Critical experiment design using OPTIMUS

The design process for critical and subcritical experiments involves weighing the advantages of many geometric and material factors while also trying to emphasize specific characteristics of the experiment. For example, a critical experiment could seek to maximize the neutron energy spectrum in a specific energy regime in order to validate different regions of the material's cross section. As these experimental designs increase in dimensionality, so does the complexity of generating the optimal critical configurations for the practitioner. There exists a need to intelligently automate the design process for such experiments. The OPTIMUS software package, developed at Lawrence Livermore National Laboratory (LLNL), addresses this need by taking a modular approach to generating the physics model, optimizing the design, and storing the results from the optimization process. This type of modular approach gives the user flexibility in how the model is created, what physics software is used, and how the results are analyzed. This work presents an overview of OPTIMUS and how it has been successfully applied to designing two Thermal/Epithermal experiment (TEX) configurations using ²³⁹Pu with alumina and ²³³U with polyethylene. Then, OPTIMUS is applied to a general ²³⁹Pu two-region curve with polyethylene dilution. OPTIMUS is shown to be up to 80% more computationally efficient than a brute force approach for this model. OPTIMUS cuts the overall number of simulations needed to determine all the critical configurations in half over the brute force approach. (author)