Nuclear power plants can run a very long time without needing more fuel, but not forever. That would make it a perpetual motion machine, and if you figure that one out you are now a billionaire, so congrats. Every 2 years or so, they have to shut down and replace some of the fuel so they can run for another 2 years (while emitting ~0 CO2). Fair warning, all these numbers vary depending on the exact plant design.
So, what does that look like? How do you get a nuclear plant from 100% power and ~600F to cold enough to pop the lid off and move highly radioactive spent nuclear fuel around?
Lets assume we start at 100% power and 600F. We don’t like to trip from 100% (it’s a pretty intense transient on the entire plant), so we need to reduce power. We do this slowly, over about a day, and cool down a little at the same time. Once the reactor is down at about 15% power and 580F, we trip it. This just means we kill power to the mechanisms holding the control rods up and let them fall into the core and stop the nuclear chain reaction.
Post-trip, the reactor settles out at about 540F. Reactor power is now less than 1% and falling steadily into what we call the source range. This is thousands of times less than what is produced at full power. Decay heat is still a thing, producing about 5% of full power right after trip, and decaying away over time. It takes years for decay heat to go away completely, which is why we always need to cool the core.
Now that the reactor isn’t actively fissioning, we need to cool it down. We do this by pulling steam out of the steam generators. Eventually, this becomes ineffective (at about 350F, the steam pressure is so low that the cooldown gets very slow), so we switch over to shutdown cooling. This uses a heat exchanger with the reactor water on one side and the cold river or bay or ocean water on the other. Don’t worry, they never mix.
We cool down all the way to ~100F. This is cold enough to not have any steam forming, and not so cold it stresses the metal parts of the reactor. Now, we need to get the lid off so we can get to the fuel. The reactor vessel head sits on top of the reactor vessel. It weighs at least 60 tons and is bolted to the top of the vessel by several dozen studs. This is a picture of a new one. You wouldn’t want to stand underneath one that just got pulled off a reactor, it’s a bit radioactive (by a bit, I mean you’d die horribly).
Once the reactor head is removed, you fill the refueling pool up with water. This keeps the core nice and cool and provides the operators with shielding from the extremely high radiation that the core is producing. There is usually about 30 feet of water over the top of the core, so riding the refueling crane you get almost no dose. The picture below if from Palo Verde in AZ, the biggest nuclear power plant in the country. I worked there for 3 years, so I have actually seen this core.
https://kjzz.org/content/472070/go-behind-scenes-palo-verde-generating-stations-nuclear-refueling
Once you fill the pool up, you have to take out a few more pieces that sit on top of the fuel. This is all highly radioactive, so it is done under water. Once the fuel is exposed, you have 2 options. You can take all of the fuel out of the core and put it in the spent fuel pool, or you can only take out the oldest assemblies and bring in the new ones.
The spent fuel pool is in its own enclosure, and is connected to the reactor side by a tunnel. You put the assembly in what’s called an upender (it tips the assembly on its side so it can go through the tunnel). then into a cart and send it through the tunnel. A crane on the other side grabs it and puts it in its storage rack.
https://www.nrc.gov/reading-rm/basic-ref/glossary/refueling-pwr.html
We often remove all the fuel so that we can work on things. If there is fuel in the core, we are required by our technical specifications (I have another post about that if you’re curious) to have certain systems working just in case something bad happens. If we put it all in the spent fuel pool, we can work on things that we wouldn’t be allowed to otherwise. The spent fuel pool has its own cooling systems, so we don’t need all the systems connected to the reactor.
Each assembly is run through the core 3 times. Every outage, we replace 1/3 of the fuel (the oldest ones that have gone through the 3 fuel cycles) with new assemblies. Civilian fuel assemblies are limited to ~5% U235. Weapons grade uranium is >90% U235. Since there is such a limited amount of the U235, we can only run 2 years. Military reactors run for as long as 20 years, but they have very highly enriched uranium in them.
Once we have finished whatever work we are doing, we reload the core. Nuclear engineers have carefully analyzed exactly where each assembly goes. We want the core to be balanced from top to bottom and from side to side. If they screw this up, you get hot spots because one part of the core has way more U235 in it than another part. If this is bad enough, you get derated, which means you can’t run at 100% power. It might even be enough to shut you down and force you to redo the core loading to get it right. This almost never happens, but it can.
Once the core is reloaded, you put it back together just like you took it apart. You get the head back on. You heat up to normal operating temperature, about 540F. You pull control rods and dilute out boron (boron absorbs neutrons, just like control rods. We put it in the water to control reactor power) to take the reactor critical. (I have a post on exactly what critical means.) Over the course of a few days, we raise power and heat up the fuel. Once we are back at 100%, we hope to stay there for 2 years before the next refueling outage.
https://www.xceed-eng.com/nuclear-reactor-refueling
I decided to post this because a discussion on the blue glow got some really good interaction on BlueSky. That’s called Cherenkov Radiation and it is really cool to see in person. It’s caused by electrons moving faster than the speed of light in water. No, that’s not impossible.
As always, this is all open source. If you have any questions, please ask in the comments.
Sleepy Neutron's Nuclear Knowledge 
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