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

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. The former study showed 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 analytical uncertainty of 1.5 %. The analytical uncertainty of CRW affects the estimation of radial power distribution (RPD) especially for large fast reactor cores with the control rod inserted. In order to eliminate the radially-dependent analytical uncertainty of CRW and RPD, this study refined the super-cell model in the RRRP method with the help of Monte-Carlo simulations. As a result, the refinement significantly decreased the radially-dependent analytical uncertainty; the analytical uncertainty of CRW turned to be negligible and that of RPD became below 0.35 %. (author)