While nuclear smuggling is possible, so are dozens of other attack scenarios. Overinvesting in countering one tactic when terrorists could easily employ another is dangerously myopic.
Unless we get to the root of the problem - religious and ideological - our free and commercial society will always be vulnerable to attack. To make it completely safe we'd have to become a totalitarian and economically dysfunctional society like North Korea.** Terrorism in North Korea is owned by the government and used against the people, but they're "safe" from outside "threats."
It's time to face reality. Mosques and Islamic centers are the problem. Saudi Arabia, among many others, but especially Saudi Arabia, fund the jihad ideology being taught to those who want to kill us. Even in the United States, Saudi taught imams control over three quarters of the mosques. That's the root of terrorism: jihad against the kufr (unbelievers). Islamic imperialism and Islamic supremacism are already here. Screening containers is an important issue but not the most important issue.
**Recent report on worldwide religious freedom, and the lack thereof.