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.