The National Nuclear Security Administration is almost finished with a proof-of-concept unit that will kick off mass production of the homogenized version of the B61 nuclear gravity bomb, an agency spokesperson said Monday.

“The B61-12 program is nearing the completion of the FPU [first production unit] milestone, and we expect to make an announcement later this calendar year,” a spokesperson at the National Nuclear Security Administration (NNSA) headquarters in Washington wrote in a Monday email.

NNSA is the part of the Department of Energy that maintains and modernizes nuclear weapons and the U.S. nuclear weapons production complex.

B61-12 will replace four existing versions of the oldest deployed U.S. nuclear weapon, including one that had what the Air Force called modest earth-penetrating capability.

After replacing unusable capacitors needed by multiple bomb components, the NNSA said last year that the B61-12 first production unit would be ready in November 2021.

The NNSA plans to refurbish some 460 of the bombs, according to estimates from the Washington-based Federation of American Scientists.

Including Air Force and NNSA work, the B61-12 will cost between roughly $11.5 billion and $13 billion over about 20 years, according to documents from the Energy and Defense departments. The NNSA’s share of the bomb’s cost is about $8 billion, the agency estimated.

Among other aircraft, the F-35A will carry B61-12, which with its Boeing-designed tail kit will have what the Air Force calls a modest standoff capability.