Small Modular Reactor
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A small modular reactor (SMR) is a nuclear fission reactor with an electrical output of up to roughly 300 megawatts (MWe) per unit, about one third the capacity of a conventional gigawatt-class plant, whose major components or entire reactor modules are fabricated in factories and shipped to a site for assembly [1]. Beginning in late 2024, SMRs moved to the center of the artificial intelligence industry's energy strategy, as hyperscale data center operators including Google, Amazon, and Meta signed agreements with developers such as Kairos Power, X-energy, Oklo, and TerraPower to secure firm, carbon-free electricity for AI workloads [3][6][11]. As of June 2026 no commercial SMR operates in the United States, and the gap between announced gigawatts and licensed, constructed plants remains the defining feature of the sector: the first American units are expected between about 2028 and the early 2030s, while China's Linglong One is on track to become the world's first land-based commercial SMR in 2026 [27].
The International Atomic Energy Agency defines SMRs as advanced reactors with a power capacity of up to 300 MWe per unit, roughly one quarter to one third the output of a conventional gigawatt-class plant; microreactors, a subset, produce up to about 10 MWe [1]. The IAEA notes that the "modular" in the name refers to "the ability to fabricate major components of the nuclear steam supply system in a factory environment and ship them to the point of use," so that plants can be "built in factories and shipped to utilities for installation as demand arises" [1]. The core premise is to trade economies of scale for economies of series production: standardized modules built on factory lines should, in principle, shorten construction schedules, lower the absolute capital cost of each project, reduce financing risk, and let buyers add capacity incrementally as demand grows [1]. Many designs also rely on passive safety systems that remove decay heat without operator action or external power, which developers argue supports smaller emergency planning zones and siting closer to industrial loads, including data center campuses.
Skeptics note the inverse of the same logic: a smaller reactor gives up scale economies, so until modules are actually produced in series, the electricity from first-of-a-kind SMR plants is projected to cost more per megawatt-hour than power from large reactors, renewables paired with storage, or gas. That tension played out in November 2023 when the first US SMR project, NuScale's contract with Utah Associated Municipal Power Systems (UAMPS), was cancelled after its target power price rose from $58 to about $89 per MWh and utility subscriptions fell short [15].
SMR designs fall into three broad families: light-water reactors that miniaturize conventional pressurized or boiling water technology, high-temperature gas-cooled reactors, and Generation IV designs cooled by liquid metal or molten salt. Several leading designs sit somewhat above the 300 MWe line but are conventionally grouped with SMRs because of their modular construction approach.
| Developer | Design | Technology | Output | Status (June 2026) |
|---|---|---|---|---|
| NuScale Power (US) | US460 / VOYGR | Pressurized light-water | 77 MWe per module, up to six modules (462 MWe) | 50 MWe module design certified by NRC January 2023; uprated 77 MWe design approved May 2025; no firm US order [14][15] |
| Kairos Power (US) | KP-FHR (Hermes) | Fluoride-salt-cooled high-temperature reactor, TRISO pebble fuel | Hermes 1 test unit 35 MWth (no power); Hermes 2 up to 50 MWe | Hermes 1 construction permit December 2023, safety-related construction began May 2025; Hermes 2 permits November 2024 [16][17] |
| Oklo (US) | Aurora | Liquid-metal-cooled fast reactor, HALEU metal fuel | Uprated to 75 MWe in March 2025 | Phased combined license application in progress; first plant targeted at Idaho National Laboratory for late 2027 to 2028 [20][22] |
| X-energy (US) | Xe-100 | High-temperature gas-cooled, TRISO-X pebble fuel | 80 MWe per unit; 320 MWe four-pack | Construction permit application for Dow's Long Mott site in Texas docketed May 2025 [23] |
| TerraPower (US) | Natrium | Sodium-cooled fast reactor with molten salt storage | 345 MWe (500 MW peak via storage) | NRC construction permit March 2026; nuclear construction began April 2026 in Kemmerer, Wyoming [18][19] |
| GE Vernova Hitachi (US/Japan) | BWRX-300 | Boiling light-water | 300 MWe | First unit approved for construction at Darlington, Ontario in May 2025 [29] |
| Rolls-Royce SMR (UK) | Rolls-Royce SMR | Pressurized light-water | 470 MWe | Selected by Great British Nuclear in June 2025 to build the UK's first three units [30] |
| CNNC (China) | ACP100 Linglong One | Pressurized light-water | 125 MWe | In commissioning at Changjiang, Hainan; commercial operation expected first half of 2026 [27][28] |
The current wave of SMR interest is driven primarily by electricity demand from AI computing. A Lawrence Berkeley National Laboratory report released by the US Department of Energy in December 2024 estimated that data centers consumed about 4.4 percent of US electricity in 2023, up from total usage of 58 terawatt-hours in 2014 to 176 terawatt-hours in 2023, and projected they could consume between 6.7 and 12 percent by 2028, with AI accelerators the main growth driver [2]. Hyperscalers face a conflict between that growth and their climate pledges: Google targets 24/7 carbon-free energy on every grid it uses by 2030, Microsoft aims to be carbon negative by 2030, and Amazon targets net-zero carbon by 2040 [3][6]. Nuclear power is attractive because it is firm and carbon-free, unlike intermittent wind and solar, and SMR vendors pitch modular plants as something that can be added in increments matched to phased campus buildouts, potentially sited near or even behind the meter at large facilities. The broader build-out of firm generation for AI compute is discussed at nuclear power for AI.
Google framed its October 2024 Kairos Power agreement explicitly around AI load. "The grid needs new electricity sources to support AI technologies that are powering major scientific advances," said Michael Terrell, Google's senior director of energy and climate, adding that nuclear "offer[s] a clean, round-the-clock power source that can help us reliably meet electricity demands with carbon-free energy every hour of every day" [3]. Amazon described its X-energy investment in similar terms: "This collaboration between Amazon and X-energy is a significant step toward accelerating advanced nuclear technologies that can help us bring new sources of carbon-free energy to the grid cost-effectively and safely," said Kevin Miller, Amazon's vice president of global data centers [6].
In practice, most announced arrangements deliver power through the grid rather than by direct connection. Kairos Power's first plant for Google, for example, will sell electricity to the Tennessee Valley Authority's grid, with Google purchasing the associated clean energy attributes to match consumption at its Tennessee and Alabama data centers [5]. US policy has also explicitly linked nuclear to AI: executive orders signed in May 2025 direct the Departments of Energy and Defense to host reactors at federal sites specifically to power AI infrastructure [24][25].
The sector also has a direct personnel link to AI. Sam Altman, CEO of OpenAI, chaired Oklo's board from 2015 and took the company public in May 2024 through his special-purpose acquisition company AltC; he stepped down as chairman on April 22, 2025, a move the company said freed it to pursue commercial partnerships with OpenAI and other hyperscalers, though no Oklo-OpenAI power agreement had been announced as of June 2026 [10].
| Announced | Buyer or partner | Developer | Scope |
|---|---|---|---|
| April 2024 | Equinix | Oklo | Letter of intent for up to 500 MW from Aurora powerhouses, with a $25 million prepayment [8] |
| September 20, 2024 | Microsoft | Constellation Energy | 20-year PPA to restart the 835 MW Three Mile Island Unit 1 (Crane Clean Energy Center), targeted for 2027; a conventional reactor restart, not an SMR [12][13] |
| October 14, 2024 | Kairos Power | Master agreement for a fleet totaling 500 MW by 2035, first plant by 2030; the first corporate order for a fleet of SMRs [3][4] | |
| October 16, 2024 | Amazon | X-energy, Energy Northwest | Amazon anchored a roughly $500 million investment in X-energy and backed the Cascade project in Washington state (initially four Xe-100 units, 320 MWe, expandable to 12 units and 960 MWe), targeting more than 5 GW deployed by 2039 [6][7] |
| December 2024 | Switch | Oklo | Non-binding master power agreement for up to 12 GW of Aurora capacity through 2044 [9] |
| August 18, 2025 | Tennessee Valley Authority and Google | Kairos Power | PPA for up to 50 MW from the Hermes 2 plant in Oak Ridge, Tennessee, the first US utility PPA for a Generation IV reactor; operations planned for 2030 [5] |
| January 9, 2026 | Meta | Oklo, TerraPower, Vistra | Agreements intended to unlock up to 6.6 GW for Meta's AI data centers by 2035, including support for a 1.2 GW Oklo campus in Pike County, Ohio [11] |
Two frequently conflated transactions are not SMR deals. Microsoft's September 2024 agreement with Constellation finances the restart of a full-size light-water reactor at Three Mile Island, and Meta's June 2025 20-year agreement with Constellation supports the existing Clinton plant in Illinois; both involve gigawatt-scale conventional reactors rather than modular new builds [11][12]. The SMR agreements themselves vary widely in firmness, ranging from equity investments (Amazon in X-energy) and signed PPAs (TVA-Google-Kairos) to non-binding frameworks (Oklo-Switch), and none was delivering electricity as of June 2026.
In the United States, the Nuclear Regulatory Commission (NRC) licenses commercial reactors. NuScale remains the only SMR developer with an NRC-certified design: its 50 MWe module was certified in a rule finalized in January 2023, and its uprated 77 MWe US460 plant design received standard design approval on May 29, 2025, after a review completed in under two years [14][15]. Kairos Power received a construction permit for its Hermes test reactor in December 2023, the first US non-light-water reactor permitted in more than five decades, followed by permits for the two-unit Hermes 2 facility on November 21, 2024; safety-related nuclear construction on Hermes began in May 2025 [16][17]. The NRC approved TerraPower's construction permit for the Natrium plant in Kemmerer, Wyoming in March 2026, its first construction permit for a commercial non-light-water power reactor in more than 40 years, and nuclear construction began in April 2026; the plant still requires a separate operating license [18][19]. Dow and X-energy submitted a construction permit application in March 2025 for four Xe-100 units at Dow's Seadrift, Texas site, which the NRC docketed in May 2025 on an 18-month review schedule [23].
Oklo's history illustrates both the promise and difficulty of novel licensing. It submitted the first combined license application for an advanced fission reactor in March 2020, but the NRC denied it without prejudice in January 2022 for failing to provide sufficient safety information [21]. Oklo restarted with a phased application strategy, completed an NRC pre-application readiness assessment for the Aurora-INL combined license application in July 2025, broke ground at Idaho National Laboratory in September 2025, and targets first power there in late 2027 or 2028, a schedule that depends on an unprecedented licensing pace [20][22].
Policy has shifted strongly in the industry's favor. The bipartisan ADVANCE Act, signed in July 2024, directed the NRC to cut fees and speed advanced reactor reviews, and on May 23, 2025 President Trump signed four executive orders seeking to quadruple US nuclear capacity to 400 GW by 2050, impose 18-month deadlines on NRC licensing decisions, deploy reactors at Department of Energy and Defense sites for AI, and run a DOE test-reactor pilot program targeting first criticality by July 4, 2026 [24][25][26]. Legal analysts noted the orders compress reviews that historically took many years and raise questions about the NRC's independence [25].
The central challenge is converting announcements into operating plants at predictable cost. No US SMR has ever been completed, and the sector's one near-commercial project, the NuScale-UAMPS Carbon Free Power Project, collapsed in November 2023 on cost grounds before construction began [15]. First-of-a-kind cost growth has appeared elsewhere too: Ontario Power Generation's four-unit Darlington BWRX-300 program was budgeted at nearly C$21 billion when construction was approved in May 2025, well above earlier expectations [31]. Most advanced designs, including Aurora, Natrium, the Xe-100, and the KP-FHR, also depend on high-assay low-enriched uranium (HALEU), a fuel historically supplied commercially only by Russia; Centrus began the first US HALEU production in Piketon, Ohio in late 2023, and the May 2025 executive orders invoked the Defense Production Act to rebuild the domestic fuel supply chain [24].
Timing is the other mismatch. AI-driven load growth is concentrated in the mid-to-late 2020s, but even on current company schedules SMR electricity arrives later: Oklo targets late 2027 or 2028 at INL, Kairos plans Hermes 2 operations in 2030, TerraPower expects Natrium completion in 2030, and Amazon's Cascade project targets construction by 2030 [5][7][19][22]. Hyperscalers are bridging the gap with gas turbines, renewables, and purchases from existing or restarted large reactors such as Three Mile Island [12]. Internationally, China is furthest ahead: the 125 MWe Linglong One at Changjiang, under construction since July 2021, completed cold functional testing in October 2025 and turbine commissioning tests in early 2026, with commercial operation expected in the first half of 2026, which would make it the first land-based commercial SMR anywhere (Russia has operated the small floating plant Akademik Lomonosov since 2020) [27][28]. Whether SMRs ultimately power AI at scale will be determined less by memoranda of understanding than by whether the 2026-2030 cohort of first plants is delivered on schedule and near budget.