US test reactors achieve milestones

Oklo's Grove reactor - located near Lockhart in Caldwell County, Texas - was one of 11 advanced reactor projects selected by the US Department of Energy (DOE) in August 2025 for the Nuclear Reactor Pilot Program, which aims to expedite the testing of advanced reactor designs that will be authorised by the DOE at sites located outside of the national laboratories. Part of the Reforming Nuclear Reactor Testing at the Department of Energy executive order signed by President Donald Trump in May last year, its goal was "to construct, operate, and achieve criticality of at least three test reactors using the DOE authorisation process by 4 July, 2026".

"Reaching criticality in less than a year is an incredible milestone for our team," said Oklo co-founder and CEO Jacob DeWitte. "Oklo developed Groves from a greenfield site on private land, completed full-scale civil excavation and construction, manufactured or commercially procured all components, including fuel, and developed its operating programmes in-house. Taken together, we believe these accomplishments establish a new benchmark for the Reactor Pilot Program and set the stage for the future of advanced nuclear deployment at scale."

Groves is a low-power test reactor designed to demonstrate reactor design, build, and operations, and to establish operating experience needed to support future isotope production facilities. The project advances Oklo's plans to establish domestic production of critical isotopes for potential use in cancer care, manufacturing, scientific research, space exploration, and national security.

"Groves demonstrates that the domestic nuclear industry can once again move from design through construction, authorisation, and startup on timelines that are measured in months rather than years when developers, suppliers, and regulators work together on an integrated deployment approach," Oklo said.

The company said the project has generated practical experience in project engineering, construction, procurement, reactor operations, startup procedures, safety readiness, training, qualification work, and deployment execution that can inform future isotope production facilities. It has also established engineering practices, operating procedures, training programmes, commissioning experience, and organisational capabilities "that will reduce uncertainty and execution risk across every Oklo facility, including the company's future isotope, powerhouse, and fuel cycle deployments".

Antares Nuclear's Mark-0 reactor became the first reactor under the Reactor Pilot Program to reach initial criticality in early June, closely followed by Valar Atomics' Ward 250 reactor. Deployable Energy's Unity demonstration reactor achieved criticality on 1 July, while Aalo Atomics' Critical Test Reactor chieved initial criticality in the early hours of 4 July.

Deep Fission reactor gets safety design approval

California-based startup Deep Fission, which aims to place small modular reactors in boreholes a mile underground, announced DOE's approval of the Nuclear Safety Design Agreement for its Gravity reactor, confirming that its design warrants advancement under the Reactor Pilot Program.

_{Gravity reactor cutaway detail (Image: Deep Fission)}

The DOE's authorisation pathway mirrors many elements of the Nuclear Regulatory Commission's licensing framework, including the development of preliminary and final safety analyses for construction and operation (read more about the DOE pathway here). The Nuclear Safety Design Agreement - also referred to as the NSDA - is the first step under the Reactor Pilot Program authorisation pathway. It covers design requirements, safety analysis approach, regulatory engagement process, applicable regulatory requirements, and identifies the key safety decisions for the design.

"With this milestone in place, Deep Fission moves forward to the next phase of the DOE authorisation pathway, advancing toward demonstration and eventual deployment of its underground nuclear reactor technology," the company said.

Deep Fission's Gravity reactor is a small modular reactor designed to be placed underground in an optimised borehole one mile (1.6 km) deep. Using traditional pressurised water reactor technology and low-enriched uranium (LEU) fuel, each reactor will generate 15 MWe, the company says, while its small footprint and dense power output means it will require a fraction of the land needed for traditional surface nuclear: ten reactors on the same site would deliver 150 MWe, or 100 reactors would produce 1.5 GWe. In this design, thermal energy is transferred through a closed-loop system from the reactor canister to a heat exchanger, then rises to the surface in a secondary closed-loop for conversion into electricity, like a geothermal system. The company says passive shielding and natural containment offered by the surrounding geology, and the combination of mature technologies from the nuclear, oil and gas, and geothermal industries, while using off-the-shelf parts and readily available LEU fuel, aims to improve safety and security and enable a faster, more cost-effective path to deployment.

Deep Fission broke ground in December at the Great Plains Industrial Park in Parsons for its pilot project and plans to build a full-scale commercial plant there following the test reactor demonstration.

"Deep Fission's Parsons project is not designed as a one-time criticality test," the company said. "Consistent with the intent of Executive Order 14301 and the Reactor Pilot Program, the company intends for its demonstration reactor to become a fully Nuclear Regulatory Commission-licensed, commercially operating unit, delivering power after initial testing and receiving DOE authorisation."