Assessment of Fast-Spectrum Blanket Lattices for Breeding Fissile Fuel From Thorium and Depleted Uranium in an Externally Driven Sub-Critical Gas-Cooled Pressure Tube Reactor

To ensure long-term nuclear energy security, it is advantageous to consider the use of externally driven sub-critical systems for producing fissile fuel to supply fleets of thermal-spectrum reactors as an alternative to using fast-spectrum or thermal-spectrum breeder reactors. Computational/analytical neutronics and heat transfer studies have been carried out for gas-cooled fuel bundle lattices with mixtures of fertile thorium and depleted uranium (DU) that could be used in the blanket region of a sub-critical fast reactor driven either by a fusion reactor in a hybrid fusion-fission reactor (HFFR) system, or an accelerator-based spallation neutron source in an accelerator driven system (ADS). The HFFR or ADS concept envisioned is one with a simple cylindrical geometry. The annular-cylindrical blanket is approximately 10 m long, can be made 2–5 m thick (1.0 m ≤ R blanket ≤ 3.0 m to 6.0 m), and is filled with a repeating square lattice of pressure tubes filled with 0.5 m long fuel bundles that are made of (DU,Th)O 2 , with various mixtures of Th and DU, and refuelled periodically online. Although using blankets made of pure DUO 2 or ThO 2 are viable options to analyze, mixing DUO 2 with ThO 2 can help alleviate any potential proliferation concerns, since any 233 U produced from breeding will be denatured by the presence of 238 U in (DU, Th)O 2 . Lattice calculations demonstrate that the total fissile content in the fuel after an extended period of burnup (50 MWd/kg) will be approximately the same, regardless of the mixture of DU and thorium used, and that the content of americium and 232 U in the irradiated fuel will be <0.01 wt%/initial heavy metal.