Griffin in the loop: A digital multiphysics test bed for next-gen reactors

As U.S. energy demand increases, advanced nuclear energy has emerged as an important option for 24/7 reliable electricity and heat generation. Nuclear engineers in national laboratories and private industry have developed dozens of nuclear reactor designs that could soon power remote communities, space exploration, mining operations, military bases and data centers. Other advanced reactors could provide high-temperature heat and electricity for communities and industry.

But developers need a way to test the safety and performance of these advanced reactors before building expensive prototypes.

Researchers at the Idaho National Laboratory and Argonne National Laboratory jointly developed the Griffin reactor physics modeling and simulation software to enable accurate predictions of reactor performance across various designs, fuels and materials under real-world conditions.

"Griffin can simulate a lot of the processes that are happening in a real-world operating reactor," said Argonne principal nuclear engineer Changho Lee. "It's closer to a real-life scenario where high temperatures, pressures and neutron flux in the harsh environment of a reactor core are causing changes to fuels and reactor materials. It's cheaper and safer to run, allowing you to explore numerous scenarios."

Griffin has played a crucial role in multiphysics modeling, simulation and core design of advanced nuclear reactors.

Now, the 2025 R&D 100 Award-winning software is a staple for researchers, industry and regulators. As a cornerstone of advanced reactor development, Griffin integrates easily into the broader modeling ecosystem, paving the way for interconnected analyses.

A master modeling and simulation tool

The U.S. Department of Energy Office of Nuclear Energy's Nuclear Energy Advanced Modeling and Simulation Program envisioned a comprehensive tool kit to support the development of advanced reactors. The Griffin tool is a critical part of this mission because it serves as the fundamental reactor physics tool.

Built on the award-winning Multiphysics Object Oriented Simulation Environment (MOOSE) platform, Griffin draws from decades of expertise in computer science and reactor physics code development from both INL and Argonne. Its architecture allows it to couple seamlessly with other MOOSE-based codes for thermal fluids, thermo-chemistry, thermo-mechanics and more.

"Griffin provides the basic nuclear physics that everything else is built on, which makes it the most important component for nuclear reactor simulation," said INL radiation transport methods development researcher Josh Hanophy. "It is built with the ability to interact with other physics within the MOOSE framework or outside of that framework as well."

A supercharged multiphysics powerhouse

To accurately model a nuclear reactor, simulations must reflect neutronics, thermal hydraulics, structural mechanics, materials behavior, fuel performance and more. Griffin meets this challenge with a comprehensive suite of capabilities. Central to this feat is the linear Boltzmann transport equation solver, which captures complex radiation behavior within the reactor core.

Griffin's algorithms track changes in isotopes over time, helping predict how fuel will evolve and how the reactor will perform. To further improve accuracy, Griffin calculates neutron interaction probabilities using nuclear data and machine learning. This allows for precise modeling of fuel usage, fission product creation and reactor efficiency.

Griffin can accelerate simulations and allow researchers to choose between fast, lower-fidelity models and slower, high-fidelity ones.

Versatile modeling possibilities

Griffin's strength lies not only in its technical depth but also in its adaptability across reactor types, capturing the underlying physics behind a wide range of phenomena such as density changes, aging effects, dimensional variations and isotopic or chemical composition shifts.

This flexibility allows Griffin to support the analysis of designs for pebble bed reactors, prismatic high-temperature reactors, molten-salt reactors, fast-sodium and lead-cooled reactors, microreactors and other experimental systems.

"To simulate the overall behavior of a given nuclear plant, you need to simulate the fuel rods, the pellets in the fuel rod, the pressure vessel, the materials, the pumps, the structures and even what happens outside the reactor core itself, including the coolant system," said INL reactor physics senior researcher Yaqi Wang.

By enabling researchers, industry and regulators to explore these complex interactions virtually, Griffin accelerates innovation while reducing the need for costly physical tests.

"Running computer models validated by experiments is significantly more cost-effective than building a physical reactor," said former INL research and development scientist Javier Ortensi.

Griffin for space exploration

Griffin's versatility even extends beyond Earth. The software has been used to help design nuclear systems for NASA, demonstrating its value in space and lunar applications.

These systems include nuclear thermal propulsion (a nuclear-powered rocket), microreactors for fission surface power on the moon and Mars, and devices that provide heat and electricity for remote applications and spacecraft.

For example, engineers used Griffin to model nuclear thermal propulsion for the Defense Advanced Research Projects Agency's Demonstration Rocket for Agile Cislunar Operation.

This project demonstrated a coupled physics system for a nuclear rocket, which allows them to operate for years with a smaller volume of fuel. For NASA, this means spacecraft can carry more equipment and personnel, travel faster and farther, and reduce crew exposure to cosmic radiation.

Another NASA project used Griffin alongside other MOOSE applications to model a semi-autonomous startup sequence for a nuclear rocket.

INL computational scientist Jackson Harter explained that Griffin, as a groundbreaking multiphysics radiation transport code, integrates smoothly with other MOOSE physics codes. This integration enabled Harter to conduct uncertainty quantification and sensitivity analysis studies.

Supporting regulatory applications

Griffin's precision and robustness position it as a valuable tool for regulatory review. The Nuclear Regulatory Commission (NRC) is implementing Executive Orders and ADVANCE Act requirements for licensing advanced reactors, which often differ significantly from the current light-water reactor fleet in design, fuel and coolant. Because Griffin can produce highly accurate results, it is an important addition to the NRC's modeling and simulation toolset for evaluating non-light-water reactor designs.

The software can provide independent verification of industry-provided simulation predictions. With support from the Griffin team, NRC and national laboratory researchers have developed reference plant models that incorporate key aspects of advanced reactor technologies. The NRC is working with these teams to refine those models as new designs come under review.

Looking forward

As computing power grows, Griffin is poised to deliver even more detailed and accurate multiphysics models. These simulations will offer deeper insights into reactor behavior, enhancing nuclear energy systems on Earth and beyond.

For one emerging area of application, fusion energy, Griffin can support safety design by modeling neutron interactions in the breeding blanket, the part of a fusion reactor where neutrons interact with lithium to produce tritium fuel. This will be essential for future fusion technologies.

Until then, the Griffin team looks to its library of validation cases, comparing simulation results against real-world measurements to strengthen the tool's predictions.

"Griffin aims to revolutionize nuclear energy by combining advanced computational power with deep scientific understanding, leading the way to a safer, more efficient energy future," said Ortensi.

Provided by Idaho National Laboratory