The US has spent billions of dollars over the past 20 years to reestablish pit manufacturing at scale. With prodding from Congress, the Department of Energy’s National Nuclear Security Administration (NNSA) will spend billions more over the next decade to build two pit factories.

But the NNSA has scrapped four different plans to establish pit factories over the past two decades, notes a January report from the Government Accountability Office (GAO). And as the agency embarks on the latest one, questions linger: How long will a plutonium pit last? How many new pits are needed and when? How many retired pits could be recycled? How long will it take, and how much will it cost to reconstitute manufacturing capability?

Pits—hollow, grapefruit-sized spheres cast from about 3 kg of plutonium—are designed to implode when compressed by the high explosive surrounding them. The prompt critical mass that results fissions and creates a...

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