An investigation on the control rod homogenization method for next-generation fast reactor cores

Abstract A Japanese next-generation fast reactor core design adopts the reaction rate ratio preservation (RRRP) method for control rod homogenization with a super-cell model in which a control rod is surrounded by fuel assemblies. An earlier study showed that the RRRP method with the conventional super-cell model could estimate the control rod worth (CRW) of a 750-MWe large fast reactor core within the error of 1.5%. The estimation of radial power distribution (RPD) tends to have relatively large error especially for large fast reactor cores with the control rods inserted. In order to reduce the radially-dependent error of CRW and RPD, this study evaluated the modeling error and refined the surrounding fuel assemblies of the super-cell model for all control rods in the RRRP method. This refinement significantly decreased the radially-dependent modeling error: the error of CRW and RPD were reduced to less than 0.1% and 0.4%, respectively.