Johnson latest DHS secretary to waive 100 percent cargo screening mandate.

Congress in the Safe Ports Act of 2007 stipulated that 100 percent of U.S.-bound shipping containers and cargo be scanned for nuclear threats by July 2012.

That would require large nuclear detection portals in every port in the world that has shipping containers headed for the United States.

The department has had two problems fulfilling the mandate.

First, the second-generation radiation detection portals the Domestic Nuclear Detection Office was tasked with developing never worked well enough to be deployed, and the program was canceled in 2012.

The second was the high cost of executing the mission. Former DHS Secretary Janet Napolitano testified that it could cost upwards of $12 billion.

Current DHS Secretary Jeh Johnson became the latest DHS leader to criticize the law.

"To be honest, it is a huge unfunded mandate. Congress told us, 'You got to go do this,' but they didn't give us the money to pay for it," he said at a Council on Foreign Relations speech.

There is a provision in the law that permits the secretary of Homeland Security to waive the requirement for two years. Napoli-tano did so in July 2012. Johnson did the same this year.

Still, despite a long string of secretaries and Customs and Border Protection commissioners speaking out against the practicality of the law, some members of Congress are still pushing DI-IS to fulfill the mandate.

Johnson has told them: "It's a great idea. But...