Many of my favorite ideas for space exploration, such as large outer solar system probes, or the Mars Direct human Mars exploration proposal, require nuclear reactors to supply the high-power requirements of the spacecraft. However I have never very much liked the idea of a large amount of refined uranium being put on top of a possible bomb (the rocket fuel). I am specifically concerned the possibility of the rocket exploding, sending radioactive uranium around the globe in a mini-Chernobyl.
Assuming we will not be getting fusion reactors anytime soon, what alternatives are there to nuclear fission power where solar power is not enough for a mission. Also, since there needs to be a high-power source on the surface of a planet (Mars) for Mars Direct, what high-power sources could be used on a planet such as Mars?