From https://www.reddit.com/r/factorio/comments/6dd7de/2x2_480mw_nowaste_nuclear_reactor/
This is a 4-reactor design heavily inspired by VestigialPseudogene's 2-reactor setup here: https://www.reddit.com/r/factorio/comments/6cy837/160_mw_nuclear_setup/
Highlights:
- 480 MW sustained power generation (~510 MW burstable)
- Zero fuel waste
- 174.6 GJ energy storage in steam tanks (~6 minutes at full load)
- Isolated electrical network for fuel cell insertion and removal
- Alarms for low/no fuel (logistics problem) and low steam storage (undersized power generation)
- Multicolored storage indicator
- Flexible water hookup (any 5 of 8 taps)
Here's what it looks like: http://imgur.com/a/j3mFQ
I used /u/VestigialPseudogene's design but quickly needed something a little bigger, but not endgame mega-factory big. So, I used his design as a prototype to make a 2x2 version at 3 times the power output. It's a 100% efficient design: spent fuel stays in the reactors until steam drops below a threshold. The inserters removing the spent cells send a pulse to the fresh fuel cell inserters, which insert exactly one new cell into each reactor simultaneously to maximize the adjacency bonus. The refueling threshold is low enough that no fuel will be wasted even if there is zero draw from the power network.
The inserters are also electrically isolated from main power generation, with dedicated solar panels and accumulators. As long as there are fuel cells in the requester chests, brownouts on the main network won't affect refueling. Alarms are copied from VestigialPseudogene's design, with alerts for low fuel, no fuel, low steam, and steam overage (which should never happen).
I used /u/nuker1110's colored lights guide (https://www.reddit.com/r/factorio/comments/6ahomg/i_made_a_guide_using_multiple_colors_for_lights/ ) to make a steam buffer indicator. The lights change from yellow to red as the stored steam drops. The water inputs are also very flexible: there are 8 taps on the sides, and any 5 of them can supply the heat exchangers at full capacity.