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.
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.