A Fast Reactor Core Concept Using an Internal Blanket

A new core concept using an internal blanket, which is one type of heterogeneous core, has been developed. The core employs a disk-shaped internal blanket at the axial central region of the core. This internal blanket extends radially all the way through the core to the external blanket and is arranged so that its thickness is greater in the radial inner core region than the outer core region. It is designated as a “hamburger” core concept. Neutronics calculations for the equilibrium burnup cycle of 2500-MW(thermal) oxide-fueled liquid-metal fast breeder reactors exhibit the following advantages for the hamburger core over the corresponding conventional twoenrichment zone homogeneous one:

1. Breeding gain (breeding ratio = 1.0) increases by 5 to 7%, and the reactor doubling time decreases by ∼10%.
2. Burnup reactivity depression is reduced by ∼30%.
3. Power peaking factor and the power swing of fuel assemblies during burnup are almost the same.
4. Sodium void reactivity for the inner core region (including the internal blanket) is reduced by ∼25%).
5. Total fuel cycle cost is almost the same.