WASHINGTON-- Still reeling from attacks in Brussels and Paris, world leaders are wrestling this week with the chilling prospect of the Islamic State of Iraq and Syria (ISIS) or other extremists unleashing a nuclear attack on a major Western city.
The terror attacks in Brussels are raising new questions about the security of nuclear plants, CBS News correspondent Jericka Duncan reported.
Months before the deadly bombings, it's believed at least two of the suspected terrorists were targeting a nuclear plant in Belgium. The El Bakraoui brothers cased the Doel Facility and secretly videotaped a top nuclear researcher there.
"If terrorists were able to successfully attack a nuclear facility, there could be a serious loss of life," said Dr. Page Stoutland of the Nuclear Threat Initiative. "It's no question that the news from Belgium will lead to countries as well as facility operators re-examining their plans."
There are more than 400 nuclear plants in the world, but no uniform security measures to keep them safe.
At the first ever nuclear summit in 2010, President Obama called for a common security approach.
"The most effective way to prevent terrorists and criminals from acquiring nuclear materials is through strong nuclear security," the president said.
Preventing terrorists from obtaining nuclear materials is the central focus as Mr. Obama hosts leaders from roughly 50 countries for a nuclear security summit starting Thursday. Despite three previous summits and six years of White House prodding, security officials warn that the ingredients for a nuclear device or a "dirty bomb" are alarmingly insecure.
"We know that terrorist organizations have the desire to get access to these raw materials and to have a nuclear device," said Ben Rhodes, President Obama's deputy national security adviser. Still, the White House said there was no indication of an imminent plot.
On Wednesday, Mr. Obama penned an op-ed in the Washington Post on nuclear security.
"Of all the threats to global security and peace, the most dangerous is the proliferation and potential use of nuclear weapons. That's why, seven years ago in Prague, I committed the United States to stopping the spread of nuclear weapons and to seeking a world without them," Mr. Obama wrote.
Decades after the Cold War, the threat of a nuclear war between superpowers has given way to growing concerns about non-state actors, including ISIS and al Qaeda offshoots operating in North Africa , Yemen and Saudi Arabia. Although the U.S. and its allies still worry about North Korea, Mr. Obama believes the threat posed by Iran has subsided due to the nuclear deal, leaving extremist groups among the likeliest perpetrators.
The havoc such an attack could wreak in an urban area like New York or London is concerning enough that leaders scheduled a special session on the threat during the two-day summit. U.S. officials said the leaders would discuss a hypothetical scenario about a chain of events that could lead to nuclear terrorism.
Those concerns have taken on heightened significant following the March 22 attacks at a Brussels airport and subway station.
On the summit's sidelines, President Obama planned to meet with the leaders of China, South Korea and Japan, who all share U.S. concerns about North Korea's nuclear program.
Yet other key players will be missing. Russian President Vladimir Putin refused to attend, as Moscow scoffed at what it deemed U.S. efforts to take control of the process. Pakistani Prime Minister Nawaz Sharif canceled his trip following an Easter bombing that killed 72 people.
Some 2,000 metric tons of highly enriched uranium and separated plutonium being used in civilian or military programs could be turned into a nuclear bomb if stolen or diverted, the White House said. And fewer than half of the countries participating in the summit have even agreed to secure their sources of radiological material, needed for a dirty bomb.
"The policies are moving in the right direction," said Joe Cirincione, who runs the nuclear security group Ploughshares Fund. "But when you're fleeing a forest fire, it's not just a question of direction, it's a question of speed."
Nuclear security experts say there are four potential scenarios for a nuclear-related attack by an extremist group. Some are more likely than others.
The most devastating but improbable scenario involves a group stealing a fully functional bomb from a nuclear-armed country. Most nuclear experts point to Pakistan as the likeliest source, though that would require cooperation with someone on the inside of Pakistan's military.
Easier to pull off would be for ISIS or another group to obtain fissile material like highly enriched uranium, then turn it into a crude nuclear device delivered by truck or ship. A third possibility is that extremists could bomb an existing nuclear facility, such as the Belgian waste plant, spreading highly radioactive material over a wide area.
The most likely scenario that security experts fear is that a group could get ahold of radioactive material, such as cesium or cobalt, for a dirty bomb that could be carried in a suitcase. Those materials are widely used in industrial, academic and hospital settings, with no consistent security standards across the globe. Last year, an Associated Press investigation revealed multiple attempts by black market smugglers to sell radioactive material to Middle East extremists.
Unlike a nuclear bomb, the only people killed instantly by a "dirty bomb" would be those close to the blast site. But the blast could spread cancer-causing substances over a vast area, triggering panic and evacuations.
"Even if it is small, such an incident would create such havoc in the world that you have to take it quite seriously," said former Ambassador Wendy Sherman, who spearheaded U.S. nuclear negotiations with Iran and North Korea before joining the Albright Stonebridge Group.
Detonated in a major city, a dirty bomb could cause tens of billions of dollars in economic damage, said Andrew Bieniawski, who studies materials security at the Nuclear Threat Initiative. People and businesses would have to be relocated -- potentially for years -- while the contamination is cleaned up. Few would be inclined to ever go back, a reality on display in Chernobyl, Ukraine decades after the 1986 accident.
Reducing the threat of nuclear terrorism has been a persistent theme for Mr. Obama, who won the Nobel Peace Prize after emphasizing nuclear disarmament. Four months into his presidency, he warned in a much-cited speech in Prague that nuclear weapons were "the most dangerous legacy of the Cold War."