Barack Obama speaks at the first plenary session of the 2012 Nuclear Security Summit in Seoul. Photo: AFP/Saul Loeb

The US President, Barack Obama, has warned global leaders that there are “still too many bad actors” in the world trying to get their hands on nuclear material, which could result in a terror attack that kills large numbers of people.

“These dangerous materials are still vulnerable in too many places,” Mr Obama said at the opening session of the Nuclear Security Summit in Seoul, which has drawn more than 40 world leaders, including the Prime Minister, Julia Gillard. “It would not take much, just a handful or so of these materials, to kill hundreds of thousands of innocent people and that’s not an exaggeration, that’s the reality that we face.”

Mr Obama, who inaugurated the first nuclear security summit in Washington DC in 2010, warned against “complacency” in preventing loose nuclear material from getting into the hands of terrorist groups. The legacy of the Soviet Union’s breakup, inadequate atomic stockpile controls and the proliferation of nuclear-fuel technology mean the world has lost precise count of atomic material, which could be used to make a weapon.

There are at least two million kilograms of stockpiled weapons-grade nuclear material left over from decommissioned bombs and atomic-fuel plants, according to the International Panel on Fissile Materials, a nonprofit Princeton, New Jersey, research institute that tracks nuclear material. That’s enough to make at least 100,000 new nuclear weapons on top of the 20,000 bombs already in weapon-state stockpiles.

Because a terrorist needs only about 25 kilograms of highly enriched uranium or eight kilograms of plutonium to improvise a bomb, the margin of error for material accounting is small.

“There is no effective way to deter terrorist groups from using nuclear materials once they have it,” South Korean President Lee Myung-bak said at the opening session. “The most optimal way to prevent nuclear terrorism is to promptly minimise and eventually eliminate excess nuclear materials, which can be used as ingredients for nuclear weapons.”

A nuclear-armed terrorist attack on the US port in Long Beach, California, would kill 60,000 people and cost as much as $US1 trillion in damage and cleanup, according to a 2006 Rand study commissioned by the Department of Homeland Security. Even a low-level radiological or dirty-bomb attack on Washington, while causing a limited number of deaths, would lead to damages of $US100 billion, according to Igor Khripunov, the Soviet Union’s former arms-control envoy to the US. He is now at the Athens, Georgi, Centre for International Trade and Security. Leaders may pledge tighter controls over nuclear materials to keep them out of the hands of terrorists, according to the draft of a communique to be released today at the end of the meeting. Securing vulnerable nuclear material before the next Nuclear Security Summit in 2014 is the top priority, according to a copy of the six-page working document.

“The threat remains,” Mr Obama said today. “That’s why what’s required continues to be a serious and sustained effort.”