A Chunk is a 16×16 block area, used internally by Minecraft for world generation, saving, and loading.

Minecraft only simulates a small part of the world at any given time, given computational limitations.

These are usually the chunks located directly around the player (or players if playing on multiplayer).

The problem is that if a machine is built inside of multiple chunks (such as a nuclear reactor + external cooling tower), you can end up in a situation where only part of the complete construction is loaded.

The situation of a working nuclear reactor with an inactive cooling tower is not one that can be safely sustained for long, for reasons that should be obvious.

As an added bonus, chunkloading your reactor also means that your nuclear reactor will continue to produce power even while you’re out exploring. (Although leaving your reactor running unattended is done at your own risk, chunkloading or not)

In singleplayer, you can simply press the “Claimed Chunk” button in your inventory,

click and drag to claim chunks,

then click and drag while holding shift to load chunks.

You can also use right click to unclaim chunks (and shift + right click to unload).

Alternatively, press the M key to bring up the map, and then enable the Claims Overlay using the highlighted button.

You can then double click to claim a chunk, and double click again to load it.
The tooltip shows further shortcuts.

The steps are the same on multiplayer, except you have to make or become part of a Team first.
You will be prompted for this when joining.

Note that there are limits imposed on both chunks claimed and how many can be loaded. (the exact values of which can be found at the bottom right of the “Claimed Chunk” menu)

These values are usually much lower when playing on multiplayer, for performance reasons.

If you are playing on Engineer Gaming and feel that your build justifies the need for more loaded chunks, please Contact me and I can look into expanding the amount of chunks you can load.
If you are playing on a different server, please contact the administrator there.

Consider this simple Zirnox setup:

By pressing F9, you can show chunk corners.

Pressing it again shows chunk borders.

This shows us that the Zirnox in this case is contained within one chunk, with the majority of the turbines in a second chunk, and the condenser in a third.

As long as these three chunks are loaded, the entire setup should be able to function normally and safely.
(Emphasis on should, I make no guarantees)