On fission: it's my understanding that this is unchanged and just IC2 nuclear power. The biggest recent change I can think of is the fluid mode that increases EU output per heat unit by (I think) 50%. It costs more and is more realistic but is worth it for a long term setup. GT adds the Large Heat Exchanger (LHE) which is a titanium multiblock that gives you a ton of steam for the heat units and is the best way to get EU out of your radioactive materials.
There's been a lot of changes over time to this so whatever you find make sure it says it's for the experimental branch if IC2 (which has been where new releases have been coming out of).
http://wiki.industrial-craft.net/index.php?title=Nuclear_Reactor
I assume you mean the mechanics and design methodology for placing the components in the reactor. You can try an existing design you can find on youtube ir something but imo it's best to understand all of the components and experiment in creative mode.