The Little Boiling Reactor That Proved the Big Idea

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.

Construction began in 1952 at the National Reactor Testing Station (now INL). This was the era when you could sketch a reactor on a napkin and then actually get funding to build it. In 9 months.

By July 1953, BORAX-I achieved initial criticality, becoming the first reactor specifically designed to test the wild idea that you could let water boil inside the core and somehow not blow the whole thing sky-high.

That question—whether boiling would destabilize a reactor—was the central mission of BORAX-I. At the time, many physicists were convinced that steam bubbles in the core would lead to uncontrollable power surges. The BORAX team, led by Samuel Untermyer II, suspected the opposite: that steam voids might actually shut the reactor down. An early hint of the negative void coefficient that all modern BWRs rely on.

To test this safely, they needed rapid, reliable control rod insertion. Enter one of BORAX-I’s most important innovations: bottom-mounted hydraulic control rod drives. Unlike top-mounted PWR drives, these rods could be inserted UPWARD into the core using hydraulic water pressure—an early version of the fast-acting systems BWRs still use today. This wasn’t just clever engineering; it was a key enabler that made boiling water reactors possible.

Throughout 1953–1954, BORAX-I conducted a series of increasingly bold power excursion tests. They drove the core into high void fractions, measured reactivity feedback, and mapped the stabilization behavior as boiling intensified. What they found stunned the skeptics: the hotter the water got, the more steam formed, and the more the reactor naturally throttled itself back. No runaway. No meltdowns. Just physics doing its job.

Naturally, this led to the final exam: the intentional destruction test. In July 1954, the operators pushed BORAX-I into a super-prompt critical power pulse—an event so intense it physically blew apart the core, scattered fuel fragments, and generated a very impressive plume of steam and debris. It was the most scientifically justified case of “we’re going to blow this thing up on purpose” in nuclear history.

And the result? Exactly the confirmation the industry needed: BWRs had strong, inherent negative reactivity feedback that made them stable under boiling conditions. This single reactor lit the path for the commercial BWR fleet.

BORAX-I died spectacularly—but it proved that boiling water reactors could live.