To assure nuclear power sustainability, ongoing efforts on advanced closed-fuel cycle options and adapted open cycles have led to investigations of various strategies involving utilization of Transuranic (TRU) nuclides in nuclear reactors. Due to favorable performance characteristics, multiple studies are focused on transmutation options using High Temperature Gas-cooled Reactors (HTGRs). Prismatic HTGRs allow for 3-Dimensional (3D) fuel shuffling and prior shuffling algorithms were based on experimental block movement and/or manual block shuffle patterns. In this dissertation, a physics based 3D multi-directional reloading algorithm for prismatic deep burn very high temperature reactors (DB-VHTRs) was developed and tested to meet DB-VHTR operation constraints utilizing a high fidelity neutronics model developed for this dissertation. The high fidelity automated neutronics model allows design flexibility and metric tracking in spatial and temporal dimensions. Reduction of TRUs in DB-VHTRs utilizing full vectors of TRUs from light water reactor spent nuclear fuel has been demonstrated for both a single and two-fuel composition cores. Performance of the beginning-of-life and end-of-life (EOL) domains for multi-dimensional permutations were evaluated. Utilizing a two-fuel assembly permutation within the two-fuel system domain for a Single-Fuel vector, the developed shuffling algorithm for this dissertation has successfully been tested to meet performance objectives and operation constraints.

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.