Metallic Combinations of Pu-Th and ²³³U-Th or Pu-Th and ²³³U-²³⁸U Fuel Cycles as Possible Alternatives to (Pu-U)O₂ in LMFBRs

Optimized breeding performances of three breeder strategies are compared. The first strategy is the normal mixed plutonium-uranium oxide fuel cycle, which is used as a reference case. The second is based on the use of the light water reactor generated plutonium in interim Pu-Th (metallic fuel) breeders cooled with sodium to build up ²³³U inventory for use in liquid-metal fast breeder reactors fueled with metallic ²³³U-Th. The third is based on a combination cycle involving two reactor types, Pu-Th and ²³³U-²³⁸U, both using metallic fuel and sodium as a coolant. These reactors will operate simultaneously; the excess ²³³U generated in the Pu-Th reactors is used to fuel the ²³³U-²³⁸U reactors and the plutonium generated in the ²³³U-²³⁸U reactors is used to fuel the Pu-Th reactors. The combination cycle has obvious antiproliferation characteristics. The breeding performance as measured by optimized compound system doubling time for nominal 1000-MW(electric) systems was 8.8 years for the combination system of Pu-Th and ²³³U-²³⁸U reactors 31.4 years for the ²³³U-Th reactor, and 14 years for the (Pu-U)O₂ reactor. The corresponding optimum fuel pin diameters were 0.30, 0.37, and 0.28 in., respectively. The Δk/k change associated with the removal of all the sodium from the inner core (inner to outer core volume ratio is 60:40) was +0.03, +1.01, +1.23, and +2.60% for the ²³³U-Th, ²³³U-²³⁸U, Pu-Th, and (Pu-U)O₂ reactors, respectively. Preliminary calculations indicate that it is possible to design the ²³³U-²³⁸U reactors to operate on an extended cycle such that once the reactor is built, it only needs natural uranium as feed fuel for the rest of the lifetime of the reactor. Estimates of the fuel cycle costs of each reactor show that the cost of the extended burnup cycle is ∼35% less than the (Pu-U)O₂ cycle.