# Small modular reactor

The small modular reactor (SMR) is a class of small nuclear fission reactor, designed to be built in a factory, shipped to operational sites for installation and then used to power buildings or other commercial operations. The term SMR refers to the size, capacity and modular construction. Reactor type and the nuclear processes may vary. Of the many SMR designs, the pressurized water reactor (PWR) is the most common. However, recently proposed SMR designs include: generation IV, thermal-neutron reactors, fast-neutron reactors, molten salt, and gas-cooled reactor models.
Commercial SMRs have been designed to deliver an electrical power output as low as 5 MW$_e$ (electric) and up to 300 MW$_e$ per module. SMRs may also be designed purely for desalinization or facility heating rather than electricity. These SMRs are measured in megawatts thermal MW$_t$. Many SMR designs rely on a modular system, allowing customers to simply add modules to achieve a desired electrical output.
Small reactors were first designed mostly for military purposes in the 1950s to power ballistic missile submarines and ships (aircraft carriers and ice breakers) with nuclear propulsion. There has been strong interest from technology corporations in using SMRs to power data centers.
The electrical output for modern naval reactors are generally limited to less than 165 MW$_e$ and dedicated to powering turboshaft props rather than delivering electricity. In addition, there are many more safety controls absent from naval reactors due to the space limitations these reactors were designed for.
Modular reactors are expected to reduce on-site construction and increase containment efficiency. These reactors are also expected to enhance safety by using passive safety features that do not require human intervention, although this is not specific to SMRs but rather a characteristic of most modern reactor designs. SMRs are also claimed to have lower power plant staffing costs, as their operation is fairly simple, and are claimed to have the ability to bypass financial and safety barriers that inhibit the construction of conventional reactors.
Working with Oregon State University (OSU), NuScale Power developed the first Nuclear Regulatory Commission approved model for the US market in 2022.

## Operational SMRs

As of 2024, only China and Russia have successfully built operational SMRs. There are more than 80 modular reactor designs under development in 19 countries. Russia has been operating a floating nuclear power plant Akademik Lomonosov, in Russia's Far East (Pevek), commercially since 2020. The floating plant is the first of its kind in the world. China's pebble-bed modular high-temperature gas-cooled reactor HTR-PM was connected to the grid in 2021.

## Background

The term was brought to wider use when U.S. Secretary of Energy Steven Chu identified "small modular reactors" as "America's new nuclear option" in a 2010 Wall Street Journal op-ed, where he stated "SMRs would be ready to 'plug and play' upon arrival [on site]" and be more affordable. He announced that President Obama had requested $39 million for a new SMR design and licensing program.

### Hope of enhanced safety and reduced costs

Economic factors of scale mean that nuclear reactors tend to be large, to such an extent that size itself becomes a limiting factor. The 1986 Chernobyl disaster and the 2011 Fukushima nuclear disaster caused a major set-back for the nuclear industry, with worldwide suspension of development, cutting down of funding, and closure of reactor plants.
In response, researchers at Oregon State University developed the first commercial SMR prototypes in the late 1990s and early 2000s. A radically different reactor than the one used by the military, OSU's SMR design decreased fabrication time, improved operational safety, and reduced the cost of operation. The goal, of course, was to make it easier for commercial and public entities to afford a traditionally cost-prohibitive form of energy. Credited as the inventor of the commercial SMR, OSU researchers believed the smaller form factor and modular design would allow manufacturers to swap economies-of-unit-scale for economies-of-unit-mass-production - lowering production costs and improving manufacturing efficiency. NuScale Power partnered with OSU to become the first to apply this manufacturing strategy starting in 2006
Proponents claim that SMRs would be less expensive due to the application of standardized modules that could be industrially produced off-site in a dedicated factory. SMRs do, however, also have economic disadvantages. Several studies suggest that the overall costs of SMRs are comparable with those of conventional large reactors. Moreover, extremely limited information about SMR modules transportation has been published. Critics say that modular building will only be cost-effective for a high number of the same SMR type, given the still remaining high costs for each SMR. A high market share is thus needed to obtain sufficient orders.

### Contribution to the net zero emissions pathways

In February 2024, the European Commission recognized SMR technology as an important contributor to decarbonization as part of EU Green Deal.
In its pathway to reach global net zero emissions by 2050, the International Energy Agency (IEA) considers that worldwide nuclear power should be multiplied by two between 2020 and 2050. Antonio Vaya Soler, an expert from the Nuclear Energy Agency (NEA), agrees that although renewable energy is essential to fight global warming, it will not be sufficient to achieve net zero CO$_2$ emissions and nuclear energy capacity should be at least doubled.
To produce the same electrical power as the ~ 400 large nuclear power reactors in the world today, BASE, the German Federal Office for the Safety of Nuclear Waste Management, warns that it would be necessary to build several thousands to tens of thousands of SMRs.
Several fleets of SMRs of exactly the same type, industrially manufactured in large numbers, should be rapidly deployed worldwide to significantly reduce emissions of CO$_2$. The Nuclear Energy Agency (NEA) launched at COP 28 an initiative Accelerating SMRs for Net Zero to foster collaboration between research organizations, nuclear industry, safety authorities, and governments, in order to reduce carbon emissions to net zero before 2050 to limit global surface temperature increase.

### Future challenges

Proponents say that nuclear energy with proven technology can be safer; the nuclear industry contends that smaller size will make SMRs even safer than larger conventional plants. This is because the main problem associated with nuclear meltdowns is the decay heat that is present after reactor shutdown, which would be much lower for SMRs because of their lower power output. Critics say that many more small nuclear reactors pose a higher risk, requiring more transportation of nuclear fuel and also increasing the production of radioactive waste. SMRs require new designs with new technology, the safety of which has yet to be proven.
Until 2020, no truly modular SMRs had been commissioned for commercial use. In May 2020, the first prototype of a floating nuclear power plant with two 30 MW$_e$ reactors – the type KLT-40 – started operation in Pevek, Russia. This concept is based on the design of nuclear icebreakers. The operation of the first commercial land-based, 125 MW$_e$ demonstration reactor ACP100 (Linglong One) is due to start in China by the end of 2026.

## Designs

SMRs are envisioned in multiple designs. Some are simplified versions of current reactors, others involve entirely new technologies. All proposed SMRs use nuclear fission with designs including thermal-neutron reactors and fast-neutron reactors.

### Thermal-neutron reactors

Thermal-neutron reactors rely on a moderator (water, graphite, beryllium...) to slow neutrons and generally use $^{235}$ U as fissile material. Most conventional operating reactors are of this type.

### Fast reactors

Fast reactors don't use moderators. Instead they rely on the fuel to absorb fast neutrons. This usually means changing the fuel arrangement within the core, or using different fuels. E.g., $^{239}$ Pu is more likely to absorb a fast neutron than $^{235}$ U.
Fast reactors can be breeder reactors. These reactors release enough neutrons to transmute non-fissionable elements into fissionable ones. A common use for a breeder reactor is to surround the core by a "blanket" of $^{238}$ U, the most easily available isotope. Once the $^{238}$ U undergoes a neutron absorption reaction, it becomes $^{239}$ Pu, which can be removed from the reactor during refueling, and subsequently reprocessed and used as fuel.

## Technologies
### Coolant

Conventional light-water reactors typically use water as a coolant and neutron moderator. SMRs may use water, liquid metal, gas and molten salt as coolants. Coolant type is determined based on the reactor type, reactor design, and the chosen application. Large-rated reactors primarily use light water as coolant, allowing for this cooling method to be easily applied to SMRs. Helium is often elected as a gas coolant for SMRs because it yields a high plant thermal efficiency and supplies a sufficient amount of reactor heat. Sodium, lead, and lead-bismuth eutectic (LBE) are liquid metal coolants studied for 4th generation SMRs. There was a large focus on sodium during early work on large-rated reactors which has since carried over to SMRs to be a prominent choice as a liquid metal coolant. SMRs have lower cooling water requirements, which expands the number of sites where a SMR could be built, including remote areas typically incorporating mining and desalination.

### Thermal/electrical generation

Some gas-cooled reactor designs could drive a gas turbine, rather than boiling water, such that thermal energy can be used directly. Heat could also be used in hydrogen production and other industrial operations, such as desalination and the production of petroleum derivative (extracting oil from oil sands, making synthetic oil from coal, etc.).

### Load following

SMR designs are generally expected to provide base load electrical power; some proposed designs are aimed to adjust their power output based on electricity demand.
Another approach, especially for SMRs designed to provide high temperature heat, is to adopt cogeneration, maintaining consistent heat output, while diverting otherwise unneeded heat to an auxiliary use. District heating, desalination and hydrogen production have been proposed as cogeneration options.
Overnight desalination requires sufficient freshwater storage capacity to deliver water at times other than when it is produced. Reverse osmosis membrane and thermal evaporators are the two main techniques for seawater desalination. The membrane desalination process uses only electricity to power water pumps and is the most employed of the two methods. In the thermal process, the feed water stream is evaporated in different stages with continuous decreases in pressure between the stages. The thermal process directly uses thermal energy and avoids the conversion of thermal power into electricity. Thermal desalination is further divided into two main technologies: the multi-stage flash distillation (MSF) and the Multi-Effect Desalination (MED).

## Nuclear safety

A report by the German Federal Office for the Safety of Nuclear Waste Management (BASE) considering 136 different historical and current reactors and SMR concepts stated: "Overall, SMRs could potentially achieve safety advantages compared to power plants with a larger power output, as they have a lower radioactive inventory per reactor and aim for a higher safety level especially through simplifications and an increased use of passive systems. In contrast, however, various SMR concepts also favour reduced regulatory requirements, for example, with regard to the required degree of redundancy or diversity in safety systems. Some developers even demand that current requirements be waived, for example in the area of internal accident management or with reduced planning zones, or even a complete waiver of external emergency protection planning. Since the safety of a reactor plant depends on all of these factors, based on the current state of knowledge it is not possible to state, that a higher safety level is achieved by SMR concepts in principle."
Negative temperature coefficients in the moderators and the fuels keep the fission reactions under control, causing the reaction to slow as temperature increases. After the shutdown of a nuclear reactor, the reactor needs to be cooled continuously in order to dissipate decay heat. A loss of emergency cooling such as in the Fukushima nuclear accident and the Three Mile Island accident can result in a nuclear meltdown when the temperature in the reactor becomes too high. Since the initial decay heat is a fraction of the reactor operating power, the lower operating power of SMRs makes them much safer since less heat needs to be dissipated.
Some SMR designs proposes cooling systems only based on thermoconvection – natural circulation – to eliminate cooling pumps that could break down. Convection can keep removing decay heat after reactor shutdown. However, some SMRs may need an active cooling system to back up the passive system, increasing cost.
Some SMR designs feature an integral design of which the primary reactor core, steam generator and the pressurizer are integrated within the sealed reactor vessel. This integrated design allows for the reduction of a possible accident as contamination leaks could be contained. In comparison to larger reactors having numerous components outside the reactor vessel, this feature increases the safety by decreasing the risks of an uncontained accident. Some SMR designs also envisage to install the reactor and the spent-fuel storage pools underground.

## Radioactive waste

The backend of the nuclear fuel cycle of SMR is a complex and challenging issue remaining disputed. The quantity and the radiotoxicity of the radioactive waste produced by SMR mainly depend on their design and the related fuel cycle. As the notion of SMR encompasses a broad spectrum of nuclear reactor types, no simple answer can be easily given to the question. SMR may comprise small light water reactors of third generation as well as small fast neutron reactors of fourth generation.
Often, the startup companies developing unconventional SMR prototypes advocate waste reduction as an advantage of the proposed solution and even sometimes claim that their technology could eliminate the need for a deep geological repository to dispose of high-level and long-lived radioactive waste. This is especially the case for companies studying fast neutron reactors of 4th generation (molten salts reactors, metal-cooled reactors (sodium-cooled fast reactor, or lead-cooled fast reactor).
Fast breeder reactors "burn" $^{235}$ U (0.7% of natural uranium), but also convert fertile materials such as $^{238}$ U (99.3% of natural uranium) into fissile $^{239}$ Pu that can be used as nuclear fuel.
The traveling wave reactor proposed by TerraPower is aimed to immediately "burn" the fuel that it breeds without requiring its removal from the reactor core and its further reprocessing.
The design of some SMR reactors is based on the thorium fuel cycle, which is considered by their promotors as a way to reduce the long-term waste radiotoxicity compared to the uranium cycle. However, using the thorium cycle also presents big operational challenges because of the production and the use of $^{232}$ U and long-lived fertile $^{233}$ U, both radioisotopes emitting strong gamma rays. So, the presence of these radionuclides seriously complicates the radiation shielding of the fresh nuclear fuel and the long-term storage and disposal of their spent nuclear fuel.
A study of 2022 made by Krall, Macfarlane and Ewing is more critical and reports that some types of SMR could produce more waste per unit of output power than conventional reactors, in some cases more than 5× the amount of spent fuel per kilowatt, and as much as 35× for other waste produced by neutron activation, such as activated steel and graphite.
These authors have identified the neutron leakage as the first issue for SMRs because they have a higher surface area with respect to their core volume. They have calculated that the neutron leakage rates are much higher for SMRs, because in smaller reactor cores, emitted neutrons have fewer chances to interact with the fissile atoms present in the fuel and to produce nuclear fission. Instead, neutrons exit the reactor core without interacting with the nuclear fuel, and they are absorbed outside the core by the materials used for the neutron reflectors and the shielding (thermal and gamma shields), turning them as radioactive waste (activated steel and graphite).
Reactor designs using liquid metal coolants (molten sodium, lead, lead-bismuth eutectic, LBE) also become radioactive and contains activated impurities.
Another issue pinpointed by Krall et al. (2022) related to the higher neutron leakage in SMR is that a lower fraction of their nuclear fuel is consumed, leading to a lower burnup and to more fissile materials left over in their spent fuel, therefore increasing the waste volume. To sustain the nuclear chain reactions in the core of a smaller reactor, an alternative is to use nuclear fuel more enriched in $^{235}$ U. This could increase the risks of nuclear proliferation and could require more stringent safeguard measures to prevent it (see also IAEA safeguards).
If higher concentrations of fissile materials subsist in the spent fuel, the critical mass needed to sustain a nuclear chain reaction is also lower. As a direct consequence, the number of spent fuels present in a waste canister will also be lower and a larger number of canisters and overpacks will be necessary to avoid criticality accidents and to guarantee nuclear criticality safety in a deep geological repository. This also contributes to increase the total waste volume and the number of disposal galleries in a geological repository.
Given the potential technical and economical importance of SMRs to supply zero-carbon electrical energy needed to fight climate change and the long-term and social relevance of the study to adequately manage and dispose of radioactive waste without imposing a negative burden onto the future generations, the publication of Krall et al. (2022) in the prestigious PNAS journal has attracted many reactions ranging from criticisms on the quality of their data and hypotheses to international debates on radioactive waste produced by SMRs and their decommissioning.
In an interview with François Diaz-Maurin, the associate editor of the Bulletin of the Atomic Scientists, Lindsay Krall, the lead author of the study and a former MacArthur postdoctoral fellow at Stanford's Center for International Security and Cooperation (CISAC) answered to questions and criticisms, amongst others, those raised by the NuScale reactor company. One of the main concerns Krall expressed in this interview is that:
    "There's definitely a disconnect between the people working on the back end of the fuel cycle—especially with geologic repository development—and those actually designing reactors. And, there is not a lot of motivation for these reactor designers to think about the geologic disposal aspects because the NRC's new reactor design certification application does not have a chapter on geologic disposal..."
The critical study of Krall et al. (2022) has the merit to have raised relevant questions that cannot be ignored by reactor designers, or decision-makers, and to have triggered open and fresh discussions on important outcomes for SMRs and radwaste management in general. Amongst the various types of SMR projects initiated today by many start-up companies, only those correctly addressing these questions and really contributing to minimize the radioactive waste they produce have a chance to be supported by the public and governmental organisations (nuclear safety authorities and radioactive waste management organisations) and their research to be funded by long-term national policies.
The high diversity of SMR reactors and their respective fuel cycles may also require more diverse waste management strategy to recycle, or to safely dispose, their nuclear waste. A larger number of spent fuel types will be more difficult to manage than only one type as it is presently the case with light water reactors only.
As previously stressed by Krall and Macfarlane (2018), some types of SMR spent fuels, or coolants, (highly reactive and corrosive uranium fluoride (UF$_4$) from molten salt reactors, or pyrophoric sodium from liquid metal cooled fast breeders) cannot be directly disposed of in a deep geologic repository because of their chemical reactivity in the underground environment (deep clay formations, crystalline rocks, or rock salt). To avoid to exacerbate spent fuel storage and disposal issues it will be mandatory to reprocess and to condition them in an appropriate and safe way before final geological disposal.
A study made by Keto et al. (2022) at the VTT Technical Research Centre of Finland also addressed the management of spent nuclear fuel (SNF) and low- and intermediate-level waste (LILW) from a possible future deployment of SMRs in Finland. It also indicates that larger masses (per GWe-year) of SNF and other HLW and larger volumes (per GWe-year) of LLW would be produced by a light water SMR compared to a large NPP.
A report by the German Federal Office for the Safety of Nuclear Waste Management (BASE) found that extensive interim storage and fuel transports are still required for SMRs. A deep geological repository is still unavoidable in any case because of the presence of highly mobile long-lived fission products that, due to their too low neutron cross section, cannot be efficiently transmuted, as it is the case with dose-dominating radionuclides such as $^{129}$ I, $^{99}$ Tc and $^{79}$ Se (soluble anions that are not sorbed onto the negatively charged minerals and are not retarded in geological media).

## Nuclear proliferation

Nuclear proliferation, or the use of nuclear materials to create weapons, is a concern for small modular reactors. As SMRs have lower generation capacity and are physically smaller, they are intended to be deployed in many more locations than conventional plants. SMRs are expected to substantially reduce staffing levels. The combination creates physical protection and security concerns.
SMRs can be designed to use unconventional fuels allowing for higher burnup and longer fuel cycles. Longer refueling intervals could contribute to decrease the proliferation risks. Once the fuel has been irradiated, the mixture of fission products and fissile materials is highly radioactive and requires special handling, preventing casual theft.
Contrasting to conventional large reactors, SMRs can be adapted to be installed in a sealed underground chamber; therefore, "reducing the vulnerability of the reactor to a terrorist attack or a natural disaster". New SMR designs enhance the proliferation resistance, such as those from the reactor design company Gen4. These models of SMR offer a solution capable of operating sealed underground for the life of the reactor following installation.
Some SMR designs are designed for one-time fueling. This improves proliferation resistance by eliminating on-site nuclear fuel handling and means that the fuel can be sealed within the reactor. However, this design requires large amounts of fuel, which could make it a more attractive target. A 200 MWe 30-year core life light water SMR could contain about 2.5 tonnes of plutonium at end of life.
Furthermore, many SMRs offer the ability to go periods of greater than 10 years without requiring any form of refueling therefore improving the proliferation resistance as compared to conventional large reactors of which entail refueling every 18–24 months.
Light-water reactors designed to run on thorium offer increased proliferation resistance compared to the conventional uranium cycle, though molten salt reactors have a substantial risk.
SMRs are transported from the factories without fuel, as they are fueled on the ultimate site, except some microreactors. This implies an independent transport of the fuel to the site and therefore increases the risk of nuclear proliferation.

## Licensing process

Licensing is an essential process required to guarantee the safety, security and safeguards of a new nuclear installation. Only NuScale Power's VOYGR SMR is fully licensed for use in the United States. However, not all countries follow the NRC or IAEA licensing standards. In the United States and IAEA adhering countries, the licensing is based on a rigorous, independent analysis and reviewing work of all structures, systems and components critical for the nuclear safety under normal and accidental conditions on the whole service life of the installation including the long-term management of radioactive waste. Licensing is based on the examination and scrutiny of the risk assessment studies and safety files elaborated by the fabricant and the exploitant of the SMR in the frame of the safety case they have to submit to the safety authority (regulatory body) when applying for a licence to construct and safely exploit the installation. For NRC and IAEA licensing, the safety and feasibility cases of nuclear installations have to take into account all processes and elements important for the operational safety, its security (access protection), the nuclear safeguard (risk of proliferation), the proper conditioning of radioactive waste under a stable physico-chemical form, and the long-term safety related to the final disposal of the different types of radwaste produced, including all the waste produced during dismantling operations after decommissioning of the installation. A particularly important point of attention for the backend of the nuclear fuel cycle is to avoid to producing poorly conditioned waste, or waste types without sustainable final destination or susceptible to generating unexpected reprocessing and disposal costs.
The most common licensing process, applied by existing commercial reactors, is for the operation of light water reactors (PWR and BWR). Early designs for large-scale reactors date back to the 1960s and 1970s during the construction of the nuclear reactor fleet currently in service. Some adaptations of the original licensing process by the US's Nuclear Regulatory Commission (NRC) have been repurposed to better correspond to the specific characteristics and needs of the deployment of SMR units. In particular, the US Nuclear Regulatory Commission process for licensing has focused mainly on conventional reactors. Design and safety specifications, human and organizational factors (including staffing requirements) have been developed for reactors with electrical output of more than 700 MWe.
To ensure adequate guidelines for the nuclear safety, while helping the licensing process, the IAEA has encouraged the creation of a central licensing system for SMRs. A workshop in October 2009 and another in June 2010 considered the topic, followed by an US congressional hearing in May 2010.
The NRC and the United States Department of Energy) are working to define SMR licensing. The challenge of facilitating the development of SMRs is to prevent a weakening of the safety regulations: the risk of lightened regulations adopted more rapidly is to lower the safety characteristics of SMRs. While deploying identical systems built in manufacturing plants with an improved quality control can be considered an advantage, SMRs remain nuclear reactors with a very high energy density and their smaller size is not per se an intrinsic guarantee for a better safety. Any severe accident with external radioactive contamination release could have potential serious consequences not so different from that of a large LWR reactor. It would also probably signify the final rejection of nuclear energy by the public and the end of the nuclear industry. The potential "proliferation" of large SMR fleets and the high diversity of their design also complicate the licensing process. The nuclear safety cannot be sacrificed for industrial or economical interests and the risk of nuclear accident increases with the number of reactors in service, small or large unit.
The U.S. Advanced Reactor Demonstration Program was expected to help license and build two prototype SMRs during the 2020s, with up to $4 billion of government funding.
In July 2024, the ADVANCE Act directed the United States Nuclear Regulatory Commission to develop a process to license and regulate microreactor designs. The Act is intended to expedite the deployment of microreactors, among other nuclear technologies.

## Flexibility

Small nuclear reactors, in comparison to conventional nuclear power plants, offer potential advantages related to the flexibility of their modular construction. It would be possible to incrementally connect additional units to the grid in the event electrical load increases. Additionally, this flexibility in a standardized SMRs design revolving around modularity could allow for a faster production at a decreasing cost following the completion of the first reactor on site.
The hypothesised flexibility and modularity of SMR is intended to allow additional power generation capability to be installed at existing power plants. A site could host several SMRs, one going off-line for refueling while the other reactors stay online as it is presently already the case for conventional larger reactors.
When electrical energy is not needed, some SMR designs foresee the direct use of thermal energy, minimizing so the energy loss. This includes "desalination, industrial processes, hydrogen production, shale oil recovery, and district heating", uses for which the present conventional larger reactors are not designed.

## Economics

A key driver of interest in SMRs is the claimed economies of scale in production, due to volume manufacture in an offsite factory. Some studies instead find the capital cost of SMRs to be equivalent to larger reactors. Substantial capital is needed to construct the factory – ameliorating that cost requires significant volume, estimated to be 40–70 units.
Another potential advantage is that a future power station using SMRs can begin with a single module and expand by adding modules as demand grows. This reduces startup costs associated with conventional designs. Some SMRs also have a load-following design such that they could produce less electricity when demand is low.
According to a 2014 study of electricity production in decentralized microgrids, the total cost of using SMRs for electricity generation would be significantly lower compared to the total cost of offshore wind power, solar thermal energy, biomass, and solar photovoltaic electricity generation plants.
Construction costs per SMR reactor were claimed in 2016 to be less than that for a conventional nuclear plant, while exploitation costs might be higher for SMRs due to low scale economics and the higher number of reactors. SMR staff operating costs per unit output can be as much as 190% higher than the fixed operating cost of fewer large reactors. Modular building is a very complex process and there is "extremely limited information about SMR modules transportation", according to a 2019 report.
A production cost calculation done by the German Federal Office for the Safety of Nuclear Waste Management (BASE), taking into account economies of scale and learning effects from the nuclear industry, suggests that an average of 3,000 SMR would have to be produced before SMR production would be worthwhile. This is because the construction costs of SMRs are relatively higher than those of large nuclear power plants due to the low electrical output.
In 2017, an Energy Innovation Reform Project (EIRP) study of eight companies looked at reactor designs with capacity between 47.5 MWe and 1,648 MWe. The study reported average capital cost of $3,782/kW, average operating cost total of $21/MWh and levelized cost of electricity (LCOE) of $60/MWh.
In 2020, Energy Impact Center founder Bret Kugelmass claimed that thousands of SMRs could be built in parallel, "thus reducing costs associated with long borrowing times for prolonged construction schedules and reducing risk premiums currently linked to large projects". GE Hitachi Nuclear Energy Executive Vice President Jon Ball agreed, saying the modular elements of SMRs would also help reduce costs associated with extended construction times.
In October 2023, an academic paper published in Energy collated the basic economic data of 19 more developed SMR designs, and modeled their costs in a consistent manner. A Monte Carlo simulation showed that none were profitable or economically competitive. For the closer to market PWR SMRs the median LCOEs ranged from $218/MWh to $614/MWh (in 2020 US dollars), with lower first quartile estimates from $188/MWh to $385/MWh. The three high-temperature gas-cooled reactor designs, which needed more development time, had lower median LCOEs from $116/MWh to $137/MWh.
The first SMR deployment project in the US was the Carbon Free Power Project, which planned to deploy six 77 MWe NuScale reactors, reduced from twelve in earlier plans. Estimated target electricity generation price after subsidies was $89/MWh in 2023, an increase from $58/MWh in 2021. The increased generation cost led to the decision to cancel the project in November 2023. Before its cancellation, the project received a $1.355 billion cost-share award toward construction costs from the US government in 2020 plus an estimated $30/MWh generation subsidy from the Inflation Reduction Act of 2022. Unsubsidized cost estimates at cancellation were a capital cost of $20,139/kW and generating cost of $119/MWe. This raised concerns about the commercial prospects in the U.S. of the other SMR designs.
In 2024, Australian scientific research body CSIRO estimated that electricity produced in Australia by a SMR constructed from 2023 would cost roughly 2.5 times that produced by a traditional large nuclear plant, falling to about 1.6 times by 2030.

## List of reactor designs

Numerous reactor designs have been proposed. Notable SMR designs:
Legend :
| Designed or under design | Seeking license | Licensed in one or more countries | Under construction |
| Operational | Canceled | Retired | |
The stated power refers to the capacity of one reactor unless specified otherwise.
List of small nuclear reactor designs :
| Name | Gross power (MW$_e$) | Type | Producer | Country | Status |
| 4S | 10–50 | SFR | Toshiba | Japan | Design (Detailed) |
| ABV-6 | 6–9 | PWR | OKBM Afrikantov | Russia | Design (Detailed) |
| ACP100 Linglong One | 125 | PWR | China National Nuclear Corporation | China | Under construction |
| AP300 | 300 | PWR | Westinghouse Electric Company | United States | Design (Detailed) |
| ARC-100 | 100 | SFR | ARC Nuclear | Canada | Design (Vendor Review) |
| ANGSTREM | 6 | LFR | OKB Gidropress | Russia | Design (Conceptual) |
| B&W mPower | 195 | PWR | Babcock & Wilcox | United States | Cancelled |
| BANDI-60 | 60 | PWR | KEPCO | South Korea | Design (Detailed) |
| BREST-OD-300 | 300 | LFR | Atomenergoprom | Russia | Under construction |
| BWRX-300 | 300 | BWR | GE Hitachi Nuclear Energy | United States/Japan | Design (Pre-licensing communications with the US NRC initiated.) |
| CANDU SMR | 300 | PWR (Heavy) | Candu Energy Inc. | Canada | Design (Conceptual) |
| CAP200 | >200 | PWR | SPIC | China | Design (Completion) |
| CAREM | 27–30 | PWR | CNEA | Argentina | Under construction |
| Copenhagen Atomics Waste Burner | 50 | MSR | Copenhagen Atomics | Denmark | Design (Conceptual) |
| DHR400 | 400 (non-electric) | PWR | CNCC | China | Design (Basic) |
| ELENA | 0.068 | PWR | Kurchatov Institute | Russia | Design (Conceptual) |
| Energy Well | 8.4 | MSR | cs:Centrum výzkumu Řež | Czechia | Design (Conceptual) |
| eVinci | 5 | HPR | Westinghouse Electric Company | United States | Design (Pre-licensing communications with the US NRC initiated.) |
| Flexblue | 160 | PWR | Areva TA / DCNS group | France | Design (Conceptual) |
| Fuji MSR | 200 | MSR | International Thorium Molten Salt Forum (ITMSF) | Japan | Design (Conceptual) |
| GT-MHR | 285 | GTMHR | OKBM Afrikantov | Russia | Design (Completed) |
| G4M | 25 | LFR | Gen4 Energy | United States | Design (Conceptual) (Company Ceased Trading) |
| GT-MHR | 50 | GTMHR | General Atomics, Framatome | United States/France | Design (Conceptual) |
| HAPPY200 | 200 MWt | PWR | SPIC | China | Design (Conceptual) |
| HTMR-100 | 35 | GTMHR | Stratek Global | South Africa | Design (Conceptual) |
| HTR-PM | 210 (2 reactors one turbine) | HTGR | China Huaneng | China | Operational (Single reactor. Station connected to the grid in December 2021.) |
| IMSR400 | 195 (x2) | MSR | Terrestrial Energy | Canada | Design (Detailed) |
| IRIS | 335 | PWR | Westinghouse-led | International | Design (Basic) |
| i-SMR | 170 | PWR | Innovative Small Modular Reactor Development Agency (KHNP and KAERI) | South Korea | Design (Basic) |
| KLT-40S Akademik Lomonosov | 70 | PWR | OKBM Afrikantov | Russia | Operational May 2020 (floating plant) |
| Last Energy | 20 | PWR | Last Energy | United States | Design (Conceptual) |
| MMR | 5-15 | HTGR | Ultra Safe Nuclear Corporation [d] purchased by NANO Nuclear Energy | United States/Canada | Company filed for Chapter 11 bankruptcy. Had been seeking licensing Design acquired by Nano Nuclear Energy. |
| MCSFR | 50–1000 | MCSFR | Elysium Industries | United States | Design (Conceptual) |
| MHR-100 | 25–87 | HTGR | OKBM Afrikantov | Russia | Design (Conceptual) |
| MHR-T | 205.5 (x4) | HTGR | OKBM Afrikantov | Russia | Design (Conceptual) |
| MRX | 30–100 | PWR | JAERI | Japan | Design (Conceptual) |
| NP-300 | 100–300 | PWR | Areva TA | France | Design (Conceptual) |
| Nuward | unknown | PWR | consortium | France | Design (Conceptual). In July 2024, existing design discontinued for a simpler redesign. |
| OPEN100 | 100 | PWR | Energy Impact Center | United States | Design (Conceptual) |
| PBMR-400 | 165 | HTGR | Eskom | South Africa | Cancelled - demonstration plant postponed indefinitely |
| RITM-200N | 55 | PWR | OKBM Afrikantov | Russia | Under construction |
| RITM-200S | 106 | PWR | OKBM Afrikantov | Russia | Under construction |
| Rolls-Royce SMR | 470 | PWR | Rolls-Royce | United Kingdom | Seeking UK GDA licensing in April 2022 A 16-month assessment was started in April 2023 |
| SEALER | 55 | LFR | Blykalla [sv] | Sweden | Design |
| SHELF-M | 10 | PWR | NIKIET | Russia | Design |
| SMART100 | 110 | PWR | KAERI | South Korea | Licensed in Korea (standard design approval) |
| SMR-160 | 160 | PWR | Holtec International | United States | US NRC pre-application suspended in favor of SMR-300 design |
| SMR-300 | 300 | PWR | Holtec International | United States | Seeking UK licensing; US NRC pre-application communications initiated |
| SVBR-100 [cs] | 100 | LFR | OKB Gidropress | Russia | Design (Detailed) |
| SSR-W | 300–1000 | MSR | Moltex Energy | United Kingdom | Design (Phase 1, vendor design review). |
| S-PRISM | 311 | FBR | GE Hitachi Nuclear Energy | United States/Japan | Design (Detailed) |
| TEPLATOR | 50 (non-electric) | PWR (heavy water) | University of West Bohemia | Czech Republic | Design (Conceptual) |
| TMSR-500 | 500 | MSR | ThorCon | Indonesia | Design (Conceptual) |
| TMSR-LF1 | 10 | MSR | China National Nuclear Corporation | China | Under construction |
| U-Battery | 4 | HTGR | U-Battery consortium | United Kingdom | Cancelled. Design archived. |
| VBER-300 | 325 | PWR | OKBM Afrikantov | Russia | Design |
| VK-300 [de] | 250 | BWR | Atomstroyexport | Russia | Design (Detailed) |
| VOYGR | 50-77 (x6) | PWR | NuScale Power | United States | Licensed in the USA (50 MWe module). Seeking NRC licensing for reactor power output upgrade to 77 MWe of 6 modules (462 MWe). |
| VVER-300 | 300 | BWR | OKB Gidropress | Russia | Design (Conceptual) |
| Westinghouse SMR | 225 | PWR | Westinghouse Electric Company | United States | Cancelled. Preliminary design completed. |
| Xe-100 | 80 | HTGR | X-energy | United States | Design (Conceptual) |
| Updated as of 2022. Some reactors are not included in IAEA Report. Not all IAEA reactors are listed there are added yet and some are added (anno 2023) that were not yet listed in the now dated IAEA report. ||||||
* ↑ Multi-unit complex based on the GT-MHR reactor design
* ↑ Urenco Group in collaboration with Jacobs and Kinectrics

## Siting/infrastructure

SMRs are expected to require less land, e.g., the 470 MWe 3-loop Rolls-Royce SMR reactor should take 40,000 m$^2$ (430,000 sq ft), 10% of that needed for a traditional plant. This unit is too large to meet the International Atomic Energy Agency's definition of a SMR being smaller than 300MWe and will require more on-site construction, which calls into question the claimed benefits of SMRs. The firm is targeting a 500-day construction time.
Electricity needs in remote locations are usually small and variable, making them suitable for a smaller plant. The smaller size may also reduce the need to access to a large grid to distribute their output.

## Proposed sites
### Argentina

In February 2014, the CAREM SMR project started in Argentina with the civil engineering construction of the containment building of a prototype reactor. The CAREM acronym means Central ARgentina de Elementos Modulares. The National Atomic Energy Commission (Spanish: Comisión Nacional de Energía Atómica, CNEA), the Argentine government agency in charge of nuclear energy research and development and Nucleoeléctrica Argentina [es], the national nuclear energy company, are cooperating to achieve the realization of the project.
CAREM-25 is a prototype of 25 MWe, the first nuclear power plant completely designed and developed in Argentina. The project was suspended several times before being resumed. In October 2022, CNEA expected that the civil construction works would be finished by 2024. If construction continues according to plan, the first criticality of CAREM-25 is foreseen by the end of 2027.

### Canada

In 2018, the Canadian province of New Brunswick announced it would invest $10 million for a demonstration project at the Point Lepreau Nuclear Generating Station. It was later announced that SMR proponents Advanced Reactor Concepts and Moltex would open offices there. One unit is scheduled for construction at Point Lepreau Nuclear Generating Station, Canada, in July 2018. Both Moltex and ARC Nuclear are vying for the contract.
On 1 December 2019, the Premiers of Ontario, New Brunswick and Saskatchewan signed a memorandum of understanding (MoU) "committing to collaborate on the development and deployment of innovative, versatile and scalable nuclear reactors, known as Small Modular Reactors (SMRs)." They were joined by Alberta in August 2020. With continued support from citizens and government officials have led to the execution of a selected SMR at the Canadian Nuclear Laboratory.
In 2021, Ontario Power Generation announced they plan to build a BWRX-300 SMR at their Darlington site to be completed by 2028. A licence for construction still had to be applied for.
On 11 August 2022, Invest Alberta, the Government of Alberta's crown corporation signed a MoU with Terrestrial Energy regarding IMSR in Western Canada through an interprovincial MoU it joined earlier.

### China

In July 2019, China National Nuclear Corporation announced it would build an ACP100 SMR on the north-west side of the existing Changjiang Nuclear Power Plant at Changjiang, in the Hainan province by the end of the year. On 7 June 2021, the demonstration project, named the Linglong One, was approved by China's National Development and Reform Commission. In July, China National Nuclear Corporation (CNNC) started construction, and in October 2021, the containment vessel bottom of the first of two units was installed. It is the world's first commercial land-based SMR prototype.
In August 2023, the core module was installed. The core module includes an integrated pressure vessel, steam generator, primary pump receiver. The reactor's planned capacity is 125 MWe.

### France

At the beginning of 2023, Électricité de France (EDF) created a new subsidiary to develop and construct a new SMR named Nuward. It was a 340 MWe design with two independent light water reactors of 170 MWe. The twin reactors were sheltered in a single containment building sharing most of their equipment. In August 2023, EDF submitted a safety case for Nuward to the autorité de sûreté nucléaire (ASN), the French safety authority.
In July 2024, EDF announced it was discontinuing the existing design process for Nuward, and will work on an SMR design based on existing rather than innovative technologies, following discussions with prospective SMR customers. In January 2025, EDF announced that the new Nuward conceptual design would be completed by mid-2026 to come to market in the 2030s, with an output of about 400 MWe and usable heat output of 100 MWt.

### Poland

Polish chemical company Synthos declared plans to deploy a Hitachi BWRX-300 reactor (300 MW) in Poland by 2030. A feasibility study was completed in December 2020 and the licensing process started with the Polish National Atomic Energy Agency.
In February 2022, NuScale Power and the large mining conglomerate KGHM Polska Miedź announced signing of contract to construct a first operational reactor in Poland by 2029.

### Romania

On the occasion of 2021 United Nations Climate Change Conference, the state-owned Romanian nuclear energy company Nuclearelectrica and NuScale Power signed an agreement to build a power plant with six small-scale nuclear reactors at the Doicești power station, on the site of a former coal power plant, located near the village of Doicești, Dâmbovița county, 90 km North of Bucharest. The project is estimated to be completed by 2026–2027, which will make the power plant the first of its kind in Europe. The power plant is expected to generate 462 MWe, securing the consumption of about 46.000 households and would help to avoid the release of 4 million tons of CO$_2$ per year.

### Russia

Russia has started to deploy on its arctic coast small nuclear reactors embarked on board icebreakers. In May 2020, the first prototype of a floating nuclear power plant with two 30 MW$_e$ reactors – the type KLT-40 – started operation in Pevek, Russia. This concept is based on the design of nuclear icebreakers.

### United Kingdom

In 2016, it was reported that the UK Government was assessing Welsh SMR sites – including the former Trawsfynydd nuclear power station – and on the site of former nuclear or coal-fired power stations in Northern England. Existing nuclear sites including Bradwell, Hartlepool, Heysham, Oldbury, Sizewell, Sellafield, and Wylfa were stated to be possibilities. The target cost for a 470 MWe Rolls-Royce SMR unit is £1.8 billion for the fifth unit built. In 2020, it was reported that Rolls-Royce had plans to construct up to 16 SMRs in the UK. In 2019, the company received £18 million to begin designing the modular system. An additional £210 million was awarded to Rolls-Royce by the British government in 2021, complemented by a £195 million contribution from private firms. In November 2022, Rolls-Royce announced that the sites at Trawsfynydd, Wylfa, Sellafield and Oldbury would be prioritised for assessment as potential locations for multiple SMRs.
The British government launched Great British Nuclear in July 2023 to administer a competition to create SMRs, and will co-fund any viable project.

### United States

The US Department of Energy had estimated the first SMR in the United States would be completed by NuScale Power around 2030, but this deal has since fallen through after the customers backed out due to rising costs. The United States has plans for several modular reactors. Dominion Energy Virginia is now accepting proposals. The U.S. has nearly 4 gigawatts in announced SMR projects in addition to almost 3 GW in early development or pre-development stages, according to Utility Dive.
SMRs differ in terms of staffing, safety and deployment time. US government studies to evaluate SMR-associated risks are claimed to have slowed the licensing process. One main concern with SMRs and their large number, needed to reach an economic profitability, is preventing nuclear proliferation.
Standard Power, a provider of infrastructure as a service to advanced data processing companies, has chosen to work with NuScale Power and ENTRA1 Energy to develop SMR-powered facilities in Pennsylvania and Ohio that will together produce nearly two gigawatts of clean, reliable energy.
NuScale Power is working with Wisconsin's Dairyland Power to evaluate VOYGR SMR power plants for potential deployment. The US leader in SMR technology believes its load-following capabilities can be used to support Dairyland's existing renewables portfolio, as well as facilitate growth. Additionally, VOYGR plants are well-suited for replacing Dairyland's retiring coal plant sites, preserving critical jobs and helping communities transition to a decarbonized energy system.
NuScale Power is working with Associated Electric Cooperative Inc. (Associated) in Missouri to evaluate deployment of VOYGR SMR power plants as part of Associated's due diligence to explore reliable, responsible sources of energy.
The Utah Associated Municipal Power Systems (UAMPS) had partnered with Energy Northwest to explore siting a NuScale Power reactor in Idaho, possibly on the Department of Energy's Idaho National Laboratory. Known as the Carbon Free Power Project, the project was canceled in November 2023 for cost reasons. NuScale said in January 2023 the target price for power from the plant was $89 per megawatt hour, up 53% from the previous estimate of $58 per MWh, raising concerns about customers' willingness to pay. Still, increased cost estimates remain well below traditional nuclear power used for commercial facilities and most other less reliable and more environmentally hazardous forms of power production.
The Galena Nuclear Power Plant in Galena, Alaska was a proposed micro nuclear reactor installation. It was a potential deployment for the Toshiba 4S reactor. The project was "effectively stalled". Toshiba never began the expensive process for approval that is required by the U.S. Nuclear Regulatory Commission.
Although the SMR now under consideration has yet to be NRC licensed, the Tennessee Valley Authority was authorized to receive an Early Site Permit (ESP) by the Nuclear Regulatory Commission for siting an SMR at its Clinch River Nuclear Site in Tennessee in December 2019. This ESP is valid for 20 years, and addresses site safety, environmental protection and emergency preparedness. This ESP is applicable for any light-water reactor SMR design under development in the United States.
In October 2024, Google agreed to commission multiple small modular reactors from Kairos Power to power its artificial intelligence processing, with the first to be operational in 2030.