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Previous issue date: 1981 === This thesis describes the development of a fuel depletion strategy that maximizes cycle length in boiling water reactor (BWR) cores. The cycle length maximization problem was formulated in terms of a core reactivity maximization scheme which provided solution to a terminal state optimization problem as well as to the optimal depletion strategy search. The nonlinear optimization problem was solved through an iterative application of linear programming involving linearization of the objective function and constraint equations. The nuclear-thermal-hydraulic model representing BWR cores was solved in a fully coupled, nonlinear form outside of the linear programming algorithm. For our numerical study, a large BWR core was modeled through a finite-difference form of the axial one-dimensional, two group neutron diffusion equation with control rods and thermal-hydraulic feedback represented. The optimal terminal state that results in maximum cycle length at the end-of-cycle for a given fuel loading is obtained through two phases, involving burnup shape optimization and cycle length extension, respectively. The optimal fuel depletion strategy is obtained through optimization of control rod pattern such that the loss in core reactivity over each depletion interval is minimized subject to power distribution constraints. The maximum cycle length obtained in our one dimensional axial depletion calculation indicates an increase of 7.4% over the corresponding Haling result, suggesting potential improvement in fuel utilization through proper control poison management. We also conclude that both the optimal terminal state and the optimal depletion strategy strongly depend upon the power distribution constraints. The fuel cycle is extended at the expense of power peaking margin. The optimal terminal state results in a bimodal bottom-peaked burnup shape and a top-peaked power distribution with the power peaking factor at the design limit. The optimal depletion calculation shows that the optimal power distribution is bimodal and time dependent with, the peaking factor at the design limit. The optimal power distribution is more skewed than the traditional Haling shape and bottom-peaked for most of the fuel cycle. For a short time interval around a coreaverage burnup of 3 GWD/T the power distribution is toppeaked reflecting the high depletion rate of the distributed burnable poison.
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