Nuclear power plants don't blow up, as in can take out a whole city. If they did no one would build them. What they can do is get really hot, parts can blow up throughing Fallout over a population. Just as Giantevilhead said, heat and radiation but no boom (mushroom cloud).
Well actually, with Chernobyl there was a very real fear of a second explosion, which would have been caused by the molten uranium seeping down into lower levels and coming into contact with water. Estimates were made that indicated that the city of Minsk would have been leveled and much of Kiev destroyed (something like an affected radius of 300km). The explosion would have also vaporized the remaining fuel in the other reactors and led to much, much more Fallout being released than was. Most of the Ukraine and Belarus would have been uninhabitable for many years. Luckily they were able to get the water drained though, but it just goes to show you the potential for destruction that Nuclear Power Plants have if they go really bad (Chernobyl was horribly mis-managed though).
But yeah, on topic, the cars blowing up makes really no sense. There isn't really any reason to have them blow up game-play wise either (and it could get really annoying on occasion). I liked how New Vegas toned down the number of explosive cars.