Multiphysics tritium transport modelling of the ARC breeding blanket with FESTIM
This paper presents a fully open-source, component-scale multiphysics framework coupling OpenMC, OpenFOAM, and FESTIM to model tritium transport in an ARC liquid immersion blanket, revealing that turbulence-enhanced diffusion dominates transport dynamics and predicting a steady-state inventory of approximately 243 mg.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Imagine trying to build a power plant that runs on the same fuel as the stars: the sun. This is the dream of nuclear fusion, a technology that promises limitless, clean energy by smashing tiny atoms together to release massive amounts of heat. But there's a catch. To keep the reaction going, the plant needs a constant supply of a rare, radioactive gas called tritium. Since tritium is hard to find in nature, the power plant has to make its own. It does this by surrounding the super-hot core with a special "blanket" filled with liquid salt. This blanket acts like a cosmic factory, catching stray neutrons from the fusion reaction and turning them into fresh tritium.
However, tritium is a master of disguise. It's incredibly small and slippery, able to sneak through solid walls, hide in cracks, or get lost in the swirling liquid. If the plant can't catch all the tritium it makes, the fuel cycle breaks, and the reactor shuts down. Worse, if too much tritium gets trapped in the wrong places, it becomes a safety hazard. So, engineers need a way to predict exactly where every single atom of tritium will go as it swims through the hot, churning liquid salt. This is a bit like trying to track a specific drop of food coloring in a hurricane; you need a super-smart computer model to see if it gets swept out safely or gets stuck in a whirlpool.
This is where the new study comes in. A team of researchers built a brand-new, open-source computer program to play a high-stakes game of "follow the tritium" inside a theoretical fusion reactor called ARC. They didn't just guess; they built a digital twin of the reactor's liquid salt blanket, connecting three different powerful computer tools to simulate how neutrons create the fuel, how the liquid salt flows and heats up, and finally, how the tritium moves through it all.
The researchers found that the movement of tritium is mostly a story of the liquid salt's flow. In their simulation, the tritium didn't just drift slowly; it was tossed around by the turbulence of the moving salt, much like a leaf caught in a rapid river. They discovered that the tritium tends to pile up in "dead zones" where the liquid stops moving, creating pockets of high concentration, while in the fast, chaotic swirls, the tritium gets mixed so thoroughly that it doesn't build up as much.
When they ran their numbers, the simulation predicted that the entire blanket would hold about 243 milligrams of tritium at any given time once it reached a steady state. It took about 30 minutes for the system to settle into this rhythm after starting up. This is a crucial finding because it tells engineers how long they might need to wait before the reactor is fully "primed" with fuel. The study also showed that the amount of tritium predicted is likely a "best-case scenario" or a lower limit. The computer model assumed the tritium was perfectly removed at the exit and didn't get stuck in the solid metal walls of the reactor. In the real world, the tritium might get trapped in the metal or leak through walls, meaning the actual amount of tritium sitting inside the reactor could be higher.
The team was very careful to check their work. They ran the same problem on two different types of computer codes—one that treats the fluid like a grid of boxes and another that treats it like a smooth mesh—and the results matched almost perfectly. This gives them confidence that their map of the tritium's journey is accurate. They also tested how sensitive their results were to the math they used, finding that while small changes in the math or assumptions about turbulence did shift the numbers slightly, the overall picture remained the same.
Ultimately, this paper doesn't claim to have solved the tritium problem forever. Instead, it provides a transparent, flexible toolkit that other scientists can use to test different designs. It highlights that the shape of the flow inside the blanket is the most important factor in keeping tritium safe and available. By showing exactly where the tritium likes to hide (in the slow, stagnant spots), the study gives engineers a clear target: design the reactor so those stagnant spots disappear, ensuring the fuel keeps flowing and the reactor stays powered.
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