Control Rod Worth and Related Nuclear Characteristics of an Axially Heterogeneous Liquid-Metal Fast Breeder Reactor Core

Abstract

An axially heterogeneous core (AHC) concept is applied to a 1000-MW(electric)-class tank-type liquidmetal fast breeder reactor (LMFBR). This AHC is characterized by a disk-shaped internal blanket with a radial thickness adjustment at the core midplane. The nuclear characteristics connected with control rod worth of the AHC are analyzed and compared with those of a homogeneous core (HOC) of the same power rating. The neutronics analysis shows that the reactivity insertion due to the vertical displacement of control rods relative to the core, which is an important safety characteristic of a tank-type LMFBR core, is significantly decreased in the AHC because of the reduced control worth requirement and smaller peakto-average differential worth in the primary control system. This allows the AHC to have a vertical displacement 50% greater than that of the HOC and may offer less rigid design conditions for roof slab stiffness and core support systems in a tank-type LMFBR. A reduction in the number of primary control rods is also possible because of a smaller control worth requirement and a better power peaking factor. The work was sponsored by a federation of Japanese electric power companies and performed under the guidance of the fast breeder reactor project office.