The United States' ultimate fear might be the sight of a mushroom cloud floating over the city of New York.

That seemed the stuff of science fiction films until the attacks of 9/11 proved the United States was vulnerable to a terrorist attack. But 9/11 would look like a mere practice run compared to the tsunami of destruction that a nuclear-armed terror group could unleash.

That is a big reason why U.S. President Barack Obama is hosting the Global Nuclear Security Summit on April 12th and 13th in Washington, D.C. Obama has on numerous occasions said he views nuclear terrorism as the most deadly threat to global security and wants to help develop a plan of action to secure vulnerable nuclear materials within four years.

And while most experts agree the odds of a terror group mounting a successful nuclear attack on the United States is low, the devastation would be so great that Obama is taking that possibility very seriously.

The summit's purpose is to figure out how to start securing vulnerable nuclear material worldwide, prevent nuclear smuggling, foil attempts at nuclear terrorism and to demonstrate commitment to the issue, and more than 40 heads of state will descend on the U.S. capital to discuss what each is willing to do.

But some nations place a higher priority on the issue than others, and that could present a hurdle.

Indeed, when it comes to nuclear security, there are roughly two camps -- the "nuclear threatened," or those nations who see nuclear terrorism as a major national security risk, and the " nuclear non-threatened," or those who do not.

The "nuclear threatened" tend to be developed nations targeted by groups such as al-Qaida, especially the United States. The " nuclear non-threatened" tend to be developing countries, although there are a number of exceptions, such as India, which has been the target of terror attacks.

"There is a significant perception gap between in particular the United States and the developing world on how serious the threat of nuclear terrorism is," said Kenneth Luongo, president of the Partnership for Global Security, a non profit promoting efforts to secure weapons of mass destruction.

"The reason is that 9/11 is seared into the American psyche and we are obviously not as well protected as we could be and we are target number one," he said.

While no one is against keeping nuclear materials out of terrorists' hands, many developing countries must contend with what they view as more pressing problems, such as food, clean water, jobs and economic development, he said.

And securing nuclear materials is no easy task for the roughly 50 nations worldwide that have them, as such operations include tightening border security at myriad ports and border crossings to make sure no bomb making materials get in or out. Unprotected border areas are also a concern.

Civilian nuclear facilities containing the type of highly enriched uranium used to construct a nuclear weapon would also have to be secured -- many are located on university campuses -- as many are not as well guarded as they should be, experts said.

Worldwide, there are enough available nuclear materials to build 120,000 nuclear bombs, and al-Qaida continues to pursue weapons of mass destruction, according to the Fissile Materials Working Group, an organization collaborating in a series of meetings designed to create consensus on controlling fissile materials.

Still, costs could inhibit practical action in some developing countries, said James Acton, associate in the nuclear policy program at the Carnegie Endowment for International Peace think tank.

Some countries are also "allergic" to policy prescriptions originating in the United States. And while Obama is working to remedy those sensitivities, the balancing act will be difficult, Luongo wrote in a recent article.

But in spite of these obstacles, the summit will be a valuable forum in which to hear every nation's priorities, said Olga Oliker, senior international policy analyst at the Rand Corporation think tank.

Luongo said the summit will likely focus on re-emphasizing mechanisms already in place, such as conventions that many countries have not yet singed onto. That includes the Global Initiative to Combat Nuclear Terrorism -- an agreement of more than 70 nations intended to prevent terrorists from obtaining a nuclear device by safeguarding nuclear materials. Another is the International Atomic Energy Agency's Additional Protocol, an agreement to strengthen safeguards.

More generally, the gathering will seek agreement that a threat exists and what it entails, ask participants to comply with existing mechanisms and to bring their own ideas to the table, he said.

Oliker said the summit should be seen as one in a series of events intended to make headway on the issue.

"I would not expect a giant leap forward from the summit but we might expect some movement toward a better understanding of each others' goals," she said.