I've got around to working out what made my fission reactor do an atomic rage quit.
After carefully reproducing my reactor build in a simulation (i.e. starting a new game slot in Creative mode), and by carefully watching all the indicators on both the reactor itself as well as the associated heavy plant such as the industrial turbine and the bank of Energy Cubes, it seems that once all the Energy Cubes become full, the energy reservoir of the industrial turbine will fill up pretty rapidly. When this is completely full, the turbine will rapidly fill up with steam. With energy storage being full, the steam is not being consumed and has nowhere to go, so the heated coolant tank within the fission reactor fills up. Once this happens, the reactor is no longer being cooled properly and buys an express ticket to Meltdown City. There is an extremely narrow window of time between the turbine filling up with steam and the reactor melting down.