This design gets you going with basic nuclear power, before getting logistics bots. (Which take a while to get in Krastorio) Feed water into all the pipe connections on the sides and feed fuel cells into the bottom.
There is an automatic disconnect switch for the inner feed logic when the monitoring accumulator falls below 75%, the red indicator will come on when this happens. The single solar panel should be enough to keep the logic and inserters running - but your water pumps, not so much. Essentially, this light warns you when you need to build another reactor block ;-)
The yellow indicator comes on when the steam level falls below 125k, this means the inserters will trigger next time the counter reaches the correct value. (5990-6000 on a counting loop of 6000) It's OK for this light to come on now and then, but if it is always on then you may be out of fuel.
Warning: Due to the pulsed nature of the loaders, the reactors will not feed in sync if the inner fuel cell loop is not full.