"dirty bomb" is a radiological weapon, not a fission/fusion weapon, and is exactly what detonating explosives (or H gas, or whatever) inside a commercial power reactor or RTG would be.
Except that a dirty bomb is designed to maximize the hazardous effects of such an event while a reactor is designed to minimize the hazardous effects of such an event.
No, a dirty bomb is mainly designed to maximize fear, not hazard. Once its gone off, telling people it was safe is probably not going to work very well.