A simple way to picture it might be to think of a cup filled with pencils and water between them. Most of the pencils are the fuel elements, some of them are the control rods that block the fuel from interacting. Removing the control rods "activates" the fuel elements and they start generating heat. The water in the cup is both the coolant and the moderator, it heats up via contact with the fuel pencils. This is basically what a VVER is.

For the RBMK it's quite similar except it's larger and the coolant and moderator are different things. So think of a laundry bowl instead of a cup - the size difference is about right when it comes to RBMK vs VVER, the RBMK is a much larger cylinder. In the RBMK the moderator is solid graphite, and the fuel rods and control rods are in the middle of such graphite blocks, so imagine pencils that are placed in the middle of wooden blocks that have a central hole in them. The coolant is also water, you can imagine it filling the space between the pencils and the wood blocks.

This is a key difference between RBMK and VVER - if the core overheats, in the VVER case the water is also the moderator, so if it boils away the reaction stops, because it needs moderator. While in an RBMK if the water between pencil & wood blocks goes away, the wood blocks are still there to provide moderation and continue the reaction and generation of heat.

Hello! Hope this helps, I am a physicist so if you have any specific questions feel free to ask, although granted I didn't specialise in nuclear.

Nuclear Reactor Schematics:

The fuel (usually uranium or plutonium) is arranged as a stack of cylindrical pellets creating a cylindrical rod and is incased in a thin walled metal container. This container is the cladding and prevents the fission products and fuel from entering or reacting to the coolant. There are normally a number of these incased cylindrical rods of fuel all side by side forming a grid. The moderator is between each of these fuel rods to absorb fast moving neutrons.

The control rods are in line with these containers and on top of them - such that they can be inserted into the fuel rods and absorb neutrons. The rods can then be 'pulled up' out of the rods if necessary. Everything so far is contained within the nuclear reactor.

The coolant (gas or liquid) flows from the source they are using into the nuclear reactor. It then flows around the fuel rods and is in contact with the cladding, not the fuel rods themselves. If the coolant is water, it can begin to boil and be directed out of the nuclear reactor and used to produce steam. A turbine outside of the reactor can then be used to generate electricity. If the coolant is gas, the gas can be pumped to a steam generator and also be converted using a turbine.

RBMK Reactors:

This link might help $1 \- if this site isn't very accessible will be happy to copy out the text. It's quite an intricate schematic and to be honest it's difficult to comprehend the diagram visually so you're not alone in not really knowing what it looks like! I only ever learnt the theory like you and have never seen one before.

More generally, visually nuclear reactors are huge metal containers with a load of rods inside or coming out the top. From the outside world, they are usually 3 or 4 massive chimneys coming out the ground with a load of steam coming out - very ominous!

The thing about the chernobyl one is that it was much huger than most. I have walked around on the top of a working nuclear reactor where they pull the fuel rods in and out and it's what, 10 metres across and a bit smaller internally. The one at chernobyl is twice that size. The top has a bunch of hatches on it where they pull the fuel rods in and out using a giant protected crane.

Thank you verry much!