The development of socially acceptable nuclear reactors requires that they are safe, clean, efficient, economical, and sustainable. Meeting these requirements calls for new approaches, driving growing interest in Small Modular Reactors (SMRs) and in Generation IV designs.  SMRs aim to improve project economics by standardising designs and shifting construction to controlled manufacturing environments, while Gen IV reactors target fundamental fuel-cycle challenges by better managing transuranics and reducing the radiotoxicity and longevity of waste. Together, these approaches offer a credible roadmap toward safer, cleaner, and more sustainable nuclear energy. However, validating new designs presents significant challenges. Due to the expense, time constraints, and inherent complexities of physical experiments, numerical simulations are fundamental to the design of nuclear reactors.…