Vendarian Reactor Corp
For when you don't have the resources or the need for a 6-chamber reactor.
These are some designs for less then 6-chamber reactors. This is for the beginning reactor engineer, so I'm also designing them not to be crazy "Run this at 50% duty cycle to make sure it gets through a cycle". Any Mark I's are modeled to not build heat, and Mark II's are modeled for very limited heat build, unless otherwise noted. Feel free to set up sufficient storage, and then let them run while you're out mining. I do recommend hooking up a redstone lever so that you can cut them off when your storage gets close to full. Reactors that have been cut off don't waste fuel, and allow cooling for Mark II's (so you can start a new cycle sooner).
No additional chambers needed, only limited water blanket required (bottom 9 blocks flooded with water should do it if most nearby cells are air, 12 blocks water total and everything else can be filled in). Only 20 EU/t, but good for a starter reactor or a small industrial complex. Can directly power a batbox, so good for low end machines without needing a transformer.
Mark I-OC v2
Next step up of first reactor, needs full water containment, but only 2 reactor chambers. Make sure you cable off of a chamber. Vertically flipped copy of Kuu's "The Harmony of Elements", but it's an independently evolved design. Seems like it's the only way to fit a 2.33 efficiency design into a 2 chamber reactor. 35 EU/t, so it is a MV design.
Will run full cycle at 3.0 efficiency and 60EU/t. Main drawback is the need for a cooldown cycle after every run, but allows very good efficiency while still only needing 5 chambers. Only real need for this reactor is to maximize EU yields. I am investigating the use of the reactor post-cycle as a breeder system.