Investigation into different core configurations of the Safari-1 research reactor
Phillips, Crystal Orion
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In-core fuel management consists of the placement of fuel in a suitable mass distribution that ensures that core performance is optimised, while technical and safety constraints are also satisfied. In addition, from an operational point of view, the selected mass distribution should result in efficient use of the fuel and tend to bring about uniformity in the neutron flux distribution. As reactor cores are designed in terms of discrete, reloadable fuel assemblies, the problem becomes one of determining the optimal locations of fuel assemblies of different burn-ups, thereby defining a core-loading pattern. In this dissertation, two core configurations are investigated in order to find the best arrangement of assemblies in the core subject to particular operational constraints. In either case, considerations regarding specific reloads include the following: the number of fresh fuel assembles to be used; whether to load fresh control rod assemblies; the remaining 235U content of the nearly depleted fuel assemblies to be reinserted; and the distribution of 235U content amongst the assemblies making up the core. Because of the current reload strategy at the SAFARI-1 research reactor, a surplus of spent fuel assemblies with between 120 g and 140 g 235U has accumulated. In view of the high premium placed on highly enriched uranium fuel utilization and the high value of fissile material it has become imperative to investigate whether these assemblies can be burned as part of normal operations in the reactor core. However, it is important to investigate the feasibility of such a fuel management proposition before implementing it. The question therefore arises as to the most suitable location in the core to place assemblies within the given 235U mass range. This objective should be achieved while maintaining the thermal neutron flux levels in the SAFARI-1 irradiation positions and without compromising fuel economy. OSCAR-3 is a 3-D neutronic computational tool that is used in this investigation to simulate the core configurations. The dissertation describes the design parameters and operational limits and conditions of the research reactor, before developing the theoretical concepts that will be used as a basis for the neutronic parameters that are generated by the code. The features of the 3-D OSCAR-3 code are described in order to establish its suitability to solve the problem that is being investigated. Finally, the results obtained from the code calculations are used to recommend an appropriate plan of action for the utilization of nearly spent fuel that has accumulated over time with the operation of SAFARI-1.
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