The Atomic-Bomb Core That Escaped World War II
https://www.theatlantic.com/technology/archive/2018/04/tickling-the-dragons-tail-plutonium-time-bomb/557006/
In 1946, shortly after the end of World War II, the physicist Louis Slotin stood in front of a low table at the Los Alamos National Laboratory, concentrating intensely on the object in front of him. His left thumb was hooked into a hole on the top of a heavy beryllium dome, fingers bracing the side as he carefully cantilevered it on its leftmost edge. In his right hand he held a flathead screwdriver, its head wedged under the right edge of the dome to keep it from closing completely. Through the gap on the right side you could just barely catch a metallic gleam, a glimpse of the 14-pound plutonium sphere that was slated to become one of the United States’ next nuclear weapons.
Slotin began slowly lowering the dome, using the screwdriver gingerly to control the opening. He had performed this feat many times before, but this time was different. As his left hand eased its hold on the dome, his right hand slipped outward just a hair too far, pulling the screwdriver out from under its edge and leaving nothing to stop it from closing.
A bright blue flash signaled that the dome had fallen into place, and Slotin felt an intense heat all across his skin. He quickly twisted his wrist to pop the dome completely off the plutonium sphere, but the damage had already been done. Nine days later, he was dead.
In the half-second that the dome was closed, the plutonium had gone supercritical, initiating a nuclear chain reaction and releasing a lethal dose of gamma rays that damaged Slotin’s cells beyond repair. Seven other people were in the room with him during the accident; three were hospitalized for acute radiation sickness. All it had taken was misplacing a screwdriver a fraction of an inch, for a fraction of a second. Richard Feynman called the technique, relatively common at the time, “tickling the tail of a sleeping dragon.”
The demon core, that 14-pound lump of plutonium that claimed the life of Louis Slotin, began its existence as rods of uranium-238, a relatively stable isotope, at the Hanford Site in Washington State. These rods were inserted into a nuclear reactor and bombarded with neutrons, tiny, uncharged subatomic particles, with the hope that some would get stuck to uranium atoms, increasing their atomic number to 239. Unlike uranium-238 (a half-life of 4.5 billion years), uranium-239 is very unstable (23 minutes); it rapidly decays into neptunium-239 (2.4 days), and after that, plutonium-239 (24,000 years). The crude plutonium for the demon core was then purified and sent to the Los Alamos Laboratory as a salt, plutonium nitrate.
Going from plutonium nitrate to a finished bomb core had proven to be a major challenge for the metallurgists at Los Alamos Laboratory. By the time they made the demon core (the third plutonium core, after those in the Trinity and Nagasaki bombs) they had worked out many of the kinks. They first converted the plutonium salt into plutonium metal by reacting it with metallic calcium, a relatively straightforward process. The metallurgists weren’t done, though: Early studies had found that plutonium produced this way is incredibly brittle, cracking with every attempt to make it into the needed shapes. To solve this, the metallurgists tried mixing the plutonium with various other metals to see if they could improve its workability as an alloy, eventually finding gallium to be ideal. The plutonium-gallium alloy of the demon core was hot-pressed into two hemispheres and then coated with a thin layer of nickel to protect the plutonium from rusting; joining these two hemispheres together completed its manufacture.
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