Mitch drew two circles on the chalkboard, one inside the other. “This,” he said, pointing at the smaller circle, “is the pit. A subcritical sphere fissionable by slow neutrons: plutonium-239, U-235, U-233, whatever. For tomorrow’s test it’s going to be an aluminum sphere a few inches across. And this,” he now pointed at the outer, bumpy circle, “is our layer of high explosive lenses. When the H-E is detonated,” he drew another diagram, “it crushes the pit.” He clapped his hands together and the fifty or so scientists in their standard conference room jumped. “Reducing its size by about eighty percent. The density goes up and the pit becomes highly supercritical.” He drew a third picture. “Not for long, generally less than ten microseconds. But that’s long enough to spawn a hundred generations of neutrons and liberate a few hundred kilotons of energy. Boom.”
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