Why does the nuclear reactor constantly consume the fuel?
Stirling engine consumes fuel only when it generates electricity into the power network; when the network is "full" (in another word, the number on a substation reaches some "cap"), stirling engine will not consume fuel and does not generate more electricity.
However, nuclear reactor does not work in this way. Whether the network is "full" or not, nuclear reactor always consumes fuel, even if it is neither producing plutonium nor generating electricity.
Alternative solution:
Use Substation, then use Panel to read its number and set threshold, then use Logic gate to block fuel into Nuclear reactor when electricity level is high.