For the reactor really all you need is 6 fuel assemblys with the fuel rods on a slider to slide them into place once the reactor starts. Put the assemblys in a 3x2 config. For the control rods, 3 will do just fine. Make sure that the dots on the fuel assembly lines up with the dots on the control rods. Keep in mind that larger temperature difference means more heat transfer but, don't have a meltdown.

I insert my control rods once the average of the temps of the fuel rods reaches about 100-125 c (faster startup). I have them insert to a constant .3 and this keeps my reactor around 200-220 c, very stable.

One six core reactor was more than enough to run 10 turbines on one boiler (i only tested up to 10 turbines, more on why later). But, if you are running a huge reactor you might consider increasing temps/adding another reactor.

Use large fluid pumps. Do not use any impeller pumps or ones that require power inputs, these will take so much power away from your net generation. For the most part, 2 large pumps is enough between segments of the generator. example layout:

boiler>p>p>turbines>p>p>condenser(s)>p>p>to boiler

More later on outputs and inputs on certain parts.

Put the boiler close to the reactor pool. one large pump in and one out to and from the reactor pool is enough for 10 turbines. For a faster startup wait for the reactor pool temp to reach at least 100 c before you begin pumping into the boiler. (I found that waiting for the pool temp to reach 125 c, then insert control rods to .3, and also pump into the boiler to start heating. Again its faster to heat up because of the large temp difference.

Wait for the boiler to reach about 100 c to begin pumping steam through the generator. Again 2 pumps for steam out and 2 pumps for water in.

Try to keep your boiler above 125 c for adequate heat transfer.

Ok, the two pumps out from the boiler are enough to power at least 10 turbines at full capacity (generally speaking). Split the pipe to reach all the turbines you need.

Steam output piping, connect it all together then use just 2 pumps to the condenser.

Turbines have a max steam flow of 60 l/s.

The generator system should work with 10+ turbines and 1 boiler on 1 condenser provided, you have enough cold water being pumped into the coolant side of the condenser (either sea water pumped in or from a sufficiently large radiator system).

The system works as long as the condenser remains under 100 c. Monitor temps. Keep in mind too that the system will be more efficient as the condenser approaches 100 c (to a point). I do not think that matters much in game.

Use as large of radiators as you can fit. You will need a lot and will just have to test your system. Keep the fluid volume flow rate as close to 80 as possible. large temp diff+faster moving fluid=more heat dissipation in radiators (to a point).

To start you will need somewhere between 2-9 gearboxes (use at least 3x3 size: don't know why but they work better). Mine has 6 3x3 gearboxes with a 3:1 ratio each (with the 3 on the output side). You'll have to test your reactor and gear ratios to get it right.

Clutch: on my particular generator, a modular clutch end piece with 3 1x1 modular flywheels after followed by another clutch end piece. The flywheels help store energy and 3 on my generator multiplied the output by at least 3x. Allow the first half of the clutch (including flywheels) turn with the turbines from the start and figure out what rps to engage the generator clutch at for a faster startup. The clutch to the generator will need to be tested. You will not need full clutch, in fact, you'll only need a small percentage.

Generator: MEDIUM GENS ARE GREAT. Small is too small and its too hard to get a large to turn fast enough. Remember generators like speed over power (torque), high speed=low torque, low speed=high torque. This is where it makes more sense to use medium gens, less torque required to turn the gen. Unless you are actually trying to drive a propeller directly from the steam turbines, you should favor high speed.

My medium generator on my system outputs almost 4300 SWatts.

My best reactor has this setup:

6- core reactor with 3 control rods
1 boiler 2 large pumps in 2 out
4 turbines (might need more pumps depending on volume flow)
1 condenser 2 large pumps in
6 3x3 gearboxes ratio 3:1 with output being 3
modular clutch with 3 1x1 flywheels
1 med gen
14 5x5 Radiators with 6 large pumps for flow rate at various points in the radiator system

reactor start (toggled)>insert fuel rods>wait>start radiators and radiator pumps>wait>once reactor pool temp reaches 125>pump to boiler>wait>boiler reaches 100 c>system pumps>wait>flywheel avg rps reaches (target, in my case 170) rps> engage clutch to 69% (nice)> wait for full spin up.

under 120 s startup time with about 4280 SWatt output.

This is an overall guide to max output on your reactor but again you'll have to play with gear ratios, temps, control rod values, how many flywheels, and clutch values.

Here is my reactor I used as an example in this guide:
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3271375260