The U.S. Department of Energy has completed a high-profile effort to bring three privately developed advanced nuclear reactors to criticality, reaching the target before a July 4, 2026 deadline.
The milestone is more precise than simply “switching on” three power plants. All three reactors completed zero-power criticality demonstrations, meaning they sustained controlled nuclear chain reactions but were not yet generating commercial electricity. Two demonstrations took place at Idaho National Laboratory, while the third occurred at a research site in Utah.
The three designs were Antares Nuclear’s Mark-0, Valar Atomics’ Ward 250 and Deployable Energy’s Unity reactor. The Energy Department said the United States became the first country to achieve criticality in three different advanced microreactor designs within a single month.
What the Energy Department Actually Achieved
The initiative followed a May 2025 executive order directing the Energy Department to authorize at least three advanced reactors and aim for criticality in each by July 4, 2026.
Antares Nuclear’s Mark-0 became the first of the three to reach the milestone on June 4. It completed a zero-power fueled criticality test at Idaho National Laboratory, becoming the first privately developed non-light-water reactor in the United States to reach criticality in more than four decades, according to the department.
Valar Atomics followed on June 18 with the Ward 250 reactor. That experiment took place at the San Rafael Energy Research Laboratory in Emery County, Utah, making it the first DOE-authorized reactor constructed outside a national laboratory.
Deployable Energy’s Unity reactor became the third design to reach criticality on June 30 at Idaho National Laboratory. The Energy Department announced the result on July 1, confirming that the three-reactor target had been completed before the deadline.
Therefore, describing all three reactors as being located in Idaho would be inaccurate. Mark-0 and Unity were tested in Idaho, while Ward 250 reached criticality in Utah.
Criticality Does Not Mean Commercial Power Has Begun
In nuclear engineering, criticality is the point at which a reactor sustains a controlled, self-supporting chain reaction. Each fission event produces enough neutrons to maintain the process at a stable level.
The word can sound alarming outside the nuclear industry, but controlled criticality is a normal requirement for reactor operation. It is different from an uncontrolled reaction or an emergency.
However, the three tests were conducted at effectively zero power. Their purpose was to confirm reactor physics, fuel behavior and the ability to establish and control the chain reaction. The reactors were not connected to local electricity grids and were not supplying homes or businesses with power.
The distinction matters because reaching criticality is an early technical milestone rather than the final step toward commercial operation. Developers must still complete additional testing, safety validation, engineering work and, where required, licensing before their designs can routinely generate and sell electricity.
Why Microreactors Are Attracting Attention
Microreactors are much smaller than conventional nuclear power plants. Depending on the design, they may produce anything from a small amount of heat and electricity to several megawatts of power.
The Idaho National Laboratory’s microreactor overview describes them as roughly 100 to 1,000 times smaller than traditional reactors. Their compact size may allow components to be manufactured in factories and transported to locations where larger nuclear plants would be impractical.
Developers are considering them for military installations, isolated communities, mines, industrial facilities, emergency operations and data centers. These users often need power that can operate continuously without depending entirely on long transmission lines or frequent fuel deliveries.
Supporters argue that microreactors could provide reliable, low-carbon energy from a small physical footprint. Their modular designs may also avoid some of the construction complexity associated with conventional nuclear projects, although that advantage still needs to be proven through repeated commercial deployments.
The Mark-0 Reactor Was the First to Reach Criticality
Antares Nuclear’s Mark-0 reactor completed the first demonstration at Idaho National Laboratory. The Energy Department said the test established a technical basis for later reactors that could produce electricity beginning in 2027 or beyond.
Antares is initially targeting military and remote-site applications. The company has said it expects to begin producing electricity with its technology by late 2027 and aims for field deployments by the end of 2028.
Those dates remain company targets rather than guaranteed commercial operating schedules. Successful criticality does not automatically resolve manufacturing, licensing, financing or customer-deployment challenges.
The Mark-0 demonstration was nevertheless significant because it moved a privately developed advanced reactor design beyond computer simulations and non-nuclear component testing into an actual fueled experiment.
Ward 250 Reached the Milestone in Utah
Valar Atomics’ Ward 250 was the second design to reach criticality. The company’s reactor had previously attracted attention when a non-fueled unit was transported from California to Utah aboard a U.S. military C-17 aircraft.
According to the Energy Department, Ward 250 successfully completed its zero-power test on June 18. The reactor is designed to eventually produce up to five megawatts, although the demonstration itself did not generate commercial power.
The Utah test was notable because it occurred outside the traditional national-laboratory environment. That may provide a model for testing future advanced reactors at privately or locally operated sites under DOE authorization.
Valar has said it hopes to begin test electricity sales in 2027 and move toward fuller commercial activity in 2028. Those plans will depend on the results of additional testing and the regulatory path followed by future deployments.
Unity Completed the Three-Reactor Goal
Deployable Energy’s Unity reactor achieved criticality at Idaho National Laboratory at the end of June. The compact design is intended as a one-megawatt water-moderated, gas-cooled “nuclear battery” for locations where conventional infrastructure may be unavailable or vulnerable.
The company envisions applications that include remote communities, defense operations, disaster response, critical infrastructure and industrial sites.
Unity’s test was completed through the Nuclear Energy Launch Pad, an initiative managed by the National Reactor Innovation Center at Idaho National Laboratory. The project reportedly reached criticality in about 150 days, illustrating the accelerated timetable the program was designed to support.
The next stage includes phased testing of reactor physics, load-following performance, inherent safety features and full-power operation. These tests will determine how closely the real reactor matches its predicted behavior under more demanding conditions.
The Regulatory Approach Has Drawn Scrutiny
The Energy Department used its own authorization authority to advance these demonstration reactors rather than requiring each experiment to follow the conventional Nuclear Regulatory Commission licensing process used for commercial nuclear plants.
Supporters say the approach allows developers to test early-stage technology more quickly while using DOE facilities, technical experts and safety reviews.
Critics argue that faster authorization should not be treated as proof that a design is commercially safe or economically viable. Edwin Lyman of the Union of Concerned Scientists told the Associated Press that achieving criticality was only a preliminary step and that substantially more safety testing would be required.
Future commercial reactors may still require approval from the U.S. Nuclear Regulatory Commission, depending on where and how they are deployed. The Energy Department’s own announcement said further testing and NRC licensing would be necessary before commercializing reactors based on the Mark-0 program.
Fuel, Cost and Waste Remain Major Questions
Technical success during a zero-power test does not answer every question facing advanced nuclear energy.
Microreactor developers must establish dependable fuel supplies, manufacture units consistently, prove that operating costs are competitive and create reliable plans for handling spent nuclear fuel and radioactive waste.
Some advanced designs require high-assay low-enriched uranium, commonly called HALEU. Domestic supplies remain limited, although the Energy Department is supporting programs intended to expand production.
Long-term waste disposal also remains unresolved across the broader U.S. nuclear industry. Commercial spent fuel is generally stored at reactor sites because the country does not have an operating permanent geological repository.
These constraints do not erase the importance of the three demonstrations, but they help explain why reaching criticality should not be confused with completing a commercially ready power system.
A Significant Step, but Not Yet Three New Power Plants
The three criticality tests demonstrate that private companies can design, construct, fuel and operate experimental microreactors on a much faster schedule than traditional nuclear projects.
They also gave developers real-world data that could support larger tests and future licensing applications.
Still, Idaho and Utah have not suddenly gained three operating commercial power plants. The reactors completed controlled, zero-power demonstrations, and their ability to generate dependable, affordable electricity at customer sites remains to be proven.
The achievement marks an important change in the American advanced nuclear program. After years dominated by design proposals and development schedules, three separate microreactor concepts have now sustained real nuclear chain reactions. Whether they become commercially successful sources of power will depend on what happens during the more difficult stages that follow.
