A concept of object-oriented intelligent CAD/CAE environment is proposed for the conceptual designing of advanced nuclear reactor system. It is composed of (i) object-oriented frame-structure database which represents the hierarchical relationship of the composite elements of reactor core and the physical properties, and (ii) object-oriented modularization of the elementary calculation processes, which are needed for reactor core design analysis. As an example practise, an object-oriented frame structure is constructed for representing a 3D configuration of a special fuel element of a space reactor design, by using a general-purpose expert system shell ESHELL/X.