"The radiation portals that were deployed in the aftermath
  of 9/11 are essentially fine, except for three problems:
  They won't find a nuclear bomb, they won't find highly
  enriched uranium, and they won't find a shielded dirty
  bomb," says Stephen Flynn, a terrorism expert and
  president of the Center for National Policy. "Other than
  that, they're great pieces of equipment."

In other words, we only have a "security theater" ability to prevent radiological weapons from entering the border. Like the TSA, this theatrical security creates jobs, consumes tax wealth, and enables politicians to say they took action to address the problem while simultaneously giving them someone to blame when an attack occurs.