Neutronic Evaluation of Breed-and-Burn Reactor Fuel Types Using an Infinite-Medium Depletion Approximation - 297

A new method of analyzing breed-and-burn reactors is developed which allows the reactor's equilibrium-cycle reactivity to be stated in terms of the depletion history of the fertile feed fuel. The method is centered on calculating the total number of neutrons produced and absorbed by a volume of feed fuel over its depletion history. The net number of neutrons produced or absorbed over the feed fuel's life is termed its neutron excess. Neutron excess is found to be insensitive to a number of factors, including reactor geometry, cycle length, discharge burnup distribution, and equilibrium-cycle shuffling path. This allows the reactivity-burnup relationship for a breed-and-burn reactor to be accurately estimated using simple one-dimensional models. One method of approximating the feed-fuel depletion history is to deplete an infinite medium of feed material in its self-produced spectrum. Performing a neutron excess calculation on the infinite-depletion case allows a reactivity estimate to be made, which is found to agree closely with reactivities calculated in actual models of a breed-and-burn system. The infinite-medium approximation is applied to compare the neutronic performance of different metal and ceramic fuels, including thorium. The minimum burnup and fast fluence required to sustain breed-and-burn operation is reported for each fuel. Uranium-zirconium alloys are found to be the best performing metal fuels while U3Si2 and uranium nitride with enriched nitrogen-15 were found to be the best performing ceramic fuels. (authors)