By Gordon Corera
BBC security correspondent
The International Atomic Energy Agency, the UN's nuclear watchdog, is holding a meeting on Wednesday to look at the security of nuclear arms and material.
The London meeting comes amid growing fears that stockpiles have not been sufficiently protected from terrorists.
The end of the Cold War left the former USSR with around 30,000 nuclear weapons and the material to build 80,000 more.
But less than 50% of this has been fully secured - through reprocessing, better protection, or destruction.
During the 1990s, international commitment was uneven.
But in the last few years a growing awareness of the desire of terrorist groups to acquire nuclear weapons has heightened efforts to protect material in the former Soviet Union as well as 40 other countries.
The conference is designed to bring together the world's experts on the subject.
The keynote speaker is former US Senator Sam Nunn, who has been leading American efforts to address the problem for more than a decade.
He told the BBC that it was a race between catastrophe and co-operation to keep material and weapons safe.
He also said he hoped that the G8 meeting this year under the British presidency would lead to an increased commitment.