[Digital Today reporter Jinju Hong (홍진주)] U.S. reactor startup Antares has set an important milestone in the race to develop next-generation small reactors. Its test reactor reached criticality for the first time, meaning it can sustain a fission reaction without outside intervention. It is also recorded as the most advanced case in the U.S. government-led plan to develop next-generation nuclear power plants.

IT outlet Ars Technica reported on June 6 that Antares said its Mark 0 test reactor, installed at Idaho National Laboratory, had reached criticality.

Reaching criticality means a fission reaction inside the reactor can be sustained on its own. It is a core step that must be passed in reactor development and is seen as the first gateway to confirming that the design works as intended in theory.

The achievement does not mean power production will begin immediately. The Mark 0 reactor now operating is a test device that is not connected to power generation equipment. The company is focusing on design verification and securing safety data rather than producing electricity.

The milestone also ties into the U.S. government's policy to foster next-generation nuclear power. The Trump administration last year announced an executive order to accelerate nuclear technology development in the United States. It also set a goal for the Department of Energy that 3 different next-generation reactor designs reach criticality within about 1 year.

Antares is considered the first company to meet that goal so far. The biggest feature of the Antares reactor is its use of TRISO fuel. TRISO is a high-performance nuclear fuel in which uranium oxide particles are wrapped in multiple layers of carbon and ceramic materials. It can withstand higher temperatures than existing reactors, and the fuel itself serves to curb the leakage of radioactive materials.

The company adopted a design that concentrates safety functions in the fuel structure itself while reducing the complexity of the reactor body. The goal is to lower the possibility of core melting and minimise the external release of hazardous isotopes if an accident occurs.

Its cooling and heat-transfer method also differs from existing nuclear plants. Antares adopted a closed Brayton cycle that transfers heat generated in the reactor to a heat exchanger through sodium, then transfers it again to pressurised nitrogen to drive a turbine. It explained that it improved safety by applying a non-water-based heat-transfer structure, unlike conventional light-water reactors that use water as coolant.

The main purpose of the ongoing tests is design verification. Antares is using the Mark 0 reactor to compare physical conditions inside the core predicted by simulation with actual data. It also plans to use it to secure safety materials needed for future licensing. The company plans to move to integrated tests that include a power generation system next year.

The business backdrop is also drawing attention. Antares is conducting tests using U.S. Department of Energy laboratories, but its business direction is closely linked to the Department of Defense's mobile reactor development project, Project Pele. The U.S. National Aeronautics and Space Administration is also supporting related research.

In the industry, the achievement is seen as meaningful progress in the competition to develop next-generation small reactors, although it remains far from commercialisation. In the United States, there is only 1 next-generation reactor design that has obtained full licensing, and none has advanced to the actual construction stage.

With Antares succeeding in reaching criticality, next-generation nuclear technology has moved beyond the laboratory stage and entered a validation phase. Future power generation test results and whether licensing procedures progress are cited as key variables that will determine the prospects for commercialisation.