Keeping the world's nuclear weapons under control and out of the hands of terrorists are top goals for more than 50 leaders who will gather in the South Korean capital.

They hope to build on the commitments they made at the first Nuclear Security Summit, hosted by President Obama two years ago in Washington.

"Two decades after the end of the Cold War, we face a cruel irony of history - the risk of a nuclear confrontation between nations has gone down, but the risk of nuclear attack has gone up," he said then.

Keeping highly-enriched uranium away from terrorists is imperative, says nuclear expert Joseph Cirincione, president of the Ploughshares Fund. He spoke recently in New York at a forum hosted by The Korea Society.

"The number-one threat to the national security of the United States is nuclear terrorism, a group getting a bomb or the material with which to build a bomb and detonating it in the United States - a "nuclear 9/11," Cirincione said.

The heavily-guarded demilitarized zone along the border with nuclear-armed North Korea is also on Obama's agenda. He is expected to meet there with U.S. soldiers.

Pyongyang has denounced the summit as an "unpardonable crime" and an "intolerable grave provocation."

North Korea's nuclear program raises fears in the West. But it will not be one of the summit's main agenda items, says Alexandra Toma, founder of the Fissile Materials Working Group. 

"There is no way you cannot not talk about North Korea, but I think that, certainly, Korean experts and the Korean government recognize that the Nuclear Security Summit is much more than that," Toma said.

The safety of nuclear power plants also will be a prominent topic of discussion in Seoul, especially after an earthquake and tsunami led to a nuclear plant meltdown last year in Japan.

At a recent discussion in Washington, Kenneth Luongo, president of the Partnership for Global Security, said the Fukushima disaster showed how much work must be done to improve nuclear power safety.

"And what Fukushima has highlighted for everybody, besides the fact that you can have a major nuclear accident in a highly developed country, is that we do not have an adequate system for dealing with radioactive dangers that cross border," Luongo said.

Before the summit itself, President Obama will meet individually with several other leaders, including Russian President Dmitry Medvedev and Chinese President Hu Jintao.