| Summary: | A new theoretical framework is introduced, the “neutron excess” concept, which is useful
for analyzing breed-and-burn (B&B) reactors and their fuel cycles. Based on this concept, a
set of methods has been developed which allows a broad comparison of B&B reactors using
different fuels, structural materials, and coolants. This new approach allows important
reactor and fuel-cycle parameters to be approximated quickly, without the need for a full
core design, including minimum burnup/irradiation damage and reactor fleet doubling time.
Two general configurations of B&B reactors are considered: a “minimum-burnup” version
in which fuel elements can be shuffled in three dimensions, and a “linear-assembly” version
composed of conventional linear assemblies that are shuffled radially.
Based on studies of different core compositions, the best options for minimizing fuel burnup
and material DPA are metal fuel (with a strong dependence on alloy content), the type of
steel that allows the lowest structure volume fraction, and helium coolant. If sufficient fuel
performance margin exists, sodium coolant can be substituted in place of helium to achieve
higher power densities at a modest burnup and DPA penalty. For a minimum-burnup B&B
reactor, reasonably achievable minimum DPA values are on the order of 250-350 DPA in
steel, while axial peaking in a linear-assembly B&B reactor raises minimum DPA to over
450 DPA. By recycling used B&B fuel in a limited-separations (without full actinide
separations) fuel cycle, there is potential for sodium-cooled B&B reactors to achieve fleet
doubling times of less than one decade, although this result is highly sensitive to the reactor
core composition employed as well as thermal hydraulic performance.
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