The fourth and final Nuclear Security Summit under President Obama has brought more than 50 world leaders to Washington. But icy relations with Russia have left Moscow out in the cold.
The two-day Nuclear Security Summit (NSS) in Washington wraps up Friday without Russia, which hosts the world's largest repository of nuclear material.
Leaders from more than 50 countries are meeting for the fourth and final time under the leadership of US President Barack Obama. (What will happen with the initiative after Obama's term ends in January is unclear).
The leaders are taking stock of their progress over the six years since the first NSS in 2010 and refocusing on the threats that remain, according to Ben Rhodes, deputy national security adviser for strategic communications in the Obama administration.
Before the summit, Rhodes pointed to the threat posed by international terrorists, such as the "Islamic State" (IS), who could obtain and use radioactive material for a dirty bomb in a future attack.
"ISIL clearly is an organization that poses a threat not just to individual countries, but to global security," Rhodes said, using a common acronym for IS.
It is believed that over the past two decades there have been 2,800 cases of nuclear material disappearing around the world, though reportedly "only" 400 involved theft.
No dirty bombs yet
Given that the material is out there and militant groups are apparently looking for it, many wonder why terrorists have not yet carried out a dirty bomb attack - one that would spread radiation across a target area without an actual atomic explosion.
Oliver Meier, a security expert at the German Institute for International and Security Affairs in Berlin, said there were a number of factors.
"Intelligence agencies have probably prevented some attacks," Meier said. "It doesn't seem to be a priority for the terrorists, but it may also be luck."
Beyza Unal, a nuclear weapons expert at Chatham House in London, said making such a bomb was easier said than done. Even though the material is out there, it is not easy to find, and some of it disintegrates quickly.
"Some materials decay in eight days, so you can't use it after that," Unal said, adding that that would create logistical problems for would-be terrorists, who often cannot pivot so quickly from securing the material to carrying out an attack.
Russia could play a key role in an agenda aimed at securing loose nuclear materia,l if for no other reason than that much of the least-guarded radioactive and fissile material is either in the country itself or its former satellite states in Central Asia and Eastern Europe.
But, Unal said, much can be accomplished at the summit even without Russia.
"There is still value added, because it's not Russia that drives the process," Unal said. "More than 50 states are talking about securing nuclear material. And if Russia wants to come back to the table, they can."
Obama's goal: Eliminate nukes
The foundation for the biennial summits was laid during a landmark speech that Obama gave in Prague in 2009. At the time, he called for the eventual elimination of all nuclear weapons from the planet.
The four pillars of Obama's speech were nuclear disarmament, nuclear nonproliferation, nuclear security and nuclear energy, and that is what drives this week's agenda.
Russia attended the three previous summits but dropped out this year as relations between Moscow and Washington and its allies have chilled.
Two years ago, following Russia's annexation of Crimea and at least tacit support for separatists in eastern Ukraine, the country was kicked out of the international club of major industrialized nations now known as the Group of Seven. The remaining members are the US, United Kingdom, Germany, Japan, France, Canada and Italy.
Despite Russia's tense relations with the United States and its allies, Meier said there had been points of significant cooperation in recent months, including the international negotiations that led Iran to decommission the weapons aspect of its nuclear program.
Despite that agreement, which led to the lifting of onerous international sanctions, Iran's government was not invited to this year's summit because the country retains close ties to militant groups widely seen as its proxy fighters across the Middle East - in places such as Lebanon, Syria, Israel and the Palestinian territories.
"Iran is still criticized for its relationships with nonstate actors such as Hamas and Hezbollah," Unal said.
Another nuclear state not invited to the summit was North Korea, arguably the most repressive and isolated country on the planet. Its nuclear material is thought to be out of reach of foreign militants.
"North Korea is a total outlier," Unal said. "They're not part of international treaties, and their nuclear material is not seen as being at risk of falling into the hands of terrorists."