Neutronic characteristics of linear-assembly breed-and-burn reactors

► Simple models used to characterize general behavior of linear-assembly B&B reactors. ► Diffusion theory model developed to explain axial distributions, height vs. reactivity. ► Neutron excess concept reformulated to include linear-assembly B&B reactors. ► Designed model of B&B reactor started using melt-refined B&B reactor used fuel. ► Computed doubling time of fuel cycle requiring no chemical separations. Linear-assembly breed-and-burn (B&B) reactors are B&B reactors that use axially connected assemblies similar to conventional LWR or fast reactor fuel assemblies. Methods for analyzing linear-assembly B&B reactors and their fuel cycles are developed and applied. General neutronic characteristics of linear-assembly B&B reactors are analyzed, including the effects that burnup, shuffling sequence, and radial and axial size have on equilibrium-cycle k-effective. The mechanisms that give rise to a highly peaked axial burnup distribution are explained, and a method for predicting peak burnup vs. k-effective based on infinite-medium depletion calculations is developed. Next, the neutron excess concept from previous studies of B&B reactors is extended to apply to linear-assembly B&B reactors, which allows the amount of starter fuel needed to establish a given equilibrium cycle to be calculated. Several example applications of the neutron excess formulation are given. First, an example model of a linear-assembly B&B reactor is analyzed to find the neutron excess cost of an equilibrium cycle. Second, simple one-dimensional models are used to predict the neutron excess value obtainable from different starter fuel configurations. Finally, these ideas are applied to design a fuel cycle consisting of linear-assembly B&B reactors and fuel recycling via a melt refining process. The neutron excess concept is used to design an appropriate starter fuel configuration made from melt refined fuel, which is then used in a transition model that shows how the desired equilibrium cycle can be established. The doubling time of this fuel cycle is calculated as an example of what is achievable using B&B reactors.