Small Modular Reactors for the European Clean Energy Transition: An Introduction and Comparison of iPWR, HTGR, and MSR Technologies

Europe's clean energy transition is rekindling interest in nuclear power, particularly in tiny modular reactors (SMRs), due to their ability to provide secure baseload electricity and flexible support for intermittent renewable energy sources. SMRs address the limitations of traditional large reactors by shortening construction time, lowering upfront costs, and enhancing operational flexibility for grid services. This article reviews three primary SMR development paths,integrated pressurized water reactors (iPWRs), high-temperature gas-cooled reactors (HTGRs), and molten salt reactors (MSRs),and assesses their suitability for Europe's energy needs. iPWRs are small, light-water reactors that prioritize near-term deployability and safety through an integrated design, while operating at moderate temperatures. HTGRs utilize helium coolant and TRISO fuel, achieving temperatures of approximately 800-950C, which enables industrial heat applications and hydrogen production while maintaining inherent safety. Molten salt reactors (MSRs) circulate liquid fuel in molten salt, allowing for high-temperature, low-pressure operation and improved fuel utilization; however, material corrosion remains a challenge. Each type of small reactor (SMR) offers unique advantages for grid integration: iPWRs can employ familiar technology for load following, HTGRs can switch between electricity and process heat, and MSRs offer the potential for integrating thermal energy storage. Given these complementary characteristics, a diversified SMR strategy can improve grid reliability, provide flexible baseload power to support renewables, and advance Europe's decarbonization goals.