AbstractA two-group, one-dimensional formulation of a coupled-core system is proposed as a revision of the one-group response function method by Shinkawa et al. The coupling coefficient of the Kyoto University Critical Assembly symmetric coupled-core loading is revised. In such a light-water-coupled system, the fast-to-fast coupling,Δ11proves the greatest, the fast-to-thermal,Δ12, the second, and the thermal-to-thermal,Δ22, the smallest component within the quantity; at the core distance of 10 cm,Δ12= 0.68Δ11andΔ22= 0.028Δ11. Beyond 20 cm, bothΔ11andΔ12decrease approximately by the fast-neutron relaxation length of water. The effectiveness of the incoming neutrons is considerably dependent on the thickness of the core that receives them.