The National Nuclear Security Administration (NNSA) said Monday it has officially OK’d the production engineering phase of the B61-12 warhead life-extension program, marking the program’s entry into the final phase of development before production.

This milestone follows four years of work in the development-engineering phase, NNSA said, noting that the first production unit is expected in fiscal 2020. Production is projected to finish by 2024.

A B61 bomb in various stages of assembly.
A B61 bomb in various stages of assembly.

The NNSA and the Air Force began the life-extension program in 2010 to consolidate four versions of the B61 nuclear bomb into a refurbished B61-12, a program estimated to cost approximately $8.9 billion. The life-extension program is expected to add no less than 20 years to the service life of the B61 system and will cut in half the number of nuclear gravity bombs in the U.S. stockpile.

The Los Alamos, Sandia, and Lawrence Livermore national laboratories contribute to the life-extension program, in addition to the National Security Campus and Pantex production plants.

“Once completed, the B61-12 LEP will allow for the retirement of the B83-1—the last megaton-class weapon in America’s nuclear arsenal—while supporting the nation’s continued commitment to our national security and that of our allies and partners,” Secretary of Energy Ernest Moniz said in a statement.

The NNSA’s budget proposal for fiscal 2017 includes $616 million to remain on track to deliver the first B61-12 production unit by March 2020, NNSA Administrator Frank Klotz said earlier this year. The House and Senate have begun a conference to reconcile differences in their respective versions of the fiscal 2017 National Defense Authorization Act; the NNSA request for the B61-12 has so far been matched in the legislation.