It depends on what level the detonation was triggered at. An accidental detonation that actually triggers the primary device detonation mechanism could result in the bomb actually going off.

If an accidental detonation means just triggering some of the explosives, then yes, you'd get a dirty bomb. However, it's actually more likely that the bomb would actually detonate than the explosives being partially set off... they're designed specifically to be hard to detonate unless the actual detonator mechanism is used.