The Lawrence Livermore National Laboratory in the San Francisco Bay area and the Los Alamos National Laboratory in New Mexico are competing to design the nation's first new nuclear bomb in two decades.
Scientists at both facilities are working around the clock on plans that will be presented to the Nuclear Weapons Council, a federal panel that oversees the nation's nuclear weapons. The council will choose a winner later this year.
"I have had people working nights and weekends," said Joseph Martz, the head of the Los Alamos design team. "I have to tell them to go home. I can't keep them out of the office."
Congress approved the new bomb, known as the reliable replacement warhead, with bipartisan support in 2005 as part of a defense spending bill. The weapon would, by law, have the same explosive power as existing warheads.
Proponents of the project say the U.S. would lose its so-called "strategic deterrent" unless it replaces its aging arsenal of about 6,000 bombs, which will become potentially unreliable within 15 years. A new, more reliable weapon, they say, would help the nation reduce its stockpile.
Critics say the project could trigger a new arms race with Russia and China, and undercut arguments that countries such as Iran and North Korea must stop their nuclear programs.
The United States and Russia signed a treaty in 2002 calling for the countries to each cut nuclear inventories to between 1,700 and 2,200 warheads by 2012.