What makes a reactor small or modular

A large U.S. reactor like Vogtle Unit 4 sends out about 1,114 megawatts of electricity. A small modular reactor is usually defined as 300 MWe or less per unit. A microreactor is smaller again, from a few kilowatts up to roughly 20 MWe. Those lines come from convention, mostly the International Atomic Energy Agency's working definitions, and no U.S. law draws them. So a company can call a 340 MWe unit "small," and Holtec does exactly that with its SMR-300.

The SMRs and advanced reactors lesson in the Nuclear Power course gave the licensing steps and a first status table. This course goes further into the designs, the Idaho work, water, fuel and waste. Start with the vocabulary, because most confusion at public meetings starts there.

Two meanings of "modular"

The word does double duty, and pitches slide between the meanings.

• Factory-built modules. Large pieces of the plant (reactor vessel sections, steel-and-concrete wall rings, piping skids) are made in a factory and shipped. The hope is assembly-line quality and speed. Ontario Power Generation's first BWRX-300 at Darlington is being built with steel composite wall rings and a basemat lifted into a shaft in April 2026.
• Multi-module plants. Several identical reactors share one site. NuScale's US460 design, approved by the NRC in May 2025, is six 77 MWe modules for 462 MWe. X-energy's Long Mott plan for Dow is four Xe-100 units totaling 320 MWe, about 80 MWe each.

A plant can be one, both or neither. A single BWRX-300 is a factory-heavy design but one reactor. A six-module NuScale plant is both. When someone says "modular," ask which one they mean, because the cost argument depends on it. Factory savings require many orders of the same design. Multi-module savings come from sharing staff, security and a control room.

The size ladder

Design	Electric output	Class	Status note, October 2026
MARVEL (DOE, at INL)	About 10 kWe (85 kWt)	Test microreactor	Primary vessel arrived in Idaho; criticality planned for 2027
Radiant Kaleidos	1 MWe (3.5 MWt)	Microreactor	Fuel delivered to INL's DOME test bed July 2026
Westinghouse eVinci	5 MWe (15 MWt)	Microreactor	Zero-power criticality experiment on a scaled mock-up of its core, in Nevada, reported August 2026
Aalo-X	10 MWe	Microreactor	Critical test reactor reached criticality at INL July 4, 2026
BWXT BANR	20 MWe (75 MWt)	Upper edge of micro	Picked for the Army's Janus program at Fort Campbell, August 2026
Oklo Aurora	Up to 75 MWe	Small	Under construction at INL under DOE authorization
NuScale module	77 MWe each, 6 per plant	Small, multi-module	NRC standard design approval May 2025
X-energy Xe-100	About 80 MWe each, 4 per plant	Small, multi-module	Construction permit decision pending at the NRC
GE Vernova Hitachi BWRX-300	Up to 300 MWe	Small	Under construction in Ontario; U.S. permit issued to TVA, September 2026
TerraPower Natrium	345 MWe, up to 500 MWe with salt storage	Small by convention	Construction began in Wyoming April 2026
Holtec SMR-300	About 340 MWe each	Called small	Partial construction permit application accepted by the NRC in 2026

Lesson 3 of this course gives the licensing status for each with dates. For now, notice how wide the range is. The smallest unit on this list would need more than 100,000 copies to match one Vogtle reactor. The largest is about a third of one.

What microreactors are trying to be

Microreactors are less about the grid and more about places the grid doesn't reach well. The design goals show up again and again in company literature: factory-assembled and shipped whole (Kaleidos fits in one standard shipping container), fueled at the factory, run for years without refueling, then shipped back. The NRC's Part 53 rule, published March 30, 2026, includes a provision for factory fuel loading into manufactured reactors (10 CFR 53.620), which is the regulatory piece that business model needs.

Customers they name: military bases that want power if the local grid goes down, remote mines and Arctic towns that now burn diesel, disaster recovery, and in the last two years, data centers. Lesson 2 sorts the designs by purpose.

Thermal versus electric

Reactors make heat. Only part of that heat becomes electricity, and the rest goes to the air or water. The ratio is the conversion efficiency, and it depends on how hot the coolant runs and what kind of turbine or engine is used. Here are the published pairs:

Sizing a fleet for a big load

Data center developers talk about reactors as if one reactor equals one campus. It doesn't work that way, because reactors stop for refueling and maintenance, and a single trip would black out a campus with no backup.

Why "supplemental"

Almost no SMR plan replaces the grid. They add to it, in roughly these arrangements:

• Grid-connected: the reactor sells into the grid like any plant (TVA's Clinch River, Darlington).
• Co-located or behind the meter: the reactor sits next to a customer and serves it directly, with or without a grid tie (Dow's Long Mott plan for industrial steam and power; Aalo's stated plan to power a co-located data center).
• Islanded: no grid at all, as with a remote mine or a base that wants to ride out an outage.

Each type raises different public questions. Grid-connected plants go through utility commissions and their costs can reach ratepayers. Behind-the-meter deals may skip much of that review, which is good for speed and bad for public oversight. The costs and timelines lesson covers who carries the risk.