  Nek5000/RS is a highly-performant open-source spectral element 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 high-order discretizations that realize the same (or lower) cost per
gridpoint as traditional low-order methods. 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 Summit,
Nek5000/RS has recently achieved an milestone in the simulation of nuclear
reactors: the first full-core computational fluid dynamics simulations of
reactor cores, including pebble beds with > 350,000 pebbles and 98M elements
advanced in less than 0.25 seconds per Navier-Stokes timestep. With carefully
tuned algorithms, it is possible to simulate a single flow-through time for a
full reactor core in less than six hours on all of Summit.
