Just to give you a bit of a primer (since I've been checking this out recently too): basically the reason this works with GT installed is because of thorium cells. Thorium and plutonium are basically what make nuclear power viable right now, and if you do a 2x or 4x to the output config on your server they're really, really, really good. You can find a million and one designs over
on the IC2 forums but you need to dig in; that thread is old and the designs get better and worse as people explore the space.
To start, you want a breeder reactor to generate a lot of fuel. I saw a bunch of designs but they didn't do what I wanted, so I sacrificed a bit of efficiency to work with more cells within my resource constraints. It's not an optimal design, but it's pretty cheap and easy to understand. You can use
this link in the reactor planner. This reactor makes enough EU to run some automation to keep it breeding, and you'll either need lots of consumption or a few MFSUs to catch all the power. You could also go for super-cheap single-purpose reactors like
this one as the epitome of the thorium single-purpose breeder. It's crazy cheap and does a great job, but cannot output any power.
Your goal is to constantly yank re-enriched cells out, craft them, centrifuge them, and continue to re-enrich. Eventually you'll be sitting on a ton of thorium and plutonium. The plutonium is a fantastic fuel, so then you use another production reactor. Plutonium by itself can make a good reactor, but the real secret seems to be in combining it with Thorium to make hybrid reactors. You can see an example from the forums
here.
Look at the stats on that. For a mere 8 plutonium cells (just a few breeder cycles!) we have a a design that can produce 220 million EU at 220 EU/t. Think about this for a second, because it bears considering what goes into this. This beast is fed by a breeder that is only consuming a handful of uranium ingots. You could produce this kind of energy with 10 thermal engines and thousands of buckets of lava; but if you've got automatic mining that self-resets it's almost impossible to run out of fuel this way: uranium is just not that rare. And if you have extrabees around, a single good, autoumated uranium bee will outproduce this consumption!
You can double up these setups (nothing I've listed is insanely expensive save on copper, but in Gregtech you can have huge sums trivially with the most rudimentary of automation) for higher voltage, but at some point you need to ask yourself about storage. After one 13 hour run you'll have so much EU on tap that only teleporters or matterfabs could really drain it entirely before another cycle completes. If you run multiples you'll want to have an AESU around, because nothing else will suffice.
Finally: every design posted here is a "Mark I" reactor with at least an "A" rating in efficiency. Meaning: it requires no thermometers, no pulsing, no worries about explosions. The designs are self-stable and a failure to automate will only result in less efficiency. You can keep these designs next to your workshop on on transformers without fear: the worst that can happen is you waste some uranium. Once you have plutonium, you can explore the Mark V space where reactors can only run for a few seconds before they have to stop. The efficiency you can reach exceeds 6, almost doubling our efficiency yet again (for example, this
simple Mark V quad plutonium design).
So there you go, a slightly windy, very rudimentary treatment on reactors. They are under-appreciated in these forums. And if you do a 2x or 4x in your config to compensate for the advent of energy tesseracts, even my humble breeder design will output a cool 44-88 eu/t which is somewhere between "wow" and "that's OP turn it back to 5, Dave."