The VISION Nuclear Fuel Cycle Simulation Model is a system dynamics model that captures all aspects of the nuclear fuel cycle, from the mining of ore to the ultimate disposal of spent fuel and high-level waste. VISION is coded in the PowerSim simulation environment, which provides both a simple user interface and a powerful set of system dynamics modeling tools. As a system dynamics model, it tracks materials and facilities as a combination of “stocks” and “flows”, where stocks represent a quantity or number of an item or facility, and flows are the time rate of change of these items as they transfer or transform from one state to the next. The input and output parameters for up to five scenarios are stored in a set of MS Excel worksheets that are distributed with the VISION model. The input worksheets specify all of the user-specified parameters, including reactor fuel recipes, reactor parameters, recycling routings, and many others. The output spreadsheets are hierarchical in nature, with a set of low-level detailed outputs of mass flows by timestep and higher-level aggregated metrics used for evaluation and comparison between scenarios.

The VISION model simulation covers a user-defined time span with customisable start and end years and a fixed quarter-year timestep. Scenarios of up to 300 years duration can be accommodated by the standard input and output worksheets.

Reactors are modelled using a fleet-averaged methodology, where the count of active reactors of each of up to ten types is tracked, with each type tracking fuel through up to six recycling passes. Reactors are built either to meet a predetermined power schedule, or to maximise usage of recyclable fissile material without incurring supply disruptions (according to a simplified heuristic algorithm). Reactors run for a user-specified period of time and are then retired, with replacement units being built in time.

The construction of reactors may be governed according to two approaches. In the first, the user defines both the annual electric generation demand and the annual fraction of each reactor type to be built. In the other, the user specifies only the total demand and a prioritisation for the construction of recycling reactors to meet this demand. In this scenario, a lightweight heuristic approach is used to forecast the amount of fissile material that will remain in each subsequent simulation year, and from that forecast estimate the number of recycling reactors that may thereby be supported. If the required power cannot be met using recycling reactors, an alternative reactor is built instead. This approach works for both burner reactors (which are always net consumers of fissile material) and breeder reactors (which provide net production of fissile material, but only after several years of operation).

In VISION, nuclear materials are generally tracked on an isotopic mass basis with 88 individual isotopes and a number of grouped streams and co-products. Nuclear material within the reactors is tracked by total mass assuming a fixed input and discharge composition given by user-defined fuel recipes. Nuclear decay transformations are tracked on select isotopes within certain key operations, such as wet and dry storage. This supports increased fidelity for recycling scenarios where the timing of separations and fabrication of spent fuel more greatly impacts the isotopics of the resulting products. Decay is not directly tracked in either interim or permanent storage, as the impacts to downstream processes are minor.

The fuelling of reactors is determined using a user-determined priority order for available materials. For example, in a closed fuel cycle where material is recycled multiple times, a user may prioritise the fuelling of the most recycled material (thereby accelerating the approach to equilibrium), or they may prioritise the fuelling with the least recycled material, thereby ensuring that all material is recycled once before any is sent for subsequent passes. In all cases, a supply of unirradiated enriched uranium (so-called Pass 0 fuel) may be specified to be available in the event of a shortage of recycled fissile supply. If such a supply is not specified, a shortage of fissile material will result in the affected reactors being unable to operate. This will be reported in the outputs as a shortage in power production, and fuel in the affected reactors will not advance through the irradiation model nor will spent fuel be produced. When material becomes available, it will once again advance through the reactor model as normal.

After discharge from the reactor, spent fuel goes into wet storage for a user-determined period of time, and is then either recycled according to a defined separations matrix, or sent to dry storage and eventually on to permanent disposal. The isotopic mass of material in each stage, along with various derived heat, non-proliferation, and radiotoxicity metrics are reported to the output spreadsheets for subsequent review and analysis.

The VISION model is free of export-controlled information and may be distributed, modified, and reused without restriction. Though development is not currently ongoing, an upcoming release of previously performed updates is planned for 2023. For information or to get the latest releases, please contact Ross Hays at the INL.