Small modular nuclear reactor reaches criticality in first US test

An 80 megawatt small modular reactor (SMR), built by TerraPower at Idaho National Laboratory, has reached criticality for the first time. Ars Technica describes the event as a "symbolic milestone" for the nuclear industry.

Criticality means that neutron multiplication in a reactor becomes self-sustaining, so the fission chain holds a steady state. SMR designs have featured in academic literature and regulator approval processes for a decade; with this test, commercial scale begins to take its first step.

TerraPower's design is called Natrium. The reactor uses sodium as a coolant and integrates a molten salt tank for energy storage. The architecture lets the reactor operate at a base of 80 MW and temporarily rise to about 350 MW for peak demand.

The US Nuclear Regulatory Commission (NRC) issued its criticality approval in March 2025; low-power tests were completed in early June this year. Full-power tests are scheduled for the first half of 2027.

Part of the project's financing comes from the Department of Energy's Advanced Reactor Demonstration Program (ARDP) grant. The programme's allocation to TerraPower is $2 billion; the company, founded with backing from Bill Gates, is contributing an additional $800 million from private capital.

The nuclear policy environment has returned to public debate in recent months as electricity demand grows. AI data-centre load is creating something like 30 GW of incremental base-load demand a year in the US over the next decade; SMRs are among the candidates to fill the gap.

The SMR technology also faces criticism. Princeton University physicist Frank von Hippel told Ars Technica: "SMRs always make the same promise: faster, cheaper. The economic benefits stay limited until the production line reaches serial output."

The cost targets are around $5,000 to $6,000 per MW of capacity. That level points to roughly a third of large-scale conventional nuclear costs. But the first units are always more expensive; the Idaho test cost roughly $4 billion, around $50,000 per MW.

In international competition, both China and Russia are advancing SMR projects. China's ACP100 Linglong One reactor was connected to the grid in 2023; Russia's Akademik Lomonosov has been operating in Chukotka since 2020. The US is arriving later but Natrium's architecture has the potential for surprising jumps.

This article is not energy investment advice. The operating cost of SMR technology, the licensing timeline and regulator approval all carry uncertainty. Investors are advised to discuss decisions with a licensed financial adviser.