Nuclear security

The alarming progress of a nuclear North Korea

Amid all the bluster and sabre-rattling, how do scientists assess North Korea's ability to launch a nuclear attack on its neighbours or on the mainland of the United States?

The fourth and fifth nuclear tests this year generated a swirl of fearsome rhetoric - both Seoul and Pyongyang said they would turn the other into a heap of ashes.

But can we see clearly what the actual scientific situation is? Stripping away the chest-thumping, what could Kim Jong-un's military actually do? And when might it be able to do do it?

There are different components to a nuclear arsenal: it needs a warhead small enough to go on a missile, and to remain intact until it hits the target. And, of course, the missiles themselves, ideally small and mobile enough to avoid being destroyed pre-emptively if preparations for launch are spotted - hence the importance of submarines.