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.

Thesis (M.Sc.(Chemistry))--University of Pretoria, 2007. / Includes bibliographical references (leaves 80-85).

The fission product releases from spherical fuel elements used in modern high temperature gas cooled reactors are one of the first source terms used in describing the safety of planned nuclear plants during normal and accident conditions. The verification and validation of the model and code used to predict the gaseous fission product behaviour and release from spherical fuel elements for the PBMR were documented in this dissertation. The PBMR is the latest design in high temperature gas cooled reactor technology utilizing spherical fuel elements based on the LEU TRISO-coated particle design. Fission products can be divided into relatively short-lived noble gas and halogens, and relatively long-lived metallic fission and activation products. Each group is described by its own models and sets of transport parameters. The noble gases and halogen fission product releases from the fuel elements are direct indications of fuel performance and are modelled by the Booth equation. The fission product release legacy code NOBLEG for noble gases and halogens was developed previously to calculate this diffusion model for high temperature reactors. The model and code are verified and validated for use in PBMR design and analyses under normal operating conditions. The history of irradiation experiments conducted on coated fuel particles and spherical fuel elements was investigated, and the most suitable irradiation tests with their post irradiation investigations were identified for the purpose of validation of the model and code. The model used to determine gaseous fission product behaviour and release from spherical fuel elements is described in detail. The application of this model in the code is verified mathematically with the Booth model, and by inspection of the source code. The thermohydraulic model used by NOBLEG to calculate fuel temperatures is verified with code to code comparisons with the core neutronics code VSOP. The irradiation tests HFR-K5 and -K6 were selected to validate the gaseous fission product code NOBLEG. An investigation was done into the development of NOBLEG to calculate gaseous fission product release under oxidizing conditions caused by water ingress events. New relationships were derived from water vapour injection tests done during the irradiation experiment HFR-K6, that allows NOBLEG to estimate the increase in gaseous fission product release under oxidizing conditions. A new model was proposed to explain peculiarities observed during the water injection tests. / Prof. P.P. Coetzee

Thesis. 1978. M.S.--Massachusetts Institute of Technology. Dept. of Nuclear Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Bibliography: leaves 176-178. / M.S.

No description available.

The heat transfer phenomena in the near-wall region of a randomly packed pebble bed are important in the design of a Pebble Bed Reactor (PBR), especially when considering the safety case during accident conditions. At higher temperatures the contribution of the radiation heat transfer component to the overall heat transfer in a PBR increases significantly. The wall effect present in the near-wall region of a packed pebble bed affects the heat transfer in this region. Various correlations exist to predict the effective thermal conductivity through a packed pebble bed, but not all of the correlations consider the contribution of radiation and some are only applicable to the bulk region. Experimental research has been done on the heat transfer through a packed pebble bed. However, most of the results are case specific and cannot necessarily be used to validate models or simulations to predict the effective thermal conductivity of a pebble bed. The objective of this study is to develop a methodology that uses experimental work together with Computational Fluid Dynamics (CFD) simulations to predict the effective thermal conductivity in the near-wall region of a randomly packed pebble bed, and to separate the conduction and radiation components of the effective thermal conductivity. The proposed methodology inter alia includes experimental tests and the calibration of a CFD model to obtain numerical results that correlate well with the experimental results. To illustrate the proposed methodology the newly constructed Near-wall Effect Thermal Conductivity Test Facility (NWETCTF) was used to gather experimental results for the temperature and heat transfer distribution through a randomly packed pebble bed. Two identical but separate experimental tests were performed and the results of the two tests were in good agreement. From the experimental results the effective thermal conductivity was derived. The effect of the near-wall region on the heat transfer and the significance of radiation at higher temperatures are evident from the results. Recommendations were made for future experimental work with the NWETCTF from the findings of the investigation. A numerically packed pebble bed that is representative of the experimental pebble bed was generated using the Discrete Element Method (DEM) and a CFD model was set up for the heat transfer through the pebble bed using STAR-CCM+.. The CFD results showed trends similar to that of the experimental results. However, some discrepancies were identified that must be addressed in future studies by calibrating the CFD model. The effective thermal conductivity for the numerical simulation was determined using the CFD results and the conduction and radiation components were separated. / MSc (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

The heat transfer phenomena in the near-wall region of a randomly packed pebble bed are important in the design of a Pebble Bed Reactor (PBR), especially when considering the safety case during accident conditions. At higher temperatures the contribution of the radiation heat transfer component to the overall heat transfer in a PBR increases significantly. The wall effect present in the near-wall region of a packed pebble bed affects the heat transfer in this region. Various correlations exist to predict the effective thermal conductivity through a packed pebble bed, but not all of the correlations consider the contribution of radiation and some are only applicable to the bulk region. Experimental research has been done on the heat transfer through a packed pebble bed. However, most of the results are case specific and cannot necessarily be used to validate models or simulations to predict the effective thermal conductivity of a pebble bed. The objective of this study is to develop a methodology that uses experimental work together with Computational Fluid Dynamics (CFD) simulations to predict the effective thermal conductivity in the near-wall region of a randomly packed pebble bed, and to separate the conduction and radiation components of the effective thermal conductivity. The proposed methodology inter alia includes experimental tests and the calibration of a CFD model to obtain numerical results that correlate well with the experimental results. To illustrate the proposed methodology the newly constructed Near-wall Effect Thermal Conductivity Test Facility (NWETCTF) was used to gather experimental results for the temperature and heat transfer distribution through a randomly packed pebble bed. Two identical but separate experimental tests were performed and the results of the two tests were in good agreement. From the experimental results the effective thermal conductivity was derived. The effect of the near-wall region on the heat transfer and the significance of radiation at higher temperatures are evident from the results. Recommendations were made for future experimental work with the NWETCTF from the findings of the investigation. A numerically packed pebble bed that is representative of the experimental pebble bed was generated using the Discrete Element Method (DEM) and a CFD model was set up for the heat transfer through the pebble bed using STAR-CCM+.. The CFD results showed trends similar to that of the experimental results. However, some discrepancies were identified that must be addressed in future studies by calibrating the CFD model. The effective thermal conductivity for the numerical simulation was determined using the CFD results and the conduction and radiation components were separated. / MSc (Mechanical Engineering), North-West University, Potchefstroom Campus, 2015

Thesis (MScIng)--University of Stellenbosch, 2008. / ENGLISH ABSTRACT: The focus of this project was the application of a passive device in the form of a loop thermosyphon as a reactor cavity cooling system (RCCS) for a Pebble Bed Modular Reactor. An extensive literature review showed that loop thermosyphons have been widely researched, both theoretically and experimentally. In the review attention has specifically been given to matters such as safety, instability, control and mathematical modelling. One of the objectives of the project was to build one of the axially symmetric sections of Dobson’s (2006) proposed full scale RCCS using a scaled down version consisting of a single loop heated by a section of the reactor pressure vessel and cooled by a tank of water. The second objective was to derive a theoretical model that could be used in a computer code to simulate the experiment. The theory and experiment would then be compared in order to verify the code. The mathematical model created used the following three major assumptions: quasistatic flow, incompressible liquid and vapour and one dimensionality. The conservation equations in the form of a set of difference equations with the appropriate closure equations were then solved explicitly. It was found that the theoretical results were heavily influenced by the surface optical properties as well as the heat transfer coefficients. The emissivity influenced the transition point from single to two-phase flow as well as the condenser outlet temperature. The single phase heat transfer coefficients influenced the condenser outlet temperature significantly while it was found that for two phase flow the combination of the available boiling and condensation heat transfer coefficients had only minor effects on the end results. A stainless steel and aluminium thermosyphon loop was built using water as the working fluid. A stainless steel heater plate provided the heat input while a 200 L water tank was the heat sink. Temperature and flow rate measurements were recorded as a function of time with various heating/cooling transients from start-up to steady state for three operating modes. The three operating modes were single phase, two-phase and heat pipe mode. It was found that the theoretical temperatures correspond reasonably well with the experimental temperatures. The time predicted by the theoretical model to reach the operating temperature was however somewhat longer than for the experimental. This is to be expected when considering that there is some uncertainty pertaining to the heat transfer coefficients as well as surface emissive properties. The correspondence of the theoretical and experimental fin temperatures was poor due to significant thermal stratification of the air separating the heater plate and fins. Several shortcomings in the theoretical model as well as the experimental setup were identified and discussed. The conclusion was reached that this exploratory study showed that the loop thermosyphon is a viable option for the RCCS and that the mathematical model is a viable theoretical simulation tool. Several recommendations were made for further study to address and overcome the shortcomings identified in the theoretical and experimental models in order to prove this conclusion. Amongst these is the determination of better material surface properties and heat transfer coefficients and improved mass flow rate measurement. Investigating scaling issues, natural convection outside the loop and updating of the computer program is also recommended. / AFRIKAANSE OPSOMMING: Die fokus van hierdie projek was die toepassing van passiewe apparatuur, in die vorm van ‘n geslote lus termoheuwel, as ‘n reaktor kamer verkoellings stelsel vir die korrel bed modulêre reaktor. Die literatuur studie wys dat hierdie tegnologie reeds breedvoerig ondersoek is teoreties sowel as eksperimenteel. In die literatuur oorsig word aandag spesifiek gegee aan veiligheid, onstabiliteit, beheer en modelleering. Een van die doelwitte van die projek was om ‘n klein skaalse model te bou van een van die aksiaal simmetriese seksies van Dobson (2006) se voorgestelde volskaalse reaktor kamer verkoellings stelsel. Die model bestaan uit n enkele lus verhit deur ‘n seksie van die reaktor drukvat en verkoel deur ‘n tenk vol water. Die tweede doelwit was die afleiding van ‘n teoretiese model wat in ‘n rekenaar program gebruik kan word om die eksperiment te simuleer. Die teoretiese en eksperimentele data kan dan vergelyk word om die geldigheid van die program te toets. Die volgende aanames is gemaak tydens die afleiding van die wiskundige model: kwasi-statiese vloei, onsamedrukbare vloeistof en gas en een dimensionalitiet. Die behouds wette is in die vorm van ‘n stel differensie vergelykings met die toepasbare sluitings vergelykings eksplisiet opgelos. Dit is bevind dat die teoretiese resultate swaar beinvloed is deur die materiaal oppervlak eienskappe sowel as die warmteoordrag koëffisiënte. Die emisiviteit beinvloed die oorgangs punt van enkel na twee fase vloei sowel as die kondenser uitlaat temperatuur. Die enkel fase warmteoordrag koëffisiënt het n beduidende invloed op die kondenser uitlaat temperatuur terwyl dit voorkom asof die spesifieke kombinasie van die koking en kondensasie warmteoordrag koëffisiënte minimale invloed op die resultate het in die twee fase gebied. Vlekvrye staal en aluminium is gebruik om die lus te bou met water as die verkoelings middel. Warmte is toegevoeg tot die stelsel deur ’n vlekvrye staal verhittings plaat terwyl ‘n 200 L water tenk die warmte onttrek het. Temperatuur en massa vloei tempo is aangeteken as ‘n funksie van tyd vir verskeie verhitting/verkoellings oorgangs gedragte vanaf begin tot bestendige toestand vir drie bedryfs modusse. Die drie bedryfs modusse was enkel fase, twee fase en hitte pyp modus. Dit is bevind dat die teoretiese temperature redelik goed ooreengekom het met die eksperimentele waardes. Die tyd wat dit neem om by die bedryfs temperatuur te kom soos voorspel deur die teorie is egter langer as wat in die eksperiment gevind is. Dit is te verstane wanneer die onsekerheid in die warmteoordrag koëffisiënte en materiaal oppervlak eienskappe in ag geneem word. Die fin temperature het ‘n swakker ooreenkoms getoon as gevolg van beduidende termiese stratifikasie van die lug tussen die fin en verhittings plaat. Verskeie tekortkominge in die teoretiese model en eksperimentele opstelling is geïdentifiseer en bespreek. Die gevolgtrekking is gemaak dat die ondersoek bewys dat geslote lus termoheuwels ‘n lewensvatbare opsie is vir ‘n reaktor kamer verkoellings stelsel en dat die wiskundige model lewensvatbaar is vir teoretiese simulasie. Verskeie aanbevelings word egter gemaak om die tekortkominge in die teoretiese en eksperimentele modelle aan te spreek om so doende die gevolgtrekking te staaf. Dit word aanbeveel dat beter waardes vir die materiaal oppervlak eienskappe en warmteoordrag koëffisiënte gevind word en verbeterde massa vloei meetings gedoen word. Dit word verder aanbeveel om skaleering asook natuurlike konveksie buite die lus te ondersoek en om die rekenaar program by te werk.

Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: Production of graphite dust inside the Pebble Bed Modular Reactor (PBMR) influences the reactor operation negatively. Graphite is used as a moderator in the reactor core and the formation and transportation of graphite dust away from the reactor core decreases the amount of moderator which in turn has a negative impact on the reactor operation. High levels of radioactive dust may also contaminate reactor components which may pose a health risk to maintenance personnel. In this study a pressure vessel was designed and used to measure the wear of a graphite pebble in helium at elevated temperatures. By means of a multi-linear regression analysis a proper mathematical function was established in order to relate graphite wear to certain tribological parameters. These parameters were identified through a literature study. Discrete Element Modelling (DEM) was used to simulate the gravitational flow of graphite pebbles through the reactor core. The experimentally determined mathematical function was incorporated into the DEM simulation to estimate the annual mass of graphite dust to be produced by the PBMR pebble bed as a result of pebble-pebble interaction and pebble-wall interaction during refuelling. / AFRIKAANSE OPSOMMING: Die vorming van grafiet stof binne die korrelbed-modulêre reaktor (PBMR) beïnvloed die werking daarvan negatief. Grafiet word gebruik as 'n moderator in die reaktor kern en die vorming en vervoer van grafietstof weg van die reaktor kern lei tot 'n afname in die hoeveelheid moderator en dit het 'n negatiewe impak op die werking van die reaktor. Hoë vlakke van radioaktiewe grafietstof kontamineer ook reaktorkomponente wat 'n gesondheidsrisiko vir onderhoudspersoneel inhou. In hierdie studie was 'n drukvat ontwerp en gebruik om die slytasie van 'n grafietkorrel in helium by verhoogde temperature te meet. 'n Multi-lineêre regressie analise is dan gebruik om 'n wiskundige funksie daar te stel wat die verband tussen grafietslytasie en die eksperimentele parameters vas stel. Hierdie parameters was met behulp van 'n literatuurstudie geïdentifiseer. Diskrete Element Modellering (DEM) was gebruik om die gravitasionele vloei van grafietkorrels in die reaktor te modelleer. Die eksperimenteel bepaalde wiskundige funksie word in die DEM simulasie ge-inkorporeer om 'n skatting te maak van die jaarlikse massa grafietstof wat gevorm sal word in die PBMR korrelbed as 'n gevolg van korrel-korrel interaksie en korrel-wand interaksie gedurende hersirkulasie.

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.