The National Nuclear Security Administration has authorized the production engineering phase for its B61-12 warhead life extension program, the administration announced Monday.

The milestone comes after four years of work in the development/engineering phase of the program and marks the final development phase before production, the NNSA said.

The warhead's first production unit is planned for Fiscal 2020, followed by full-scale production.

The B61 currently contains some of the oldest components in the U.S. arsenal, and the life extension program will add at least 20 years, the administration said.

NNSA and the U.S. Air Force jointly run the life extension program, seeking to preserve a critical element of the U.S. nuclear triad and the extended deterrent.

The life-extension program refurbishes nuclear and non-nuclear components to extend a bomb's service life while improving security, safety and reliability.

The B61-12 will replace existing B61-3, -4, -7 and -10 bombs and will allow for the retirements of the B83-1, the last megaton-class weapon in the U.S. nuclear arsenal.