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Fugen was one of those reactors that makes you stop and say, “Well…THAT’s different.”

Built at Tsuruga, Japan, Fugen was a prototype 557 MWth/148 MWe Advanced Thermal Reactor (ATR)

Germany tried to push a BWR to superheated glory for extra efficiency, but like its American cousins Pathfinder and BONUS, the fancy upgrade proved too hot to handle – and the whole club ran out of steam fast.

Nuclear history has an awkward "middle child" phase that the big-utility PR machines usually pretend didn't happen.

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Some reactors fail after years of operation.

Marviken failed before the first fuel assembly ever entered the core—an awkward milestone for something advertised as a power plant.

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Imagine a 1970s Lada Niva. It’s boxy, it’s loud, and it lacks modern airbags. Now, imagine that Lada is a nuclear power plant parked on a tectonic fault line. Every time the neighbors try to tow it to the scrapyard, it just coughs a cloud of steam and keeps on chugging.
Welcome to Metsamor, the Armenian Nuclear Power Plant (ANPP).

In the world of nuclear engineering, we often talk about proven technology, but the TRR takes that concept to a level that would make a Maytag repairman weep.

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If most reactors are clean, disciplined engineering decisions… Atucha I is what happens when a design meeting refuses to end.

In a remote region of Gabon, near the town of Oklo (from which the site takes its name), lies perhaps the most extraordinary reactor ever discovered—not engineered by man, but assembled by nature with the quiet confidence of a master craftsman who never needed blueprints.

In the late 1950s, Oak Ridge National Laboratory followed up on HRE-1 with a bold experiment: the Homogeneous Reactor Experiment No. 2 (HRE-2).

Rancho Seco wasn’t killed by a meltdown, a design flaw, or some exotic physics problem. It was killed by a ballot box.

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Every decade or so, someone in nuclear engineering decides water is just a little too… ordinary.

Back in 1963, France had a bold plan. They wanted a nuclear fleet running on natural uranium—no enrichment plants, no begging Washington for fuel, total energy independence. That was the UNGG program: Uranium Naturel Graphite Gaz (Gas).

In the 1970s, most countries shopping for their first nuclear program just took what Westinghouse or GE was selling. South Korea bought the Westinghouse reactors — Kori-1 and Kori-2 — but then also bought a CANDU 6 from Canada.

Every few years, thorium comes back.
A new article.
A new startup.
A new wave of enthusiasm explaining why this time, finally, it’s going to change everything.

The Westinghouse Testing Reactor (TR2) at Waltz Mill, Pennsylvania—about 30 miles southeast of Pittsburgh—earned bragging rights as the first privately owned research and test reactor in the U.S.

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Most reactors were built to generate electricity.
Halden was built to answer a different question: what actually happens to fuel when you push it to its limits?

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While Australia debates nuclear power like it’s a dangerous exotic animal that might escape its enclosure, the country quietly operated a high-performance reactor for nearly 50 years… without a single headline-grabbing tantrum.

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Before the USS Nautilus sailed, the ORNL team was building something entirely different – liquid nuclear fuel at extreme temperatures.

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Before Chernobyl made denial fashionable, the Soviets already had a master class in radioactive misadventure: the Kyshtym disaster.

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One of the most serious Soviet naval reactor accidents didn’t occur at power — it occurred during refueling — and it remained largely hidden until after the Soviet Union collapsed.

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Let’s talk about Fast Flux Test Facility - accurately and in context.

Construction of FFTF began in 1968 at the Hanford Site. The reactor achieved first criticality in 1980 and entered routine operation in 1982. It was a 400 MW-thermal, liquid-sodium-cooled fast reactor, and by most technical measures, it performed very well in the role it was designed for.

And that role matters.

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When people talk about “alternative” reactor paths, the conversation usually jumps straight from water to sodium, gas, lead (oh no!) or molten salt. Almost nobody remembers that, in the 1950s, the Atomic Energy Commission took a serious run at something else entirely.
Oil.

The X-10 Graphite Reactor at Oak Ridge doesn’t get the attention it deserves—and that’s a mistake.
Construction began in 1943, in the middle of the Manhattan Project. By that point, the United States had already proven that a nuclear chain reaction was possible with Chicago Pile-1. What it had not proven was whether that physics could be scaled into a reliable, continuously operating production reactor.
That was X-10’s job.

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In the mid-1950s, nuclear propulsion was still new enough that nobody fully knew where the nuclear dragons lived. The USS Nautilus had already proven that a pressurized water reactor could push a submarine around the globe with quiet competence.
Naturally, the next step was: Let’s try something harder. MUCH harder.
Enter the USS Seawolf (SSN-575), the Navy’s second nuclear submarine—and the only one ever powered by a liquid sodium–cooled reactor.

Before “nuclear-powered spacecraft” became a punchline in sci-fi movies, it was a very real engineering program quietly taking shape in Southern California. Meet the SNAP reactors — Systems for Nuclear Auxiliary Power — America’s early attempt at putting fission reactors where solar panels feared to tread.

Long before nuclear power plants lit cities or powered submarines, there was Hanford B Reactor—a machine built for one purpose only: make plutonium, fast, in total secrecy, during a world war.
Construction of B Reactor began in October 1943 at the remote Hanford Site in Washington State. Just eleven months later—at a pace that feels almost reckless by modern standards—the reactor achieved first criticality on September 26, 1944.

If you’ve ever wondered what happens when you mix brilliant nuclear engineering with Cold War optimism and a dash of reckless enthusiasm, look no further than the Soviet Project 705 Alfa-class submarines.

If you’ve ever wondered where modern boiling water reactors got their start, you have to go all the way back to a sandy patch of Idaho desert in the early 1950s—where a small group of scientists armed with courage, curiosity, and suspiciously short safety guidelines built Boiling Water ReActor Experiment or BORAX-I, the world’s first experimental BWR.

If you’ve ever wondered where today’s pebble-bed reactor hype machines got their origin story, look no further than Germany’s AVR — a quirky little high-temperature gas-cooled reactor that ran from 1967 to 1988 and taught the world a lot about what not to do with a sphere full of uranium.
Let’s rewind.

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South of Rincón, Puerto Rico—tucked behind surf breaks and palm trees—sits a lonely concrete dome that once symbolized the atomic future of the Caribbean. Today, it’s a quiet relic. But in the early 1960s, that dome housed one of the most ambitious nuclear experiments ever attempted.

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