Here's my take on the old problem. I've adapted a few things according to my preferences, since I wanted a nice clean setup for starting nuclear power:
Clean fit up against a bus (yes I put nuclear processing on my bus)
Low cost to build (relatively speaking)
High efficiency (respectable neighbor bonus)
Auto-start (no priming necessary)
Automated control (no fuel wastage)
Manual overrides (for debugging/servicing)
Deep throttling (can throttle between 0~480MW without loss)
Failsafe-ing (handles low/no fuel, power outages, etc. gracefully)
Customization (interface at steam/water pipes)
I intentionally excluded turbines and pumps, since I wanted flexibility in how to pipe in water, and I didn't want to invest in 40-something turbines right off the bat (since I run very low-power operations). I've therefore made the water and steam pipes accessible, and as long as 5 pumps are connected to any 2 sides of this thing (see picture for example), water flow appears stable for at least a few full cycles (and probably more; my base doesn't gobble enough power to test the pipe balancing requirements for continuous operation). It's also possible to lop off at least 6 exchangers without loss for further build cost reductions, as long as you don't try to tax the plant above its new power limit (-10MW per exchanger excluded).
I did, however, design and test for all the weirdness that may occur during normal operation of a base (fuel starvation, power outages, waste backup, etc.) and this setup will simply stop whenever a blocking condition occurs, and resume operation normally once that condition is removed. Finally, the logic is designed to start from the blueprint state without priming, so I think I've achieved my goal of set-and-forget; once you hook this up you shouldn't have to futz with it. (See circuitry section for details.)
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The current setup is designed for belting use, but it can easily be converted for bots by removing the belts, replacing the iron chests with fuel chests, and placing depleted cell chests under the long-arm inserters.
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The control circuit tests for 4 conditions before it activates the fuel cell inserters. In order, it checks that ① all 4 of the fuel cell chests have fuel, ② there are no fuel cells outstanding in any of the reactors, ③ manual overrides are not active, and ④ steam is low. Once all 4 conditions are fulfilled, all fuel cell inserters grab fuel cells simultaneously. (Aside: I discovered that inserters will grab from entities with just 1 tick of enabling signal, but they cannot do the same from belts, so the fuel chests are necessary even with belting.) All inserters are set to read in pulse mode, and feed fuel cell counts to memory for tracking.
Combinators by dot color: Red: Condition checkers. From left to right, fuel in chests, reactor cell counter, manual override reader, steam detector. Steam detector is set to activate below 20k steam per tank. Green: Fuel chest checkers. They check chests individually to prevent uneven distribution of fuel (e.g. fuel on belt running out, or player dumps fuel cells in a random chest) from causing activation when only some of the reactors have fuel in their chest. Blue: Spent cell signal converter. Converts pulses from spent fuel inserters to negative pulses of fuel cell signal to track fuel cell counts. Black: Memory cell. Keeps track of fuel cells currently in reactors by counting and remembering pulses from all reactor inserters; fresh cell inserters increment, spent cell inserters decrement. White: Sends activation signal to fuel cell inserters if all 4 conditions are met. Orange: Provides holding signal for memory cell, and resets it if manual memory reset is triggered. Constant combinator: Provides manual override access. H signal halts operation, while R signal resets memory (shouldn't be necessary unless messing with circuitry or manually adding to / removing from reactors)
To make sure none of the circuitry gets into a weird state during construction, the blueprint is recorded with the manual halt active. Switch the H signal on the constant combinator to 0 once you have everything hooked up.
Last edited by Theikkru on Mon Jun 17, 2019 2:53 am, edited 4 times in total.
Finally did some heat tests, and adjusted the steam trigger to 20k/tank to improve steam buffering and flow behavior. Even with 6 exchangers missing and no steam drain, reactors do not overheat. Blueprint and text in first post updated.
The blueprint doesn't include the turbines; that image was just an example where I was using it and I didn't need that much power (which is why I put in the exclusion lines). As I wrote earlier, you can knock off heat exchangers to save on building costs if you don't need that much power initially, but I'm putting them all in the blueprint so you can repaste it later to expand. The pump is definitely overkill at the new steam trigger level. You can just connect turbines directly to the steam pipes around the edges if you wish.