Why don't they just attach a radiation detector to each crane or something?
So the port only checks containers likely to have radioactive contamination, like scrap metal, which they did and they found this - system worked precisely as it should.
You could have monitors on the cranes to detect very high levels of radiation, so high you don't even want to check it manual - as in this case, But these happen so rarely it's not worth it. And with 250 Million container movements a year what sort of error rate would you need to achieve?
That said, the eventually used a much more precise germanium-based detector that was able to identify the radiation by source. Such a detector could distinguish the decay of shielded U-235 from a shipment of bananas. I wonder how much those detectors cost...
"The team then brought in one of the most sensitive portable detectors on the market, an $80,000 Ortec HPGe Detective DX-100T. Inside the unit, a 1.65-pound chunk of germanium..."
If you'd want to be on the cheap side, there's a used Ge on eBay for $3k right now, add a HV supply anda suitable ADC card for your PC and you can go recording γ-spectra in no time, possibly on a $5k budget.
As you'll have to identify the isotopes yourself, check http://www.inl.gov/gammaray/catalogs/catalogs.shtml for reference spectra.