Blueprint Preview (Thanks /u/demodude4u for the amazing Blueprint Bot!):
The ratios are NOT perfect (horror!), but they're as close as I could realistically get them without compromising the design. I wanted everything to fit in horizontally tilable columns that consumed the equivalent amount of energy to the reactors directly beneath them. I eventually settled on a 9-tile wide column design which produces 279.36 MW of power. 9 tiles worth of reactor is theoretically capable of outputting 288MW of energy, so this is a very healthy chunk of that (97% yield). Here's a close-up of the reactor layout and heat-exchanger portion of the column design:
I'm quite proud of the water unbarrelling setup which I'm using here:
Since the water and steam pipes have to have the same throughput (1 unit of water = 1 unit of steam), they both suffer from the same dropoff bottleneck. Every pipe which I added caused hundreds of units of fluid per second to disappear.
![Sad :(](./images/smilies/icon_e_sad.gif)
In the end, it did end up meeting the 20GW target goal while maintaining horizontal scalability:
Edit: Fixed spoiler image tags