Lightwater SMR

Light-water small modular reactors ("LW-SMRs") are a class of advanced nuclear reactors with electrical outputs of up to 300–470 MWe that use proven pressurized water reactor technology in compact, factory-fabricable configurations with enhanced passive safety systems. The dominant design approach is the integral pressurized water reactor ("iPWR"), which integrates all major primary coolant system components within a single reactor pressure vessel to eliminate large-break loss-of-coolant accident scenarios. LW-SMRs use standard low-enriched uranium fuel (<5% U-235) compatible with existing fabrication and waste management infrastructure — a significant practical advantage over non-LWR SMR concepts requiring higher-assay fuel with limited supply chains. NuScale Power achieved the first-ever NRC design certification for an SMR in January 2023 (50 MWe), with an uprated 77 MWe design approved in May 2025. However, the technology has not yet been commercially deployed under the factory-production model that underpins its economic thesis. NuScale's flagship UAMPS demonstration project was cancelled in November 2023 due to cost escalation. Leading LW-SMR designs now advancing globally include Westinghouse AP300, Rolls-Royce SMR (470 MWe), Holtec SMR-300, and GE Hitachi BWRX-300, all competing in the UK's Great British Nuclear SMR selection. The core economic proposition is that factory fabrication, shorter construction timelines, and serial production learning will offset the inherent per-kW cost penalty of smaller reactors. Techno-economic analyses show estimated LCOE in the range of $80–$220/MWh depending on assumptions, with significant uncertainty that will only resolve as FOAK units are built and operated. Near-term deployment is expected in the late 2020s to early 2030s, with the critical question being whether sufficient order volumes can materialize to drive costs down the learning curve.

The concept of small nuclear reactors has roots extending back to the earliest days of nuclear power. The U.S. Army Nuclear Power Program (1954–1977) built and operated eight transportable small reactors for remote military installations, providing foundational operating experience with compact nuclear systems, though these units proved expensive and were ultimately discontinued. Naval propulsion programs in the United States, United Kingdom, France, and Russia developed highly compact pressurized water reactors ("PWRs") for submarines and aircraft carriers beginning in the 1950s, accumulating thousands of reactor-years of operating experience with small light-water reactor ("LWR") technology. The modern concept of a civilian light-water SMR emerged in the late 1990s and early 2000s. The International Reactor Innovative and Secure ("IRIS") project, led by Westinghouse with an international consortium of universities and laboratories, proposed an integral PWR design in 1999 that integrated all primary reactor coolant system components within a single pressure vessel. The IRIS concept influenced subsequent iPWR-type SMR designs. Between 2000 and 2003, the U.S. DOE funded the Multi-Application Small Light Water Reactor ("MASLWR") project at Oregon State University, which developed a natural-circulation-cooled iPWR concept with passive safety systems. This research directly led to NuScale Power, which was spun out as a commercial entity to develop the MASLWR concept into a licensable reactor design. In parallel, South Korea's Korea Atomic Energy Research Institute ("KAERI") developed the System-integrated Modular Advanced ReacTor ("SMART"), a 100 MWe iPWR that received standard design approval from Korea's Nuclear Safety and Security Commission in 2012 — making it the first SMR to receive a design certification from any national regulator. Argentina's CAREM-25, a 32 MWe integral PWR, began construction in 2014 as the country's first domestically designed reactor, though the project has experienced significant delays with completion now expected by 2027. In the United States, the DOE's 2012 SMR Licensing Technical Support ("LTS") program selected two designs for cost-shared development: Babcock & Wilcox's 195 MWe mPower reactor and NuScale Power's modular iPWR. The mPower project was terminated in 2017 after Babcock & Wilcox was unable to secure additional investors or customer contracts, despite over $486 million in combined industry and DOE spending. NuScale persisted and submitted its Design Certification Application to the NRC in March 2017. In January 2023, the NRC certified NuScale's 50 MWe design — making it the first SMR and only the seventh reactor design ever cleared for use in the United States. NuScale subsequently submitted an application for an uprated 77 MWe module in January 2023, which the NRC approved in May 2025 — becoming the second SMR design approved by the regulator. However, NuScale's flagship demonstration project — the Carbon Free Power Project with UAMPS at Idaho National Laboratory — was cancelled in November 2023 due to escalating cost estimates (from $58/MWh in 2021 to $89/MWh in 2023, with unsubsidized estimates reaching $119/MWh) and insufficient subscriber commitments. Globally, only two SMRs have entered commercial operation: Russia's KLT-40S floating reactors aboard the Akademik Lomonosov (two 35 MWe PWRs, operational since 2020) and China's HTR-PM high-temperature gas-cooled reactor (operational since 2023, though this is not a light-water design). China's ACP100 land-based iPWR (125 MWe) began construction at the Changjiang site in Hainan in 2021. The current generation of leading LW-SMR designs includes NuScale's VOYGR (77 MWe per module), Westinghouse's AP300 (300 MWe, derived from the operating AP1000 design), Rolls-Royce SMR (470 MWe PWR), Holtec's SMR-300 (300 MWe PWR), GE Hitachi's BWRX-300 (300 MWe boiling water reactor), and France's NUWARD (170 MWe iPWR). The UK's Great British Nuclear SMR selection process, as of 2025, has shortlisted all four light-water contenders for its national programme.

Reactor Core and Fuel

LW-SMRs use the same fundamental nuclear fission process as conventional pressurized water reactors: low-enriched uranium ("LEU") fuel, typically UO₂ ceramic pellets loaded into zircaloy-clad fuel rods arranged in assemblies, undergoes controlled fission chain reactions in the presence of light water, which serves as both neutron moderator and primary coolant. Fuel enrichment is typically below 5% U-235, consistent with existing fuel fabrication infrastructure and nonproliferation norms (the threshold for low-enriched uranium). This is a significant practical advantage over many non-LWR SMR designs (such as high-temperature gas-cooled or molten salt reactors) that require higher-assay LEU ("HALEU") enriched to 5–20%, which currently has limited supply chain availability.

Reactivity is controlled through a combination of control rod drive mechanisms ("CRDMs"), soluble boron in the primary coolant (in some designs), and solid burnable absorbers integrated into the fuel assemblies. Some newer LW-SMR designs (such as NUWARD) are designed to be boron-free, relying entirely on solid burnable poisons and CRDMs for reactivity control, which simplifies the primary coolant chemistry system and avoids the liquid radioactive waste associated with boron management. Refueling cycles typically range from 18 months to 3+ years depending on the design, with target design lifetimes of 60 years.

Integral Reactor Pressure Vessel Design

The most common LW-SMR architecture is the integral pressurized water reactor ("iPWR"), which integrates all major primary reactor coolant system components — steam generators, pressurizer, and control rod drive mechanisms — within a single reactor pressure vessel ("RPV"). This is a fundamental departure from conventional PWR architecture, where the steam generators, pressurizer, and reactor coolant pumps are located outside the RPV and connected by large-diameter piping (the primary coolant loop).

The iPWR approach eliminates the large external piping that connects these components, which directly eliminates the possibility of a large-break loss-of-coolant accident — historically one of the most severe design-basis accident scenarios for conventional PWRs. By confining the entire primary coolant boundary within the RPV, the number and size of vessel penetrations are minimized, reducing the probability of coolant leaks. The steam generators in iPWR designs are typically helical coil or once-through heat exchangers located within the upper portion of the RPV, where primary coolant flowing upward through the core transfers heat to the secondary (steam) circuit without the primary water ever leaving the vessel.

Some LW-SMR designs use forced circulation via reactor coolant pumps integrated into the RPV (e.g., ACP100, SMART), while others rely entirely on natural circulation of the primary coolant (e.g., NuScale, CAREM-25). Natural circulation designs eliminate the need for coolant pumps entirely, removing a potential failure mode and simplifying the system, but at the cost of lower thermal power density and a taller RPV to maintain sufficient driving head.

Passive Safety Systems

LW-SMRs are designed with multiple independent passive safety systems that can bring the reactor to a safe shutdown state and maintain long-term cooling without any external power supply, operator intervention, or additional water. These systems exploit natural physical phenomena — gravity, natural convection, compressed gas pressure, and condensation — to perform safety functions. The key passive safety systems include:

Passive Decay Heat Removal System ("PDHRS"): After reactor shutdown, fission products continue to generate decay heat (initially ~6–7% of full power, declining exponentially). The PDHRS transfers this residual heat from the reactor core to an ultimate heat sink (typically an external water pool, a cooling tank, or the containment atmosphere) via natural circulation loops. Heat exchangers immersed in cooling pools absorb the decay heat without any mechanical assistance.

Passive Safety Injection System: In the event of a loss-of-coolant accident, gravity-driven water injection systems deliver borated water from elevated tanks into the RPV to maintain core coverage and shutdown reactivity. These systems operate solely on the force of gravity and require no pumps or electrical power.

Reactor Depressurization System: Automatic depressurization valves reduce primary system pressure to enable gravity-driven safety injection and to transition to long-term passive cooling. These are typically actuated by compressed gas or spring-loaded mechanisms.

Containment System: iPWR-type SMRs employ compact, high-pressure steel containment vessels, often submerged in or surrounded by large water pools that serve as the ultimate heat sink. Heat transferred to the containment atmosphere condenses on the containment wall, which is cooled by the surrounding pool water. Some designs (e.g., NuScale) submerge the entire reactor module and containment in a shared reactor pool, enabling passive heat rejection for extended periods (72+ hours) without any external intervention.

The combination of these passive systems is designed to achieve core damage frequencies ("CDF") substantially lower than conventional reactors — in the range of 10⁻⁷ to 10⁻⁸ per reactor-year, compared to approximately 10⁻⁵ for older Generation II designs and ~10⁻⁶ for Generation III+ large reactors.

Power Conversion and Output

At the surface, the secondary steam circuit receives heat from the steam generators within the RPV. The generated steam drives a conventional Rankine-cycle turbine-generator set to produce electricity. Because LW-SMRs operate at similar primary coolant temperatures and pressures to conventional PWRs (typically 280–330°C at 10–16 MPa), they achieve comparable thermal efficiencies of approximately 30–33%. Individual module outputs range from 50 MWe (NuScale) to 470 MWe (Rolls-Royce SMR), with multi-module plant configurations scaling from ~300 MWe to ~900+ MWe. For cogeneration applications, steam can be extracted from the secondary circuit at various stages for district heating, desalination, or industrial process heat, with the diversion reducing electrical output proportionally.

Modular Construction and Deployment

The "modular" in SMR refers to a manufacturing and construction philosophy, not just the reactor size. Complete reactor modules — including the RPV, steam generators, and containment vessel — are designed to be fabricated in dedicated factory facilities under controlled quality conditions, then transported to the installation site as finished or near-finished units. On-site construction is limited to civil works (foundations, seismic isolation, spent fuel storage) and balance-of-plant systems. Multi-module plants are designed so that individual modules can be installed, commissioned, and brought online sequentially, with each module operating independently. This approach targets a construction timeline of approximately 3–4 years for a single module, compared to 7–15 years typical of conventional large reactor projects.

LW-SMRs offer several structural innovations over the incumbent approach to nuclear power: conventional gigawatt-scale light-water reactors (Gen II and Gen III/III+), which have dominated the global nuclear fleet since the 1960s.

Reduced Financial Risk and Capital Requirements

Conventional nuclear power plants require capital investments typically exceeding $10 billion per unit and construction periods of 7–15+ years, creating enormous financial exposure and construction risk. The history of nuclear construction in Western economies is marked by chronic cost overruns and schedule delays — most recently illustrated by the Vogtle Units 3 and 4 project in Georgia, which came in at roughly double its original budget. LW-SMRs reduce this risk through smaller unit size (typically 50–470 MWe vs. 1,000–1,600 MWe for large reactors), enabling customers to purchase capacity in increments that match utility or industrial demand profiles. The smaller scale also reduces the absolute capital at risk and makes projects financeable by a broader range of investors. In multi-module configurations, the first unit can begin generating revenue while subsequent units are still under construction — a phased commissioning approach impossible with large reactors that must be fully completed before producing any electricity.

Factory Fabrication and Modular Construction

The defining innovation of the SMR concept is the shift from bespoke, site-built construction to standardized factory production. Reactor modules and major components are designed to be manufactured in controlled factory environments with high quality standards, then transported to the installation site by road, rail, or barge for assembly. This approach is intended to capture the manufacturing learning curve effects that drove cost reductions in industries like aerospace and shipbuilding — disciplines where mass production of complex, safety-critical systems is routine. A PNAS expert elicitation found consensus among nuclear engineering experts that factory fabrication and shorter construction schedules are the primary factors that could make LW-SMRs economically viable. An Applied Energy techno-economic analysis (2023) found through Monte Carlo simulation that LW-SMR construction cost and schedule distributions have lower mean and standard deviation than conventional large reactors, reflecting reduced project risk.

Enhanced Passive Safety

LW-SMRs, particularly integral PWR designs, incorporate passive safety systems that rely on natural physical laws — gravity, natural convection, and compressed gas — rather than active pumps, diesel generators, or operator intervention. In iPWR designs, all major primary coolant system components (steam generators, pressurizer, and control rod drive mechanisms) are integrated within a single reactor pressure vessel, which eliminates large-bore piping penetrations and thereby eliminates the possibility of large-break loss-of-coolant accidents ("LB-LOCAs"). Passive decay heat removal systems use natural circulation to transfer residual heat to ultimate heat sinks (water pools or containment structures) without any external power supply. NuScale's design achieves a coping time of more than 3 days without outside intervention and is rated as "walk-away safe" — meaning the reactor can safely shut down and cool indefinitely with no operator action, no AC or DC power, and no additional water.

Siting Flexibility

The smaller footprint and enhanced safety case of LW-SMRs enable deployment in locations where large reactors are impractical: small or islanded electrical grids, industrial sites requiring process heat, remote communities, and brownfield sites such as retired coal plants. Several designs feature underground installation for enhanced protection against external hazards. The ability to deploy nuclear power at scales matching actual demand — rather than requiring a grid capable of absorbing 1+ GWe — opens nuclear energy to a far broader global market, including developing nations and energy-intensive industrial users.

Cogeneration and Non-Electric Applications

While conventional large reactors are optimized almost exclusively for baseload electricity generation, LW-SMRs are designed for versatility: electricity generation, district heating, desalination, hydrogen production, and industrial process steam. Some designs also offer load-following capability to complement intermittent renewable generation on modern grids.

The deployment of LW-SMRs faces interconnected challenges spanning economics, manufacturing scale-up, licensing, supply chain development, and public acceptance.

Capital Cost and Diseconomies of Scale

The fundamental economic tension in SMR design is the tradeoff between smaller size and diseconomies of scale. On a per-kilowatt basis, SMR capital costs are higher than conventional large reactors because the reactor vessel, containment, turbine, and safety systems cannot be scaled down proportionally with power output — a 300 MWe reactor does not cost one-third as much as a 1,000 MWe reactor. A 2024 Renewable and Sustainable Energy Reviews study found that average capital costs for SMRs are approximately 7,031 €/kW, roughly 41% higher per kW than large reactors. Proponents argue that these diseconomies will be offset by learning effects, factory production efficiencies, shorter construction schedules, and reduced financing costs. However, these cost reductions depend on achieving high-volume serial production — a condition that has not yet been demonstrated for any civilian SMR design.

LCOE Uncertainty

Estimates of SMR levelized cost of electricity vary dramatically depending on assumptions about construction cost, schedule, learning rates, and capacity factor. An Applied Energy techno-economic analysis (2023) estimated the LCOE for an LW-SMR at $89.6/MWh. A Steigerwald et al. (2023) Monte Carlo analysis produced a median LCOE of $218/MWh for PWR-type SMRs. The CSIRO estimated AU$230–382/MWh for SMRs by 2030. The NuScale UAMPS project's cost escalation from $58/MWh (2021) to an estimated $119/MWh unsubsidized at cancellation underscored the fragility of early cost projections. These figures compare unfavorably to utility-scale solar ($26–50/MWh), onshore wind ($26–50/MWh), and natural gas combined cycle ($45–74/MWh). However, proponents note that SMRs provide firm, dispatchable, carbon-free baseload power — a capability that intermittent renewables cannot match without storage. If carbon pricing or clean energy mandates are factored in, the competitive position improves significantly.

First-of-a-Kind Risk and the "Order Book" Problem

The cost reduction thesis for SMRs depends critically on achieving sufficient order volumes to drive manufacturing learning and amortize factory investment. However, potential customers are reluctant to commit to orders without proven cost data, and developers cannot demonstrate costs without orders — creating a classic chicken-and-egg problem. The cancellation of NuScale's UAMPS project in 2023 and the termination of the mPower program in 2017 illustrate the difficulty of converting regulatory approval into commercial deployment. As of 2026, no light-water SMR has been built under the factory-production model that underpins cost projections. The first-of-a-kind ("FOAK") units will inevitably carry a significant cost premium, and the pathway from FOAK to competitive nth-of-a-kind ("NOAK") costs requires sustained investment through a potentially lengthy period of above-market pricing.

Supply Chain and Manufacturing Infrastructure

The factory-fabrication model that underpins SMR economics requires purpose-built manufacturing facilities capable of serial production of reactor modules, RPVs, steam generators, and containment vessels to nuclear quality standards. These facilities do not currently exist at scale for most LW-SMR designs. Building them requires significant upfront capital investment predicated on a pipeline of orders that has not yet materialized. The nuclear supply chain has also contracted significantly since the height of nuclear construction in the 1970s–1980s, with limited capacity for large forgings, nuclear-grade welding, and specialized component manufacturing.

Licensing and Regulatory Harmonization

While NuScale achieved NRC design certification in the U.S., each country requires its own licensing review, creating a fragmented and expensive regulatory landscape. There is no international mechanism for mutual recognition of reactor design approvals. The IAEA's Nuclear Harmonization and Standardization Initiative ("NHSI") and the NEA SMR Regulators' Forum are working to address this, but progress is slow. The time and cost of obtaining design certifications in multiple jurisdictions represents a significant barrier to the global deployment model that SMR economics require. Rolls-Royce SMR's UK Generic Design Assessment, for example, is a multi-year process running in parallel with the firm's development timeline.

Spent Fuel and Waste Management

A 2025 comprehensive review in Nuclear Engineering and Technology covering 141 SMR designs emphasized that backend nuclear fuel cycle considerations — including spent fuel management, radioactive waste disposal, decommissioning planning, and nonproliferation safeguards — must be addressed from the earliest design phases. On a per-MWh basis, SMRs may produce more spent fuel and radioactive waste than large reactors due to higher neutron leakage from smaller cores. The spent fuel from LW-SMRs uses standard LEU and produces waste streams compatible with existing storage and disposal infrastructure, which is a practical advantage over non-LWR SMR concepts that would introduce novel fuel forms and waste types into an already challenged disposal system.

Public Perception and Political Risk

Despite enhanced safety features, SMRs still face public opposition related to nuclear technology broadly — including concerns about radioactive waste, proliferation, and the legacy of accidents at Three Mile Island, Chernobyl, and Fukushima. Siting decisions for SMR plants can trigger local opposition even when technical safety margins are substantially improved. Political risk is also a factor: shifts in government policy, subsidy regimes, and energy market regulation can alter the economic viability of projects that require decades-long investment horizons.