Nuclear Steam Generation is a touchy, tricky business. The easy way out is just use Stirling Generators, but there is an advantage of something like 50% more power from Steam.
I'm going to make a step-by step "How-to" on steam power on the forum, but I wanted to give the "shorthand" version in the form of a picture. NOTE: I'm pretty sure this will work, but I did not have time to test yet.
NOTES: Every red or blue arrow, split-tail or not, needs an ejection port, at the block that has (or would have) the TAIL. Each of these ports needs to be made DIRECTIONAL in that direction by SHIFT-RIGHT clicking on the OPPOSITE face that the arrow is pointing to.
FOR EXAMPLE, an arrow pointing down is ejecting to the bottom. it needs to be placed in the hand, then shift-right-clicked onto the bottom face of a block.
The Green and White arrows will happen automatically. Do not put, or try to put, ejector ports for these arrows.
The small arrows in the tanks immediately next to the Liquid Heat Exchangers are pointing to (or from) the next group. They only need to be there is there is a next group.
The Liquid Heat Exchangers NEED to have the square pointed to the Boiler. They are only pointed differently to make it clear what they are.
The tanks next to the reactor ports only need to be there in the first group, to get fluid to or from the tanks next to the Liquid Heat Exchangers.
This can be flipped upside-down and still work, for example if you have more space below your reactor than above.
I suppose this could also be used to minimize the material for a Stirling setup, but the point here was to get enough heat to the Boilers to make steam constantly.
The point is you can stack these next to each other, one for every 200 HU or portion of 200 HU your reactor outputs. If you have a 1340 HU reactor, you stack 7 of these next to each other and have all of your heat maximally used. Otherwise, I think you need to put multiple liquid ports on your reactor and it gets Frankenstein's monster-looking and huge.