By Humphrey Hawksley
Los Angeles police officers take part in a simulation of a dirty bomb explosion
The proliferation of weapons of mass destruction is once again at the top of security agenda - and not only from hostile states such as Iran and North Korea.
President Barack Obama has turned his focus to stockpiles of dangerous chemical and radiological materials that remain at risk from being stolen by terror groups planning to make what is known as a dirty bomb.
Much of it is in the former Soviet Union, but it's also being kept in other parts of the world by countries that have nuclear power plants or had plans to build a nuclear bomb, but abandoned them.
American intelligence agencies believe a dirty bomb strike is almost inevitable in a major city within the next five years, and last month the British government conceded that such an attack was highly likely and could happen without warning at any time.
In his Nato speech in Prague, Mr Obama set a deadline and pledged to clear up all unsecured nuclear material within four years.
"We must ensure that terrorists never acquire a nuclear weapon. This is the most immediate and extreme threat to global security. One terrorist with one nuclear weapon could unleash massive destruction," he said.
"We know that there is unsecured nuclear material across the globe. To protect our people, we must act with a sense of purpose without delay."
But it might not be that easy.
The sight of nuclear installations in the former Soviet Union protected only by a broken fence and rusting padlock was common in the 1990s. Plans were put in place to make it all secure, but they were never fully carried out.
Figures vary, but it's thought that between 40% and 60% of all dangerous materials remain unsecured and at risk of theft. Much of it is still hidden or governments are not allowing international inspectors in to check on the safety measures they have put in place.
US Senators Sam Nunn and Richard Lugar during a trip to Moscow in 2007
"No-one knows the extent of the danger because we don't know the extent of the proliferation," says US Senator Richard Lugar who, together with former Senator Sam Nunn, initiated the main American programme to make nuclear material secure.
"Some nations are not co-operative and are not prepared to make available their material that might have been left over."
Senator Lugar's office wall is covered with photographs on the subject. One is of him in Russia with then Senator Barack Obama looking at nuclear armed submarines. In another, he is holding a briefcase. Inside this is a small chemical warhead - precisely the type of weapon that it's feared terror groups could get hold of.
"In the event that you used this one correctly, you could kill all the people in an 80,000 seat stadium," he says. "If something that could kill that many people could be carried around at that size, this is a very difficult thing to stop."
Most countries, friend or foe, guard their nuclear secrets closely, with governments claiming they can be responsible for their own lethal material.
"There are nuclear materials that are inadequately secured in dozens of countries all over the world," says Matthew Bunn of Harvard's Belfer Center and author of an annual report called Securing the Bomb.
"My belief is that pretty much every country where there are nuclear weapons or plutonium, or highly enriched uranium that is used to make nuclear weapons, has more to do to improve security."
2004 Satellite image shows the Pelindaba nuclear research facility. Picture: GeoEye
The issue was highlighted in South Africa in November 2007 when raiders broke through the perimeter fence of the nuclear site in Pelindaba near Pretoria. While South Africa gave up its nuclear weapons programme many years ago, it kept its dangerous, highly enriched uranium.
"One of the raiding teams went through a 2000-volt security fence, disabled the intruder detector so no alarm went off, reached the emergency control centre where they got into a struggle with a worker and shot him in the chest," says Mr Bunn.
"They spent 45 minutes inside the guarded perimeter of the site without ever being engaged by the site security force. Then they went back through the fence at the same point they went in and they've never been caught. The United States government offered to help the South Africans install better security, but South Africa said they didn't need help because they knew how to handle these materials."
There lies the crux of President Obama's problem. If all nuclear and radiological material is to be made safe, there has to be complete international cooperation. So far that is not forthcoming.
"There's way too much of it," says Tim Roemer, a close Obama adviser who was on the commission that predicted a dirty bomb attack within five years. "This needs to be the number one problem of the president and vice president to work on every day."