This is from June 2005, so it must have been implemented by now:
Nation’s Busiest Seaports to Have Complete Radiation Detection Coverage by End of 2005
For Immediate Release
Office of the Press Secretary
Contact: 202-282-8010
June 3, 2005
Department of Homeland Security (DHS) Secretary Michael Chertoff today announced that the nation’s busiest seaports -- Los Angeles/Long Beach, California -- will have complete Radiation Portal Monitor (RPM) coverage by year’s end.
Three terminal locations, at Piers 400, 300 and Trans Pacific, within the Port of Los Angeles are scheduled to go on-line by the end of June. A total of ninety RPMs, which will screen all international container traffic and vehicles exiting the facility for nuclear materials or hidden sources of radiation, will be operational by December 2005.
http://www.dhs.gov/dhspublic/display?content=4530
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The point is that they look to find the high risk containers, BEFORE they are shipped to the US, inspect the high risk containers, mostly also BEFORE they get here, then have additional screening in the US ports, for radiation.
And right in this press release:
"Use of Cutting-Edge Technology: CBP is currently utilizing large-scale X-ray and gamma ray machines and radiation detection devices to screen cargo. Presently, CBP operates over 680 radiation portal monitors at our nation’s ports (including 181 radiation portal monitors at seaports), utilizes over 170 large scale non-intrusive inspection devices to examine cargo, and has issued 12,400 hand-held radiation detection devices. The President’s FY 2007 budget requests $157 million to secure next-generation detection equipment at our ports of entry. Also, over 600 canine detection teams, who are capable of identifying narcotics, bulk currency, human beings, explosives, agricultural pests, and chemical weapons are deployed at our ports of entry. "
it's what they had at the RNC convention. You had to walk through radiation detectors to get into the convention. Similar.