The US could spend more than $1 trillion (£675bn) over the next 30 years modernising its arsenal of nuclear weapons.
It wants to make them faster and more accurate.
Other nuclear states are trying to do the same, raising questions about their commitment to disarm.
Are we entering a new nuclear arms race?
The BBC World Service’s The Inquiry programme hears from four expert witnesses.
John Mecklin: ‘Technological advance is the race’
John Mecklin is editor of The Bulletin of Atomic Scientists
“It is an arms race; it’s just a different kind of arms race. It’s one which technological advance is the race. Nuclear countries are trying to make sure that the other nuclear countries don’t get some sort of technological edge.
“There’s a reasonable argument to make that 40-year-old weapons systems are unreliable and that you don’t want weapons that are this dangerous to be unreliable so modernisation doesn’t seem like an inherently crazy thing to pursue.
“For instance in the United States, the current plan is to replace all three legs of what is called the nuclear triad with entirely new weapons: a new land-based missile, a new long range nuclear bomber and new nuclear submarines. It’s estimated to be something on the order of a trillion dollars over 30 years. Russia is undergoing the same sort of rebuild too.
“The United States is upgrading its nuclear bombs to be more precision guided weapons that can be dropped from farther from target.
“Because they can be more accurate the yield on the weapon can be reduced. This does create in some the fear that you can use them and not start a worldwide war…”