A fission reactor, by necessity, has a large minimum size, which makes it rather unwieldy, not to mention heavy.
Work Step by Step
A fission reaction requires a critical mass to start. As stated on page 642, besides the nuclear fuel, a fission reactor also requires a moderator, control rods, coolant, and shielding. This makes it far too large to directly power a car. Indirectly, fission can power automobiles by generating electricity to run plug-in cars.