June 8, 2022 — Small modular reactors (SMRs) are advanced nuclear reactors that can be incrementally added to a power grid to provide carbon-free energy generation to match increasing energy demand.[1]^,[2] Their small size and modular design make them a more affordable option because they can be factory assembled and transported to an installation site as prefabricated units.

Compared to existing nuclear reactors, proposed SMR designs are generally simpler and require no human intervention, external power, or the application of external force to shut down. SMRs are designed to rely on passive systems that utilize physical phenomena, such as natural circulation, convection, gravity, and self-pressurization to eliminate or significantly lower the potential for unsafe releases of radioactivity in case of an accident.[3] Computer models are used to ensure that the SMR passive systems can safely operate the reactor regardless of the reactor’s operational mode—be it at idle, during startup, or running at full power.

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 generates datasets of virtual reactor design simulations based on high-fidelity, coupled physics models for reactor phenomena that are truly predictive and reflect as much ground truth as experimental and operational reactor data.[4]

An Integrated Toolkit

The Exascale Computing Project’s (ECP’s) ExaSMR team is working to build a highly accurate, exascale-capable integrated tool kit that couples high-fidelity neutronics and computational fluid dynamics (CFD) codes to model the operational behavior of SMRs over the complete reactor lifetime. This includes accurately modeling the full-core multiphase thermal hydraulics and the fuel depletion. Even with exascale performance, reduced-order mesh numerical methodologies are required to achieve sufficient accuracy with reasonable runtimes to make these simulations tractable.

According to Steven Hamilton (Figure 2), a senior researcher at Oak Ridge National Laboratory (ORNL) and PI of the project, ExaSMR integrates the most reliable and high-confidence numerical methods for modeling operational reactors. Specifically, ExaSMR is designed to leverage exascale systems to accurately and efficiently model the reactor’s neutron state with Monte Carlo (MC) neutronics and the reactor’s thermal fluid heat transfer efficiency with high-resolution CFD.[5] The ExaSMR team’s goal is to achieve very high spatial accuracy using models that contain 40 million spatial elements and exhibit 22 billion degrees of freedom.[6]

Hamilton notes that high-resolution models are essential because they are used to reflect the presence of spacer grids and the complex mixing promoted by mixing vanes (or the equivalent) in the reactor. The complex fluid flows around these regions in the reactor (Figure 1) require high spatial resolution so engineers can understand the neutron distribution and the reactor’s thermal heat transfer efficiency. Of particular interest is the behavior of the reactor during low-power conditions as well as the initiation of coolant flow circulation through the SMR reactor core and its primary heat exchanger during startup.

To make the simulations run in reasonable times even when using an exascale supercomputer, the results of the high accuracy model are adapted so they can be utilized in a reduced order methodology. This methodology is based on momentum sources that can mimic the mixing caused by the vanes in the reactor. [7] Hamilton notes, “Essentially, we use the full core simulation on a small model that is replicated over the reactor by mapping to a coarser mesh. This coarser mesh eliminates the time-consuming complexity of the mixing vane calculations while still providing an accurate-enough representation for the overall model.” The data from the resulting virtual reactor simulations are used to fill in critical gaps in experimental and operational reactor data.  These results give engineers the ability to 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.[8]

“The importance,” Hamilton states, “is that many different designs are being studied for next-generation reactors. Investing in computer design capability means we can better evaluate and refine the designs to come up with the most efficacious solutions. Exascale supercomputers give us a tool to model SMRs with higher resolution than possible on smaller supercomputers. These resolution improvements make our simulations more predictive of the phenomena we are modeling. We are already seeing significant improvements now on pre-exascale systems and expect a similar jump in performance once we are running on the actual exascale hardware.” He concludes by noting, “Many scientists believe that nuclear is the only carbon-free energy source that is suitable for bulk deployment to meet primary energy needs with a climate-friendly technology.”

To read the entirety of Rob Farber’s technical highlight, visit this link.

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Source: Rob Farber, contributing writer for the Exascale Computing Project