A shelter is counterproductive and dangerous, though, with a single terrorist weapon or dirty bomb.
If it goes off, there's no warning. Then you have people who are contaminated coming into an enclosed space with people who weren't, and now everyone's inhaling radioactive isotopes from dust on clothing, and they're all dead. Same with a single warhead. The idea of a shelter was to get in and lock the door before it hit. Not after.
A dirty bomb will not create fallout. A dirty bomb would be mostly psychlogical, and require expensive cleanup of the immediate area.
Decontamination stations and sensors for fissionables would make much more sense.
This is a soundbite solution, busywork, "duck and cover 2007", and it makes no sense at all in terms of the realities of a terrorist nuclear attack, where the only proper response would be shielded and trained first-responders triaging and decontaminating survivors.
The money would have been better spent on portable decontamination shower units.