system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What made nuclear submarines so special?

What makes them different from traditional submarines and why does nuclear energy impact that? Do nuclear subs have nuclear reactors on the submarine? What do they do with the waste?
gpt
Traditional submarines use a combination of diesel motors and electric batteries to travel. When the submarine is on the surface, it burns diesel fuel to move and also recharge the electric batteries. This ruins a big disadvantage of submarines, which is that they... can go *under* the water and remain hidden to sneak around and catch enemies by surprise. When the submarine *does* go under water, it cannot burn diesel any more, because burning diesel makes toxic gases that need to be vented outside or else you'll suffocate the crew. So it uses those electric batteries, except the electric batteries don't have the longest lifespan (like, only a couple of days, though newer systems with air-independent propulsion can last up to several weeks underwater).

With nuclear-powered submarines, though, a submarine can stay underwater *forever*, because a little bit of nuclear fuel produces a ton of energy. Literally the one thing that limits how long a nuclear-powered submarine can stay underwater is the amount of food you have onboard for your crew, because the crew's gotta eat. And it can even stay out to sea forever (minus the food thing) - a diesel-electric submarine still needs liquid fuel to run. 

And yes, they do have a nuclear reactor on board.