Natrium, explained: TerraPower's reactor with a built-in battery

Natrium is TerraPower's answer to the question nuclear has never handled well: what if the grid doesn't want constant output? The design couples a 345 MWe sodium-cooled fast reactor with molten-salt thermal storage that can push delivered power to 500 MWe for more than five hours — a nuclear plant that behaves like baseload and a battery at once. In 2026 it stopped being a concept: the NRC issued its construction permit on March 4, and ground broke at Kemmerer, Wyoming on April 23.

Key facts

• 345 MWe sodium-cooled fast reactor + molten-salt storage, boostable to 500 MWe for 5.5+ hours
• The March 4, 2026 NRC construction permit was the first for a commercial non-light-water US power reactor in more than 40 years — completed in 18 months against a 27-month schedule
• Construction at Kemmerer, Wyoming began April 23, 2026, on the site of a retiring coal plant
• The owner expects construction complete by February 2031, with roughly 1,600 workers at peak and about 250 permanent jobs
• In July 2026 TerraPower became the first advanced-reactor company admitted to INPO, the operator-standards body of the existing US fleet

How it works

Sodium instead of water changes almost everything. Liquid sodium runs near atmospheric pressure — no massive pressure vessel or containment sized for steam explosions — and carries heat so well that the reactor can shed decay heat passively. A fast neutron spectrum burns fuel more efficiently, though it requires HALEU (high-assay low-enriched uranium), a supply chain still being built in the West.

The signature move is separating the nuclear island from the power island. The reactor heats a salt loop; the salt banks energy in storage tanks; the turbine draws on the tanks. The reactor never load-follows — the tanks do. That means the most expensive part of the plant runs at full capacity around the clock while the output flexes with electricity prices, which is exactly the profile a wind-heavy grid like Wyoming's pays a premium for.

Why Kemmerer matters

Kemmerer 1 is a stack of firsts: first US utility-scale advanced reactor in construction, first commercial fast-reactor permit of the modern era, and the flagship test of coal-to-nuclear transition — the plant sits next to a retiring coal unit whose grid connection and workforce it inherits. It is also, technically, slightly above the IAEA's 300 MWe SMR threshold: Natrium is best read as the leading edge of the broader advanced-reactor wave rather than a classic SMR — which is also why its milestones are watched by everyone in the SMR field anyway.

From one plant to a fleet

The 2026 milestones read like a company preparing to operate, not just build. Excavation and early works at Kemmerer Unit 1 progressed through the summer with the energy-storage island moving ahead of the nuclear island. In July, TerraPower joined the Institute of Nuclear Power Operations — the first advanced-reactor developer inside the body that sets operating standards for the existing US fleet, and a signal regulators and buyers read as operational seriousness. The order pipeline runs through a January 2026 agreement with Meta for up to eight plants serving data centers, PacifiCorp's forecast of two more units by 2033, utility agreements in Utah and Kansas, and entry into the UK's Generic Design Assessment in February 2026. Fuel is the constraint TerraPower manages most publicly: four parallel HALEU tracks spanning Centrus enrichment, fuel fabrication in North Carolina, a Framatome metallization pilot that produced its first uranium pucks in November 2025, and a long-term supply deal starting 2028.

Common misconceptions

"Sodium reactors are experimental." Fast reactors have decades of operating history (EBR-II in Idaho ran from 1964 to 1994); what's new is the commercial licensing and the storage pairing, not the physics.

"It's a bigger bet than water-cooled SMRs." Different bet: water-cooled designs like the BWRX-300 minimize licensing risk; Natrium accepts more first-of-a-kind risk in exchange for storage, flexibility, and fuel efficiency. The comparison is the point — see BWRX-300 vs AP300 vs Natrium.

Current state (August 2026)

Construction is underway at Kemmerer with the energy-storage island proceeding ahead of the nuclear island and the owner's stated construction-completion expectation holding at early 2031. TerraPower is now an INPO member — the first advanced-reactor company inside the fleet's operating-standards body — and its commercial pipeline (Meta, PacifiCorp, Utah, Kansas, the UK) makes Natrium the advanced design with the most credible path from one plant to many. HALEU supply remains the program's watch item.