An Axially Heterogeneous Core Concept for Large LMFBRs and Its HCDA Behavior

One type of axially heterogeneous core concept for liquid-metal fast breeder reactors has been developed in which a disk-shaped internal blanket with its thickness radially changed is introduced at the driver core midplane except for a few outside rows of the driver core fuel subassemblies. This core configuration gives a high breeding ratio and short doubling time, as well as flat and stable power distribution throughout the operation. The hypothetical core disruptive accident (HCDA) behavior of this core is considerably mitigated, and the mechanical work energy released in energetic events during the initiating phase of the severest HCDA is a few times less than that of an equivalent conventional two-enrichment-zone homogeneous core. This effect is attributed to a smaller sodium-void reactivity worth around the core center and an axially flattened fuel worth distribution in the presented heterogeneous core.