I would guess that it's entirely possible for a nuclear "accident" aboard a sub to create the kind of impact that you would expect from a dirty bomb. All it takes is for a Loss of Coolent Accident (LOCA) for the core to become exposed, melt down and cause a Prompt Critical Rapid Disassembly which would breach the pressure hull and cause a **** storm (that's a technical term
). If the sub was surfaced at the time then this could be a major problem.
Even the safety systems can inadvertantly cause problems. If the ship is at the pier, in a hot standby state and power on the vital bus is lost (the main coolant pump is run from vital power), and assuming that the crew has been incapacitated then the Emergency Cooling (XC) system should kick in to pump cold water round the core to remove the decay heat. Good news. This would actually be really crap news if the XC system kicked in when the ship was rigged for sea and the reactor was critical, because the cold water, which is a great moderator, would cause the fission rate to shoot up and you've got a nasty cold water accident. This is why the XC is valved out when further than 50nm from shore.
Basically there are plenty of ways for a reactor to make a mess.
Nick
/edit - or so I've read. I must stress that I don't know any of this first hand and I'm sure there are plenty of people far more qualified than me reading this.