Rogue elements within a state’s security forces enrich dozens of kilograms of uranium. It’s then assembled into a crude nuclear bomb. The bomb is transported on a civilian aircraft to Washington D.C, and loaded onto a delivery truck. The truck is driven by an American citizen midway between the White House and the Capitol Building. The driver casually steps out of the vehicle, and detonates the weapon. There are more than 80,000 instant deaths. There are also at least 100,000 seriously wounded, with nowhere left to treat them.

It’s likely that one of those immediately killed would be Samantha Pitts-Kiefer, who works only one block away from the White House.

Samantha serves as Senior Director of The Global Nuclear Policy Program at the Nuclear Threat Initiative, and warns that the chances of a nuclear terrorist attack are alarmingly high. Terrorist groups have expressed a desire for nuclear weapons, and the material required to build those weapons is scattered throughout the world at a diverse range of sites – some of which lack the necessary security.

When you combine the massive death toll with the accompanying social panic and economic disruption – the consequences of a nuclear 9/11 would be a disasterare almost unthinkable. And yet, Samantha reminds us – we must confront the possibility.

Clearly, this is far from the only nuclear nightmare. We also discuss:

* In the case of nuclear war, what fraction of the world's population would die?

* What is the biggest nuclear threat?

* How concerned should we be about North Korea?

* How often has the world experienced nuclear near misses?

* How might a conflict between India and Pakistan escalate to the nuclear level?

* How quickly must a president make a decision in the result of a suspected first strike?

* Are global sources of nuclear material safely secured?

* What role does cyber security have in preventing nuclear disasters?

* How can we improve relations between nuclear armed states?

* What do you think about the campaign for complete nuclear disarmament?

* If you could tell the US government to do three things, what are the key priorities today?

* Is it practical to get members of congress to pay attention to nuclear risks?

* Could modernisation of nuclear weapons actually make the world safer?