Electech put his finger on it, if you can get your hydrogen using waste heat from a nuclear reactor you might have a sustainable source but the volume we would need to put a dent in gasoline use is so great I doubt we will ever have that amount of reactors.
If there's enough waste heat from a reactor to do something with it seems like making electricity with it is the thing to do since that's what the reactor is there for. Bleeding off some of the available heat to make hydrogen instead of electric isn't really saving anything.
