Advanced fission and fusion energy hold promise as a reliable, carbon-free energy source capable of meeting the United States' commitments to addressing climate change. A wave of investment in fission and fusion power within the United States and worldwide indicates an important maturation of academic research projects into the commercial space. Nonetheless, the design, certification, and licensing of novel reactor concepts pose formidable hurdles to successfully deploying new technologies. Because of the high cost of integral-effect nuclear experiments, high-fidelity numerical simulation is poised to play a crucial role in these efforts. This talk explores recent pioneering large-scale fluid flow high-fidelity simulation of fusion energy systems. First-of-a-kind, full-core simulations of fission reactors have been conducted on Frontier. Simulations of unprecedented scale have also been conducted for fusion energy systems. In particular, we model high Reynolds number flow with heat transfer in the CHIMERA facility designed to study fusion breeding blankets. Simulations have been performed on Frontier, the world's fastest supercomputer, with up to 9000 nodes. The simulations performed are significantly larger than prior work in our field. We emphasize that only exascale resources make these simulations possible. We employ NekRS, a GPU-oriented version of the Nek5000 code, which is a highly scalable open-source spectral element code for Computational Fluid Dynamics (CFD) simulation. Far from being a purely academic pursuit, the capabilities provided by NekRS enable scientific discovery for a range of applications crucial to fission and fusion energy deployment. We spend the latter part of the talk on this aspect, emphasizing the importance of bridging the gap between supercomputing and scientific and engineering practice.

As the US Department of Energy (DOE) computing facilities began deploying petascale systems in 2008, DOE was already setting its sights on exascale. In that year, DARPA published a report on the feasibility of reaching exascale. The report authors identified several key challenges in the pursuit of exascale including power, memory, concurrency, and resiliency. That report informed the DOE's computing strategy for reaching exascale. With the deployment of Oak Ridge National Laboratory's Frontier supercomputer, we have officially entered the exascale era. In this paper, we discuss Frontier's architecture, how it addresses those challenges, and describe some early application results from Oak Ridge Leadership Computing Facility's Center of Excellence and the Exascale Computing Project.

ENRICO is a coupled application developed under the U.S. Department of Energy's Exascale Computing Project (ECP) targeting the modeling of advanced nuclear reactors. It couples radiation transport with heat and fluid simulation, including the high-fidelity, highresolution Monte-Carlo code Shift and the Computational fluid dynamics code NekRS. NekRS is a highly-performant open-source code for simulation of incompressible and low-Mach fluid flow, heat transfer, and combustion with a particular focus on turbulent flows in complex domains. It is based on rapidly convergent high-order spectral element discretizations that feature minimal numerical dissipation and dispersion. State-of-the-art multilevel preconditioners, efficient high-order time-splitting methods, and runtime-adaptive communication strategies are built on a fast OCCA-based kernel library, libParanumal, to provide scalability and portability across the spectrum of current and future high-performance computing platforms. On Frontier, Nek5000/RS has recently achieved an unprecedented milestone in breaching over 1 billion spectral elements and 350 billion degrees of freedom. Shift has demonstrated the capability to transport upwards of 1 billion particles per second in full core nuclear reactor simulations featuring complete temperature-dependent, continuous-energy physics on Frontier. Shift achieved a weak-scaling efficiency of 97.8% on 8192 nodes of Frontier and calculated 6 reactions in 214,896 fuel pin regions below 1% statistical error yielding first-of-a-kind resolution for a Monte Carlo transport application.

Nek5000/RS, a highly-performant open-source spectral element code, has recently achieved an unprecedented milestone in the simulation of nuclear reactors: the first full core computational fluid dynamics simulations of reactor cores, including pebble beds with 352,625 pebbles and 98M spectral elements (51 billion gridpoints), advanced in less than 0.25 seconds per Navier-Stokes timestep. The authors present performance and optimization considerations necessary to achieve this milestone when running on all of Summit. These optimizations led to a fourfold reduction in time-to-solution, making it possible to perform high-fidelity simulations of a single flow-through time in less than six hours for a full reactor core under prototypical conditions.