Geological disposal is a method widely accepted as safe and feasible for the permanent disposal of spent fuel. There is not yet an operating geological disposal facility, however, there are various projects ongoing in many countries. KBS-3 disposal concept developed by Sweden is the basis of most of the geological repositories under development but there are differences among the concepts mainly in the geometry and material of disposal canisters, geological formations of host rock and vertical or horizontal emplacement of canisters. The aim of this study is to design disposal canisters for spent fuels of two representative new generation reactors, VVER-1200 and ATMEA1, and determine the disposal densities of designed canisters when emplaced horizontally in a granitic rock formation. In the first part of the study, characteristics of spent fuels discharged from VVER-1200 and ATMEA1 reactors and cooled for various time periods before disposal are determined by utilizing the MONTEBURNS 2.0 code. Then, canister designs for different spent fuel loading patterns (various spent fuel numbers, geometric orientations, precooling periods) are developed by taking into consideration the dimensional and thermal limitations. In the last part of the study, suitable disposal distances between the designed canisters are determined for horizontal emplacement in a geological repository by performing thermal analysis in the ANSYS and most favorable canister designs for VVER-1200 and ATMEA1 spent fuels are determined with respect to disposal density. Results showed that canisters consisting of 5 VVER-1200 spent fuel assemblies with 60 years pre-cooling period and 5 ATMEA1 assemblies with 70 years pre-cooling period require less spacing and disposal area than other canisters investigated. (C) 2020 Elsevier Ltd. All rights reserved.