The High Temperature Integrated Multi-Modular Thermal Reactor is a small modular reactor that uses an enhanced conductivity BeO-UO2 fuel with supercritical CO2 coolant to drive turbo-machinery in a direct Brayton cycle. The core consists of several self-contained pressurized modules, each containing fuel elements in pressurized channels surrounded by a graphite moderator, and Brayton cycle turbo-machinery. Each module is subcritical by itself, and when several modules are brought into proximity of one another, a single critical core is formed. The multi-modular approach and use of BeO-UO2 fuel with graphite moderator and supercritical CO2 coolant leads to an inherently safe system capable of high efficiency operation. The pressure channel design and multi-modular approach eliminates engineering challenges associated with large pressure vessels. The subcriticality of the modules ensures inherent safety during construction, transportation, and after decommissioning. Serpent, a continuous-energy Monte-Carlo reactor physics burnup calculation code, was used to develop a critical configuration of the subcritical modules using UO2 fuel enriched with 5 wt% 235U with a 5 wt% BeO additive. The core lifetime was found to be 14.6 years operating at 10 MWth, though the U enrichment and power can be altered to achieve desired core lifetimes. Negative fuel and moderator temperature coefficients of reactivity were found that could maintain safety during operation. The multi-modular design was found to be beneficial compared to a core with all fuel elements in one module. Batch battery type refueling was found to be beneficial and the feasibility of controlling the reactor was demonstrated through the use of control shells that surround each module. The HT-IMMTR design is an inherently safe, highly efficient, economically competitive, and most important, feasible reactor design that takes advantage of proven technologies to facilitate the demonstration of a successful commercial deployment.

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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

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.