Axially heterogeneous thorium fuel designs for transuranic burning in reduced-moderation BWRs

Reduced-moderation Boiling Water Reactors (RBWRs) can allow sustained burning of transuranics (TRU), such that full actinide recycle can be achieved. However, the void coefficient (VC) tends to become positive with reduced moderation and high TRU loading, which can severely limit the design feasibility and performance. This motivates use of thorium (Th) as an alternative feed to uranium (U), as this tends to result in a more negative VC, leading to much improved neutronic performance. While axially homogeneous fuel design is preferable for ease of fuel fabrication, it is valuable to evaluate axially heterogeneous fuel designs to see if improved neutronic performance can be realised, which could lead to a reduction in fuel reprocessing and fabrication throughput. Multi-recycled Th-TRU fuel contains Th, U, Pu and MAs, leading to a wide range of possible fuel designs. Axially heterogeneous designs are considered using 3D pincell calculations using the Monte Carlo code Serpent. Radially heterogeneous assembly designs are considered in a companion paper. Spatial separation of Th-TRU and Th-U3 into regions of the order of a few thermal neutron diffusion lengths greatly improves neutronic performance. This can be accomplished radially or axially, but radial separation results in significantly easier fuel fabrication. Axial seed-blanket heterogeneity improves neutron economy at the expense of high power peaking, such that the radially heterogeneous assembly design is preferred. Separation of Th-TRU and Th-U3 into larger regions is not effective, due to increased power density in the Th-TRU region with voiding and with burn-up, leading to a more positive VC. It is therefore concluded that there is no motivation to pursue axially heterogeneous Th-TRU RBWR burner designs