A dirty bomb is in no way shape or form a genuine nuclear bomb.If you're trying to build a real bomb like Little Boy, then you're right. If you just want to make a big mess and piss off a lot of people using something that fits in a suitcase, it's not nearly as difficult. I'm not going to discuss the details here because I think I hear black helicopters, but anyone with an internet connection, a lot of money, and access to some decent engineers could pretty easily turn an urban area into a hot mess.
The impact, especially using something like regular uranium, would actually not be that great at all physically, with the psychological impact and related economic inconvenience being the real effects. To be clear, a dirty bomb on the scale you're talking about is not even on the same planet as the damage done by any genuine remotely decent sized nuclear explosion.
