Small modular reactors (SMRs) and advanced reactor concepts (ARCs) will deliver clean, flexible, reliable, and affordable electricity while avoiding the traditional limitations of large nuclear reactor designs, including high capital costs and long construction timelines. Current advanced reactor design approaches leverage decades of experimental and operational experience with the US nuclear fleet and are informed by calibrated numerical models of reactor phenomena. The exascale SMR (ExaSMR) project will generate virtual reactor simulation datasets with high-fidelity, coupled physics models for reactor phenomena that are truly predictive, filling in crucial gaps in experimental and operational reactor data. The ExaSMR virtual designs can accelerate the currently cumbersome advanced reactor concept-to-design-to-build cycle that has constrained the nuclear energy industry for decades. ExaSMR can also provide an avenue for validating existing industry design and regulatory tools.

Project Details

ExaSMR integrates the most reliable and high- confidence numerical methods for modeling operational reactors: the reactor’s neutron distribution with Monte Carlo (MC) transport and the reactor’s thermal fluid heat transfer efficiency with high-resolution computational fluid dynamics (CFD)—and all for efficient execution on exascale systems. ExaSMR builds on a base of simulation applications that have demonstrated high efficiency on current petascale-class leadership computing systems. The ExaSMR effort also provides value to US nuclear fuel providers and the broader nuclear community through the generation of highly detailed virtual datasets of operational and advanced concept nuclear reactors. Although ExaSMR is nominally focused on SMRs, the majority of the approaches are generic and have the potential to impact a wide range of reactor designs from microreactors to existing commercial reactors.

ExaSMR’s exascale challenge problem will open the door to high-confidence prediction of reactor conditions, including low-power conditions at startup via the initiation of natural circulation of the coolant flow through a small modular reactor core and its primary heat exchanger. The exascale software orchestrating this simulation, known as ENRICO, ensures intimate coupling of CFD (NekRS) and MC neutron transport modules through a common interface that supports multiple exascale simulation technologies: one targeting the exascale Frontier architecture at ORNL (Shift) and another targeting the exascale Aurora system at Argonne National Laboratory (OpenMC).

Exascale neutron transport simulations for ExaSMR will accommodate a full-core SMR model, which typically has ~40 fuel assemblies (each with ~300 fuel rods). The MC portion of the simulation will simulate over 10B particles per eigenvalue iteration with pin-resolved reaction rates having 3 radial tally regions and 20 axial levels. Within each spatial region, reaction rates for several different reaction channels in each of more than 200 isotopes will be computed. These calculations will exceed the size of any previous known simulations by a substantial factor.

Exascale CFD requirements for ExaSMR will include a full-core mesh  assembly bundle mesh models with at least 40M spatial elements and 22B degrees of freedom, including assembly bundle mesh models with momentum sources from representative resolved spacer grid simulations.

Software products

Total neutron interaction rate throughout SMR core computed by Shift.

Total neutron interaction rate throughout SMR core computed by Shift.

Principal Investigator(s):

Steven Hamilton, lead, Oak Ridge National Laboratory; Paul Romano, Argonne National Laboratory; Ben Forget, Massachusetts Institute of Technology


Argonne National Laboratory, Massachusetts Institute of Technology, Pennsylvania State University, North Carolina State University

Progress to date

  • Completed novel implementation of the Shift MC neutron transport algorithm on GPUs with nearly linear weak-scaling behavior on Summit and a 20× performance speedup over Titan.
  • Contributed to creation of NekRS, a GPU-enabled version of the Nek5000 CFD code using OCCA performance portability library and optimized computational kernels from libParanumal library developed by the Center for Efficient Exascale Discretizations (CEED). Demonstrated greater than 160× performance improvement over Titan using full Summit machine.
  • A new windowed multipole method that provides on-the-fly temperature dependence in continuous-energy nuclear data implemented in both the OpenMC and Shift applications for use on accelerated hardware and a multiphysics driver capable of performing coupled neutronics and fluid flow simulations.

Exascale reactor modeling capabilities delivered by ExaSMR can help inform the design and licensing of advanced and SMRs with unprecedented resolution by improving the fidelity of the modeling of complex physical phenomena occurring within operating nuclear reactors.

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