After reading the real information on dirty bombs it is a surprise to me how clean they are. You would be more dangerous if you lit a pile of old tires on fire.
A dirty bomb could well be a properly designed thermonuclear weapon design that has just fizzled or one that has been designed to fizzle. Take a look for example at the purple passion war head which is designed a 10 ton yield nuclear sub demolition munition used for blowing up buildings, bridges, tunnels etc. Its about the same size as a bowling ball.