I've recently been playing with establishing a satellite base on a mushroom island linked (via nether-based teleporters) to my main base some 16,000 blocks away.
I recently returned to my main base (using said teleporters) to discover my breeder reactors had all used their fuel up and started to cool. (Mine are set to automatically maintain 8k heat - long story, but it works well.)
I also noticed that on loading up the game at the mushroom island site, I heard the sound of a reactor for a fraction of a second.
It seems as if the chunk isn't being unloaded despite the distance. Relevant factors may be....
There may be quite a lot of block updates going on as the reactors are often above the temperature where they evaporate water.
There's another teleporter (overworld based this time) only a few chunks away.
I have plenty of memory available to minecraft, and there's probably no compelling reason to unload the chunks....
Any thoughts about what might be happening?