Here’s what you’ll learn when you read this story:

• On October 10, 1957, technicians at the Windscale reactor complex discovered a fire raging in the heart of one of its reactors.
• The fire released plumes of iodine-131 and polonium-210 throughout the English countryside. Thankfully, 95 percent of these radioactive elements were captured by a smokestack filter, which was installed years earlier.
• This filter, in combination with quick thinking to douse the fire, saved countless lives and prevented Windscale from reaching the notoriety of Three Mile Island and Chernobyl.

Before Fukushima, Chernobyl, or Three Mile Island, there was the Windscale fire—the worst nuclear disaster in British history. And its origins actually begin with an atomic betrayal.

U.K. scientists were critical in developing the first atomic bomb as part of the Manhattan Project, but the end of World War II ushered in a new era of secrecy. In 1946, President Harry Truman signed the Atomic Energy Act, which restricted atomic weapons to foreign countries—including close allies.

As a result, in a race to acquire its own atomic bomb in the early 1950s, the U.K. government quickly constructed the Windscale Piles No. 1 and No. 2 in Cumberland to create the necessary weapons-grade plutonium. These piles were made of massive blocks of graphite interspersed with uranium rods, much like Chicago Pile-1—the world’s first artificial nuclear reactor—and were air-cooled through two 410-foot-tall chimneys.

But the structures had an inherent problem: Reactors built with graphite are susceptible to what’s known as Wigner energy. First discovered by Hungarian-American theoretical physicist Eugene Wigner in 1942, Wigner energy is caused by high-energy neutrons rearranging the atomic structure of graphite itself. And because of radio silence from the U.S. on all matters nuclear, U.K. engineers didn’t know about this energy source. So, they developed a solution that raised the temperature of the core in a process known as “annealing,” which allowed the atoms to revert back to their original state and release their energy.

On October 7, 1957, technicians at the Windscale Piles were preparing to anneal Pile 1 for the ninth time in its seven-year-long operation. However, instead of an even heating of the entire reactor core, the temperature dropped in all but one channel, labeled 20/53. On the following day, a second Wigner release caused the reactor to rise as expected. It wasn’t until October 10 that technicians started to get really worried.

Typically, as atoms settled back into place, the core temperature steadily decreased, but on the 10^th, experts at the pile noticed that one thermocouple (a sensor for measuring temperature) recorded a reading of 400 degrees Celsius—twice the reactor’s operating temperature. To get a closer look at what was going on, plant workers in radiation-protective clothing inspected the fuel elements at the hottest recorded part of the reactor and found they were glowing red. In other worlds, the atomic heart of Pile 1 was on fire. Meanwhile, radiation levels were spiking in the smokestacks, and iodine-131 and polonium-210 were escaping and blowing across the English landscape.

Such an accident should have left this corner of England inhabitable for decades. And it would have, if it wasn’t for the foresight of Nobel Prize-winning scientist Sir John Cockcroft (who, along with Ernest Walton, was the first to split the atomic nucleus in 1932). Cockcroft advocated for the installation of filters on the smokestacks in case of a fire. At the time, engineers nicknamed these last-minute additions “Cockcroft’s Follies,” as they were seen as a waste of time and money. Years later, those filters saved an untold number of lives.

The story of the Windscale fire does have one more hero: Tom Tuohy, deputy to the general manager at Windscale, who doused the fire with water on the morning of October 11. This was a gutsy move, as it was possible that the molten metal would oxidize the water on contact create an explosion.

“We were quite honestly frightened of the water because we didn’t know whether there would be an explosion or not,” Tuohy told the Penney inquiry during the subsequent investigation into the Windscale fire, but he felt they had no other choice. Luckily, an explosion was averted, and the fire was officially doused.

The U.K. government largely covered up the disaster, though they did ban the sale of milk produced in a 200-mile radius of the site, worrying that cows in the pastures surrounding the Windscale Piles might’ve been exposed to dangerous levels of iodine. Eventually, some 30 years after the fire, the investigation into the Windscale disaster was fully declassified, bringing fully to light the U.K.’s worst nuclear disaster for the first time.