Blueprint contains 20GW variation. Heat exchanger/turbine columns and reactor block both expand evenly so heat pipes will never be too far away from the reactors for further expansion.
Utilizes water barrels in lieu of pipes due to throughout issues (requires over 200,000 water per second to operate).
Ratios are a few percentage points away from optimal in favor of the scalable design.
Accumulators and solar panels are included in some of the empty internal spaces in order to help jumpstart the reactor and keep the inserters/pumps operating.
Contains some room for barrel belts between heat exchangers, though at some point during expansion (~60GW) this space would run out and you'd have to start stacking the belts along the outsides of the reactor.
Detailed screenshots: http://imgur.com/a/HHmWn
Reddit discussion: https://www.reddit.com/r/factorio/comments/6jykvc/infinitely_scaling_nuclear_reactor_design_tested/
Factorio forum discussion: https://forums.factorio.com/viewtopic.php?f=208&t=50293