ball, referred to as \fuel spheres". The reactor core consists of approximately 360,000 fuel pebbles distributed randomly. From a reactor design perspective it is important to be able to understand the fluid flow properties inside the reactor. However, for the case of PBMR the sphere packing inside the core is random. Unknown flow characteristics defined the objective of this study, to understand the flow properties in spherically packed geometries and the effect of turbulence models in the numerical solution. In attempt to do so, a steady state computational study was done to obtain the pressure drop estimation in different packed bed geometries, and describe the fluid flow characteristics for such complex structures. Two out of the three Bravais lattices were analyzed, namely, simple cubic (symmetric) and body centered cubic (staggered). STARCCM commercial CFD software from CD- ADAPCO was used to simulate the flow. To account for turbulence effects several turbulence models such as standard k-epsilon, realizable k-epsilon, and Reynolds stress transport model were used. Various cases were analyzed with Modified Reynolds number ranging from 10,000 to 50,000. For the simple cubic geometry the realizable k-epsilon model was used and it produced results that were in good agreement with existing experimental data. All the turbulence models were used for the body centered cubic geometry. Each model produced different results what were quite different from the existing data. All the turbulence models were analyzed, errors and drawbacks with each model were discussed. Finally, a resolution was suggested in regards to use of turbulence model for problems like the ones studied in this particular work.

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

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The focus of this project was on the potential application of waste heat recovery and utilisation (WHR&U) systems in pebble bed modular reactor (PBMR) technology. The background theory provided in the literature survey showed that WHR&U systems have attracted the attention of many researchers over the past two decades, as using waste heat improves the system overall efficiency, notwithstanding the cost of extra plant. PBMR waste heat streams were identified and investigated based on the amount of heat rejected to the environment. WHR&U systems require specially designed heat recovery equipment, and as such the used and/or spent PBMR fuel tanks were considered by the way of example. An appropriately scaled system was designed, built and tested, to demonstrate the functioning of such a cooling system. Two separate and independent cooling lines, using natural circulation flow in a particular form of heat pipes called thermosyphon loops were used to ensure that the fuel tank is cooled when the power conversion unit has to be switched off for maintenance, or if it fails. A theoretical model that simulates the heat transfer process in the as-designed WHR&U system was developed. It is a one-dimensional flow model assuming quasi-static and incompressible liquid and vapour flow. An experimental investigation of the WHR&U system was performed in order to validate the theoretical model results. The experimental results were then used to modify the theoretical heat transfer coefficients so that they simulate the experiments more accurately. Three energy conversion devices, the dual-function absorption cycle (DFAC), the organic Rankine cycle (ORC) and the Stirling engine (SE), were identified as suitable for transforming the recovered heat into a useful form, depending on the source temperatures from 60 ºC to 800 ºC. This project focuses on a free-piston SE with emphasis on the thermo-dynamic performance of a SE heat exchanger. It was found that a heat exchanger with a copper woven wire mesh configuration has a relatively large gas-to-metal and metal-to-liquid heat transfer area. Tube-in-shell heat exchanger configurations were tested, with the working fluid flowing in ten copper inner pipes, while a coolant flows through the shell tube. A lumped parameter model was used to describe the thermo-fluid dynamic behaviour of the SE heat exchanger. In order to validate the theoretical results, a uni-directional flow experimental investigation was performed. The theoretical model was adjusted so that it simulated the SE heat exchanger. It was found that after this correction the theoretical model accurately predicts the experiment. Finally, a dynamic analysis of the SE heat exchanger experimental set-up was undertaken to show that, although vibrating, the heat exchanger setup assembly was indeed acceptable from a vibrational and fatigue point of view. / AFRIKAANSE OPSOMMING: Die hoofoogmerk met hierdie projek was die moontlike aanwending van afvalhitteherwinningen- benutting-(WHR&U-) stelsels in modulêre-gruisbedreaktor-(PBMR-) tegnologie. Agtergrondteorie in die literatuurondersoek toon dat WHR&U-stelsels al menige navorser se belangstelling geprikkel het, hetsy vanweë die moontlike ekonomiese voordele wat dit inhou óf vir besoedelingsvoorkoming, bo-en-behalwe die koste van bykomende toerusting. Die PBMRafvalhittestrome is ondersoek en bepaal op grond van die hoeveelheid hitte wat dit na die omgewing vrystel. Om in die prosesbehoeftes van WHR&U-stelsels te voorsien, moet goed ontwerpte, doelgemaakte hitteherwinningstoerusting in ʼn verkoelings- en/of verhittingsproses gebruik word, dus is die PBMR as voorbeeld gebruik vir die konsep. ʼn Toepaslik geskaleerde WHR&U-stelsel is dus ontwerp, gebou en getoets om die geldigheid van die stelselontwerp te toon. Twee onafhanklike verkoelingslyne, wat van natuurlike konveksie gebruik maak, in die vorm van hitte-pype of termoheuwel lusse, was gebruik om te verseker dat verkoeling verskaf word wanneer die hoof lus breek of instandhouding nodig hê. ʼn Teoretiese model is ontwikkel wat die hitteoordragproses in die ontwerpte WHR&U-stelsel simuleer. Dié model was ʼn eendimensionele vloeimodel wat kwasistatiese en onsamedrukbare vloeistof- en dampvloei in die WHR&U-stelsel-lusse veronderstel. ʼn Eksperimentele ondersoek is op die WHR&U-stelsel uitgevoer ten einde die teoretiese model se resultate te bevestig. Die eksperimentele resultate was dus geneem om die teoretiese hitteoordragkoëffisiënte aan te pas sodat dit die eksperimente kon simuleer. Drie energieomsettingstoestelle, naamlik die dubbel funksie absorpsie siklus (DFAC), die organiese Rankine siklus (ORC) en die Stirling enjin (SE), is as geskikte toestelle uitgewys om die herwonne hitte op grond van brontemperature tussen 60 ºC en 800 ºC in ʼn bruikbare vorm om te sit. Hierdie tesis het op vryesuier-SE’s gekonsentreer, met klem op die hitteruiler. Meer bepaald is die termodinamiese werkverrigting van ʼn SE-hitteruiler ondersoek. Daar is bevind dat ʼn hitteruiler met ʼn geweefde koperdraadmaas-samestelling oor ʼn betreklik groot gas-totmetaal- en metaal-tot-vloeistof-oordragoppervlakte beskik. Die verhitter en verkoeler is in ʼn buis-in-mantel-vorm ontwerp, met die werksvloeistof wat deur tien koperbinnepype vloei en ʼn koelmiddel deur die mantelbuis. ʼn Saamgevoegde-parameter-model is gebruik om die termodinamiese gedrag van die SEhitteruiler te beskryf. Ten einde die teoretiese resultate te bevestig, is ʼn eenrigtingvloeiproefondersoek uitgevoer. Die teoretiese model is aangepas sodat dit die SE-hitteruiler kon simuleer. Ná die nodige verstellings is daar bevind dat die teoretiese model die proefneming akkuraat voorspel. Laastens was ʼn dinamiese ontleding van die SE-hitteruiler ook onderneem om te toon dat, hoewel dit vibreer, die hitteruiler proef samestel inderdaad veilig is.

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: A high temperature reactor (HTR) generates heat inside of the reactor core through nuclear fission, from where the heat is transferred through the core and heats up the reactor pressure vessel (RPV). The heat from the RPV is transported passively through the reactor cavity, where it is cooled by the reactor cavity cooling system (RCCS), through the concrete confinement structure and ultimately into the environment. The concrete confinement structure can withstand temperatures of up to 65°C for normal operating conditions and temperatures of up to 125°C during an emergency. This project endeavours to research the heat transfer between an HTR’s RPV and the outside of the concrete confinement structure by utilising three investigative approaches: experimental, computational fluid dynamics (CFD) and analytical. The first approach, an experimental analysis, required the development of an experi- mental model. The model was used to perform experiments and gather temperature data that could be used to verify the accuracy of the CFD simulations. The second approach was a CFD analysis of the experimental model, and the external concrete temperatures from the simulation were compared with the temperatures measured with the experimen- tal model. Finally, an analytical analysis was performed in order to better understand CFD and how CFD solves natural convection-type problems. The experiments were performed successfully and the measurements taken were com- pared with the CFD results. The CFD results are in good agreement with the Dry experiments, but not with the Charged experiments. It was identified that the inaccurate results for the CFD simulations of the Charged experiments arose due to convective heat leakage through gaps in the heat shield and between the heat shield and the sides of the experimental model. A computer program was developed for the analytical analysis and it was established that the program could successfully solve the natural convection in a square cavity - as required. / AFRIKAANSE OPSOMMING: ’n Hoë temperatuur reaktor (HTR) genereer hitte binne die reaktor kern deur kernsplyting en die hitte word dan deur die kern versprei en verhit die reaktor se drukvat. Die hitte van die reaktor drukvat word dan passief deur die reaktorholte versprei, waar dit deur die reaktorholte se verkoelingstelsel afgekoel word, en deur die beton beskermingstruktuur gelei word en uiteindelik die omgewing bereik. Die beton beskermingstruktuur kan temperature van tot 65°C onder normale operasietoestande van die reaktor weerstaan, en temperature van tot 125°C tydens ’n noodgeval. Hierdie projek poog om die hitte-oordrag tussen ’n HTR-reaktor drukvat en die buitekant van die beton beskermingstruktuur te on- dersoek deur gebruik te maak van drie ondersoekbenaderings: eksperimenteel, numeriese vloei dinamika (NVD) en analities. Die eerste benadering, ’n eksperimentele analise, het die ontwikkeling van ’n eksper- imentele model vereis. Die model is gebruik om eksperimente uit te voer en temperatu- urmetings te neem wat gebruik kon word om die akkuraatheid van die NVD simulasies te bevestig. Die tweede benadering was ’n NVD-analise van die eksperimentele model, en die eksterne betontemperature verkry van die simulasies is vergelyk met die gemete temperature van die eksperimente. Uiteindelik is ’n analitiese analise uitgevoer ten einde NVD beter te verstaan en hoe NVD natuurlike konveksie-tipe probleme sal oplos. Die eksperimente is suksesvol uitgevoer en die metings is gebruik om die NVD resultate mee te vergelyk. Die NVD resultate van die Droë eksperimente het goeie akkuraatheid getoon. Dit was nie die geval vir die Gelaaide eksperimente nie. Daar is geïdentifiseer dat die verskille in resultate tussen die NVD en die eksperimente aan natuurlike konveksie hitte verliese deur gapings in die hitteskuld en tussen die hitteskuld en die kante van die eksperimentele model toegeskryf kan word. ’n Rekenaarprogram is geskryf vir die analitiese ontleding en die program kon suksesvol die natuurlike konveksie in ’n vierkantige ruimte oplos.

The Pebble Bed Modular Reactor (PBMR) is one of the most technologically advanced developments in South Africa. In order to build a commercially viable demonstration power plant, all the specifically and uniquely designed equipment must first be qualified. All the prototype equipment is tested at the Helium Test Facility (HTF) at Pelindaba. One of the largest components that are tested is the Core Unloading Device (CUD). The main function of the CUD is to unload fuel from the bottom of the reactor core to enable circulation of the fuel core. The CUD housing vessel forms part of the reactor pressure boundary. Pebble-directing valves and other moving machinery are installed inside its machined inner surface. It is essential that the interior surfaces of the CUD are case hardened to provide a corrosion- and wear-resistant layer. Cold welding between the moving metal parts and the machined surface must also be prevented. Nitriding is a case hardening process that adds a hardened wear- and corrosion-resistant layer that will also prevent cold welding of the moving parts in the helium atmosphere. Only a few nitriding furnaces exist that can house a forging as large as the CUD of the PBMR. Commercial nitriding furnaces in South Africa are all too small and have limited flexibility in terms of the nitriding process. The nitriding of a vessel as large as the CUD has not yet been carried out commercially. The aim of this work was to design and develop a custom-made nitriding plant to perform the nitriding of the first PBMR/HTF CUD. Proper process control is essential to ensure that the required nitrided case has been obtained. A new concept for a gas nitriding plant was developed using the nitrided vessel interior as the nitriding process chamber. Before the commencement of detail design, a laboratory test was performed on a scale model vessel to confirm concept feasibility. The design of the plant included the mechanical design of various components essential to the nitriding process. A special stirring fan with an extended length shaft was designed, taking whirling speed into account. Considerable research was performed on the high temperature use of the various components to ensure the safe operation of the plant at temperatures of up to 600°C. Nitriding requires the use of hazardous gases such as ammonia, oxygen and nitrogen. Hydrogen is produced as a by-product and therefore safety was the most important design parameter. Thermohydraulic analyses, i.e. heat transfer and pressure drop calculations in pipes, were also performed to ensure the successful process design of the nitriding plant. The nitriding plant was subsequently constructed and operated to verify the correct design. A large amount of experimental and operating data was captured during the actual operation of the plant. This data was analysed and the thermohydraulic analyses were verified. Nitrided specimens were subjected to hardness and layer thickness tests. The measured temperature of the protruding fan shaft was within the limits predicted by Finite Element Analysis (FEA) models. Graphs of gas flow rates and other operation data confirmed the inverse proportionality between ammonia supply flow rate and measured dissociation rate. The design and operation of the nitriding plant were successful as a nitride layer thickness of 400 μm and hardness of 1 200 Vickers hardness (VHN) was achieved. This research proves that a large pressure vessel can successfully be nitrided using the vessel interior as a process chamber. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2010.

The Pebble Bed Modular Reactor (PBMR) is one of the most technologically advanced developments in South Africa. In order to build a commercially viable demonstration power plant, all the specifically and uniquely designed equipment must first be qualified. All the prototype equipment is tested at the Helium Test Facility (HTF) at Pelindaba. One of the largest components that are tested is the Core Unloading Device (CUD). The main function of the CUD is to unload fuel from the bottom of the reactor core to enable circulation of the fuel core. The CUD housing vessel forms part of the reactor pressure boundary. Pebble-directing valves and other moving machinery are installed inside its machined inner surface. It is essential that the interior surfaces of the CUD are case hardened to provide a corrosion- and wear-resistant layer. Cold welding between the moving metal parts and the machined surface must also be prevented. Nitriding is a case hardening process that adds a hardened wear- and corrosion-resistant layer that will also prevent cold welding of the moving parts in the helium atmosphere. Only a few nitriding furnaces exist that can house a forging as large as the CUD of the PBMR. Commercial nitriding furnaces in South Africa are all too small and have limited flexibility in terms of the nitriding process. The nitriding of a vessel as large as the CUD has not yet been carried out commercially. The aim of this work was to design and develop a custom-made nitriding plant to perform the nitriding of the first PBMR/HTF CUD. Proper process control is essential to ensure that the required nitrided case has been obtained. A new concept for a gas nitriding plant was developed using the nitrided vessel interior as the nitriding process chamber. Before the commencement of detail design, a laboratory test was performed on a scale model vessel to confirm concept feasibility. The design of the plant included the mechanical design of various components essential to the nitriding process. A special stirring fan with an extended length shaft was designed, taking whirling speed into account. Considerable research was performed on the high temperature use of the various components to ensure the safe operation of the plant at temperatures of up to 600°C. Nitriding requires the use of hazardous gases such as ammonia, oxygen and nitrogen. Hydrogen is produced as a by-product and therefore safety was the most important design parameter. Thermohydraulic analyses, i.e. heat transfer and pressure drop calculations in pipes, were also performed to ensure the successful process design of the nitriding plant. The nitriding plant was subsequently constructed and operated to verify the correct design. A large amount of experimental and operating data was captured during the actual operation of the plant. This data was analysed and the thermohydraulic analyses were verified. Nitrided specimens were subjected to hardness and layer thickness tests. The measured temperature of the protruding fan shaft was within the limits predicted by Finite Element Analysis (FEA) models. Graphs of gas flow rates and other operation data confirmed the inverse proportionality between ammonia supply flow rate and measured dissociation rate. The design and operation of the nitriding plant were successful as a nitride layer thickness of 400 μm and hardness of 1 200 Vickers hardness (VHN) was achieved. This research proves that a large pressure vessel can successfully be nitrided using the vessel interior as a process chamber. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2010.