Graduate Thesis Or Dissertation
 

Feasibility study on a soluble boron-free small modular reactor

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  • The elimination of soluble boron creates several advantages for Small Modular Reactor (SMR) operation. Most of these advantages are realized through significant core simplification (removal of pipes, pumping, and purification systems), the removal of the corrosive effects of soluble boron, and from improved safety effects. However, removing soluble boron creates its own set of specific challenges that must be overcome. Traditional pressurized water reactors employ soluble boron for uniform power suppression throughout the core. Thus any boron-free SMR design requires increased dependence on control rods and burnable poisons, where both are discrete neutron absorbers that locally impact the core where they are inserted. Since control rods are partially inserted, their presence negatively impacts the axial power profile and this distortion creates undesirable power peaks, leading to a reduced operating margin and a significant economic burden. Thus, the main challenge in any boron free design concerns excess reactivity suppression and active reactivity control while maintaining a proper axial power profile and reduced power peaks. The goal of the feasibility study is to investigate the physical effects of removing soluble boron, and to investigate and identify an effective strategy for containing power peaks in a boron-free SMR. Studsvik's CASMO-4E was employed to solve 2-D Transport equation for infinite lattice analysis, and SIMULATE-3K was employed to solve 3-D nodal diffusion equation for full core analysis. The study identified improved reactivity feedback mechanisms associated with the removal of soluble boron, arising from a softened neutron flux and a decreased production of plutonium. An analysis of strategies for soluble boron-free operation that involved axially grading burnable poisons and U²³⁵ enrichment percentages was found unable to be able to control the axial power profile throughout core lifetime. The inherent limitations in the lifetime of burnable poisons resulted in an inability to control the axial power profile through middle and end of cycle. Investigations of additional strategies involving an advanced control rod algorithm produced significantly improved results that met the prescribed criteria for success. The advanced control rod algorithm is thus recognized as a viable strategy for boron-free operation for SMRs.
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