An updated argument against nuclear power relates to our renewed fears of the proliferation of weapons of mass destruction. In the case of Iran, for example, it's very hard to distinguish a peaceful program from a weapons program. But along with concerns that more nuclear power now means more fissionable material available later for nuclear weapons, a number of commentators worry about the accumulation of radioactive material that could find its way into a "dirty bomb". This article in MIT's Technology Review puts that threat into proper perspective and is well worth your time.
All I would add is that, when it comes to setting our security priorities, we need to be very clear that actual nuclear weapons must be at the top of the list, with first call on funding. We should be much more aggressive in securing Russia's cooperation in decomissioning their old bombs and reactors, and getting the fissionable material under airtight security. When Russia was eager for help in this area in the 1990s, our response, though well-intentioned, was far too modest.