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The Verifiable Fuel Cycle Simulation (VISION) Model is a comprehensive simulation of the nuclear fuel cycle from mining through disposal using a system dynamics framework. It is capable of modeling fleets of any scale from individual reactors up to multi-region international cooperative scenarios. VISION takes certain scenario parameters (duration, growth rate of power demand, fuel recycling pathways, etc.) as input from which is computed a set of outputs (long-term radiotoxicity in disposal, actinide recycling rates, etc.) that can be used to evaluate the fitness of the scenario according to the user's metrics. The model itself does not specify any of the properties associated with a given reactor type (e.g., light-water reactor, sodium-cooled fast reactor). Instead, all parameters are provided as input from a spreadsheet and so may be generalized or specified as desired by the user. VISION is implemented within the Powersim Studio modeling environment and so has a host of preconfigured user interface and control elements along with additional built-in capabilities that the user may leverage for their own analyses. This page gives a high-level overview of how to obtain and run the VISION model. Details regarding the design of the underlying models, customization of scenarios, input and output files, and best practices for modifying the model itself are included in subpages linked in the document tree to the right.
The VISION model incorporates advanced facility deployment logic to support simulations of various upset conditions and contingency plans, including:
- Active forecasting of used fuel supplies to support startup of new reactors using recovered materials
- Adjustments to the rates of deployment to ensure adequate supplies of recycled material to support continuous operations
- Backfilling of fuel shortages with new enriched product or other contingent fuel sources
- Temporary shutdown of active reactors if fuel supplies fall short.
Nuclear fuel is modeled as a vector of mass fractions for 88 selected isotopes and groups for both charge and discharge from the reactor. Core mass and refueling requirements are computed based on specified discharge burnup, core residency time, and reactor power. Partitioning of fuel through recycling processes is determined by user-defined matrices, which permit modeling of arbitrary processes with allowances for specific waste and reuse streams. New fuel can be fabricated from combinations of fresh and recycled feedstock with sources chosen based on availability and a user-specified priority order.
At first glance, fuel cycle scenario modeling would appear to be a simple matter of tracing material from one point to another. The meshing of causal loops feeding back against each other, combined with variable nuclear decay rates, and time-of-use constraints can result in a rich variety of behaviors that are not always apparent from the initial conditions. Models of nuclear fuel cycle scenarios focus on predicting material flows to provide insights into the viability of proposed alternatives. These include assessing the sufficiency of available resources, the efficacy of proposed waste reduction techniques, or the net attractiveness of material stocks for malign diversion.
Most fuel cycle scenarios are centered on the deployment and operation of nuclear reactors, where the thermal energy or a derivative thereof (hydrogen, electricity, etc.) is the primary product produced. In VISION, reactors are ordered, built, operated, and retired based on either a prescribed schedule set out directly by number or computed to meet a given energy demand curve. Functionality embedded in the model is able to forecast the net amount of fuel provided to or extracted from a given reactor utilizing recycled fuel. This forecast can then predict when the construction of a proposed reactor will result in a shortfall in available fuel, thereby causing the reactor to fail to operate. In these situations, alternative, nonrecycling reactors can be built instead, thereby increasing the inventory of used fuel to support subsequent recycling reactors at a later date.
Fuel discharged from a reactor is initially stored for a fixed period of time corresponding to typical wet pool storage. After a fixed period, the fuel may then be moved to another buffer representing dry cask storage. Once in dry storage, fuel may either remain in place, be moved to a retrievable storage buffer, be moved to permanent disposal, or be sent to a recycling facility where the constituent elements can be separated for recombination into new fuel or alternative waste products.
Material recovered from separations processes is discharged after a user-defined fixed processing time into so-called separated material buffers. It is from these buffers that the constituent materials for recycled fuel are drawn. One problem faced by fuel scenario models is that the precise isotopic composition of material extracted from used fuel rarely, if ever, matches with the isotopic composition specified for the recycled fuel. This problem is further complicated by the ongoing radioactive decay of the separated materials and the complicated time-phasing of the arrival and usage of the fuel material at the separations plant. This is often referred to as the Winery problem (i.e., the incoming material has a range of vintages and flavors that affect their mixing and usability for certain applications). Simplifying assumptions are often applied at this point so that a tractable (if approximate) result can be obtained. In the VISION model, it is assumed that all isotopes within a control set may be treated interchangeably. These so-called control isotopes are defined in the input spreadsheet as being either Pu-239, All transuranics (TRU), All Pu, All U except U-238, All fissile isotopes, or All transthorium (details can be found here)[/02-The-VISION-Model#managing-the-winery-problem]. Thus, a spent fuel stream containing 3 kgs of Pu-239 and 1 kg of Pu-241 could be utilized to create 30 kgs of fuel requiring 10% Pu-239 under the Pu-239 control group or 40 kgs under the All Pu control group. In the latter case, the model will convert the 1 kg mass of Pu-241 into Pu-239. Though this of course violates physics, it maintains the tractability of the calculations. In either situation, an accounting is kept of the net masses of materials that are spontaneously created or destroyed through this process. Net mass is conserved, however masses of specific isotopes are not.
When all of these factors are considered, the initial setup of a fuel-cycle scenario can be potentially quite complex. A set of documented example input files are provided to simplify this process for users.
The VISION model runs in the Powersim Studio modeling environment and utilizes a set of Microsoft Excel workbooks for managing input and output data. Powersim is available in several editions with varying features and limitations. Powersim Professional, Expert, or Premium is required to support the large number of variables within the VISION model. Powersim itself requires a Microsoft Windows operating system with at least 2 GB of RAM and 50 MB of available disk space. The VISION model itself requires 100–200 MB of file space, depending on how the user chooses to manage their files (i.e., if they keep parallel working copies or overwrite old data as they go). As is often the case, computational performance improves with greater memory availability.
Once Powersim has been installed, it is recommended that new users work through some of the included tutorials to familiarized themselves with the general features, user interface, and operations of the software.
The VISION model (as updated) consists of at least three files: the input workbook, the model file itself, and the output workbook. These are available for download from this site. Additional output data processing can be performed by linking additional spreadsheets to the primary output files. Once the user has obtained these files, they begin by opening Powersim and using it to open the VISION.sip model file. When Powersim does this, it will also open the corresponding input and output workbooks.
Once the model has opened, press the 'reset' button (circled above) to ensure that all variables are initialized to their proper starting values from the input spreadsheet.
Navigate through the click-thru agreement to the Interface tab to begin using the VISION model.
Due to the complexity of both the formatting and data of the VISION input spreadsheet, it is recommended that users start from an existing spreadsheet and make changes rather than attempting to recreate the entire input spreadsheet from scratch. The user specifies which of their input spreadsheets to use by navigating to the 'Simulation Settings' option under the 'Simulation' menu; the input spreadsheet file name and path are listed under the 'INPUT_DECK' value in the 'Placeholder Values' tab. The output data file is defined in a similar way.
The VISION model is configured around sets of five input scenarios (sometimes referred to as 'base cases'). Each input spreadsheet has five parallel sets of inputs corresponding to the base cases populated in the dropdown list on the main VISION interface.
Once a scenario is selected, the user starts the computation process via the 'play' button on the Powersim menu. It will advance through the simulation until the designated end time, at which point the results will be written to the first run of the output spreadsheet (the writing process may take some time). At this point, the user has three options. If the results are as desired and they have no further calculations to do, they may close the model and use the saved results for further analysis. If the results are not as desired, they may make changes to the input workbook, save those changes, and then reset the model and try again. Finally, if they have multiple consecutive simulations they wish to compare in a single output file (each saved as a different base case), they may press the 'advance' button on the Powersim menu to shift the output to the next entry (denoted by the Powersim RUNINDEX value), select the next base case, and rerun the model. This may be repeated for a total of five runs in a single output file. At any time, if the user presses the 'reset' button on the Powersim menu, the RUNINDEX value will be reset to 1 and any saved data may be overwritten.
The only input values shown explicitly in the VISION model are the base case names and descriptions along with the starting and ending dates of the simulation. All user manipulations of the input parameters occur within the input spreadsheet.
A variety of embedded output graphs are provided within the VISION model so that users can track scenario progress and diagnose simple issues. They may be found on the 'Output Graphs' tab, as shown below.
For more detailed examination of the simulation results, the user should consult the 'Output Data 1' spreadsheet. This workbook contains over 40 sheets per RUNINDEX value, and as such provides a comprehensive view into the simulation results. More details can be found on the sidebar to the right.
The size and complexity of the 'Output Data 1' workbook renders it unwieldy for most routine analytical work. For most uses, it is better to generate a separate linked workbook that will draw a relevant selection of values from the main 'Output Data 1' workbook. 'Output Data 2' is one such workbook. It offers the important capability to store and retain values across multiple VISION runs, where the values within 'Output Data 1' are reset every time the VISION model is reset. This spreadsheet is utilized independently of the VISION model and does not have any direct linkages to Powersim.
Once you are familiar with how to open and run the model, you can proceed to the pages listed on the right to gain an understanding of how the model is formulated, how the scenarios are defined in the input files, and how the output is reported in the output files. Finally, best practices for modifying the model itself are included for those who wish to explore scenarios beyond the current capabilities.