[en] Osiris is a general purpose software tool which has been developed for performing studies of material usage, economics and environmental impacts associated with the nuclear fuel cycle. It is particularly suited to the analysis of complex multiple recycling scenarios featuring combinations of both existing and new reactor systems. A discrete event model is used to represent the movement of material batches around the fuel cycle due to the operations and timings of process plants. Nuclear materials are represented by state vectors, which undergo compositional changes due to radioactive decay and irradiation. A library of generic plant types is provided, which are declared and configured in order to represent specific plants. Reactors, front-end and back-end process plants, buffers, stores and material sources can be modelled. Fuel cycle scenarios are then constructed by defining the material flow paths between the plants. The plant parameters and flow paths can be altered dynamically over the course of a scenario in order to represent changes in recycling strategies and retirement and replacement of process plants. Discounted electricity costs can be determined by assigning unit costs to all capital investments, processes and raw materials. Radiotoxicity levels of waste streams can also be evaluated. The software was developed in C++ using objected oriented analysis and design methods. The use of abstraction and inheritance have enabled an open-ended fuel cycle modelling environment to be established, into which new reactor or plant models can readily be integrated. An overview of the object model, numerical modelling assumptions and the design and implementation of the software is presented. (author)