OpenMC is part of the small modular reactor (SMR) simulation subproject "ExaSMR" in the broader Exascale Computing Project (ECP), which is a US DOE project. The goal of the ExaSMR project is to enable extreme-fidelity simulation of SMRs, which have been a recent focus of engineering development. These reactors are of great interest due to their ability to economically provide consistent baseload power while avoiding many of the downsides of traditional (large) reactor designs. Part of the ExaSMR project was to define a representative challenge problem. The problem defined in this input dataset to OpenMC is a realistic SMR geometry that represents the NuScale reactor design recently licensed by the US Nuclear Regulatory Commission. The SMR problem is also defined with the goal of depletion calculations in mind, where hundreds of nuclides are present in the reactor fuel, and isoptic content lists are maintained for hundreds of thousands of unique fuel burnup regions throughout the reactor core. While the SMR problem is defined in-depth in Reference [1], we give an overview of additional key input parameters below that illustrate the difficulty of the simulation: Parameter Value Number of unique materials 195,368 Number of unique fuel materials 195,360 Number of depletion tally regions 195,360 Number of nuclides in each fuel region 244 Number of nuclides in simulation 281 Overall, the ExaSMR challenge problem forms an ideal basis for performance analysis due to its relevance to the nuclear industry and its prominence as a high performance computing "hero problem" as part of ECP. For these reasons, we will use the SMR problem as the main basis of our performance analysis and scaling challenges. The model is also available in python format at: https://github.com/mit-crpg/ecp-benchmarks/tree/fom-model References: [1] Kord Smith. 2017. NuScale Small Modular Reactor (SMR) Progression Problems for the ExaSMR Project. Milestone Report WBS 1.2.1.08 ECP-SE-08-43. Exascale Computing Project.