Nuclear reactors are in essence boilers. Reactor fuel goes in, hot water comes out. But I propose three additional mechanics for reactors to make them behave quite differently:
- Reactors don't heat water directly, instead they consume fuel to increase their internal buffer, which is in turn consumed to heat water. The power output can only change slowly, so if the reactor is running full-tilt and then suddenly your power consumption drops away, your reactor will begin to decrease its output but won't be able to do so fast enough, and the internal buffer will fill up, which leads to...
- Reactor overheat and shutdown. I see a lot of suggestions that reactors should explode or melt down, but I'm not a fan of this. Everything else in Factorio is fail-safe, it's consistent and reasonable to assume that reactors should be too. What I suggest is that when the reactor buffer fills, the reactor enters emergency shutdown state, and stops consuming fuel until its internal buffer (i.e. core temperature) drops back to 0. It should decrease over time very slowly from passive air cooling, but the best way to cool down is by pumping more water through it to consume the core heat, which leads to...
- Water discharge pipes or (preferably) cooling towers. These provide a drain for hot water when engines are not using enough.
You have a sustained power draw that suddenly falls off, for whatever reason (biter attack cuts power to a factory? bottleneck causes it to grind to a halt? end of a biter attack means your laser turrets stop firing? massive accumulator bank fills up?). Your reactor core is boiling away, but your engines are not consuming as much hot water, so the water in the reactor reaches max temperature, and not enough cold water is coming in to consume the heat being produced. The reactor drops its output, but it can't drop it fast enough, and the internal temperature rises.
At this point, a naive setup would quickly reach the cut-off temperature and shut down for however long it took to cool down to ambient temperature, and this could be quite a few minutes.
However, your setup is not naive, and uses a circuit-controlled pump to detect the increase in core heat, and start pulling hot water out of the reactor. This water goes through a cooling tower, and is injected back into the reactor (the pump that usually brings water from the offshore supply has been switched off). This continues until the temperature drops to a safe level, and then they emergency coolant pump switches off and the reactor is once more fed from the offshore supply.
All of the above would also mean that if your power demand suddenly spiked, your reactor wouldn't be able to increase fast enough to keep up with the draw, and the water temperature going to the engines would decrease. Perhaps you might build auxiliary coal boilers on the output to cover this eventuality.