Forgotten Reactors SeriesHanford’s B Reactor: The Original Plutonium Factory

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. From dirt to operating reactor in under a year, using technology no one had ever run before. No prototypes. No pilot plant. No pressure… except all of it.

Technically, B Reactor was simple on paper and mildly terrifying in reality. It was a graphite-moderated, water-cooled plutonium production reactor, eventually operating at roughly 250 MWth.

There was no turbine hall and no electricity generation. Instead, an enormous graphite pile was penetrated by hundreds of aluminum process tubes, each containing uranium fuel slugs. Columbia River water was pumped straight through the core to carry away heat—once through, no second chances.

What made B Reactor truly unusual was how it was fueled and defueled. Fresh uranium slugs were pushed into one end of each process tube, while irradiated fuel—now containing newly formed plutonium—was pushed out the other end into shielded collection basins.

The reactor stayed running while fuel quietly moved through it like an assembly line. No vessel head removal. No outages. Just continuous, industrial-scale plutonium production.

Operational history was equal parts brilliance and improvisation.

Shortly after startup, operators watched reactor power mysteriously collapse. The culprit turned out to be xenon-135, a powerful neutron absorber no one had fully accounted for yet.

The fix? Add more fuel channels. This episode essentially created practical reactor physics as a real-time operational science.

Cooling constraints were constant companions. A pump trip meant a scram. Warm river water mattered. Instrumentation was crude, margins were thin in places, and procedures were being invented daily—often mid-shift.

Yet the core physics was sound. The reactor did exactly what it was designed to do, relentlessly and quietly. And yes—this reactor produced the plutonium used in the Trinity and Fat Man bombs.

B Reactor operated from 1944 until 1968, becoming the first successful large-scale nuclear reactor in history. Not experimental. Not theoretical. Fully industrial.

Today it stands as a National Historic Landmark—the factory floor where the nuclear age truly began.

Before power reactors.Before licensing bases.Before best practices.

There was B Reactor—pushing fuel in one end, plutonium out the other, and history along with it.