I just built my first save in a while, Check it out here: https://powdertoy.co.uk/Browse/View.html?ID=2176096

If the fuel runs out replace it, Note that if the temperature drops below about what it is the reactor will stop producing power, This is intentional as a safety feature as normally in nuclear reactors if there was a blackout and the reactor stopped producing power the operators would lose control of the control rods causing them to drop, which over time if the reaction was powerful and there was enough residual heat would cause the rods to melt resulting in a runaway reaction. This happened at Chernobyl in 1986, although instead of a blackout they were testing the safety features of the reactor.