A good understanding of the flow of nuclear material through a fuel cycle is a vital aspect of non-proliferation activities. For both the design of new reactors and monitoring of extant fuel cycles, modelling and tracking the flow of material throughout all phases of a fuel cycle may make it possible to highlight possible diversion risks and predict trends in the material volumes and compositions. ORION is a fuel cycle modelling code developed by the UK’s National Nuclear Laboratory for over 20 years. ORION tracks the flow of around 2,500 nuclides throughout the fuel cycle through user-defined enrichment facilities, fabrication processes, reactors, storage facilities and reprocessing techniques. In modelling reprocessing operations, ORION can model any co-extraction or single-element extraction process. In addition, ORION is capable of calculating the effective fissile mass of fuel required for a reactor to achieve a set cycle length, therefore, enabling prediction of the drawdown rate of stockpiled fissile material utilised in a closed fuel cycle. Minor actinide recycling can be assessed in addition to the total impact for a closed vs open fuel cycle in terms of effective fissile mass, heat generation and radiotoxicity of spent fuel. The capabilities of ORION are discussed along with examples demonstrating how ORION could be used to model a country or region’s fuel cycle providing a predictive tool for modelling and tracking the flow of material in each phase of the cycle - information which can be used to inform safeguards assessments, thereby aiding the peaceful use of nuclear material.