Calutron

The calutron emerged because Ernest Lawrence understood that his cyclotron's principle—charged ions deflected by magnetic fields—could separate uranium isotopes. Lighter atoms curve more tightly than heavier ones. Applied at industrial scale, this could concentrate uranium-235 from natural uranium's 0.7% to weapons-grade enrichment. The name CALUtron derived from CALifornia University cycloTRON.

The adjacent possible aligned days before America entered World War II. When Lawrence first operated the calutron on December 2, 1941—five days before Pearl Harbor—a uranium beam intensity of 5 microamperes reached the collector. A nine-hour run on January 14, 1942 with a 50 microampere beam produced just 18 micrograms of uranium enriched to 25% U-235. The principle worked. Scaling it would require something unprecedented.

The Y-12 plant at Oak Ridge, Tennessee, housed 1,152 calutrons arranged in oval 'racetracks.' The facility cost $673 million—equivalent to $10.8 billion in 2024—matched only by the Manhattan Project's other facilities. At peak, Y-12 employed 22,000 people. Since the Berkeley cyclotron had been operated by scientists, wartime labor shortages meant young women—many recent high school graduates—operated the calutrons. These 'Calutron Girls' were vital to enrichment production, adjusting controls they didn't understand to optimize a process they weren't told about.

The calutrons demanded so much copper for magnet windings that Manhattan Project administrators borrowed nearly 15,000 tons of silver from Treasury Department vaults at West Point. Silver substituted for copper throughout the electromagnets—a solution possible only because the project operated outside normal economic constraints.

Over two years, Y-12's calutrons produced about 140 pounds (64 kg) of weapons-grade uranium-235—enough for one bomb. Little Boy, detonated over Hiroshima on August 6, 1945, contained calutron-enriched uranium. A second bomb's worth would have been ready by December 1945.

The calutron proved a technological dead end. Electromagnetic separation was slow, energy-intensive, and could not scale efficiently. Gaseous diffusion, though more complex, proved more economical for sustained production. Most calutrons were dismantled after the war, though some remained in use producing isotopically enriched samples for medical, scientific, and military purposes.

Iraq's clandestine nuclear program in the 1980s attempted to revive calutron technology, reasoning that the principle was proven and within their industrial capacity. The discovery of these 'Baghdadtrons' after the 1991 Gulf War demonstrated how a wartime expedient could resurface when nations sought the path of least resistance to enrichment.