The B61, a thermonuclear weapon originally produced in the 1960s as a strategic bomb, underwent an eight-year process to extend the lifecycle of the weapon past its original design, the NNSA reported.

As a result of international protocols banning nuclear weapons testing, refurbishing the B61 arsenal had to be conducted strictly through science-based research and development. Officials say the United States has not produced any new nuclear bombs since the end of the Cold War.

"This is the culmination of an ambitious continuing effort which helped to ensure that the nation's aging nuclear weapons stockpile continues to be reliable," Robert Smolen, NNSA deputy administrator for defense programs, said in a statement.

"Nuclear scientists, engineers and technicians across NNSA's national security enterprise contributed to this effort."

The U.S. Air Force's B-52H and B-2A bombers are equipped to use the B61.