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. During this Master Thesis most of the time has been spent developing, structuring and classifying the functions composing the Feed Water Backup System of the reactor named Oskarshamn 3. A considerable amount of time was also spent to find a general structure for typical motor and valve circuits in the plant, which are also applicable for the configuration of the Feed Water Backup System. This general structure will then be used to support maintenance and to get faster decisions when new systems are designed. Effectuating the modernization of the nuclear power plants in Sweden, an ever- increasing use of highly software intensive systems will be introduced, which also leads to the need of finding other ways to describe those systems. A suitable method is to use Rational Rose, where the entire process, from description to final product, will be done in an integrated way. Use cases are generated and together with their related documentation they will form the description of the desired system functions.

<p>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. </p><p>During this Master Thesis most of the time has been spent developing, structuring and classifying the functions composing the Feed Water Backup System of the reactor named Oskarshamn 3. A considerable amount of time was also spent to find a general structure for typical motor and valve circuits in the plant, which are also applicable for the configuration of the Feed Water Backup System. This general structure will then be used to support maintenance and to get faster decisions when new systems are designed. </p><p>Effectuating the modernization of the nuclear power plants in Sweden, an ever- increasing use of highly software intensive systems will be introduced, which also leads to the need of finding other ways to describe those systems. A suitable method is to use Rational Rose, where the entire process, from description to final product, will be done in an integrated way. Use cases are generated and together with their related documentation they will form the description of the desired system functions.</p>