Improved Treatment of the Neutron Streaming Through Control Rod Followers in a Sodium-Cooled Fast Reactor

A method is presented for determining by numerical integration a diffusion coefficient, D_z, applicable to the axial direction of a control rod follower region in a sodium-cooled fast reactor. By comparing criticality values from transport and diffusion theory for a simplified reactor model, we are able to show that this D_z applies well to the followers from the viewpoint of the calculated criticality factor, k_eff. By use of the same model, an inter comparison is also made between the present and other definitions of D_z. By using the present D_z, as a conclusion, we show that the currently used D (= 1/3Σ) for control rod followers leads to an underestimation of k_eff by ∼0.3% for a typical 300-MW(electric) class liquid-metal fast breeder reactor with one-third of the control rods inserted, which may increase to 0.4 to 0.5% for a case where all control rods are withdrawn.