In this work, the conceptual design supporting tools for unclear power plants have been developed. One is made for system synthesis and reliability analysis, and the others are made for numerical simulation. The system synthesis program conbined with the reliability analysis program accomplishes the system synthesis. This new design strategy can reduce mistakes, effort and time. This design tool based on Prolog language is applied to the AFWS. A logic based fault tree analysis program (LOFT), was also developed using Prolog language. As LOFT performs symbolic computation during the fault tree analysis, its linking with knowledge-base systems is very easy and the partial usage of the program is possible. The importance measure of components obtained from the system reliability analysis and the complexity measure of system give very important information to the system designer. A Object-Oriented Simulation tool (OSIM) and a interactive dynamic simulation tool (InterSim) were developed for numerical analysis of the designed system. OSIM is wirtten in C language and uses FORSIM package as a dirving routine which was developed in AECL for continuous simulation. This system makes it possible to perform quick and inexpensive simulation. A system, which has redesigned components or newly made components, is easily simulated only developing MACROs of the new components. It is applied to the secondary loop simulation of a nuclear power plant. The simulation tool, InterSim, written in Prolog language is very convenient. It automatically discretizes the given equations and constructs a symbolic matrix of the discretized equations. the InterSim uses FORTRAN libraries for inversion of a matrix. A demonstration is accomplished for the two-temperature-region reactor control. Although the current developed system can treat only first-order ordinary differential equations, it can be extended to treat partial differential equations.