Antares' advanced reactor, Mark-0, hit criticality this month at Idaho National Laboratory, meaning it reached a self-sustaining chain reaction.

• Another chain reaction within the national security community quickly followed.

Why it matters: Today's battlefields and bases are rife with energy hogs. Drone batteries need charging; computers and servers and networks and radars need to stay online.

• Now imagine tomorrow.
• Nuclear power of all shapes and sizes could ease the burden.

Driving the news: Antares was the first of 10 companies tapped for the Energy Department's Reactor Pilot Program to run through the tape. (Valar Atomics, another entrant, in February had a reactor airlifted on C-17s.)

• "We're months to years out from being able to start deploying this technology to military installations," CEO Jordan Bramble told reporters.
• "Thousands and thousands of assets here in the U.S., on military installations, need resilient power," he added. "Couple that with the fact that we now have an adversary that's actually capable of disrupting the civilian grid."

State of play: Both the Army and Air Force have expressed significant interest in nuclear power, with their respective Janus and Advanced Nuclear Power for Installations initiatives.

• The Air Force this year paired Antares with Joint Base San Antonio in Texas. ANPI seeks to have at least one reactor running on at least one installation by 2030.
• "There is no better high-energy-density source that can keep the lights on on a bad day for a long time," Rian Bahran, the deputy assistant secretary of energy for nuclear reactors, told reporters.
• Diesel generators, a mainstay of U.S. power projection, "can get you weeks to months," Bahran said. With a micro-reactor, it's "years without the need to refuel."

Zoom in: BWX Technologies' tri-structural isotropic fuel, or TRISO, powered the Antares reactor. Enough was made to power multiple reactors in the future, according to Bramble.

• BWXT is also deeply involved in Project Pele, a mobile nuclear reactor championed by the Pentagon's Strategic Capabilities Office.

The bottom line: "I think June 4 will be a historic day in the American nuclear renaissance," Energy Secretary Chris Wright told reporters. "The team at Antares and their partners in the DOE have shown America can do bold things."

More from Axios:

South Korea sets sights on nuclear submarine

Defense-tech diversifies as "voracious" investors pour in

Nuclear fuel recycling startup Project Omega raises $12M

Skyler Shuford, a Hermeus co-founder, has a new startup, Reaxiomatic. He's keeping much of it out of the public eye.

The big picture: Reaxiomatic is dedicated to aerospace and defense, Shuford confirmed in an interview. It could easily be associated with the growing American reindustrialization and supply-chain-awareness crowds.

• "At Hermeus, we built an organization that can design, build and fly brand-new jets from scratch in about a year, which is about eight times faster than any near-peers. A similar playbook can, and must, be applied to other critical defense systems," Shuford told me.
• "I think a lot of companies may be hamstringing themselves or distracting themselves by trying to be dual-use too early."

Driving the news: Reax will today announce a $7.25 million pre-seed led by Scout Ventures and NVP Capital. (Bling Capital participated.) Shuford said his startup's "prototype product" will be shown off "within the scope of this round."

• Exactly what that product is remains a mystery to outsiders. The money will also be used to build out the team.
• "Most hardware companies might be demonstrating a subsystem or something minor," he said. "I want to under-promise and over-deliver."

Between the lines: Reax has a hangar and office space in San Diego. That provides it plenty of room for big projects — or what Shuford calls "heavy attritables for critical defense infrastructure."

Friction point: The Defense Department is often chided for its unwieldiness. Bureaucracy slows progress, project asks bloat, prices creep up, and when kit finally arrives, it's roasted for being outdated or unnecessary.

The bottom line: "When I look at challenges that the department faces, a lot of people are looking at it through the lens of 'we need to get more technology to the warfighter.' I, actually, don't think that's true," Shuford said.

• "A multiyear bureaucratic process didn't tell the world that RC planes dragging fiber-optic cables was the right architecture in Ukraine. It was tight cycles, fast improvement, available hardware, doubling down on what works ... and ripping out what doesn't," he added.
• "That philosophy needs to scale up. That's what we're doing at Reax."

Go deeper: Defense-tech diversifies as "voracious" investors pour in

Harbinger is ready to produce thousands of unmanned hybrid-electric vehicles a year for national-security customers, chief executive John Harris told me.

• The news follows the rollout of Harbinger Praesidia, the business' dedicated defense division.

Why it matters: The vehicles are autonomy-ready, easily exportable and can be configured for a wide range of missions, including resupply, communications, troop transport, patrol and counter-drone, according to specs shared with me.

• "We are selling what is still, fundamentally, a truck platform. The difference is we are now offering it in a way that is really well-suited to a lot of defense use cases," Harris said.
• "The point is that our capacity is fully available for this," he added. "It's not a bespoke product."

Zoom in: The vehicle, available in 158-, 178- and 208-inch wheelbases, combines an electric motor with a gasoline engine and a generator that can recharge batteries or pump out external power.

• It can hit speeds around 65 miles per hour and is designed to work in extreme temperature ranges.
• "We can choose to run silently or not," said Harris, who previously worked on the Anduril Industries Sentry Tower. "There are so many advantages to a hybrid-electric architecture."

The intrigue: In-Q-Tel now backs Harbinger. A dollar figure was not immediately available.

• In-Q-Tel's support is an "important conduit for us," Harris said. "They serve as a pipeline into the defense community."
• Harbinger also recently announced a partnership with American Rheinmetall.
• "The contracting path to get one of our vehicles into the hands of" the Defense Department will "almost always run through a prime," Harris said.

Catch up quick: Harbinger employs around 500 people, mostly in California. It raised $160 million in November, bringing its total to $358 million. Investors include FedEx.

German defense company Helsing armed U.S. soldiers with HX-2 drones for drills in Lithuania, where they successfully spotted and struck targets.

Why it matters: This could be a sign of Helsing breaking into American markets. It's also a symptom of European rearmament.

Driving the news: At least one HX-2 launch was visible in the Army's own footage from Project Flytrap, a counter-drone exercise at Pabradė Training Area.

• In a separate photo, Gen. Christopher Donahue, the head of U.S. Army Europe and Africa, can be seen huddling over an HX-2 transport box.

What they're saying: "Helsing was at Flytrap in force with their hardware and software engineers, as were many other companies," Alex Miller, the Army's chief technology officer, told me.

• "The HX-2 was originally used as a one-way attack and counter-drone system," he said.

Read the full story.

The crew of a U.S. Apache attack helicopter shot down by Iran was rescued by a Saronic-made Corsair drone-boat.

Why it matters: It was a first-of-its-kind operation, with incredibly high stakes.

• It also offers a glimpse at future warfare, in which humans and smart, militarized machinery operate alongside one another.

Driving the news: A spokesperson for Central Command, which oversees American military action in the greater Middle East, told Axios the Corsair "picked up" the crew and "transported them to another location on the water," where they were then "hoisted up to a helicopter."

• Both crew members are in stable condition.
• President Trump blamed Iran for shooting down the AH-64 Apache, which is made by Boeing, and vowed a military response.

Read the full story.

President Trump's envoys Steve Witkoff and Jared Kushner traveled to the national lab in Oak Ridge, Tennessee, last week for consultations with a team of technical experts that could play a role in nuclear negotiations with Iran.

Why it matters: The White House is trying to reach a memorandum of understanding (MOU) with Iran to end the war and begin in-depth nuclear negotiations, and wants to have experts at the ready should those talks take off.

• The U.S. and Iran are still at odds on several details of the MOU, according to U.S. officials and regional sources involved in the mediation.
• "This meeting in Oak Ridge doesn't mean that a deal is going to happen, but it is a sign that the negotiations are in a very serious phase and that there is a good chance to get it done, and we want to be prepared," a U.S. official said.

The intrigue: Axios was alerted last week that Witkoff made an unannounced trip to eastern Tennessee. Two U.S. officials later confirmed he and Kushner were visiting Energy Department facilities at Oak Ridge.

• Some of the country's foremost experts in uranium processing and centrifuge technology are based at Oak Ridge National Laboratory and Y-12 National Security Complex.
• In the past, nuclear materials and equipment, including from Kazakhstan and Libya, were routed through Tennessee.

Read the full story.

🛰️ Apex Space is now valued at $2.3 billion, just months after breaking the $1 billion mark.

• Why it matters: "Space is undergoing a fundamental transition from bespoke programs to scalable, proliferated infrastructure," Paul Hudson, founder of Glade Brook Capital Partners, said in a statement. "Apex has built exactly the kind of business needed to support that shift."
• 💭 My thought bubble: I talked with CEO Ian Cinnamon and toured the factory, just north of LAX, in December. Check that out, here.

🇩🇰 The State Department approved a potential $842 million sale of Joint Air-to-Surface Standoff Missiles with Extended Range and related equipment to Denmark. The weapons are made by Lockheed Martin.

• Why it matters: The deal will strengthen the Royal Danish Air Force and its F-35s, according to the department.
• 💭 My thought bubble: Two hundred missiles are on the table. A decent lot.

🤖 A bipartisan group of U.S. lawmakers introduced the GUARD Act, legislation that requires scrutiny of humanoid and quadruped robots made by China and other countries deemed adversarial.

• Why it matters: "Robotics may sound like science fiction, but the security risks are very real," Craig Singleton at the Foundation for Defense of Democracies told me. "Platforms built in adversarial tech ecosystems can collect data, map terrain and connect to U.S. networks in ways that create exposure or leverage."
• 💭 My thought bubble: When I started here two years ago, I wrote about a gun-toting Chinese robo-dog. Time flies.

🪖 The U.S. Army is buying 82 AeroVironment P550 drones to conduct reconnaissance, surveillance and targeting, according to Pentagon contract announcements.

• Why it matters: The deal is worth a little more than $117 million.
• 💭 My thought bubble: The service also scooped Switchblade 600 Block 2s and armor-busting 300 Block 20 Explosively Formed Penetrators earlier this year.

Airbus this week unveiled the U145, an uncrewed version of the widely used twin-engine H145 helicopter.

The big picture: Footage shared by the company on X shows the U145 lacking a cockpit.

• Instead, the nose doors swing wide — peep the handles — and a small platform folds out; cargo is then loaded via forklift through the front.

By the numbers: The U145 has a maximum takeoff weight around 8,400 pounds.

What's next: A maiden flight, with a safety pilot aboard, is expected by year's end.