It is hard to imagine, but it could happen anywhere in the world - an atomic blast set off by terrorists.  The U.S. military is training for that possibility in the United States.  More than 3,000 Army troops and National Guard - the state militias - are in Indiana to learn how to respond to a nuclear explosion.  Our correspondent visited a training center that is set up to simulate an attack.

The Muscatatuck Urban Training Center in southern Indiana simulates the aftermath of an atomic bomb blast in the largest city in Indiana, Indianapolis.  Buildings are on fire and massive debris has piled on roads.  Survivors are in shock and need medical help.

National Guard, Army and other emergency groups are working together to respond to the chaos where 100,000 people would likely be killed and up to one million more forced to evacuate the city. 

Army Colonel James Larsen is the chief of training exercises:

"The most important thing that they can learn is how to operate in this environment," he said. "We don't often train to do it in an environment, what we call a dirty environment, where there is a radiological enemy here."

In a real nuclear attack, hundreds of thousands of people would have to be decontaminated from radiation.  During this mock exercise, water is used instead of chemicals to decontaminate people who play the role of survivors.

Army Lieutenant Zacharia Davis, a nurse, says it was sometimes difficult to provide help quickly during the exercise, because of a lack of communication among military groups.

"A lot of units from different areas, different parts of the country have to integrate, work as a team, and that's why we come here to practice because we need to be ready in case something does happen," she said.

The training was staged to be as realistic as possible and the soldiers did not know what would happen next. 

Helicopters fly overhead and real local ambulances respond to emergencies.   A speaker sends out emergency messages. 

Police capture a criminal who broke out of jail.  There are even mock journalists and photographers.

Michael Reimer is with Safety Solutions, a company that created the rubble piles.

"We've brought in tons of rubble that we get from other abandoned buildings," he said. "We've poured some concrete on site and then vehicles we get from a local scrap yard.  We crush them with the victims in place, such as mannequins."

Soldiers search the debris pulling out the mannequin victims.

Zachary Thomas, with the South Carolina National Guard, says it was tough.

"There's a lot of things to clear and get over," he said. "Cars turned over.  A couple of fires."

During the exercises, military observers stay in the background taking notes with suggestions for improvement.  Besides the actual performance, they look at how long it takes the soldiers to complete their tasks, since time can make the difference between survival or death for those who are lucky enough to be alive after the nuclear blast.