Technology Review reports researchers at NASA and the Department of Energy recently tested key technologies for the creation of a nuclear fission reactor that could power a human base on the Moon or Mars. The tests provide evidence that it's feasible to build a safe, reliable and efficient system by 2020. Check it out over here.
Now nuclear power is being considered for lunar and Mars missions because, unlike alternatives such as solar power, it can provide constant energy, a necessity for human life-support systems, recharging rovers, and mining for resources. Solar power systems would also require the use of energy storage devices like batteries or fuel cells, adding unwanted mass to the system. Solar power is further limited because the moon is dark for up to 14 days at a time and has deep craters that can obscure the sun. Mars is farther away from the sun than either the Earth or the moon, so less solar power can be harvested there..