Can be quite scary...
I was busy building another nuclear reactor and planning for it when I decided I best to check on my other reactor. I get up to the top of the mountain where I had it built and noticed that blocks around it were turning into lava!
OH SHIT! Quick, cool it down! COOL IT DOWN!
Luckily, I had an emergency chest full of water. I took out the uranium and shoved water in it like crazy until water quit evaporating.
*looks up threshold for lava*
It had gotten above 85% of it's max heat. 15% more and all my hard work would have gone BOOM!
With a "violent explosion", which would have left a massive crater and of course all 8mil+ of my EU would have also gone :(
These nuclear reactors sure are dangerous when left alone for too long (pending on setup)