A State-of-the-Art Review on Nuclear Reactor Concepts and Associated Advanced Manufacturing Techniques

Abstract

The political commitment to reaching carbon-free energy generation by the year 2050 has led to an increased expansion of renewable energy power plants. As renewable energy generation is intermittent and current energy storage options are limited, a diversified energy grid including nuclear power is the preferable choice for most nations. Many innovative reactor concepts are being pursued in research and development, aiming to supplement fluctuating energy sources. However, it is yet unclear if these technologies can be economically deployed in time. This paper presents the current political views and events concerning the global expansion of nuclear energy, focusing on Europe and the USA. Further, the most important safety aspects of large nuclear power plants are discussed. Moreover, knowledge and definition gaps regarding the applicability of established procedures for innovative reactor concepts are included. The authors highlight that advanced manufacturing techniques play a key role in the economic and technical realization of innovative reactor concepts. The present work is intended to provide insight into current developments in nuclear technology while providing more detail on safety aspects and innovative manufacturing methods.

1. Introduction

By 2050, the global population is predicted to grow by about 1.7 billion [1], inherently leading to an increased energy demand. Some countries aim to cover this demand with carbon-free energy sources only. In a report about the long-term strategy of the USA published by the U.S. States Executive Office of the President in 2021, benefits and specific goals regarding the nation’s greenhouse emissions are named. It includes a commitment to reduce the country’s net greenhouse gas emissions by 50–52% by 2030 in order to reach net-zero emissions by 2050 [2]. Overall, 120 countries committed to achieving net-zero emissions by that time. Canada, for example, launched different initiatives such as the net-zero accelerator fund and the net-zero challenge to support and motivate local industries and businesses taking part in this transition [3]. The European Union also shares this ambitious goal and strives to be the first climate-neutral continent by 2050 [4]. As a first step, all 27 member states committed to reduce carbon emissions by at least 55% by 2030 compared to 1990 levels [5].

For that, energy resources such as wind, water, solar, geothermal, bioenergy and nuclear are meant to replace fossil fuel power plants over time in the generation of electricity and heat. Nuclear energy especially is seen as a reliable, carbon-free option to provide affordable base load energy. China’s ambitious plan of building 150 new nuclear reactors by 2035 is a prominent example of the importance of nuclear power in the energy transition [6].

The importance of nuclear power for reaching the net-zero goal was also highlighted during the 2023 Scientific Forum hosted by the International Atomic Energy Agency (IAEA) in Vienna. Here, it was emphasized that nuclear energy is the only available option to meet the energy demand when and where needed [7]. Another argument in favor of nuclear power can be revealed by comparing the carbon dioxide emissions produced in total by different energy sources. The National Renewable Energy Laboratory (NREL) conducted a systematic review of published life cycle assessment (LCA) studies of electricity generation technologies in 2012. Here, 2165 references were screened following strict criteria, leaving 296 references to initiate a database. Based on this database, the greenhouse gas (GHG) emissions from the different energy sources were assessed. A first overview can be found in the SRREN report, Table A.II.4 of Annex II, published by the IPCC in 2011 [8]. The median values of the published estimates of GHG emissions for a chosen set of energy sources can be found below in

Table 1. Estimates of life cycle greenhouse gas emissions from different energy sources. The values are in grams of carbon dioxide equivalent per kilowatt–hour (${\mathrm{gCO}}_2\mathrm{e}$/kWh) and represent the median value published in the respective references. Data published by the NREL represent the harmonized estimates.

Reference	Year	Wind	Solar	Photovoltaic	Nuclear	Hydropower
IPCC SRREN [8]	2011	12	22	46	16	4
NREL	2012	11 [11]	12 [12]	33 [13,14]	12 [15]	-
NREL [16]	2021	13	28	43	13	21

. The NREL applied harmonization to this dataset in order to adjust the published GHG emission estimates to a consistent set of methods and assumptions, specific to the technology under investigation. The harmonized GHG emission estimates were published in a special issue called Meta-Analysis of Life Cycle Assessments by the Journal of Industrial Ecology in 2012. The harmonized median values are integrated in Table 1 too. Further, Workgroup III of the IPCC contributed information about the LCAs of electricity-generating technologies to the fifth assessment report (AR5) in 2014. Taking a closer look at the data displayed in Figure 7.6 of the report reveals that no updated estimates were taken into account [9]. To the best of the authors’ knowledge, no comparative data was included in the latest assessment report (AR6) published in 2023 by the IPCC or any reports contributing to it. The latest publicly available update was published by the NREL in 2021. Here, 835 additional references were systematically reviewed. The updated harmonized median values, which can be found in Table 1, show for most technologies only slight changes. The overall trend in the GHG emissions shows that nuclear power has about the same carbon footprint as wind power and less than other renewable energy sources. Nevertheless, focusing on an economic perspective, the most cost-effective way of decarbonizing electricity generation is to pair nuclear energy with wind and solar energy. This conclusion was drawn in a study by Vibrant Clean Energy in 2022 [10]. This study further stated that the higher the percentage of advanced nuclear deployment in the grid by 2050, the lower the retail spending for customers.

So-called “advanced reactors” are seen to play a key role in the challenge of decarbonizing energy-intensive industries and providing for district heating [7]. The term advanced nuclear reactor refers to a certain class of nuclear reactors designed to offer improvements and more opportunities regarding flexible operation and deployment scenarios as traditional reactors [17]. Additionally, the term “advanced design” is frequently used. It refers to a design that is currently of interest and has undergone significant improvements compared to its predecessor and/or conventional commercial power reactor designs [18]. Advanced reactor designs include newer water-cooled reactor designs and designs relying on different coolants such as molten salt or gas. Further, small modular reactors (SMRs) and microreactors (MRs) fall into the category of advanced reactor designs. Both concepts are not new but received an increasing amount of interest from industry and politics since the early 2000s [19]. Unlike conventional large-scale nuclear power plants with an electricity gross output of up to 1.7 GWe, SMRs are designed for an electricity output of up to 300 MWe and even MRs only up to 10 MWe. Further, both designs consist of modular, factory-premade components that can be assembled on-site or in the factory and transported fully constructed to the deployment site. This stands in stark contrast to the construction of larger nuclear reactors, which require many components to be manufactured and assembled on site. Many different SMR concepts are currently under development, promising a more flexible operation. The European Commission has specifically recognized SMRs as a possibility for helping decarbonize the energy mix since 2023. In order to accelerate the deployment of the first SMR projects in the early 2030s, the European Commission launched the SMR Industrial Alliance in February 2024 [20].

There are different perspectives on nuclear energy; however, political statements and global investments in nuclear power plants indicate that this energy source is considered a critical component of many nations’ energy strategies. What remains uncertain is the extent to which nuclear energy will be utilized and what type of nuclear power plants will be deployed.

This review article aims to provide an overview of recent developments in the field of nuclear energy, offering insights into important safety aspects and the transition towards smaller reactors. To achieve this, key information about large-scale nuclear power plants (NPPs), including the various reactor types currently in operation and those with the most active development, are presented. Here, safety considerations are addressed. Subsequently, this article discusses the advancements in small modular reactors (SMRs) and advanced modular reactors (AMRs), with a focus on evaluating the applicability of existing safety requirements to both water-cooled and non-water-cooled designs. Furthermore, this article examines the impact of advanced manufacturing techniques for the nuclear sector. The authors would like to emphasize that broader issues such as nuclear waste management, public perception and grid integration are beyond the scope of this work and are not discussed in detail.

2. Nuclear Energy

After the design of the first operational nuclear reactor in 1942 by Enrico Fermi [21], many different concepts evolved in the hope of replacing fossil energy in the energy generation sector completely. Although nuclear energy expanded in some countries during the 1970s and 1980s, supplying approximately 20–25% of their electricity [22], the anticipated global expansion of this promising technology did not materialize. A few exceptions exist, such as France, where nuclear energy accounts for roughly 70% of electricity production as of 2024 [23]. On a global scale, however, nuclear power contributed only 5% to the total energy production in 2023, as shown in Figure 1. Two major challenges can be named as a reason for why the nuclear sector does not dominate power generation today: safety regulations and economics.

Regarding the first reason, let us take a look back into history. Three big nuclear accidents can be named which raised questions and public concern about as well as resistance towards nuclear technology. The first one is the Three Mile Island accident in 1979, where a pressure water reactor (PWR) partially melted down in Pennsylvania, USA. Fortunately, no health effects on plant workers or the public caused by the small amount of released radioactivity were detected [24], unlike with the Chernobyl disaster in 1986, which led to the death of numerous people [19]. Here, the uranium fuel overheated and melted through the protective barriers after control of the nuclear power plant was lost during a test. The following fire and explosion set large amounts of radiation free [25]. Each incident led to substantial enhancements in nuclear safety requirements, which in turn extended permitting and construction durations. Lovering et al. reviewed historical reactor-specific overnight construction cost data for 349 reactors in seven countries covering builds from 1954 to projects that had been completed by the end of 2015. In the USA, a rapid cost escalation and spike in construction time after the Three Mile Island accident can be observed. Interestingly, Lovering et al. found that other countries experienced only modest cost escalation, and South Korea even saw a decrease in construction costs [26]. Massive protests against nuclear power after the accident in Fukushima in 2011 and falling costs for other energy sources put new nuclear projects on hold in the end [19]. A comparison of U.S. adults’ responses in Gallup polls regarding support for the construction of a new nuclear power plant (NPP) within a five-mile (approximately eight-kilometer) radius of their residence revealed a marked increase in opposition following these major nuclear accidents. In 1967, 45% of respondents indicated they would oppose the construction of a new NPP nearby. This proportion rose to 60% following the Three Mile Island accident, and further increased to 73% after the Chernobyl disaster [27]. A more recent Gallup poll in 2019 assessed whether respondents favored or opposed the use of nuclear power as an energy source in the U.S., and it was found that Americans were evenly divided, with 49% in favor and 49% opposed [28].

As a result of the factors discussed above, nuclear power currently accounts for only a small share of global energy production. Its growth has stagnated over the past two decades, as illustrated in Figure 1. In contrast, fossil fuels such as natural gas and petroleum have seen increased use, driven by their relatively well-understood and predictable processes, to meet society’s growing energy demands. Further, great efforts have been invested in developing and optimizing renewable energy. Thanks to these efforts and a mostly positive taking of the public towards renewable technology, the sum of hydro, geothermal, solar PV, solar thermal, tide, wind, municipal waste, biofuel and biogas amounted to about 14% of the global energy production in 2023 [22].

Figure 1. On the left-hand side is the world energy production by fuel in Mtoe (million tonnes of oil equivalent) from 1995 to 2021. On the right-hand side is the global energy mix at the end of 2021. The illustrations were generated based on publicly available IEA material on world energy statistics and balances [29].

Figure 1. On the left-hand side is the world energy production by fuel in Mtoe (million tonnes of oil equivalent) from 1995 to 2021. On the right-hand side is the global energy mix at the end of 2021. The illustrations were generated based on publicly available IEA material on world energy statistics and balances [29].

It may seem that renewable energy is meant to replace all other kinds of fuel types used for energy production and that nuclear energy has no future. But while there are many benefits to renewables, there is currently reasonable doubt about their capability to produce enough energy whenever needed. Renewable energy sources exhibit significant intermittency in their energy supply due to their reliance on specific technological factors, which are often influenced by geographical location as well as seasonal and diurnal variations. Therefore, energy storage systems are essential to capture and retain surplus energy generated during windy and sunny periods for use during times when wind and sunlight are insufficient. Concerns about reliable and steady energy production to cover the increasing base load without carbon dioxide emissions after the net-zero goal for 2050 support a newly awakened interest in nuclear energy. This interest was fueled further by the latest conflict in Europe, which led to a strong discussion about national energy security. Around 30 newcomer countries are either considering nuclear power or moving forward in their plans to construct their first nuclear power plant; examples are Egypt and Bangladesh [30].

In order to avoid past errors and ensure the highest safety standards for not only new constructions but for the operation and maintenance of existing power plants too, international guidelines and recommendations are being followed. This helps to ensures the general international obligation that a state must not pursue activities that cause damage in another state with or without nuclear power plants on their soils [31]. Internationally recognized guidelines and standards are being published by the IAEA. For example, fundamental principles, requirements and recommendations to ensure nuclear safety are established in the IAEA safety standards [32]. They state that whatever the condition of the plant, the fulfillment of the following fundamental safety functions must be ensured: control of reactivity; removal of heat from the reactor and the fuel pool; and confinement of radioactive material, shielding against radiation and controlling planned releases of radioactive materials, including the limitation of accidental releases of radioactive materials. Different countries or regions have further, more specified, requirements in addition to the IAEA standards. For example, member states of the European Union have to analyze the effect of new nuclear installations on other EU countries. What information has to be submitted in order to correctly evaluate the potential health impact of a new project is defined in a recommendation (2010/635/Euratom) from the European Commission [33]. This recommendation states that, among others, a study of accidents of internal or external origin that could lead to an unplanned release of radioactive substances has to be included in the safety report [34].

The majority of operating nuclear power plants are based on light water reactor technology. Due to their widespread deployment and experience in handling them, most of the planned new constructions (in fact: roughly 88%) are based on this same technology [35]. That being said, this is not the only nuclear technology being pursued. So-called “Generation IV” or “GEN-IV” nuclear energy systems are meant to offer comparative advantages regarding capital cost, nuclear safety and nuclear waste generation. In 2002, multiple countries including Canada, France, Japan and the United States (and more) prepared with the IAEA, the Organization for Economic Co-operation and Development (OECD) and the Nuclear Energy Agency (NEA) a technology roadmap for the most promising GEN-IV nuclear systems and the necessary research and development undertakings. The nuclear systems named were the following: gas-cooled fast reactors (GFRs), very-high-temperature reactors (VHTRs), supercritical water-cooled reactors (SCWRs), sodium-cooled fast reactors (SFRs), lead-cooled fast reactors (LFRs) and molten salt reactors (MSRs) [36].

Subsequently, the technical aspects and challenges associated with light water reactors and GEN-IV reactors are presented. Following this, important guidelines about the safety assessment of a nuclear system are elaborated.

2.1. Technical Aspects of GEN-IV Nuclear Systems

2.1.1. Light Water Reactors

Light water reactors (LWRs) are the most common reactors in operation, accounting for about 84% of the global nuclear fleet at the end of 2023. Moreover, they have been the best-performing reactors since 2013, with a capacity or performance factor of about 88% [35]. The performance or capacity indicates how much electrical output was generated over a period of time compared to the maximum possible output. There are two types of LWRs: pressurized water reactors (PWRs) and boiling water reactors (BWRs). In both cases, water (more specifically, light water) is used as a coolant and moderator. A schematic of a PWR is displayed in Figure 2. The water in the primary circuit exits the pressure vessel at roughly 320 °C [37], having been heated by the process of nuclear fission. To prevent boiling, it is maintained at a pressure of around 160 bar. In contrast, in the secondary circuit, water begins to boil upon entering the heat exchanger and absorbing thermal energy from the primary circuit. The resulting steam drives a downstream turbine connected to a generator, thereby producing electrical energy. Subsequently, the steam is condensed back into water by means of a third cooling circuit. In contrast to the PWR design, the BWR employs only two cooling circuits. In this configuration, the water within the primary circuit is allowed to boil, with the generated steam directly driving the turbine. This results in lower thermal losses compared to the PWR system. But, as the nuclear contamination is limited to the primary circuit, maintenance is less complicated for a PWR than a BWR [38].

The reactivity of the reactor core (refer to Section 2.2.1) in a PWR is controlled in two ways: (i) supplementation of boric acid to the primary circuit, and (ii) by the electrical retraction of the control rods into the reactor core. The boric acid and the control rods are neutron-absorbent. Therefore, they enable controlling the nuclear reaction and, with that, the reactor power. If the power supply of the plant is stopped for any reason, the control rods fall due to their own weight into the reactor core and end the nuclear reactions [39]. The surrounding steel reactor pressure vessel (RPV) has a wall thickness of about 25 cm, weighs up to 400 to and acts as part of the containment system of the radioactive material as well as a protective barrier against external effects [40]. Although LWRs are based on a mature design, their technology faces several persistent and emerging challenges. A primary concern among these challenges is their aging infrastructure. Many reactors operate well beyond their initial design lifespans, causing increased incidences of material degradation such as stress corrosion, cracking and embrittlement. This necessitates advanced material research and improved monitoring systems to ensure safety and efficiency [41,42]. Additionally, the sector must modernize its digital infrastructure as many existing plants still rely on outdated analog control systems. The adoption of digital instrumentation and artificial intelligence for predictive maintenance and operational optimization is increasingly crucial [43].

2.1.2. Very-High-Temperature Gas-Cooled Reactors

The very-high-temperature gas-cooled reactor (VHTR) is a helium-cooled, graphite-moderated nuclear fission reactor designed for electricity and heat cogeneration. The reactor outlet temperature can reach up to 1000 °C [37], which is significantly higher than the outlet temperature of a PWR and of a high-temperature gas-cooled reactor (HTGR) with 750–900 °C [44]. Due to the high operation temperature, all-ceramic fuel cells, so-called TRISO (TRIstructural–ISOtropic) particles that are coated with ceramic, are required to be used [45]. As displayed in Figure 3, two different fuel systems are in use. For the first fuel system (a), the fuel particles are embedded in cylindrical compacts that are integrated into blocks that resemble hexagonal ‘prisms’ made of graphite. The second fuel system (b) involves encasing the fuel particles in silicon carbide within graphite in pebbles approximately 60 mm in diameter. The coated fuel particles can withstand high internal gas pressure without releasing their fission products. Their accident fuel temperature limit is typically 1600 °C [46]. An important difference between both designs is the capability of pebble bed reactors to do on-line refueling. This means that no shutdown is needed during the refueling [46]. Producing TRISO-coated fuel particles is more complex as it demands precision, scalability and rigorous quality assurance. An industry report highlights that only three qualified suppliers currently produce nuclear-grade silicon carbide (SiC) coating powder, a critical component of TRISO fuel. Supply constraints have led to 31% year-over-year price increases for SiC powder (around USD 38/kg), contributing approximately USD 0.70 per MWh to the overall fuel cost. Ongoing shortages are projected through 2028 unless new suppliers are certified [47]. If future scaling does not lower the cost, fuel production will be more costly than for water-cooled reactors.

Figure 4 presents a schematic of a VHTGR power plant. In this configuration, helium gas passes through the pebble bed or reactor block before entering an intermediate heat exchanger or steam generator. The thermal energy is subsequently transferred to a secondary heat medium, which is then utilized for external applications such as electricity generation or high-temperature industrial processes.

The high outlet temperature characteristic of VHTGRs enables higher thermal-to-electric conversion efficiencies, offering notable economic advantages. In addition to their efficiency, VHTGRs incorporate several intrinsic safety features. These include passive heat removal mechanisms facilitated by a low power density and a large height-to-diameter ratio of the reactor core. A key safety attribute is the negative temperature coefficient of reactivity, which ensures that any increase in reactor temperature leads to a corresponding decrease in the rate of nuclear fission. This self-regulating behavior prevents the reactor core from reaching its melting temperature, thus supporting the classification of the VHTGR as inherently safe [40]. In contrast to light water reactors, the VHTGR design eliminates concerns regarding chemical interactions between the coolant, fuel and moderator. This is primarily due to the use of helium as a single-phase, inert coolant that does not react with other reactor materials. Additionally, the low power density of the VHTGR enables effective passive heat removal through the large graphite volume, which serves as both moderator and heat sink.

The VHTGR technology is considered a promising solution for reducing carbon dioxide emissions in sectors that are challenging to electrify, particularly those requiring high-temperature process heat [49]. Due to the reactor’s high outlet gas temperature, it can also be employed as a thermal heat source for endothermic chemical processes, such as coal chemistry and hydrocarbon upgrading [46]. A market assessment showed that the greatest potential over the next few decades is in applications requiring a modest outlet temperature of 750 °C to 850 °C [50]. Overall, the VHTGR concept represents significant progress in nuclear innovation but still faces substantial technical and market barriers before large-scale deployment can be realized. For example, the response to accident scenarios in VHTGR systems is shaped by several critical technical challenges that are the focus of ongoing research and safety assessments. A central issue is the management of air and water ingress events, where breaches in reactor pressure boundaries can introduce oxidants into the core, leading to graphite oxidation, corrosion and potential release of fission products. A white paper about the safety assessment of VHTGR published in 2018 by the GEN-IV Forum emphasized that fast and reliable detection, coupled with effective mitigation systems, is essential. However, this remains technically demanding due to the complex interplay between reactor materials and elevated temperatures [51].

In addition, as with other new nuclear technologies, public acceptance and the demonstration of safe, reliable operation will impact market adoption rates [52].

2.1.3. Molten Salt Reactors

Nuclear reactors using molten salts as the reactor fuel, coolant and/or moderator are considered molten salt reactors (MSR). There are numerous design variations in MSRs. The Generation IV International Forum categorizes them into three primary types based on the specific role of molten salt within the reactor system: (i) molten salt fuel (pumped); (ii) molten salt fuel (natural circulation); and (iii) molten salt coolant (only). These reactors may operate with a thermal neutron spectrum (using graphite as a moderator), a fast spectrum (without moderation) or an epithermal spectrum, depending on the specific design [40,53]. MSRs can also be categorized into various classes and families according to their technical characteristics, as illustrated in Figure 5.

The heat produced in the reactor’s molten salt is transferred via an intermediate heat exchanger to a secondary coolant circuit, and subsequently through an additional heat exchanger to the power conversion system [54].

Although the first MSRs were conceived and tested decades ago, no commercial deployment has taken place. Significant obstacles such as materials durability, complex fuel processing, regulatory uncertainty and economic risks have prevented commercialization of this technology [55]. As a result, operational experience with MSRs remains limited, and standardized safety assessment procedures are lacking.

Nevertheless, the technology’s ability to operate at temperatures exceeding 600 °C makes it attractive for industrial applications requiring high-grade heat—such as hydrogen production. However, among the technical challenges of this technology is that molten salt is highly corrosive [40], particularly when exposed to impurities or radiation. A 2023 consensus study report by the National Academies of Sciences, Engineering, and Medicine emphasized that such chemical complexity, coupled with high-temperature operation, demands substantial material R&D before MSR technology can move toward deployment and commercialization [56].

MSRs also offer inherent safety features. The coolant itself can chemically bind many fission products even at elevated temperatures and acts as a barrier to the release of nuclear material. As the fuel is not surrounded by a cladding tube, the molten salt resembles the first barrier containing the radioactivity (refer to Section 2.1.1), unlike in LWR concepts. Additionally, the molten salts have low chemical reactivity and can absorb some of the decay heat after a reactor shutdown. In the event of overheating, a valve located beneath the reactor core enables the contaminated molten salt to flow into a passively cooled storage tank. The valve is made from a material specifically designed to dissolve upon reaching a critical temperature, ensuring a passive safety mechanism [55].

Figure 5. Classification of MSRs by the IAEA [55].

Figure 5. Classification of MSRs by the IAEA [55].

2.1.4. Fast Reactors (Gas-Cooled, Sodium-Cooled, Lead-Cooled)

Reactors featuring a fast neutron spectrum and a closed fuel cycle are considered fast reactors. By design, these reactors do not include a neutron moderator. Fast reactors can be differentiated depending on the coolant as gas-cooled (GFR), sodium-cooled (SFR) or lead-cooled (LFR) fast reactors [54]. As these coolants have significantly different physical, thermal and chemical properties, there are major core design differences, as can be seen in Figure 6. However, they all have a fast neutron spectrum in common, for which you need a coolant with heavy atoms that do not slow down the generated prompt neutrons too much [57]. Each fast reactor technology, labeled as GEN-IV, is going to be presented briefly below.

SFR: Sodium possesses several favorable properties as a reactor coolant, primarily due to its exceptionally high thermal conductivity, which is approximately five times greater than stainless steel and more than ten times that of water. It has a low melting point of about 98 °C and remains liquid over a wide temperature range, not boiling until 883 °C under ambient pressure. One of sodium’s notable advantages is its excellent chemical compatibility with stainless steel, allowing its use in reactor systems operated at ambient pressure. Typically, sodium enters the reactor core from below at around 400 °C and exits at the top at approximately 550 °C.

In all sodium-cooled reactor designs, including modern ones, a secondary sodium loop is consistently implemented as a safety measure. Heat is first transferred from the primary sodium system to at least one intermediate sodium circuit, which then delivers the heat to a steam generator located in an adjacent compartment.

The SFR is the most widely constructed Generation IV design to date. However, all existing examples have been experimental or prototype reactors, with no commercial deployment achieved thus far.

Despite its favorable thermal properties, sodium presents significant safety challenges. It reacts violently with water, forming sodium hydroxide and hydrogen, and burns when exposed to air. Consequently, plant designs incorporate a sump tank into which sodium can be rapidly drained in the event of a leak, ensuring enhanced safety and containment. An additional operational challenge arises from the fact that liquid sodium is opaque, complicating visual inspection and control of plant systems at any operational state [57].

LFR: Lead-cooled fast reactors address two major safety concerns associated with sodium-cooled fast reactors. First, in the event of a leak, liquid lead solidifies and, at most, forms a superficial oxide layer, thereby minimizing the risk of fire or explosive reactions. Second, lead has an extremely high boiling point, such that the fuel cladding would fail before the coolant begins to boil. This reduces the risk of coolant vaporization under normal operating conditions.

However, a significant drawback is the corrosive nature of liquid lead at elevated temperatures, which can dissolve iron and nickel from steel components. To mitigate corrosion, LFRs typically operate in a temperature range between 400 °C and 550 °C. Further, the oxygen concentration in the coolant must be tightly controlled to maintain a protective oxide layer on structural materials. This oxygen control is technically complex, particularly in large-scale LFR power plants, and operational experience remains limited [58]. Despite these challenges, LFR technology remains of interest due to its potential suitability for both remote locations and centralized power generation [61].

GFR: Gas-cooled fast reactors use inert gas, most commonly helium, as a coolant. Unlike sodium, helium does not react with air or water, and unlike lead or lead-bismuth, it does not cause corrosion. Helium is particularly well suited for fast reactors as it neither moderates nor absorbs neutrons, supporting a hard neutron spectrum. Further, its transparency allows for visual control of the reactor interior. However, helium’s low density significantly limits its heat transfer capability compared to liquid metal coolants. To overcome this limitation, GFRs must operate with higher coolant volumes and elevated system pressures. This necessitates a higher coolant flow rate and a reduction in the specific power output of fuel rods to maintain safe cladding temperatures. Core outlet temperatures typically exceed 750 °C. Although no helium-cooled fast reactor has been constructed to date, a reference design named ALLEGRO was developed under the Generation IV program to assess the technical and economic feasibility of the GFR concept. This design features a thermal power output of 2400 MW and achieves a cycle efficiency of approximately 48%. In ALLEGRO, heat from the primary helium coolant circuit is transferred to a secondary gas cycle containing a helium–nitrogen mixture, which drives a closed-cycle gas turbine [57,61].

For all three fast reactor technologies, further research and development is needed before a demonstrator can be built. It is therefore hard to estimate a timeframe for the deployment of these technologies.

2.1.5. Supercritical Water-Cooled Reactors

Supercritical water-cooled reactors (SCWRs) operate with water at conditions above its thermodynamic critical point (374 °C, 22 MPa), where it becomes a single-phase superheated fluid with unique thermophysical properties [40]. At these conditions, the coolant no longer exists as a water–steam mixture, eliminating phase changes within the core.

Several SCWR concepts have been developed globally, often to support advanced reactor R&D and to train the next generation of nuclear engineers. One notable example is the High-Performance Light Water Reactor (HPLWR) developed in the European Union in the early 2000s. It is designed to operate at 25 MPa with an inlet temperature of 280 °C (see Figure 7). SCWRs can be designed for thermal, fast or hybrid neutron spectra, offering flexibility in reactor physics and fuel cycle options [62].

A key advantage of SCWRs, similar to HTGRs, is their higher thermal efficiency, exceeding 43%, compared to approximately 35% for conventional LWRs. This improvement enables up to 20% more electricity to be generated from the same fuel input. Additionally, operating above the critical point eliminates bubble formation, simplifying flow dynamics and allowing the elimination of bulky components like steam separators and dryers. This can significantly reduce the reactor’s overall size and containment requirements. For instance, the containment height of the HPLWR is about 25 m, compared to 83 m for the AP1000 PWR from Westinghouse, highlighting its potential for cost savings [57].

Although the safety systems proposed for the different SCWR concepts are similar to the ones used in conventional LWRs, their performance under supercritical conditions remains to be fully validated [57,62]. Ongoing research and development efforts are being conducted by several international institutions to address the technical challenges of SCWR technology. For instance, the Canadian Nuclear Laboratories, in collaboration with universities and industry partners, are actively developing SCWR designs with a focus on fuel behavior, corrosion-resistant materials and thermal–hydraulic testing under supercritical conditions. Complementing these efforts, the IAEA Coordinated Research Project I31034 brings together experts from multiple member states to develop predictive modeling tools and compile experimental data essential for validating SCWR thermal hydraulics and system behavior.

2.2. What About Safety?

2.2.1. Fundamental Safety Functions

According to the specific safety requirements listed by the IAEA for the safety of NPP No. SSR-2/1 (Rev.1) [63], three fundamental safety functions need to be fulfilled for all plant states: (a) control of reactivity, (b) removal of heat from the reactor and from the fuel store and (c) confinement of radioactive material, shielding against radiation and controlling planned radioactive releases, including the limitation of accidental radioactive releases.

The reactivity of a nuclear reactor describes its deviation from the critical state, which represents the normal operating condition, characterized by a self-sustaining chain reaction at a constant rate. For each reactor, a reactivity coefficient can be defined to quantify how its criticality changes in response to various time-dependent parameters such as temperature. A reactor with a negative reactivity coefficient is considered inherently safe, as an increase in temperature leads to a decrease in reactivity, thereby reducing the rate of the chain reaction and enhancing stability. Neutron-absorbing elements, such as control rods, and neutron-decelerating elements, such as the coolant, are incorporated into the reactor design to control the nuclear fission reaction.

The cooling medium is further necessary to transfer the heat generated due to the fission from the reactor core to the heat exchanger. Even after the nuclear fission ceases (e.g., after a reactor shuts down), decay heat is generated and needs to be removed to maintain the integrity of the nuclear system. Different coolants such as water, gas (e.g., helium) or liquid metals (e.g., lead) can be utilized. The choice of coolant depends on the nuclear physical properties of the neutron capture cross section and the scattering cross section of the nuclear reactor. Moreover, the choice of the cooling medium determines what type of radiation develops and influences the necessary safety measures for the core-surrounding components.

Depending on the nuclear system, to have a thermal reactor or a fast reactor, the coolant needs to fulfill further requirements. In a thermal reactor, the coolant is either integrated as a part of the moderator or is even the only moderator used. The moderator in a nuclear reactor decelerates the neutron velocity and, with that, the nuclear fission process, yielding slower heat generation. A suitable medium has to be chosen [64]. In a fast reactor, no deceleration of the neutron’s motion is desired, i.e., each nuclear fission is followed by the next nuclear fission. Hence, the coolant used in a fast reactor must have as little a moderator effect as possible. This can be achieved, for example, by keeping the amount of coolant as small as possible, which would avoid collisions between neutrons and nuclei other than those of the fuel [40].

Concerning the third fundamental safety function of an NPP, different physical barriers are integrated in the reactor and plant design with the objective of containing radioactive material. Either the fuel cladding or the core-surrounding components usually act as the first physical barrier, as can be seen in Figure 8. The rest of the physical barriers depend on the nuclear system in place but, in general, multiple barriers are installed to prevent any leakage of radioactive material or to minimize the leaked amount due to an accident. Each barrier is part of the containment system of the NPP. Besides the confinement of radioactive substances in operational states and accident conditions, the containment system has to protect the reactor against natural external and human-induced events. Moreover, it ensures radiation shielding in operational states and accident conditions [63]. Therefore, the containment system is an integral part of the Defense in Depth (DiD) concept. This concept is meant to prevent accidents in an NPP and, in the case of accidents, to mitigate the consequences. According to its fundamental safety principles [65], DiD consists of consecutive and independent levels of protection that would have to fail before people or the environment are harmed. The different levels of the DiD concept are displayed in Figure 9.

The design of a series of physical barriers for a nuclear power plant combines active, passive and inherent safety features, all relevant aspects for the implementation of the DiD concept. Incorporated into the five levels of the DiD are requirements from the safety standards of the IAEA. These include, for example, steps that are meant to prevent any challenges to the integrity of physical barriers, the failure of a barrier when challenged and the failure of one barrier as a consequence of the failure of another barrier.

2.2.2. Safety Assessment

According to the IAEA, the term safety includes the protection of people and the environment against radiation risks, as well as the safety of facilities and activities that give rise to radiation risks [65]. Comprehensive safety assessments of the NPP design are required to be carried out to demonstrate the achievement of the fundamental safety objectives. The safety assessment is a systematic process that is carried out throughout the design process (and throughout the lifetime of the facility or the activity) to ensure that all the relevant safety requirements are met by the proposed (or actual) design [67].

The safety assessment involves analyzing and evaluating all potential events that could lead to an emergency or accident, along with an indication of the associated risk. The risk-informed process of classifying events for an accident analysis involves two steps: (i) classification by frequency of occurrence and (ii) grouping of events by type or event sequence. The classification of plant events is carried out only after a thorough analysis and comprehensive review of the plant design for all subsequent event initiators and event sequences. A plant probabilistic safety assessment (PSA) is meant to be used for developing a comprehensive analysis of the possible event scenarios and its evaluation [68]. For example, for modular HTGR systems, the IAEA proposes three major event frequency-based categories, which are depicted in Figure 10.

During the second step of accident analysis, two types of accidents are primarily distinguished during the design phase of a nuclear reactor: reactivity-initiated accidents where over-reactivity is accidentally introduced into the core, and cooling accidents where the core is insufficiently cooled [21]. Depending on the technology of the nuclear power plant and the conducted accident analyses, the corresponding DiD concept (refer to Figure 9) is developed [65].

It is mandatory to define hazards for each nuclear system in order to assess accidents. A hazard is defined as the potential for harm or other detriment, especially for radiation risks, or as a factor or condition that might operate against safety [67]. A distinction between internal and external hazards is made. Events such as fire, explosion, flooding, missile impact, collapse of structures and falling objects, pipe whip, jet impact and release of fluid from failed systems or from other installations on the site are considered internal hazards. In contrast, an external hazard originates from outside the site boundary and outside the activities that are under the control of the operating organization. For such specific hazards, specific requirements exist. Some are well detailed, like the ones for considering seismic activities. For example, German safety requirements for (LWR) NPPs include that the design needs to withstand seismic activities of at least intensity VI EMS/MSK independent of the siting. Consideration regarding the oscillation excitation of structural components, systems and plant parts, as well as changing underground conditions, is needed. The necessary earthquake parameters to assess seismic hazards for site evaluation can also be chosen based on the IAEA Safety Standards Series No. SSG-9 [69]. These safety requirements specifically state, as some generation IV reactor systems include multiple reactor modules, that the number of modules must be considered in the calculation of event frequencies [68].

Another important scenario that needs to be considered during safety assessment is the transport of spent nuclear fuel (SNF). Uranium-bearing fuel elements that have been utilized at commercial nuclear reactors but no longer produce sufficient energy for power generation, as their efficiency to sustain a nuclear reaction mitigates over time, are referred to as SNF. The transportation of SNF as well as radioactive material is regulated and monitored by the nuclear regulatory commission of each country (national policy). The U.S. nuclear regulatory commission (NRC), for example, demands certain safety and security requirements in conjunction with the certification of the transportation casks and inspections [70]. About 3 million packages of radioactive materials are shipped each year in the United States, either by highway, rail, air or water. Regulating the safety of these shipments is therefore the joint responsibility of the NRC and the Department of Transportation. The Department of Transportation regulates the shipments while they are in transit and sets standards for labeling and smaller quantity packages [71]. Using rails, the transport of high-level radioactive material in North America is regulated by the Association of American Railroads (AAR). Specifically, the AAR’s S-2043 standard applies [72]. Transport of radioactive material by rail is still subject to research and development efforts. The U.S. Department of Energy invested approximately USD 33 million [73] in the development of a special railcar for the future large-scale transport of SNF during the 10-year Atlas railcar project. This railcar can weigh between 82 tons and 210 tons while carrying up to 17 different SNF containers.

These strict transport regulations consider the safety requirements of the packaging. Since safety is primarily assured by the packaging, regardless of the mode of transport, each package containing radioactive material or SNF needs to be certificated [74]. An overview of all the necessary information for an application for package approval from the U.S. NRC can be found in NUREG-2216, published in 2020 [75]. This document states that after a new transportation design is proposed, it needs to be reviewed. During this review, compliance with applicable regulatory requirements and acceptance criteria is assessed. These factors are evaluated as part of the structural analysis and must be satisfied under normal transport conditions, hypothetical accident conditions and air transport accident conditions. For example, the test conditions that a package designed to transport radioactive material must withstand under normal transportation conditions are outlined in 10 CFR 71.71, a section of the Code of Federal Regulations (CFR) published by the U.S. NRC [75]. Furthermore, numerical models are used to analyze the package behavior under major transport accidents. This approach allows the evaluation of new package proposals under load conditions resembling severe real-world accidents such as the “MacArthur Maze” accident in 2007 [76] or the Baltimore railroad tunnel fire in 2001 [77]. In conjunction with the NRC regulations (e.g., 10 CFR 71), the American Society of Mechanical Engineers’s (ASME) Boiler and Presser Vessel Code (BPVC), Section III, Division 3, provides rules for the construction of containment systems for high-level radioactive material transport, in particular for transport packages for SNF [75].

2.3. Global Trends

Although some countries plan on increasing their nuclear power capacity by building new large NPPs, an opposing trend is visible in the shutdown data sheets globally. The U.S. Department of Energy (DOE) noted in 2021 that due to challenging market conditions, 12 reactors had to shut down since 2013. Additionally, seven units are to be shut down by 2025. Reasons for this are challenging market conditions, high operating and maintenance costs for older NPPs and limited political and regulatory support to keep uneconomic plants despite their environmental and grid benefit [78]. Economic concerns can also be seen with one of the latest large-scale nuclear power plant projects, Hinkley Point C in Somerset, UK, still under construction. The initially determined construction budget of around EUR 21 billion was estimated in 2024 to be EUR 37 billion, and the operating start of the plant is now set at its earliest for 2031, 15 years after start of the construction [79].

Apart from a few recent proposals in China, new proposals for large reactors have become increasingly rare, with the U.S. program having largely been phased out [19]. Reference data by the IAEA show that by the end of 2023, 59 reactors were under construction globally. The majority (roughly 40%) were located in China [35]. Figure 11 presents the number of NPPs that are currently operable, under construction, planned or proposed in selected countries and across Europa. Here, the term “planned” refers to projects with approvals, funding or commitments in place, with most expected to become operational within the next 15 years. The term “proposed” denotes projects associated with specific programs or site proposals, though their timeline remains highly uncertain [80].

Facing the goal of reaching net-zero by 2025 and uncertainties regarding cost competitiveness and guarantee of supply at the same time, an increasing number of countries are currently prioritizing the long-term operation of their nuclear power plants [81]. The decision to prolong the operation time of an existing NPP involves considering a series of factors, with safety being the most important. Following the safety standards and guidelines of the IAEA, a peer review analyzing the safety aspects of long-term operation of a power plant can be requested by the plant operators and owners if they wish to expand their operation time. By the end of 2023, 193 nuclear reactors were authorized to operate beyond 40 years [82]. But investments in the current nuclear fleet can only happen with adequate political motivation like the Bipartisan Infrastructure Law signed by the U.S. president in 2021 [78].

Despite the challenges new nuclear projects and existing nuclear fleets are facing, international studies conducted by the NEA, the French grid operator RTE and the consulting firm Enco for the Dutch government identified nuclear energy as an important way of reducing the costs of decarbonizing the electricity supply. The expansion costs for renewable energy plants, grid expansion, back-up power plants and storage can be saved since nuclear power can be easily integrated into the existing infrastructure [83]. Another important aspect in favor of nuclear energy is its capacity factor. The capacity factor compares the net energy generation with available capacity over a certain timeframe [84]. Since power plants or other forms of energy generation never produce the maximum amount of energy due to maintenance, refueling, challenging weather conditions, etc., the capacity factor is therefore always below 100%. See a comparison of the capacity factor for different energy sources in the USA in Figure 12.

3. Bigger = Better?

To facilitate the integration of nuclear energy into a carbon-free energy grid, a previously established concept has gained increasing attention over the past decade. The idea of smaller reactor designs began to experience a revival in the early 2000s [19]. These reactor types can be grouped into (i) small modular reactors (SMRs) and (ii) advanced modular reactors (AMRs). The term small modular reactor refers to a reactor that is smaller, both in terms of power output and physical size, than conventional gigawatt-scale nuclear reactors. The electrical power output of an SMR is typically less than 300 MWe. SMRs with an electrical power output up to only 10 MWe are being called microreactors [85]. The components of SMRs are designed for serial manufacturing, allowing them to be either fully assembled and transported to the deployment site or transported as individual components for on-site assembly. Reactor concepts with a higher power output but still less than the conventional 1000 MWe are referred to as AMRs.

Research and development on more than 80 SMR/AMR concepts are currently conducted on an international scale. A comprehensive overview of these technologies’ progress can be found in the NEA SMR Dashboard. The latest publication, released in 2024, includes information up to November 2023 [85]. SMRs are envisioned for niche electricity or energy markets where large reactors would not be viable. They can support cogeneration for heat, produce hydrogen, enable desalination and provide power for small electricity grids and remote or off-grid areas, while also facilitating hybrid nuclear–renewable energy systems. According to the NEA SMR Dashboard, the majority of SMR concepts are in design phase II or III, making progress towards construction. The design phases mark specific milestones that an NPP design must pass through sequentially before receiving a license and being authorized for construction. Figure 13 displays the design classification model proposed by the IAEA [18]. So far, only three SMRs are operating, and several designs are under construction; see Table 2 for a more detailed list.

The increased interest in smaller reactor concepts is not only visible by the number of concepts being pursued. A systematic literature review of nuclear safety systems in small modular reactors, published in 2021, showed that there has been a drastic increase in research in the past fifteen years [86].

3.1. Why Shift to Smaller Nuclear Reactors

Besides their inherent safety features, SMRs also leverage design features that have the potential to bring safety-related advantages such as (a) smaller reactor cores with smaller quantities of nuclear material, (b) the use of accident-tolerant fuels and advanced fuels that can maintain their structural integrity even at higher temperatures and (c) operation at lower pressures and the use of passive safety systems that do not require external sources of electricity or human intervention to maintain safety. Removal of heat from the core is one of the three fundamental safety functions that need to be considered in the design of the core. In certain SMR concepts, manufacturers claim that the surrounding ambient medium, specifically air, provides sufficient cooling to dissipate decay heat under passive conditions. An example of this concept is the microreactor eVinci, developed by Westinghouse. This design features a solid graphite monolith into which fuel rods, shutdown rods and heat pipes are integrated. The fuel rods generate heat through nuclear fission, while the heat pipes facilitate passive high-temperature heat transfer to cool the monolith. These long, narrow pipes contain liquid metal that vaporizes within the section enclosed by the graphite monolith. Beyond this region, the pipes extend into a heat exchanger where circulating fluid dissipates the heat. In emergency situations, decay heat is conducted to the reactor containment and subsequently released into the surrounding atmosphere. Due to its compact size, transportability in shipping containers and scalable 5 MWe output, the manufacturer has identified potential deployment scenarios that include remote communities, mining operations and hydrogen production, among others [87]. In 2022, the Saskatchewan Research Council (SRC) and Westinghouse Electric Canada declared their collaboration for a project to locate an eVinci microreactor in Saskatchewan, Canada, for the development and testing of industrial, research and energy use applications [88].

Table 2. SMR concepts approved for construction and operation. Data from [85] is used, with additional sources incorporated where appropriate.

SMR	Country and Thermal Power, Outlet Temperature	License to Construct	Under Construction?	License to Operate	Operating?
CAREM SMR	Argentina 100 MWth, 326 °C	Yes	Yes	-	-
ACP100	China 385 MWth, 320 °C	Yes	Yes	-	-
RITM-200N	Russia 396 MWth, 318 °C	Yes	Yes [89]	-	-
BREST-OD-300	Russia 700 MWth, 350 °C	Yes	Yes	-	-
HTR-PM	China 500 MWth, 700 °C	Yes	Yes	Yes	Yes
KLT-40S	Russia 300 MWth, 316 °C	Yes	Yes	Yes	Yes
HTTR	Japan 30 MWth, 950 °C	Yes	Yes	Yes	Yes

Table 2. SMR concepts approved for construction and operation. Data from [85] is used, with additional sources incorporated where appropriate.

SMR	Country and Thermal Power, Outlet Temperature	License to Construct	Under Construction?	License to Operate	Operating?
CAREM SMR	Argentina 100 MWth, 326 °C	Yes	Yes	-	-
ACP100	China 385 MWth, 320 °C	Yes	Yes	-	-
RITM-200N	Russia 396 MWth, 318 °C	Yes	Yes [89]	-	-
BREST-OD-300	Russia 700 MWth, 350 °C	Yes	Yes	-	-
HTR-PM	China 500 MWth, 700 °C	Yes	Yes	Yes	Yes
KLT-40S	Russia 300 MWth, 316 °C	Yes	Yes	Yes	Yes
HTTR	Japan 30 MWth, 950 °C	Yes	Yes	Yes	Yes

Further reasons for the growing interest in smaller reactors include manufacturers’ claims of flexible operation, which would make them well suited to complement energy grids with intermittent renewable sources like wind and solar, or to provide base load power as needed. This flexibility could enhance energy reliability and grid stability, thereby contributing positively to overall energy security. In early 2025, the US microreactor developer Last Energy announced plans of constructing 30 microreactors based on pressurized water reactor technology in northwest Texas to satisfy a demand from Texas-based data center developers. The governor of Texas emphasized the importance of microreactors as due to their scalability and siting flexibility, they are the best solution to meet Texas’s energy demand quickly in his opinion [90]. Another key advantage of SMRs is their high energy density, particularly when compared to other carbon-free energy sources in terms of land use. As illustrated in Figure 14, even conventional nuclear power plants require significantly less land than solar or wind energy installations. SMRs can have a plant footprint of up to approximately 0.04 km², while microreactors may require as little as 0.008 km², which is roughly equivalent to the size of 1.5 soccer fields. In addition, smaller emergency planning zones could be allowed for SMRs. Findings from Monte-Carlo analyses conducted by Carless et al. showed that airborne radioactivity is drastically reduced due to the characteristic large lateral surface area-to-volume ratio of SMRs. A comparison of two large LWRs and a conceptual SMR based on PWR technology under a loss-of-coolant accident showed that the radioactive particle concentration surrounding the LWRs was 40–50% higher than for the SMR [91].

SMR and AMR concepts based on either light water reactors or high-temperature gas-cooled reactors are considered near-term deployable, i.e., commercialization is projected within the next 5–10 years. One example is the NuScale VOYGR SMR, which falls into that category. This SMR features a natural circulation modular light water reactor that integrates a reactor core, a pressurizer and two helical coil steam generators, all enclosed in a common reactor pressure vessel that is housed in a compact cylindrical steel containment. One reactor or power module has a power output of 77 Mwe, and multiple modules can be assembled within a single power plant to achieve scalable electricity production. This SMR from the US company NuScale is so far the only SMR design certified by the U.S. Nuclear Regulatory Commission (NRC) [92,93]. Although the reactor design was approved, their first commercial project, featuring 12 power modules at the U.S. Department of Energy’s Idaho National Laboratory, was canceled in 2023. Rising interest rates and inflation led to a 50% increase in projected electricity costs, prompting some communities to withdraw their commitments [94]. Media coverage at the time around the incident reflected a pessimistic outlook for advanced reactor technologies. The Financial Post described the cancelation as a major setback for next-generation nuclear technology [95], and Wired magazine highlighted that even a USD 1.4 billion DOE grant could not prevent escalating costs and the project’s demise [96], while The Telegraph in the UK called it a blow to green energy, noting that the partnership between an inexperienced developer (NuScale) and operator (Idaho National Laboratory) contributed to the failure [97]. However, international interest and trust in NuScale’s technology seemed to remain strong despite this setback. Two examples to be named are Romania’s ongoing plans to deploy a nuclear power plant with six VOYGR modules by 2030 [98] and Poland’s 2023 decision-in-principle to construct a NuScale VOYGR modular nuclear power plant for copper and silver production [99]. Moreover, in 2023, NuScale entered into a Memorandum of Understanding (MoU) with its South Korean partners Samsung C&T Corporation, Doosan Enerbility and GS Energy to explore the potential deployment of NuScale’s VOYGR power plant in South Korea [100]. Although no further public media reports or official press releases have been issued since late 2023 regarding the Uljin VOYGR-6 project, evidence suggests that cooperation between NuScale and its Korean partners has continued to strengthen. Notably, in mid-2024, Doosan Enerbility secured a substantial contract to supply key nuclear components to NuScale, with the order reportedly valued at approximately USD 1.46 billion [101]. Keeping track of NuScale’s homepage reveals a shift in their communication strategy: instead of providing a summary of ongoing projects, the company now primarily lists its strategic partners. The history of NuScale’s SMRs shows the importance of robust financial planning and the maintenance of regular communications with stakeholders in order to sustain their commitment throughout a project’s lifetime.

Figure 14. Land footprint of different energy sources to generate 1800 MWe [102].

Figure 14. Land footprint of different energy sources to generate 1800 MWe [102].

SMR and AMR concepts, including microreactors with a fast neutron spectrum or molten salt coolant, are considered viable for mid- to long-term deployment. Operational readiness is anticipated within the next 10–20 years (mid-term) or beyond 20 years (long-term). Just like their larger GEN-IV counterparts, they are applicable for industries that are considered hard to decarbonize.

A joint study conducted by ULC-Energy, Topsoe, Rolls Royce and KYOS demonstrated that hydrogen can be produced for less than 3.50 EUR/kg using an SMR nuclear power plant and Topsoe’s proprietary Solid Oxide Electrolysis Cell (SOEC) technology, offering an alternative hydrogen production strategy that is cheaper [103]. Since this study is not publicly available, further information can be requested from ULC [104]. In Canada, a molten salt-based small modular reactor concept successfully completed a crucial pre-licensing vendor design review in 2023, marking the first time an MSR has achieved this milestone necessary for commercial deployment [105]. Political momentum for SMR deployment is also evident in Eastern Europe. In 2023, Poland issued a decision-in-principle to construct 24 BWRX-300 reactors, manufactured by GE Hitachi Nuclear Energy (Wilmington, NC, USA), across six locations in Poland [106]. This SMR design, based on boiling water reactor technology, is estimated by the manufacturer to be constructed within 24–36 months, which is significantly shorter than the typical 5–10-year timeframe required for larger nuclear power plants. Additionally, GE Hitachi claims that its SMR design reduces the plant footprint by approximately 90% and, therefore, utilizes 50% less concrete per megawatt [107]. While manufacturers present promising quantitative data, the commercial-scale cost-competitiveness of SMRs remains unproven. As noted by Vinoya et al. in their review of small modular reactors in 2023, various theoretical financial analyses indicate that SMRs possess economic potential. However, this viability depends upon achieving mass production. Among the numerous SMR designs currently under development, China’s HTR-PM and Russia’s KLT-40S stand out as the most advanced toward serial production, being the only designs with multiple units already constructed or operational [108].

In general, massive challenges need to be overcome before the deployment of SMRs. The lack of standardization in safety assessments and regulations as well as missing experience with non-established technologies are just a few of the drawbacks. Step-by-step possible challenges are more precisely defined in international groups and committees aiming to close the gap for a fast deployment of small and advanced nuclear reactors. The IAEA published two reports about the applicability of existing safety standards. The first report considered near-term deployable concepts, namely those based on LWRs or HTGRs [109]. This report identified necessary changes in safety requirements given in a previous publication about the Safety of nuclear power plants: Design [63] for LW-SMRs and HTG-SMRs amounted to around 10% and 35%, respectively. The second report included the applicability to non-water-cooled concepts, published in 2023 [110]. The report identified preliminary areas of novelty specific to both the design and construction processes, as well as the transportation, of radioactive materials. Regarding the first area, the IAEA concluded that existing nuclear safety standards do not yet fully address novel or advanced manufacturing techniques (AMTs), nor the implications of factory-based manufacturing approaches. These gaps are particularly relevant as AMTs, such as additive manufacturing or advanced welding, are increasingly considered foundational for the accelerated and cost-effective deployment of next-generation reactors. A 2023 IAEA report further noted that, despite their growing relevance, probabilistic safety assessments related to these technologies and their integration into multi-unit SMR facilities remain underexplored [111]. To increase the share of nuclear energy in the power grid, research and development initiatives span all disciplines involved in constructing and operating nuclear power plants. The following section examines recent developments in the manufacturing of nuclear components, with particular attention to advanced manufacturing techniques, as these are considered an enabling technology for the nuclear industry [112].

3.2. Advanced Manufacturing Techniques for the Nuclear Industry

The NEI considers a manufacturing method that has not yet been utilized by the nuclear industry but potentially in other industries as an advanced manufacturing technique. A report published by the NEI in 2019 highlights the motivation for utilizing these techniques for nuclear component fabrication. AMTs promise to produce high-quality components more rapidly and cost-effectively than traditional methods like forging or casting, which come with long lead times. In addition to accelerating the production of new components, AMTs enable the quick replacement of obsolete parts, thereby reducing warehouse inventories and offering a chance to enhance functionality due to increased design freedom [113]. The report further identifies several AMTs that are likely to be deployed earliest in the nuclear sector. These include additive manufacturing (e.g., powder bed fusion, direct energy deposition (DED), binder jetting), near-net-shape manufacturing (e.g., powder metallurgy–hot isostatic pressing (PM-HIP), investment casting), joining/cladding techniques (e.g., adaptive feedback welding, diode laser cladding, electron beam welding (EBW), friction stir welding, hybrid laser arc welding) and surface modification or coating processes (e.g., chemical vapor deposition, cold spray (CS) additive manufacturing, laser peening, physical vapor deposition). In the following sections, a selected subgroup of three AMTs are presented in more detail. Afterwards, current challenges regarding standardization are addressed and activities to overcome these challenges reviewed.

3.2.1. Direct Energy Deposition

Directed energy deposition is an advanced additive manufacturing process primarily used to repair or enhance existing components. It uses a multi-axis nozzle to deposit the metal, typically in wire or powder form, which is simultaneously melted by a laser or electron beam and solidifies on the target surface. Unlike fixed-axis systems, DED machines offer four- or five-axis movement, enabling material deposition from various angles, making it ideal for complex geometries and part refurbishment. While compatible with polymers and ceramics, DED is most commonly used with metals.

Although DED is considered a cutting-edge manufacturing method today, its foundational principles have long-standing roots in industrial applications. Notably, the nuclear industry has been leveraging wire-fed DED techniques for nearly a century, particularly to repair and build up critical structural and pressure-containing applications. For example, during the 1980s, segregation in cast and forged metals posed a major challenge, especially in large-scale industry applications. Alternatively, wire-fed DED has been utilized to manufacture larger safety-relevant components such as reactor pressure vessels. But, during most of that time, the process was known as weld metal buildup or shape welding [113,114]. A specific form of wire-fed DED is wire arc additive manufacturing (WAAM), which uses an electric arc as a heat source. WAAM is further characterized by its fast deposition rate (up to 600 ${\mathrm{cm}}^3/\mathrm{h}$ [115]), allowing the manufacturing of very large, robust parts quickly and for cheap. Potential applications of WAAM in the nuclear sector are named in Table 3.

A comprehensive understanding of how process parameters, such as energy input, wire feed rate, welding speed and deposition pattern, influence the dimensional accuracy, thermal history, microstructure and resulting mechanical and surface properties of WAAM-produced parts remains limited [116]. Hence, this poses a key technical challenge that must be addressed.

The Oak Ridge National Laboratory (ORNL) in the USA focuses on the development of WAAM for nuclear-grade stainless steel and nickel-based alloys. In 2022, the ORNL successfully demonstrated the fabrication of functional components from 316L stainless steel using WAAM as part of a feasibility study focused on the production of spent nuclear fuel canisters. The resulting components met key mechanical and corrosion performance criteria in accordance with nuclear standards, thereby confirming the viability of WAAM for critical nuclear applications [117]. Building upon these findings, ORNL published a comprehensive report in 2023 detailing the large-scale additive manufacturing of nuclear-relevant components, including 316L stainless steel valves, pumps and impellers. The report underscores several technical challenges, notably the significant residual stresses induced by the localized heat input inherent to WAAM processes. This can lead to part distortion or cracking, issues that are especially critical in the context of pressure boundary applications in nuclear systems. To address these challenges, the report emphasizes the integration of advanced in situ monitoring tools throughout the fabrication process. These monitoring technologies are essential not only for real-time process control and quality assurance but also for the development and validation of process–structure–property relationships. Furthermore, the data acquired through in situ monitoring provides a foundational basis for the rigorous qualification and certification protocols required for the deployment of additively manufactured components in nuclear environments [118]. Complementing this, the French national research institution CAE contributed to the Europe-wide Grade2XL project (2020–2024), which was primarily aimed at the fabrication of extra-large structures via WAAM. Within this project, CAE concentrated on the development and implementation of quality assurance methodologies applicable both during the additive manufacturing process and for the evaluation of completed build structures. Notably, as a non-destructive technique for detecting anisotropic microstructural features, Resonant Ultrasound Spectroscopy was identified as a particularly promising tool [119].

3.2.2. Powder Metallurgy–Hot Isostatic Pressing

Hot isostatic pressing is an advanced manufacturing process used to densify metal powders, as well as cast or sintered components, by applying high temperature and isostatic gas pressure within a sealed furnace. Typical process conditions range from 100 to 200 MPa and temperatures between 900 °C and 1250 °C. The uniformly applied gas pressure ensures isotropic mechanical properties and facilitates near- or full-density consolidation of the material. In the case of PM-HIP, powder is compacted through pressure inside a container. HIP-manufactured parts can be found in many industry sectors including energy, transportation and aerospace, nuclear and scientific, and oil and gas [120].

Similar to DED, PM-HIP is not new for nuclear applications. Early research about using PM-HIP in the nuclear industry began in the mid-1950s, when the HIP process was invented at Battelle Memorial Institute in the United States for fuel element production [121]. A technical report published in 1960 describes the successful preparation of ceramic fuel elements using HIP [122]. Since then, PM-HIP has been used to manufacture components for nuclear power plants. For example, large valve bodies made from 316 stainless steel using PM-HIP have been successfully produced while maintaining or surpassing the mechanical properties of conventionally manufactured counterparts [121]. In general, PM-HIP is qualified as an acceptable fabrication method alongside traditional casting and forging techniques for producing ferritic and austenitic steels, as well as nickel-based alloys, primarily for non-nuclear applications under the ASME Boiler and Pressure Vessel Code Section II. According to ASME Code Case N-834, PM-HIP components fabricated from 316L are qualified to be used in non-irradiation-facing nuclear applications. Concurrently, ongoing efforts are advancing to extend this qualification to include 316L PM-HIP components intended for irradiation-facing nuclear applications. In a joint presentation by EPRI and GE Hitachi during the NRC Advanced Manufacturing Virtual Workshop in 2020, several additional alloys requiring qualification were identified. Notably, this list included low-alloy steels as well as high-performance nickel-based alloys such as Alloy 625 and Alloy 718 [123].

One challenge of extending qualifications to irradiation-facing nuclear applications is that PM-HIP components need at least comparable irradiation tolerances to forged components in order to be qualified for nuclear applications under irradiation. In a 2024 study by Jiang et al., a direct comparative analysis was conducted on SA508 Grade 3 Class 1 low-alloy reactor pressure vessel steel, fabricated both by conventional forging and by PM-HIP, subjected to two distinct neutron irradiation conditions. The results demonstrated that although the PM-HIP variant exhibited increased irradiation-induced hardening and a greater reduction in total elongation relative to the forged material, its ductility at its load-bearing capacity, reflected by comparable uniform elongation, and overall toughness remained on par with the forged counterpart. Jiang et al. concluded that the irradiation performance of PM-HIP SA508 is promising. However, they emphasized the necessity of further fracture toughness testing to fully satisfy nuclear code qualification requirements [124].

Moreover, Rolls Royce highlighted that good-quality powder fabrication, a comprehensive material test program and ensuring competitiveness are essential requirements for PM-HIP to be adopted in place of traditional manufacturing techniques in the nuclear sector [125].

3.2.3. Laser Powder Bed Fusion

The laser powder bed fusion (LPBF) process allows the fabrication of intricate geometries that are impossible or extremely difficult to produce using traditional manufacturing like conformal cooling channels for heat transfer applications [126]. Moreover, the integration of topology optimization and generative design techniques enables the efficient manufacturing of lightweight structures. The process itself is characterized by the layer-by-layer melting of metal powder particles. A thin layer of metal powder is evenly distributed on a build plate inside a closed machine set-up. Most commonly, one laser beam is projected onto the metal powder layer, selectively melting the particles according to the underlying CAD data. Repetition of this process step yields the desired 3D geometry. During this process, multiple geometries can be printed parallel. After the printing, the non-melted powder particles are extracted from the build chamber, filtered and reused [127]. While a high density can consistently be achieved using the LPBF process, several studies have shown that the microstructure, and with that the material properties, highly depends on the chosen process parameters [128,129,130,131].

Therefore, extensive testing might be necessary before producing safety-critical parts. Sewalski et al. studied the robustness of the LPBF process for fabricating a 316L specimen. After varying different process parameters, they concluded that the hardness of the samples was not influenced by this variation, but a noticeable effect on the tensile strength was observed. It was further highlighted that the change in process parameters had a lesser influence on the mechanical properties than variations in the chemical composition [132]. Furthermore, Argonne National Laboratory has been actively engaged in optimizing additive manufacturing for three key reactor materials: an austenitic stainless steel (A709) and two ferritic/martensitic steels (G91 and G92). A comprehensive report published in 2024 emphasized the critical role of tailored LPBF process parameters combined with rigorous post-process treatments to achieve the stringent mechanical and microstructural performance required for nuclear applications. Importantly, this study also demonstrated the feasibility of employing LPBF for these iron-based reactor alloys, indicating a promising pathway for their qualification and deployment within current reactor environments [133]. Li et al. emphasized the need for standards, qualification and robust testing under nuclear conditions including radiation, high temperatures and corrosive environments in order to accelerate the adoption of AM for safety-critical reactor applications [134].

Table 3 summarizes the key weight and size limitations of these advanced manufacturing techniques, along with their potential applications in the nuclear sector, highlighting the feasibility of each method for different nuclear components.

3.2.4. Standardization

In order for AMTs to be successfully implemented in the manufacturing process of structural nuclear components, they need to be included in codes and standards, such as the ASME Boiler Pressure Vessel Code (BPVC) [135]. Section III of the BPVC addresses rules for the construction of nuclear facility components. For example, work on a code case aiming to include the material 316 stainless steel manufactured using the LPBF process began in 2019 and is still ongoing [136]. Additionally, work to incorporate DED in BPVC Section III has begun too. Supplementing these efforts, the 2025 edition of the ASME Construction Codes and Product Standards is meant to include requirements for additive manufacturing (AM) processes. Further, code cases are planned to be published preceding the 2025 edition. They include AM of pressure equipment using the DED process with wire feedstock and the PBF process [137]. After attending two ASME BPVC weekly meetings in 2023, the Oak Ridge National Laboratory (ORNL) emphasized the necessity of many more code cases in order to prove the reliability and safety of the wide variety of AM nuclear components [136]. Extensive material datasets are necessary to support the incorporation of not only PM-HIP but all ATMs. However, data generation is expensive and lacks suitable standardization approaches. As a countermeasure, ASTM officially launched a Global Consortium for Material Data and Standardization (CMDS) in 2022 to accelerate adoption of AM technologies through standardization [138]. More international standards are being developed by ISO/ASTM. In 2023, the standard ISO/ASTM 52920:2023 was published. It indicates requirements for industrial AM processes and production sites and can further be used to establish a suitable quality management system [139]. One year later, in 2024, the standard ISO/ASTM 52909:2024 was published, which provides guidelines for evaluating mechanical properties of AM metal parts [140].

Table 3. Capabilities of selected AMTs and their potential nuclear applications.

AMT	Weight Limit	Size Limit	Potential Use in the Nuclear Industry
DED/WAAM	230 kg [113]	Limited by the handling system [141] structural thickness ≥ 3 mm [115]	Heavy shielding, pressure vessel sections, piping, valves, components of nozzles or heat exchanger [118]
PM-HIP	larger HIP unit: 30 t [120]	Larger HIP unit: diameter up to 2.2 m, height of appr. 4 m [120]	Larger or thicker components [121]
LPBF	23 kg [113]	Standards systems: 250 × 250 × 250 mm [141]	Fuel assembly components [113], channel fasteners, pump impellers, thimble plugging devices or valve bodies [142]

Table 3. Capabilities of selected AMTs and their potential nuclear applications.

AMT	Weight Limit	Size Limit	Potential Use in the Nuclear Industry
DED/WAAM	230 kg [113]	Limited by the handling system [141] structural thickness ≥ 3 mm [115]	Heavy shielding, pressure vessel sections, piping, valves, components of nozzles or heat exchanger [118]
PM-HIP	larger HIP unit: 30 t [120]	Larger HIP unit: diameter up to 2.2 m, height of appr. 4 m [120]	Larger or thicker components [121]
LPBF	23 kg [113]	Standards systems: 250 × 250 × 250 mm [141]	Fuel assembly components [113], channel fasteners, pump impellers, thimble plugging devices or valve bodies [142]

Current European efforts addressing the technical and standardization challenges of applying AMTs in the nuclear sector include the NUCOBAM project. Bourgeois et al. describe the project’s goal of developing qualification methodologies for additively manufactured reactor components that comply with nuclear codes (e.g., RCC-M), with promising initial results [143]. In a related study in the scope of the NUCOMAB project, Bertelle et al. evaluated an ex-core valve produced by LPBF using AISI 316L, which successfully passed non-destructive, static and burst testing, confirming its structural integrity [144]. Work published by Mally et al. demonstrated that the LPBF-produced ferritic steel 22NiMoCr3-7 closely replicates the microstructure and mechanical properties of conventionally forged material after post-process heat treatment [145]. Further research carried out by Werz et al. in that context includes multiple AMTs (LPBF, WAAM, PM-HIP, EBW and cold gas spray) and materials such as AISI 316L, Inconel 718 and 22NiMoCr3-7. The corresponding research project incorporates the assessment of respective manufacturing feasibility and the performance of advanced manufactured components under demanding conditions such as hydrogen exposure, high-temperature stress, liquid metal corrosion or dynamic loading in accident scenarios [146].

Current global activities further underscore the potential of AMTs in nuclear manufacturing. At the 2023 NRC Workshop on Advanced Manufacturing Technologies for Nuclear Applications, Rolls Royce reported the world’s first reactor vessel produced using PM-HIP and EBW, reducing the welding time for a 2 m diameter, 80 mm thick vessel from 120 days (using narrow-gap TIG welding) to just 2 days [125]. Similarly, EPRI demonstrated early-stage EBW welds for a 1.82 m diameter flange-to-cylinder assembly manufactured in just 47 min [147]. Additionally, Lincoln Electric showcased the capability of DED for producing replacement parts faster than with casting and forging. For example, manufacturing nuclear reactor door hinges using DED with wire feedstock takes less than 2 weeks instead of several months [148]. Westinghouse Electric Company is certain that development and implementation of AMTs will improve industry competitiveness. In 2020, the company installed their first additively manufactured thimble plugging device in a commercial reactor. Using, again, the PBF process, Westinghouse further produced bottom nozzles that were installed in two commercial reactors in northern Europe. According to the company’s own testing, the AM bottom nozzle performed better than existing bottom nozzles [149].

In general, AMTs play a significant role in reducing costs and enhancing the efficiency of nuclear component production, especially for SMRs and AMRs. A study published by the NEA in 2020 about construction cost reduction strategies in the nuclear sector identified eight levers for that purpose. Advanced construction and manufacturing methods were named as one of the main short-term technology approaches [150]. In 2024, the Saskatchewan Industrial and Mining Suppliers Association contracted March Consulting Associates Inc. to conduct a study in which the feasibility of AMTs for manufacturing SMRs was investigated. For this, the response of five active SMR vendors in Canada was analyzed. Listed reasons for considering AMTs in the fabrication of small and advanced modular reactors were the following: cost reductions, schedule reductions, faster turnaround to the market, improved product features and increased quality. The interest of Canadian SMR vendors in using selected AMTs for current or future nuclear designs is displayed in Figure 15. This study highlights in general the importance of implementing AMTs for cost-saving and efficiency improvements in SMR manufacturing [151].

4. Discussion

This review underscores the evolving landscape of nuclear power, emphasizing the critical role of advanced reactor designs and innovative manufacturing technologies in enabling a low-carbon future. As the global push toward net-zero emissions by 2050 accelerates, next-generation nuclear technologies are often cited as promising solutions to overcome the limitations of conventional light water reactors (LWRs). Generation IV systems sum up the most promising next-generation technologies, including VHTRs, MSRs, SFRs, LFRs, GFRs and SCWRs. However, many aspects of these systems, such as the development of advanced nuclear fuels, remain under active research. As a result, the timeline for commercial deployment is uncertain. Additionally, the authors stress the need to raise public awareness. Addressing key concerns such as nuclear waste and cost will be essential to maintaining public support. Whether these advanced technologies become established will depend heavily on political will and the willingness of the industry to invest in unproven systems.

Beyond Gen IV reactors, advanced concepts like small modular reactors (SMRs) and advanced modular reactors (AMRs) are receiving increased attention. These designs are engineered for improved safety, operational flexibility and siting versatility. SMRs and microreactors, in particular, introduce modular construction, smaller physical footprints, passive safety features and scalability. These qualities make them attractive for remote locations, off-grid areas and integration with variable renewable energy sources. The authors agree that smaller reactors are well justified, especially given the impracticality of replacing fossil-fueled generators in remote regions with full-scale nuclear power plants. However, most SMR designs remain in early development, and real-world performance data is limited. Their economic viability depends on mass production, supply chain readiness and regulatory alignment. Although political interest has helped accelerate their development, the authors believe conventional LWRs will continue to play a central role in national energy strategies. This is reflected by many countries prioritizing the long-term operation and life extension of existing NPPs.

Advanced manufacturing techniques (AMTs) such as directed energy deposition (DED), powder metallurgy–hot isostatic pressing (PM-HIP) and laser powder bed fusion (LPBF) have emerged as enablers for modern reactor construction, especially for SMRs and AMRs. These methods offer reduced lead times, increased design flexibility and potential cost savings. DED, including WAAM, enables rapid production of large components, yet challenges such as residual stress and dimensional accuracy persist, especially in pressure-retaining applications. While PM-HIP provides near-net-shape parts with isotropic properties, its qualification for irradiation-facing environments is ongoing. Similarly, LPBF enables intricate geometries but exhibits sensitivity to process parameters and material feedstock, requiring extensive testing. Despite these hurdles, the widespread research on and industrial activity toward optimizing AMTs and qualifying materials underscore their potential. The authors view 316L stainless steel as especially promising, given its current qualification for non-irradiation-facing components and broad applicability across AMTs. Expanding its qualification for irradiation-facing applications would be a meaningful step toward mainstream AMT adoption in nuclear manufacturing.

The integration of AMTs into advanced reactor construction introduces novel safety, regulatory and quality assurance challenges. While institutions like the IAEA and NEA are assessing the applicability of current safety standards, important gaps remain—particularly regarding multi-unit modular designs and probabilistic safety assessments. Encouraging demonstration projects, such as Westinghouse’s LPBF-produced in-core components and ORNL’s WAAM-fabricated spent fuel canisters, will confirm the feasibility of AMTs in safety-relevant contexts. Nonetheless, widespread deployment will require harmonized regulatory frameworks, standardized qualification protocols and robust material data. The convergence of advanced reactors with AMTs holds substantial promise for accelerating the energy transition, provided that technical, regulatory and public acceptance challenges are proactively addressed.

5. Conclusions

This state-of-the-art review analyzes the progression of nuclear energy systems by systematically evaluating technical aspects of conventional and advanced reactor designs in parallel with emerging advanced manufacturing techniques (AMTs), which are increasingly integral to their design optimization, component fabrication and deployment feasibility. As the global demand for reliable, low-carbon energy grows, the role of nuclear power becomes ever more critical, particularly in the context of achieving long-term decarbonization goals. Generation IV reactor systems, along with small modular reactors (SMRs) and advanced modular reactors (AMRs), offer promising solutions for overcoming the technical and logistical limitations of conventional light water reactors. These advanced designs feature improved safety, operational flexibility, scalability and compatibility with diverse deployment scenarios. This work highlights the significance of innovation in manufacturing as a key contributor for the successful implementation of these reactor concepts. The analyzed literature shows that innovative fabrication methods are thought of as a transformative tool for the nuclear sector. These AMTs not only significantly reduce production lead times and costs but also enable the fabrication of complex, high-performance components with enhanced material efficiency. Furthermore, they provide new pathways for producing custom, modular and site-adapted components critical to the deployment of SMRs and AMRs in remote or decentralized environments. Three of the AMTs said to be deployed earliest in the nuclear sector by the NEI are reviewed in more detail, including directed energy deposition, metallurgy–hot isostatic pressing and laser powder bed fusion. A short overview of these technologies, complemented with important manufacturing constraints and their possible nuclear applications, is integrated in this review. Despite ongoing research and successful demonstrations, several challenges still require resolution. Key issues include material qualification for irradiation-facing applications, dimensional stability, residual stress management and the integration of in situ monitoring systems. Moreover, regulatory frameworks and nuclear codes continue to lag behind these technological advancements. The lack of standardized data and harmonized qualification protocols limits the broader adoption of AMTs across the nuclear supply chain. Nevertheless, the growing involvement of international research institutions, industry stakeholders and standardization bodies underscores the increasing momentum behind these technologies. Materials such as 316L stainless steel, which is already partially qualified, demonstrate the feasibility of transitioning AMTs from prototyping to commercial reactor applications. Continued investment in material testing, data standardization and code development proves essential to unlocking the full potential of AMTs. In conclusion, the synergy between advanced reactor concepts and state-of-the-art manufacturing techniques presents a powerful opportunity to modernize nuclear energy systems. If technological, regulatory and public acceptance challenges are effectively addressed, this convergence holds the potential to significantly accelerate the global transition to a carbon-free yet resilient energy infrastructure.