Thorium nuclear reactors are real, same kind of thing as a Uranium reactor. A few countries have projects actually happening to test Thorium (India, and maybe China I think).
No idea why you'd want to put a fission reactor in a car though, probably be safer and more efficient to generate electricity and then use batteries or some other form of energy storage (hydrogen). A battery probably weighs less than a nuclear reactor, though that's just a guess
