How are nuclear weapons maintained and modernised in the 21st century? America stopped explosively testing its warheads and bombs in 1992. Now the country relies on sophisticated computer simulations and energetic lasers to understand how these devices work and to keep them safe as they age. For the first time ever, America’s nuclear scientists are also having to design a brand new warhead using simulations alone.

This four-part series traces the scientific story of nuclear weapons. We go behind the scenes at America’s nuclear-weapons laboratories to find out how the country is pushing the frontiers of extreme physics, materials science and computing to modernise its stockpile.