Damien LaVera, a spokesman for the National Nuclear Security Administration, said the department had not lost its crucial skills, but he acknowledged that retaining experienced weapons scientists and training a new generation of scientists were "an ongoing concern."
At issue with the W76, at least in part, is a classified component that was used in the original weapon but that engineers and scientists at the Energy Department's plant in Oak Ridge, Tenn., could not duplicate in a series of efforts over the last several years.
The component, known by the code word "fogbank," is thought to be made of an exotic material and is crucial to a hydrogen bomb reaching its designed energy level in the microseconds before it blows apart.
The W76 is designed to release energy equal to about 100 kilotons of TNT, through both fission and fusion of atoms.
When it came time to make new batches of fogbank for the refurbishment program, the current workforce was unable to duplicate the characteristics of the batches made in the 1970s and 1980s, according to a March report by the Government Accountability Office.
"I don't know how this happened that we forgot how to make fogbank," Coyle said. "It should not have happened, but it did."
Given the problems, the technical staff at the Pantex plant was stunned by the Energy Department announcement in February that the warhead had been delivered to the Navy, according to an engineer who spoke on condition of anonymity.
B&W Pantex, the private company that operates the plant, was still awaiting delivery of a classified part from another facility and cannot assemble the warhead, the engineer said.
Navy spokesman Lt. Clay Doss told The Times on Thursday: "We have not received delivery of any refurbished W76 warheads. The answer is none."
LaVera defended the accuracy of the February announcement, saying a federal council had decided to accept the final design of the weapon and therefore it was technically a part of the stockpile.
The failure of the Energy Department to actually deliver a W76 was brought to the attention of The Times by the Project on Government Oversight, a Washington-based watchdog group that has long expressed concern about poor performance at the nation's weapon sites.
"NNSA gets away with producing shoddy work . . . and even lying to the public," said Danielle Brian, the group's executive director. "Our confidence in the stockpile cannot depend on lies."
