The National Nuclear Security Administration finished building the secondary stage for the first war-ready B61-12 nuclear gravity bomb on Dec. 6, the semi-autonomous Department of Energy agency said late last week.

The canned subassembly is the uranium-fueled stage of the future gravity bomb, which as part of a 20-year life-extension program will homogenize four previous versions of the venerable weapon. The subassembly is only part of the first war-ready weapon, which the semiautonomous Department of Energy nuclear-weapon steward says it will build by September 2020.

Consolidated Nuclear Security, the Bechtel-led joint venture managing the Y-12 National Security Complex in Oak Ridge, Tenn., finished the B61-12 canned subassembly four months ahead of schedule, the NNSA said in a press release.

Including the one finished in December, Y-12 is scheduled to ship eight canned subassemblies to the Pantex Plant in Amarillo, Texas, by March, the agency said. Consolidated Nuclear Security also manages that facility, where it assembles and disassembles nuclear weapons.

The NNSA plans to finish the final B61-12 in 2024. The weapon is designed to fly aboard versions of the B-2, the planned B-21, F-15, F-16, F-35, and the German-made PA-200, according to the Air Force Nuclear Weapons Center. The nonprofit Federation of American Scientists estimates the NNSA will build some 480 B61-12 bombs.

The B61-12 will be an adjustable-yield gravity bomb. The DoE Los Alamos National Laboratory in New Mexico designed and built the first B61 in 1968. Including Air Force and NNSA work, the B61-12 and its new guided tail kit will cost between $11.5 billion and $13 billion over about 20 years, according to NNSA and Pentagon documents. The NNSA’s share of the work comes to a little over $8 billion, the agency estimated in its 2019 Stockpile Stewardship and Management Plan.

B61-12 is part of the ongoing, 30-year nuclear modernization and maintenance plan started by the Barack Obama administration in 2016 and augmented by the Donald Trump administration last year in the 2018 Nuclear Posture Review.