This paper assesses the feasibility of Seed-and-Blanket (S&B) Sodium-cooled Fast Reactor (SFR) cores that are made of once-through subcritical blankets driven by recycled TRU seeds. All S&B cores are to generate 1000 MW_(th) and fit within the S-PRISM reactor vessel. Two fuel options are evaluated for the blanket - depleted uranium and thorium; both in metallic form. The design objective is to maximize the power generated by the blanket. As the blanket fuel cost is significantly lower than the cost of the seed fuel and does not need reprocessing, the larger the fraction of the reactor power generated by the blanket, the smaller will be the reactor fuel cycle cost and the smaller will be the fuel reprocessing capacity required per unit of electricity generated. The seed (or "driver") is of a prolate (or "cigar") shape to maximize the fraction of neutrons that radially leak into the subcritical blanket. Both seed and blanket contain multiple fuel batches; the blanket batches are gradually shuffled inward and not recycled, while the seed fuel is reprocessed and recycled. The preliminary study found that the fraction of power generated by the blanket is between 30% and 50% without exceeding the 200 DPA (Displacement per Atom) radiation damage constraint on the HT-9 clad; this fraction increases when the seed is designed to have a smaller conversion ratio (CR). For a seed with same CR, the depleted uranium blanket can generate approximately 10% (absolute) higher fractional power than the thorium blanket. The low CR S&B cores feature significantly smaller burnup reactivity swing relative to conventional TRU burning SFR cores with identical CR. This enables to design the S&B cores to operate for longer cycles and discharge their fuel at higher burnup. These features are expected to improve the SFR economics.
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