IBM's Sequoia has taken the top spot on the list of the world's fastest supercomputers for the US.
It is the first time the US claim poll position since it was beaten by China two years ago.
Sequoia will be used to carry out simulations to help extend the life of aging nuclear weapons, avoiding the need for real-world underground tests.
It is installed at the US Department of Energy's Lawrence Livermore National Laboratory in California.
"While Sequoia may be the fastest, the underlying computing capabilities it provides give us increased confidence in the nation's nuclear deterrent," said National Nuclear Security Administration (NNSA) administrator Thomas D'Agostino.
"Sequoia also represents continued American leadership in high performance computing."
The computer is capable of calculating in one hour what otherwise would take 6.7 billion people using hand calculators 320 years to complete if they worked non-stop.
Although the US's efforts helped secure it the lead, its overall tally of three computers in the top 10 was worse than six months ago when it had five.
China and Germany both have two supercomputers, while Japan, France and Italy have one.
But IBM proved to be the leading manufacturer claiming five out of the top 10 spots.