CBO Paper: Scanning and Imaging Shipping Containers Overseas: Costs and Alternatives [open pdf - 3MB]
"Each year, about 12 million shipping containers enter U.S. ports. After the September 11, 2001, attacks, concern arose that terrorists might use containers to smuggle weapons of mass destruction--particularly nuclear weapons--into the country. To reduce that threat, the federal government implemented several security measures. Among them, Customs and Border Protection (CBP), an agency of the Department of Homeland Security (DHS), scans every container entering the United States by sea or land to detect radiation. CBP also identifies about 5 percent of all incoming seaborne containers as high risk, and it inspects those containers with X-ray or gamma-ray imaging systems. The agency opens and examines containers if the images suggest that the cargo is potentially dangerous or does not match the manifest. In 2007, the Congress mandated that DHS use both radiation detectors and imaging systems to scan and image all incoming seaborne containers before they are loaded onto a U.S.-bound ship. That approach would shift the radiation scanning and nonintrusive imaging from U.S. ports to overseas ports, with the goal of detecting any serious threats before they arrive. The approach also would aim to image all containers instead of limiting the use of expensive imaging resources to high-risk containers. The law gave DHS until 2012 to fully implement this system, but the deadline has been extended three times and is now 2018. The Congressional Budget Office examined five options that illustrate the cost and implications of meeting the mandate as well as alternative approaches to increase the scanning and imaging of containers."
Congressional Budget Office: http://www.cbo.gov/