In this paper, we present APACS, Advanced Plant Analysis and Control System, a multi-agent system (MAS) with repository support. APACS has been designed and implemented for monitoring and diagnosing real-time nuclear power plant failures. Specifically, we demonstrate the importance of repository technology in achieving knowledge communication within a multi-agent system. In this paper, we will outline important design and conceptual problems such as the choice of data model, the portability problem, the transparency problem and the maintenance problem, encountered during our practical experience that will no doubt be encountered by other designers of MAS systems, as well as presenting our practical solutions and their benefits. It is hoped that this experience will be of interest to researchers due to its innovative solutions and their impacts in the conceptual frameworks of next generation MASs, and of interest to practitioners who may benefit from the demonstration of both the feasibility and the economical and technical benefits of knowledge sharing using repositories in the design and implementation of MASs.