Southeastern Alabama is the epicenter of an exercise this week testing officials’ ability to respond to a major nuclear accident — one almost on a scale of the nation’s worst nuclear disaster in 1979 at the Three-Mile Island Nuclear Power plant in Pennsylvania.

The National Nuclear Security Administration kicked off the exercise early Monday with a scenario involving hundreds of federal, state and local officials:

A truck carrying radioactive cesium, a common chemical with many medical and industrial uses, collided with a car at a rural southeastern Alabama intersection, triggering an explosion that unleashed a drifting plume of the potentially cancer-causing substance over towns, forests and agricultural fields.

In response, officials ordered mock evacuations, arranged emergency medical care for people who may have been severely contaminated and monitored the spread of the radioactive cloud.

“This is a massive exercise,” said Kevin Rohrer, a spokesman for the nuclear security administration, a part of the Department of Energy.

To add to the realism, the exact amount of the release won’t be announced until Friday, the final day of the exercise, but officials said it was significant.

“It’s enough to cause a lot of damage,” said Rohrer said. “It will have a projected plume that could affect the three states.”

As the mock disaster unfolds, federal monitoring teams will go out to test radioactive levels in soil, water, air, fish and vegetation and report back to a command center, where scientists will analyze the information and make recommendations to state and local emergency officials.

Instruments simulate all radioactive measurements, so the teams don’t have to use the real stuff, officials said.

Although no one was killed in the TMI accident, the incident led to dramatic changes in the response to nuclear emergencies, the planning for such disasters and the operation of the nation’s nuclear reactors.

“These kinds of exercises are an important part of what we do,” Rohrer said. “One of the lessons learned in response to the Three Mile Island accident was that there were multiple organizations at the local, state and federal level using a variety of different instruments, making different corrective-action recommendations, so the governor and the decision-makers were getting inconsistent advice.”

In the command center on Tuesday, representatives of several federal agencies conferred, peered at laptop screens and gazed at the ever-changing information on a large-screen television. They included members of the FBI, the Environmental Protection Agency, the Centers for Disease Control and the Coast Guard.

Federal employees will do the field testing and analysis, then they’ll make recommendations to state and local officials, including the state emergency management agencies of all three states.

The control center was set up in a couple of warehouse-sized buildings at the Houston County Farm Center, where farmers bring fresh tomatoes and cantaloupes to sell beneath striped awnings. They seemed oblivious to the drama unfolding in the warehouses.

Cesium was one of the radioactive materials that spewed from what is considered the world’s worst nuclear power accident — the 1986 Chernobyl disaster that heavily contaminated parts of three countries, produced radioactive fallout as far away as eastern North America and forced the resettlement of over 336,000 people.

“This is a national exercise, testing the capacity of the federal government to respond,” said Phil May, executive officer of the Alabama Emergency Management Agency. “It also tests the ability of the states of Alabama, Florida and Georgia to respond.”