system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:We can power an aircraft carrier for 25 years on nuclear reactors - why not spacecrafts?

So we have these ships that can run for 25 years without refueling through the use of nuclear reactors.. Why can't we use these to power space ships?
gpt
We can and have done so.

There are a few problems. Nuclear reactors tend to be big, and produce lots of heat. At sea, you can use water. In space, you need radiators.

In addition, people tend to be upset when you tell them you're going to shoot nuclear reactors into space.

In general preference is for RTG's(Radiothermal generator. Basically a bit of radioactive material) as those are simpler and can be smaller.