Pebble-fueled high temperature gas-cooled reactor (HTGR) technology was first developed by the Federal Republic of Germany in the 1950s. More recently, the design has been embraced by the People's Republic of China and the Republic of South Africa. Unlike light water reactors that generate heat from fuel assemblies comprised of fuel rods, pebble-fueled HTGRs utilize thousands of 60-mm diameter fuel spheres (pebbles) comprised of thousands of TRISO particles. As this reactor type is deployed across the world, adequate methods for safeguarding the reactor must be developed. Current safeguards methods for the pebble-fueled HTGR focus on extensive, redundant containment and surveillance (C/S) measures or a combination of item-type and bulk-type material safeguards measures to deter and detect the diversion of fuel pebbles. The disadvantages to these approaches are the loss of continuity of knowledge (CoK) when C/S systems fail, or are compromised, and the introduction of material unaccounted for (MUF). Either vulnerability can be exploited by an adversary to divert fuel pebbles from the reactor system. It was determined that a solution to maintaining CoK is to develop a system to identify each fuel pebble that is inserted and removed from the reactor. Work was performed to develop and evaluate the use of inert microspheres placed in each fuel pebble, whose random placement could be used as a fingerprint to identify the fuel pebble. Ultrasound imaging of 1 mm zirconium oxide microspheres was identified as a possible imaging system and microsphere material for the new safeguards system concept. The system concept was evaluated, and it was found that a minimum of three microspheres are necessary to create enough random fingerprints for 10,000,000 pebbles. It was also found that, over the lifetime of the reactor, less than 0.01% of fuel pebbles can be expected to have randomly the same microsphere fingerprint. From an MCNP 5.1 model, it was determined that less than fifty microspheres in each pebble will have no impact on the reactivity or temperature coefficient of reactivity of the reactor system. Finally, using an ultrasound system it was found that ultrasound waves can penetrate thin layers of graphite to image the microsphere fingerprint.

High Temperature Gas-cooled Reactors (HTGRs) can provide clean electricity,as well as process heat that can be used to produce hydrogen for transportation and other sectors. A prototypic HTGR, the Next Generation Nuclear Plant (NGNP),will be built at Idaho National Laboratory.The need for HTGR analysis tools and methods has led to the addition of gas-cooled reactor (GCR) capabilities to the light water reactor code MELCOR. MELCOR will be used by the Nuclear Regulatory Commission licensing of the NGNP and other HTGRs. In the present study, new input techniques have been developed for MELCOR HTGR analysis. These new techniques include methods for modeling radiation heat transfer between solid surfaces in an HTGR, calculating fuel and cladding geometric parameters for pebble bed and prismatic block-type HTGRs, and selecting appropriate input parameters for the reflector component in MELCOR. The above methods have been applied to input decks for a water-cooled reactor cavity cooling system (RCCS); the 400 MW Pebble Bed Modular Reactor (PBMR), the input for which is based on a code-to-code benchmark activity; and the High Temperature Test Facility (HTTF), which is currently in the design phase at Oregon State University. RCCS results show that MELCOR accurately predicts radiation heat transfer rates from the vessel but may overpredict convective heat transfer rates and RCCS coolant flow rates. PBMR results show that thermal striping from hot jets in the lower plenum during steady-state operations, and in the upper plenum during a pressurized loss of forced cooling accident, may be a major design concern. Hot jets could potentially melt control rod drive mechanisms or cause thermal stresses in plenum structures. For the HTTF, results will provide data to validate MELCOR for HTGR analyses. Validation will be accomplished by comparing results from the MELCOR representation of the HTTF to experimental results from the facility. The validation process can be automated using a modular code written in Python, which is described here.

There are two basic needs globally and that is the control and supply of reliable electricity and clean water. However, one of the biggest challenges the world is facing today is the lack of fresh water resources. Lower rainfall, together with population and industry growth, are only a few factors contributing to the fast increasing strain on existing water supplies around the world. This fast increasing need therefore necessitates the investigation into finding alternative sources. One such option is that of desalination. In the last 50 years desalination technologies have been applied to produce high quality fresh water from brackish and seawater resources. In the 1980's a breakthrough was made with the introduction of the membrane desalination technology, known as the reverse osmosis (RO) process. Today newly developed technologies are improving the competitiveness of the reverse osmosis process against the traditional distillation processes. There are a number of options to increase the efficiency of a reverse osmosis plant and one option is to use warm industrial waste water as the feed water to the desalination plant. It is known that the viscosity of water is inversely proportional to its temperature. Therefore, if the feed water temperature of a reverse osmosis plant is increased the membranes will become more permeable. This will result in a higher production volume or in a lower energy demand. South Africa is on the edge of building the first fourth generation nuclear power plant, called the Pebble Bed Modular Reactor (PBMR) at Koeberg. The PBMR will produce a cooling water outlet temperature of 40°C which can be used as feed water to a reverse osmosis plant. In this study design guidelines of a reverse osmosis plant are given in nine steps. These steps were then used during a basic component design of a reverse osmosis plant coupled to the waste water stream of a PBMR nuclear power plant. Furthermore design software programs were used to simulate the coupling scheme in order to validate the outcome of the design guidelines. The results of the two design approaches compared well to one another. It furthermore showed that by using the waste water from the PBMR nuclear power plant the efficiency of the RO plant is increased and the operating cost is decreased. Fresh water can be produced at a cost of R 5.64/m3 with a specific electricity consumption of 2.53 kWh/m3. / Thesis (M.Ing. (Nuclear Engineering)--North-West University, Potchefstroom Campus, 2009.

There are two basic needs globally and that is the control and supply of reliable electricity and clean water. However, one of the biggest challenges the world is facing today is the lack of fresh water resources. Lower rainfall, together with population and industry growth, are only a few factors contributing to the fast increasing strain on existing water supplies around the world. This fast increasing need therefore necessitates the investigation into finding alternative sources. One such option is that of desalination. In the last 50 years desalination technologies have been applied to produce high quality fresh water from brackish and seawater resources. In the 1980's a breakthrough was made with the introduction of the membrane desalination technology, known as the reverse osmosis (RO) process. Today newly developed technologies are improving the competitiveness of the reverse osmosis process against the traditional distillation processes. There are a number of options to increase the efficiency of a reverse osmosis plant and one option is to use warm industrial waste water as the feed water to the desalination plant. It is known that the viscosity of water is inversely proportional to its temperature. Therefore, if the feed water temperature of a reverse osmosis plant is increased the membranes will become more permeable. This will result in a higher production volume or in a lower energy demand. South Africa is on the edge of building the first fourth generation nuclear power plant, called the Pebble Bed Modular Reactor (PBMR) at Koeberg. The PBMR will produce a cooling water outlet temperature of 40°C which can be used as feed water to a reverse osmosis plant. In this study design guidelines of a reverse osmosis plant are given in nine steps. These steps were then used during a basic component design of a reverse osmosis plant coupled to the waste water stream of a PBMR nuclear power plant. Furthermore design software programs were used to simulate the coupling scheme in order to validate the outcome of the design guidelines. The results of the two design approaches compared well to one another. It furthermore showed that by using the waste water from the PBMR nuclear power plant the efficiency of the RO plant is increased and the operating cost is decreased. Fresh water can be produced at a cost of R 5.64/m3 with a specific electricity consumption of 2.53 kWh/m3. / Thesis (M.Ing. (Nuclear Engineering)--North-West University, Potchefstroom Campus, 2009.

Many countries worldwide experience water shortages on a daily basis and this water crisis is expected to increase even more in the near future due to limited fresh water resources. Alternative sources of fresh water such as desalinated seawater are becoming an attractive option for many developing countries. Although various desalination technologies exist today, interest in multi-effect distillation (MED) is growing rapidly worldwide. Today various energy power sources are utilized in MED plants, but the use of nuclear power as a clean and effective heat source for the MED process seems to be gaining interest. Implementation of HTGR technology, such as the Pebble Bed Modular Reactor being developed in South Africa is ideal for MED desalination purposes. In these types of reactors high temperature water is available as waste heat as opposed to high temperature steam from conventional steam power plants. Currently conventional MED plants utilize steam as the process heat source, to drive the MED process. In this study a system simulation model was developed in the computer language C++. It evaluates different MED process flow configurations in order to identify an optimum MED plant configuration for both water and steam as process heat source. Simulation results indicate that a steam-heat-source (SHS) MED plant produces approximately 25-30% more product water than a water-heat-source (WHS) MED plant while utilizing less plant stages. Plant layout and economics are impacted by the available process heat source. Results also indicate that a parallel feed configuration (PFC), which incorporates preheating of feed water, seems to be the optimum process flow configuration type for both the SHS and WHS type plants. Product water costs for optimized SHS and WHS MED plants were also compared. Various system parameters influence plant performance, but the serie effect temperature difference seems to be the most influential parameter in terms of water production. Preheating of feed water increases production levels up to 30%. Results from the C++ model have been compared to results calculated with MEE-TVC, a desalination system design program and were generally in good agreement. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2008.

Many countries worldwide experience water shortages on a daily basis and this water crisis is expected to increase even more in the near future due to limited fresh water resources. Alternative sources of fresh water such as desalinated seawater are becoming an attractive option for many developing countries. Although various desalination technologies exist today, interest in multi-effect distillation (MED) is growing rapidly worldwide. Today various energy power sources are utilized in MED plants, but the use of nuclear power as a clean and effective heat source for the MED process seems to be gaining interest. Implementation of HTGR technology, such as the Pebble Bed Modular Reactor being developed in South Africa is ideal for MED desalination purposes. In these types of reactors high temperature water is available as waste heat as opposed to high temperature steam from conventional steam power plants. Currently conventional MED plants utilize steam as the process heat source, to drive the MED process. In this study a system simulation model was developed in the computer language C++. It evaluates different MED process flow configurations in order to identify an optimum MED plant configuration for both water and steam as process heat source. Simulation results indicate that a steam-heat-source (SHS) MED plant produces approximately 25-30% more product water than a water-heat-source (WHS) MED plant while utilizing less plant stages. Plant layout and economics are impacted by the available process heat source. Results also indicate that a parallel feed configuration (PFC), which incorporates preheating of feed water, seems to be the optimum process flow configuration type for both the SHS and WHS type plants. Product water costs for optimized SHS and WHS MED plants were also compared. Various system parameters influence plant performance, but the serie effect temperature difference seems to be the most influential parameter in terms of water production. Preheating of feed water increases production levels up to 30%. Results from the C++ model have been compared to results calculated with MEE-TVC, a desalination system design program and were generally in good agreement. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2008.

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 effect of water ingress in two pebble bed high temperature gas-cooled reactors i.e. the PBMR-200 MWthermal and the PBMR-400 MWthermal were simulated and compared using the VSOP 99/05 suite of codes. To investigate the effect of this event on reactivity, power profiles and thermal neutron flux profiles, the addition of partial steam vapour pressures in stages up to 400 bar into the primary circuit for the PBMR-400 and up to 300 bar for the PBMR- 200 was simulated for both reactors. During the simulation, three scenarios were simulated, i.e. water ingress into the core only, water ingress into the reflectors only and water ingress into both the core and reflectors. The induced reactivity change effects were compared for these reactors. An in-depth analysis was also carried out to study the mechanisms that drive the reactivity changes for each reactor caused by water ingress into the fuel core only, the riser tubes in the reflectors only and ingress into both the fuel core and the riser tubes in the reflectors. The knowledge gained of these mechanisms and effects was used in order to propose design changes aimed at mitigating the reactivity increases, caused by realistic water ingress scenarios. Past results from simulations of water ingress into Pebble Bed Reactors were used to validate and verify the present simulation approach and results. The reactivity increase results for both reactors were in agreement with the German HTR-Modul calculations. / Thesis (MSc (Engineering Sciences in Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013

The effect of water ingress in two pebble bed high temperature gas-cooled reactors i.e. the PBMR-200 MWthermal and the PBMR-400 MWthermal were simulated and compared using the VSOP 99/05 suite of codes. To investigate the effect of this event on reactivity, power profiles and thermal neutron flux profiles, the addition of partial steam vapour pressures in stages up to 400 bar into the primary circuit for the PBMR-400 and up to 300 bar for the PBMR- 200 was simulated for both reactors. During the simulation, three scenarios were simulated, i.e. water ingress into the core only, water ingress into the reflectors only and water ingress into both the core and reflectors. The induced reactivity change effects were compared for these reactors. An in-depth analysis was also carried out to study the mechanisms that drive the reactivity changes for each reactor caused by water ingress into the fuel core only, the riser tubes in the reflectors only and ingress into both the fuel core and the riser tubes in the reflectors. The knowledge gained of these mechanisms and effects was used in order to propose design changes aimed at mitigating the reactivity increases, caused by realistic water ingress scenarios. Past results from simulations of water ingress into Pebble Bed Reactors were used to validate and verify the present simulation approach and results. The reactivity increase results for both reactors were in agreement with the German HTR-Modul calculations. / Thesis (MSc (Engineering Sciences in Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013

Thesis (MSc (Physics))--Stellenbosch University, 2010. / ENGLISH ABSTRACT: This dissertation reports on the impact of a set of selected nuclear fuel management parameters on reactor operations of the PBMR core. This is achieved by performing an assessment of the impact of nuclear fuel management parameter variations on the most important safety and economics issues for the PBMR core. These include the maximum fuel temperature at steady state and during Depressurized Loss of Forced Cooling (DLOFC) accident conditions. The reactivity worth of the Reactor Control System (RCS which determines the shutdown capability of the reactor core and the average discharge burn-up of fuel are also established. The fuel management parameters considered in this study include different enrichment levels, heavy metal loadings and fuel sphere circulation regimes. The impact and importance of these parameters on plant safety and economics is assessed. The dissertation will report the effects on the standard core physics parameters such as power peaking, multiplication factor, burn-up (safety and economics) and derive the benefits and drawbacks from the results. Based upon the findings from this study, and also experimental data, an optimum fuel management scheme is proposed for the PBMR core. / AFRIKAANSE OPSOMMING: Hierdie verhandeling beskryf die uitwerking van ‘n gekose stel kernbrandstofparameters op die bedryf van die PBMR reaktor. Die impak wat variasies in kernbrandstofparameters op belangrike veiligheids- en ekonomiese oorwegings het, is tydens hierdie studie ondersoek. Van die belangrikste oorwegings is die maksimum brandstoftemperatuur tydens normale, konstante bedryf, asook gedurende ‘n “Depressurized Loss of Forced Cooling (DLOFC)” insident waar alle verkoeling gestaak word. Ander belangrike fasette wat ondersoek is, is die reaktiwiteitwaarde van die beheerstelsel (RCS), wat die aanleg se vermoë om veilig af te sluit bepaal, asook die totale kernverbruik van die brandstof. Die kernbrandstofparameters wat in ag geneem is, sluit die brandstofverryking, swaarmetaalinhoud en die aantal brandstofsirkulasies deur die reaktorhart in. Die belangrikheid en impak van elk van hierdie parameters is ondersoek en word in die verhandeling beskryf . Daar word verslag gelewer oor die voor- en nadele, asook die uitwerking van hierdie variasies op standaard reaktorfisika-parameters soos drywingspieke in die brandstof, neutronvermenigvuldigingsfaktore en kernverbuik van die brandstof, vanaf ‘n veiligheids- en ekonomiese oogpunt. Gebaseer op die gevolgtrekkings van hierdie studie, tesame met eksperimentele data, word ‘n optimale kernbrandstofbestuurprogram voorgestel.