The decarbonization of the energy sector is essential for meeting global climate targets and reducing greenhouse gas emissions. Nuclear power, as a reliable and low-carbon energy source, is poised to play a pivotal role in the transition to a sustainable energy future. This aligns with key United Nations Sustainable Development Goals (SDGs), particularly SDG 7 (Affordable and Clean Energy) and SDG 13 (Climate Action) [https://sdgs.un.org/goals].
In this context, advanced nuclear technologies such as Generation IV (Gen IV) reactors and Small Modular Reactors (SMRs) are emerging as transformative technologies. These innovations promise enhanced safety, efficiency, and adaptability, addressing both current energy demands and future sustainability.
Generation IV Reactors
Generation IV (Gen IV) reactors represent a new class of reactors designed to overcome the limitations of current technologies in terms of safety, sustainability, and efficiency. At the forefront of these efforts is the Generation IV International Forum (GIF) [https://www.gen-4.org/], which has defined six advanced reactor concepts. These designs are differentiated by the type of coolant (gas, liquid metal, supercritical water, or molten salt), neutron spectrum (thermal or fast), operating conditions (high temperature, pressure), and fuel type (see Table 1 and Figure 1).
Table 1: Six generation IV reactor technologies: Very High Temperature Reactor (VHTR), Molten Salt Reactor (MSR), Sodium-cooled Fast Reactor (SFR), Supercritical Water-cooled Reactor (SCWR), Gas-cooled Fast Reactor (GFR) and Lead-cooled Fast Reactor (LFR) [GENIV, WNA].
Fast-spectrum reactors (GFR, LFR, MSR-fast, SFR) are particularly promising for transmutation of long-lived radioactive isotopes and improving fuel utilization via breeding, while thermal-spectrum designs (VHTR, SCWR, MSR-thermal) prioritize efficient hydrogen production and high thermal conversion efficiency.
However, these technological advances are not without challenges. The development of new nuclear fuels (such as TRISO particles [USDE], metallic or ceramic fuels), the behavior of materials under extreme conditions, and the use of unconventional coolants pose complex scientific and engineering questions.
In particular, low-Prandtl-number coolants—such as liquid metals and molten salts—exhibit high thermal conductivity, enabling efficient heat transfer. However, this same property complicates the modeling of turbulent phenomena, buoyancy effects, and thermal anisotropy. These aspects still demand fundamental research and the development of advanced simulation tools [Roelofs, F., et. al., 2021]. Significant efforts are currently underway in collaboration with research centers, universities, and international initiatives such as Euratom and the CCP-NTH programme [https://ccpnth.ac.uk/].
Small Modular Reactors (SMRs)
Small Modular Reactors (SMRs) represent a significant evolution in nuclear technology and are suitable for GEN IV reactors. With power outputs typically below 300 MWe, SMRs are designed for modular construction, enabling off-site manufacturing and on-site assembly. Their key advantages include:
- Scalability: Modules can be added incrementally to match energy demand.
- Reduced Construction Time and Costs: Factory production streamlines the building process.
- Enhanced Safety: Many SMRs rely on passive safety systems that use gravity, natural convection, and condensation for cooling—eliminating the need for active safety systems.
- Versatility: Beyond electricity generation, SMRs can support desalination, district heating, and hydrogen production.
Most SMRs use low-enriched uranium (LEU) and feature extended refueling cycles, enhancing operational efficiency. For example, Seaborg Technologies' Compact Molten Salt Reactor (CMSR) can operate for up to 24 years without the need for physical refueling (https://www.seaborg.com/ ).
Global Initiatives and Investments
According to the IAEA, by the end of 2022, over 20 countries had active SMR development programs targeting commercial deployment by 2035. More than 80 SMR designs are currently under development worldwide. Some notable initiatives include:
- United States:
- $900 million DOE (Department of Energy) funding to accelerate SMR commercialization [USDE2025].
- Idaho National Laboratory (INL) advancing next-generation reactors and infrastructure with $150 million in federal investment [INL, USDE2022].
- NuScale Power's VOYGR: 77 MWe pressurized water SMR [Nuscale].
- X-energy’s Xe-100: High-Temperature Gas Reactor (HTGR), 80 MWe per unit [Xenergy].
- EUROPE:
- The ESNII initiative coordinates efforts for Gen IV reactors, such as ASTRID (SFR), ALFRED (LFR), MYRRHA (ADS), and ALLEGRO (GFR).
- France: Jimmy Energy developing SMRs for industrial decarbonization [Lemonde2024].
- Denmark: Seaborg developing CMSRs for floating nuclear power plants (100 MWe)
- Rusia:
- The KLT-40S, a 70 MWe floating SMR, supplies electricity and heat to remote Arctic communities
- United Kingdom:
- The UK government could allocate up to £20 billion for the deployment of up to 20 SMRs, with Rolls-Royce and GE Hitachi positioned as leaders of the industrial consortium [Thetimes].
- Holtec International selected South Yorkshire for a £1.5 billion SMR manufacturing facility with the goal of 5GW of SMRs deployed in serial production by 2050 [Nucnet2024].
- China:
- Connected the world’s first operational Gen IV reactor, the HTR-PM, to the grid (210 MWe).
- The ACP100 is under construction.
- Argentina:
- Active participant in the Gen IV program through CNEA and INVAP.
- INVAP has patented the ACR–300, a 300 MWe SMR concept for domestic and international markets.
Conclusion
Generation IV reactors and Small Modular Reactors (SMRs) represent the cutting edge of nuclear reactor innovation, integrating advanced passive safety systems, improved fuel cycle efficiency, and operational flexibility. In the context of the global energy transition, both technologies are emerging as critical solutions for deep decarbonization of the energy sector, while also reinforcing energy security and industrial competitiveness.
By 2050, the strategic deployment of Gen IV and SMR technologies could form a foundational pillar of a low-carbon energy infrastructure—particularly in hard-to-electrify sectors. However, realizing this vision will require sustained coordination between research and innovation, the development of adaptive and risk-informed regulatory frameworks, and transparent, evidence-based public engagement.
