In order to facilitate design and maintenance of such a large and complex site as a nuclear power plant, all system functions must be described in a stringent way. In the past, these descriptions consisted of text documents and logical diagrams, but today there are an increasing number of object-oriented programs available on the market which might be used for this purpose. This Master Thesis has made a closer study of one of these programs named Rational Rose. The principal of the program is to facilitate software design and development, not to create models of plants. However, using the program the same way as developing software, specifying actors then gradually extend the model with use cases, use cases diagrams etc, the same methods can be used when modelling plants.