I've melded down reactors. I've messed up reactors. Apparently I have never run one with no heat escape... until now.
Completely inadequate energy storage means I was running the reactor to refill an MFE, a not more than 5-minute operation. I was SURE I had enough time to shut it off before it melted, and I had it nearly enough heat dissipation.
Apparently that goes out the window if the energy has nowhere to go. This screenshot is where my reactor used to be. Apparently lack of heat dissipation means it exploded, rather than melting the reactor core like it has always done in the past.
I've decided I'm going to make a new world and turn on cheats. this was something like 1-2 week's worth of work, all gone, with not enough resources to rebuild. I am going to cheat and get me back to this point, I can't afford the time lost 
Interesting side note: my machines are as strong as reinforced concrete!