Steven Hamilton. Image credit: Carlos Jones, ORNL
As renewable sources of energy such as wind and sun power are increasingly added to the country’s electrical grid, old-fashioned nuclear energy is also being primed for a resurgence.
For the past 20 years, fission reactors have produced a nearly unchanging portion of the nation’s electricity: around 20 percent. But that percentage could start increasing soon. The advent of small modular reactors, or SMRs, and advanced reactor concepts, or ARCs, signals a new generation of fission power. SMRs are substantially smaller than most commercial nuclear reactors today, and they use standardized designs, thus reducing construction costs and production time. Meanwhile, ARCs explore new technologies to produce fission power more efficiently and safely.
Exascale Small Modular Reactor, or ExaSMR, is a suite of exascale-optimized simulation codes that aims to provide the nuclear industry’s engineers with the highest-resolution simulations of reactors to date and, in turn, help advance fission power. Supported by DOE’s Exascale Computing Project since 2016, the ExaSMR project endeavors to leverage the next-generation power of exascale supercomputers — capable of at least a quintillion calculations per second — to make large-scale nuclear reactor simulations easier to access, cheaper to run, and more accurate than the current state of the art.
ExaSMR’s performance on ORNL’s Frontier supercomputer — an HPE Cray exascale system currently ranked fastest in the world — showed a 100 times speedup of its codes compared to baseline simulations performed on Titan, the U.S.’s most powerful supercomputer in 2016. That’s when ECP set out to develop advanced software for the arrival of exascale-class supercomputers, which occurred with Frontier’s debut in 2022.
“By accurately predicting the nuclear reactor fuel cycle, ExaSMR reduces the number of physical experiments that reactor designers would perform to justify fuel use,” said ExaSMR project leader Steven Hamilton. “In large part, that’s what simulation is buying companies: a predictive capability that tells you how certain features will perform so that you don’t need to physically construct or perform as many experiments, which are enormously expensive.”
Coupling physics codes
Commercial nuclear reactors generate electricity by splitting uranium nuclei to release energy in a process known as fission. This energy turns water into steam, which spins electricity-producing turbines. ExaSMR integrates the most accurate computer codes available for modeling the physics of this operation, creating a toolkit that can predict a reactor design’s entire fission process. The toolkit includes the Shift and OpenMC codes for neutron particle transport and reactor fuel depletion and the NekRS code for thermal fluid dynamics.
Although most of these codes are already well established in science and industry, the ExaSMR team has given them a complete exascale makeover. For the past seven years, researchers from ORNL, Argonne National Laboratory, the Massachusetts Institute of Technology and Pennsylvania State University have been optimizing the codes for exascale supercomputers such as Frontier and Argonne’s forthcoming Aurora.
“What we’re doing in ExaSMR is a coupled physics simulation between the neutron transport and the fluid dynamics — these two physics codes talk back and forth,” said Hamilton, an R&D scientist in ORNL’s HPC Methods for Nuclear Applications group.
“The neutron transport is telling you where the heat is generated. That heat becomes a source term for the fluid dynamics calculation. The fluid dynamics calculates the temperature resulting from that heat source. And then you can adjust the parameters in the simulation until the neutron transport and the fluid dynamics are in agreement.”
ExaSMR’s ability to accurately model in high resolution the whole reactor process — the amount of heat produced by nuclear fission, the ability of the reactor to transfer that heat to power generators, and the life expectancy of the entire system — provides engineers with key insights to ensure the safety and efficiency of their reactor designs.
What’s ahead for ExaSMR?
Partnering with Westinghouse, a producer of commercial nuclear power technology, the ExaSMR team applied for a Leadership Computing Challenge grant from DOE’s Office of Advanced Scientific Computing Research. Westinghouse wants to evaluate the impact of using fuel enriched to higher levels of fissile uranium-235 than that currently used in its reactors. Running ExaSMR on Frontier will allow the company to perform high-fidelity simulations to predict how different types of fuels would perform if used in an operating reactor.
Likewise, Hamilton wants to apply ExaSMR to current ARC technologies being explored in the power industry. The DOE Office of Nuclear Energy’s Advanced Reactor Demonstration Program provides funding for commercial companies to accelerate the demonstration of advanced reactors. Two such reactors are slated for near-term deployment by 2027: X-energy’s Xe-100 pebble-bed reactor and TerraPower’s Natrium sodium-cooled fast reactor. Five additional designs from Kairos, Westinghouse, BWX Technologies, Holtec International and Southern Company are ramping up for longer-term deployment.
Hamilton foresees ExaSMR becoming an indispensable tool for companies entering a new era of nuclear power.
“Various companies are exploring different types of reactor design, and the high-performance, high-fidelity simulations that we’re developing have a lot of appealing features for designers,” he said. “It’s unlikely, in the near future, that we’ll have enough confidence in simulations that they would fully replace experiments, but if we can reduce the number of experiments that are performed, then there can be huge gains for these companies.”
