Those are bad designs for multiple reasons:
- You can get the same cooling output for a lot less cost
- In GregTech 2.90h, the "hybrid effect" bug that caused thorium and plutonium to multiply each other's power output was fixed, and thus the numbers in the reactor planner are not what you get ingame.
- In GregTech 2.90h, plutonium was changed to have completely different stats, and thus the numbers in the reactor planner are not what you get ingame.
- In GregTech 2.90h, thorium is severely bugged and only scale up in heat when you group them up but not in output, and thus the numbers in the reactor planner are not what you get ingame.
(Riuga's suggestions are invalid for the same reasons.)
2.90h is a beta builld that was meant to be a preview/testbed for 1.5.x features, and not meant for regular gameplay. It's broken in several places, nuclear reactors being the most egregious case.
If you want to build a thorium reactor in that version, you need to build one with as many thorium quad cells as you can fit without any of them touching each other. You're not going to see more than the most miniscule extra power from grouping them up, but your heat output will go through the roof if you do. That means that sadly, efficiency 3 is the best you're realistically going to get, but at least the reactors will be reasonably cheap because thorium's heat output is fairly low. You may be able to get away with just spamming OC vents and thorium cells in a grid pattern; let me check ingame if that works.
EDIT: unfortunately the grid pattern doesn't work as it makes the cells spread their heat in a manner too unfocused, and that overloads some of the OC vents. You can however do this:
http://www.talonfiremage.pwp.blueyo...zmnt7uua0qr9saax3uwfwcn9imn6uvu803xmg2gwk8d1c
Shows up as failing in the reactor planner, but ingame it is stable because the base values for thorium have also changed. This is the cheapest possible setup that can cool this many thorium quad cells, costing just 580 copper, 183 iron and 36 gold for an output of 216 EU/t. You'll notice that it's a repeating pattern; you can also use it in 0-chamber or 3-chamber reactors, but from a build cost perspective one six-chamber is cheaper than three 0-chambers (unless space is a concern).
It's not suited for automation as is - you'll need to fill up the empty spaces with something cheap that doesn't interfere with the heat transfer, such as reactor plating or component vents.