system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't spacecrafts use nuclear fuel?
gpt
Some do. Some spacecraft use radioactive decay as a power source. The craft is equipped with a radioactive material, often plutonium, but some others work as well. This material slowly decays and the energy that is released in the decay is collected and used for the spacecraft. The power output of such a system is quite low, but it lasts for a long time. It's therefore primarily useful for long duration missions, such as probes that are sent to distant planets.

Actual nuclear (fission) reactors have been used as well, primarily by the Soviet Union in satellites. 

The problem with nuclear reactors in spacecraft is that they can be quite complex to design and build and there is the ongoing public concern of potential fallout should a spacecraft explode shortly after launch.