The possibility of terrorists smuggling radioactive materials into the United States has prompted the government to beef up countermeasures at America's ports. The Port of Charleston, S.C., is the first in the Southeast to use radiation portal monitors (search), otherwise known as RPMs.

"This is just one other step in making sure that there is nothing that is going to be leaving our ports with any kind of weapon of mass destruction or any kind of radiological threat," explained Pamela Zaresk (search), U.S. Customs and Border Protection director of the Port of New Orleans.

Previously, shipping containers were subject to random inspection. But containers had to be physically opened, making it impractical to search all cargo passing through the port. With the new radiation monitors, every container is scanned as it moves out of the terminals.

"The balance here is making sure the commerce moves while we protect our nation," said Byron D. Miller of the South Carolina State Ports Authority (search). "With $120 million in cargo every day moving through this port, it's vital that we keep that commerce on the move."

The RPMs are large enough for 18-wheelers to drive through. If an RPM detects radiation, customs agents open the suspect container and use handheld monitors to determine the source — in one case, naturally-occurring radiation in a harmless shipment of granite from South Africa.

Officials say the chances of smuggling an actual nuclear weapon or weapon-making materials into the United States are reduced by a system of checks that includes offshore boarding of ships, X-ray machines and improved security at international ports.

"Cargo coming to the United States is boarded on ships in secure facilities where the enforcement is roughly equivalent to what it is here in the United States," said Capt. John Cameron of the U.S. Coast Guard.

RPMs give inspectors one more tool in preventing illegal radioactive materials from reaching their intended targets.

Click in the video box above to watch a full report by FOX News' Jonathan Serrie.