ORNL uses ORION, a systems dynamics fuel cycle code developed and maintained by the National Nuclear Laboratory (NNL) in the United Kingdom, to model fuel cycle scenarios. ORION can simulate the full range of nuclear-related facilities, including storage facilities, fabrication and enrichment plants, reprocessing facilities, and reactors. A GUI allows the user to define these facilities and connect them to form a fuel cycle model. The code can track over 2,500 nuclides and can model decay and in-reactor irradiation. While ORION can track an extremely large number of nuclides, it can also follow a much simpler series of decay chains involving any number of nuclides or even idealized (lumped) material types (e.g., fission product, uranium, transuranic material). The simulations defined in ORION are time dependent, so the evolution of these quantities can
be tracked over time, including transition time from one fuel cycle to another.
Cross sections used in ORION are generated using lattice physics models in SCALE. With recent ORION developments, direct coupling to SCALE/ORIGEN has been developed, and the ORIGEN application
programming interface (API) has been integrated to provide flexibility during model development when the specifics regarding reactor operations and fresh fuel compositions are not fully known. A dynamic reactor control functionality in ORION enables automated deployment of reactors based on various conditions such as fissile material in store, growth rate of nuclear energy, and commissioning or decommissioning profiles of reactors in the model. ORION can predict the availability of fissile material and assess which reactor type to build. The user can also set stream priorities to favor specific fissile material pathways into a reprocessing facility with various fractions of feed from different streams.
Finally, recent interest in modeling MSRs prompted an effort to include capabilities in ORION to simulate the continuous flow of material through MSRs, as recipes cannot be applied to analyze MSRs in ORION.