Spartan, if one of our nuclear subs or carriers goes down, theres a few trillion gallons of water just waiting to soak up the heat and absorb the neutrons, effectively killing the reaction. Water is very effective at dealing with radioactive materials because of its density and it's high specific heat coefficient.
If a nuclear reactor were to be breached on land, it would very likely become uncontrollable and melt down. Granted, a nuclear meltdown is nowhere near as catastrophic as newsies would have you believe, but it most definitely wouldn't be good for anyone in the immediate area or downwind of the crash site.