@HikingVet The W54, developed in the 1950s, was a fully functional fission device weighing 23.1-26.5 kg, and had versions that one person could carry.

A dirty bomb could be much smaller and lighter, and would definitely fit in a briefcase.

The actual direct harm it might cause depends on many factors, especially immediate circulation of its products, but it's more than enough to terrorize a whole city, which is its purpose.