|MadSci Network: Physics|
Nuclear power plants have been used for many unmanned space satellites. For the manned capsules, they pose a problem because they require much more shielding than does any of the unmanned orbiters. Unlike the submarines, weight is much more of a problem in capsule construction because of the large amount of energy it takes to get that weight into orbit. So anything that is manned depends primarily on solar power cells. Solar cells weigh less and have no radiation shielding problems of the type associated with nuclear power. The submarines can make drinking water from sea water by distilling it. Their reactors give them plenty of heat and electricity to do that. The only water on a space capsule is what it brings with it, and though that is recycled as much as possible, it is still limited. Given, sea water, the submarine could conceivably, through electrolysis, produce breathable Oxygen from the water, but usually they just surface, or use their snorkle to change the air. Oxygen could be extracted from water in space, but isn't because there is almost as much of a need for water as water as there is need for air. Since people exhale carbon dioxide, the air can be replenished by removing the carbon from the carbon dioxide freeing the Oxygen for reuse, and that is the preferred method in space. It's a good idea and perhaps some day it will happen.
Try the links in the MadSci Library for more information on Physics.