Inside a nuclear warhead, a plutonium pit is crucial to setting off the sequence of reactions that make a thermonuclear explosion. Inside the pit is a gas, like deuterium/tritium, and around the pit is chemical explosive. When the chemical explosive detonates, it compacts the plutonium around the gas until the core is dense enough to trigger a fission reaction. What makes a warhead thermonuclear, as opposed to just atomic, is that this is combined in the same warhead with a uranium core, which creates a fusion explosion.
”The pits we’re making now are part of that proving out of the processes, the development of techniques that we’ll use starting in 2023 for war reserve pit production,” lab director Thom Mason said in a December 1 forum.
So far, Los Alamos has reportedly produced six prototype pits, as part of the process of relearning and fine-tuning plutonium core production. This is hard work, and it was once done at an industrial scale.
“Fabricating a pit requires a number of operations: melting down an old pit, casting the metal, machining, and finishing the formed piece,” writes Cheryl Rofer, a retired nuclear scientist who worked at Los Alamos labs when its pit production facility was set up in the late 1970s. “There may be other operations as well, particularly if impurities are to be removed from the metal. Each operation requires at least one glovebox with equipment. A glovebox has a window and gloves to reach into a working space that is separate from the room atmosphere. The toxicity of plutonium requires it be contained.”
For most of the duration of nuclear warhead production, plutonium pits were made at the Rocky Flats Plant in Golden, Colorado. The facility, which ran from 1952 until it was closed down in 1989, is now a superfund site.
Source : https://www.popsci.com/technology/los-alamos-makes-new-plutonium-pits/