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Neutrons by the trillions: Using computational physics to understand nuclear reactors

Jun 18, 2025

3 min

Zeyun Wu, Ph.D.

Zeyun Wu, Ph.D., associate professor in the mechanical and nuclear engineering department at VCU Engineering, is reshaping the future of nuclear power.


Nuclear reactors are among the most complex engineered systems on earth, with different physical processes interacting simultaneously across various scales. Even the world's most powerful computers cannot simulate every detail of an operating reactor at once.


With a background in computational reactor physics, Wu’s research develops modeling and simulation techniques crucial to understanding next-generation nuclear reactors. By creating these advanced tools, his research eliminates the need for costly physical experimentation while ensuring the safety, efficiency and environmental sustainability of future nuclear power plants.


Wu's research focuses on understanding reactor behavior through two aspects: multi-physics and multi-scale modeling. The multi-physics approach integrates various physical phenomena, such as nuclear physics reactions, fluid dynamics, heat transfer and structural mechanics, into a unified simulation framework. The multi-scale modeling technique addresses the vast range of physical scales involved, from subatomic neutron interactions to meter-sized reactor components.


Wu’s research can simulate the complex phenomena within reactors at different scales. These models, developed using advanced numerical methods, help predict reactor behavior under various conditions.


One of the models Wu uses tracks neutron behavior, a fundamental aspect to understand nuclear reactions. His simulations track trillions of neutrons as they move through various reactor materials, cause fission events and generate power.


"What drives power is actually the neutron," explained Wu. "Once an atom splits, along with the nuclear energy release, lots of neutrons come out. We're talking about 1012 to 1013 neutrons per second. Our code tracks each neutron to understand where it comes from and where it goes."


By understanding neutron distribution across space, time and energy domains, Wu's team can predict power distribution throughout the reactor core. This helps identify potential hotspots – areas of heightened thermal activity that could pose safety challenges. Beyond neutron behavior, Wu's research also explores how cooling fluids interact with neutrons and carry away thermal energy, a field known as thermal hydraulics, because how the reactor components are cooled significantly affects the neutron behavior as well. This also explains why the multi-physics modeling becomes essential for nuclear reactor simulations.


Wu founded the Computational Applied Reactor Physics Laboratory (CARPL) to continue his research in nuclear reactor modeling and simulation. Undergraduate and master’s students learn to use established nuclear simulation codes to analyze reactor problems – skills highly valued in the industry and national labs. Ph.D. students build on theoretical foundations to deepen their understanding, enhance existing models, and develop new ones.


“My area of research has been continually evolving for the past 60 years or so,” said Wu. “Most of the codes we use have been developed by national labs, like Oak Ridge National Lab, but these codes aren’t perfect. National labs hire Ph.D. level students with this niche to identify deficits in the code, correct errors and even add new functions and improve them.”


Looking forward, Wu hopes his research will have a real-world impact on the upcoming shift in nuclear power in America. Over the next 20 to 30 years, the nation's approximately 90 light-water-cooled nuclear reactors reach the end of their operational lifetimes. Light water refers to ordinary water (H₂O), used in most existing reactors to both cool the system and slow down neutrons to sustain the nuclear reaction. To replace them, experts are looking toward advanced, non-light-water-cooled reactors, such as the Molten Uranium Breeder Reactor (MUBR) shown in the figure. Computational methods and tools like Wu’s research lab developed will be essential to their development and implementation.


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Non-light-water cooled reactors offer significant advantages over the older designs. Some can operate at higher temperatures while others produce substantially less nuclear waste, addressing one of the industry's persistent challenges.


"Unlike traditional water reactors, where we have decades of operational experience and established analysis tools, these new designs present unique challenges," explained Wu. "Companies like Dominion employ large teams of analysts who use well-tested computational tools to maintain their existing reactors, but those same tools aren't calibrated for these next-generation reactors. Our research is developing the computational methods and simulations these advanced reactors will need. When these new reactors come online, the methodologies we're creating now can be quickly converted into production-level nuclear codes, providing immediate practical value to industry.”


Connect with:
Zeyun Wu, Ph.D.

Zeyun Wu, Ph.D.

Associate Professor, Department of Mechanical and Nuclear Engineering

Reactor physics, computational methods, sensitivity and uncertainty analysis, advanced data analytics, machine learning

Reactor PhysicsReactor core design and analysisComputational method for neutron transportUncertainty and sensitivity analysisAdvanced data analytics

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