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

Social and environmental justice for a growing and developing global population requires significant increases in energy use. A possible means of contributing to this energy increase is to incinerate plutonium from spent fuel of pressurised light water reactors (Pu(PWR)) in high-temperature reactors such as the Pebble Bed Modular Reactor Demonstration Power Plant 400 MWth (PBMR-DPP-400). Previous studies showed that at low temperatures a 3 g Pu(PWR) loading per fuel sphere or less had a positive uniform temperature reactivity coefficient (UTC) in a PBMR DPP-400. The licensing of this fuel design is consequently unlikely. In the present study it was shown by diffusion simulations of the neutronics, using VSOP-99/05, that there is a fuel design containing thorium and plutonium that achieves a negative maximum UTC. Further, a fuel design containing 12 g Pu(PWR) loading per fuel sphere achieved a negative maximum UTC as well as the other PBMR (Ltd.) safety limits of maximum power per fuel sphere, fast fluence and maximum temperatures. It is proposed that the low average thermal neutron flux, caused by reduced moderation and increased absorption of thermal neutrons due to the higher plutonium loading, is responsible for these effects. However, to fully understand the mechanisms involved a detailed quantitative analysis of the roll of each factor is required. A 12 g Pu(PWR) loading per fuel sphere analysis shows a burn-up of 180.7 GWd/tHM which is approximately double the proposed PBMR (Ltd.) low enriched uranium fuel burn-up. The spent fuel has only a decrease of 24.5 % in the Pu content which is sub-optimal with respect to proliferation and waste disposal objectives. Incinerating Pu(PWR) in the PBMR-DPP 400 MWth is potentially licensable and economically feasible and should be considered for application by industry. / MIng (Nuclear Engineering), North-West University, Potchefstroom Campus, 2012

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

Social and environmental justice for a growing and developing global population requires significant increases in energy use. A possible means of contributing to this energy increase is to incinerate plutonium from spent fuel of pressurised light water reactors (Pu(PWR)) in high-temperature reactors such as the Pebble Bed Modular Reactor Demonstration Power Plant 400 MWth (PBMR-DPP-400). Previous studies showed that at low temperatures a 3 g Pu(PWR) loading per fuel sphere or less had a positive uniform temperature reactivity coefficient (UTC) in a PBMR DPP-400. The licensing of this fuel design is consequently unlikely. In the present study it was shown by diffusion simulations of the neutronics, using VSOP-99/05, that there is a fuel design containing thorium and plutonium that achieves a negative maximum UTC. Further, a fuel design containing 12 g Pu(PWR) loading per fuel sphere achieved a negative maximum UTC as well as the other PBMR (Ltd.) safety limits of maximum power per fuel sphere, fast fluence and maximum temperatures. It is proposed that the low average thermal neutron flux, caused by reduced moderation and increased absorption of thermal neutrons due to the higher plutonium loading, is responsible for these effects. However, to fully understand the mechanisms involved a detailed quantitative analysis of the roll of each factor is required. A 12 g Pu(PWR) loading per fuel sphere analysis shows a burn-up of 180.7 GWd/tHM which is approximately double the proposed PBMR (Ltd.) low enriched uranium fuel burn-up. The spent fuel has only a decrease of 24.5 % in the Pu content which is sub-optimal with respect to proliferation and waste disposal objectives. Incinerating Pu(PWR) in the PBMR-DPP 400 MWth is potentially licensable and economically feasible and should be considered for application by industry. / MIng (Nuclear Engineering), North-West University, Potchefstroom Campus, 2012

In this thesis advanced fuel cycles for the incineration, i.e. deep–burn, of weapons–grade plutonium, reactor–grade plutonium from pressurised light water reactors and reactor–grade plutonium + the associated Minor Actinides in the 400 MWth Pebble Bed Modular Reactor Demonstration Power Plant was simulated with the VSOP 99/05 diffusion code. These results were also compared to the standard 9 g/fuel sphere U/Pu 9.6% enriched uranium fuel cycle. The addition of the Minor Actinides to the reactor–grade plutonium caused an unacceptable decrease in the burn–up and thus an unacceptable increase in the heavy metal (HM) content in the spent fuel, which is intended for direct disposal in a deep geological repository, without chemical reprocessing. All the Pu fuel cycles failed the adopted safety limits in that either the maximum fuel temperature of 1130°C, during normal operation, or the maximum power of 4.5 kW/sphere was exceeded. All the Pu cycles also produced positive Uniform Temperature Reactivity Coefficients, i.e. the coefficient where the temperature of the fuel and the graphite moderator in the fuel spheres are varied together. these positive temperature coefficients were experienced at low temperatures, typically below 700°C. This was due to the influence of the thermal fission resonance of 241Pu. The safety performance of the weapons–grade plutonium was the worst. The safety performance of the reactor–grade plutonium also deteriorated when the heavy metal loading was reduced from 3 g/sphere to 2 g or 1 g. In view of these safety problems, these Pu fuel cycles were judged to be not licensable in the PBMR DPP–400 reactor. Therefore a redesign of the fuel cycle for reactor–grade plutonium, the power conversion system and the reactor geometry was proposed in order to solve these problems. The main elements of these proposals are: v 1. The use of 3 g reactor–grade plutonium fuel spheres should be the point of departure. 232Th will then be added in order to restore negative Uniform Temperature Reactivity Coefficients. 2. The introduction of neutron poisons into the reflectors, in order to suppress the power density peaks and thus the temperature peaks. 3. In order to counter the reduction in burn–up by this introduction of neutron poisons, a thinning of the central reflector was proposed. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.

In this thesis advanced fuel cycles for the incineration, i.e. deep–burn, of weapons–grade plutonium, reactor–grade plutonium from pressurised light water reactors and reactor–grade plutonium + the associated Minor Actinides in the 400 MWth Pebble Bed Modular Reactor Demonstration Power Plant was simulated with the VSOP 99/05 diffusion code. These results were also compared to the standard 9 g/fuel sphere U/Pu 9.6% enriched uranium fuel cycle. The addition of the Minor Actinides to the reactor–grade plutonium caused an unacceptable decrease in the burn–up and thus an unacceptable increase in the heavy metal (HM) content in the spent fuel, which is intended for direct disposal in a deep geological repository, without chemical reprocessing. All the Pu fuel cycles failed the adopted safety limits in that either the maximum fuel temperature of 1130°C, during normal operation, or the maximum power of 4.5 kW/sphere was exceeded. All the Pu cycles also produced positive Uniform Temperature Reactivity Coefficients, i.e. the coefficient where the temperature of the fuel and the graphite moderator in the fuel spheres are varied together. these positive temperature coefficients were experienced at low temperatures, typically below 700°C. This was due to the influence of the thermal fission resonance of 241Pu. The safety performance of the weapons–grade plutonium was the worst. The safety performance of the reactor–grade plutonium also deteriorated when the heavy metal loading was reduced from 3 g/sphere to 2 g or 1 g. In view of these safety problems, these Pu fuel cycles were judged to be not licensable in the PBMR DPP–400 reactor. Therefore a redesign of the fuel cycle for reactor–grade plutonium, the power conversion system and the reactor geometry was proposed in order to solve these problems. The main elements of these proposals are: v 1. The use of 3 g reactor–grade plutonium fuel spheres should be the point of departure. 232Th will then be added in order to restore negative Uniform Temperature Reactivity Coefficients. 2. The introduction of neutron poisons into the reflectors, in order to suppress the power density peaks and thus the temperature peaks. 3. In order to counter the reduction in burn–up by this introduction of neutron poisons, a thinning of the central reflector was proposed. / Thesis (PhD (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012.