A conceptual design tool for three types of liquid metal reactors (LMRs), ultra long life, accelerator-driven subcritical transmutation and large scaled LMRs has been proposed. This tool is developed for system design with artificial intelligence, scaling of design parameters, and validation analysis. The system design consists of design synthesis and design analysis. System design decides the optimal structure and layout of a LMR using design synthesis with rule bases and databases, and design analysis with design constraints, The designed system is scaled by scale laws to be optimal with desired power level, and then a specific design basis accident is analyzed in validation part. Design synthesis contains the data about each component and general LMRs, and the rules about selection of each component and general LMRs, Design analysis contains several design constraints for formulation and solution of a design problem. In these two parts, a designer's intention can be externalized through emphasis on design requirements. For the purpose of demonstration, the conceptual design tool was applied to an ultra long life LMR with 35 MWe power level, This ultra long life LMR was designed from knowledge structure based on all the characteristics of ultra long life, accelerator-driven subcritical transmutation, and large scaled LMRs, and then optimally scaled to the 35 MWe power level. (C) 1999 Elsevier Science Ltd. All rights reserved.