Nuclear medicine is one of the branches of clinical medicine that consumes 50% of radioactive isotopes supply in the world. Multi-purpose synthesis system is used to produce such radiopharmaceuticals. The system is made of Commercial off-the-shelf (COTS) parts, that function in radioactive operational conditions. During its life, the synthesis system is exposed to radiation, therefore it must have high reliability so that it can fulfil its mission without failures. Purpose of the work presented in this paper is to carry out a probabilistic assessment of system reliability. To achieve the task following steps should be done: first step is to develop a reliability model of the system, second step is to carry out reliability assessment of the system based on previously developed model. In order to model, asses and analyse the reliability of the synthesis system, reliability block diagram (RBD) and Hasse diagram have been used. Also, there is opportunity to use an approach based on graph models issued from Hasse diagrams to compare the results with a classical aprraoch such as RBD.