I currently operate an EU reactor with a Mark I setup, which vents away all heat produced during operation, and can meet the variable power needs of my base by being operable at any time for any length of time, so long that the fuel lasts. This works perfectly with uranium fuel rods, as the reactor is fully stable on 0 heat, however I'm confused about how to allow some heat to build up without compromising stability too much.
If I build heat up slowly controlled by vents, then the MOX rods will operate below optimal for most of their life cycle, and during low power demand, the vents in the idle reactor (shut down by the MFE it's feeding if it can't accept anymore EU) completely vent away all heat within seconds.
If I take a few vents out of the equation to let heat build up more aggressively, then I run the risk of a runaway chain reaction as exponentially more heat builds up every second, and I find a quick heat - quick cool timer set up a bit like playing games with fire.
So my question is, what's the best way to reliably keep a reactor core at reasonably high heat without risking meltdown, but also without losing all the heat quickly during downtime through a series of vents?