For example: Can I place them next to a wall and get the same efficiency?
I usually place coolant in all around the yellorium rods so they only touch coolant.
It's a little more complicated than that, since fuel rods also fertilize each other so it's not good to isolate the rods from each other completely, but the rule that no fuel rod should touch the casing is pretty hard. Ideally, as mentioned above, there should be 4 blocks of coolant between the rods and the casing, but only in a straight line, i.e. the blocks that form a "+" around the rod should be coolant or other fuel rods, while the blocks that form an "X" around it can be anything, unless there's another rod in a straight line of them of course.
My last reactor looked like this (C=casing, O=Coolant, Y=Fuel rod, space=anything, but should be solid if you're using liquid coolant):
Code:
CCCCCCCCCCCCCCC
C OOOOO C
C OOOOO C
C OOOOO C
C OOOOO C
COOOOYYYYYOOOOC
COOOOYOYOYOOOOC
COOOOYYYYYOOOOC
COOOOYOYOYOOOOC
COOOOYYYYYOOOOC
C OOOOO C
C OOOOO C
C OOOOO C
C OOOOO C
CCCCCCCCCCCCCCC
I'm not quite sure if it was good to concentrate so many fuel rods as I did, it's possible the reactor would be more efficient if I removed the centre rod and maybe the four at the centres of the sides of the block. I didn't get around to testing this.