I've just been sat here digesting the loss of at least many hours, if not several days' worth of play time. I had a beautiful multi-block nuclear reactor constructed in the basement of a vast tower that had previously been filled with monster spawners and environmental hazards. Connecting this reactor to 3x3 array of fully-upgraded water pumps was an intricate manifold of highest-tier fluid pipes, steam tubes and energy cables that I took several days to meticulously lay down.
It was all going so well at first. I had a uranium hexafluoride production line feeding a centrifuge keeping the reactor topped up with fissile fuel, the pipes and coolant reservoirs were fully set up and at 100% capacity, so I decided that now was the time to finally activate the reactor, and drag the lonely wilderness of modded Minecraft kicking and screaming into the Atomic Age. I played around with the fuel burning rate a bit, before deciding to leave it at a nice and minimal 0.1mB per tick. I observed the reactor temperature for a few minutes, and noted that it was holding steady. My dedicated work in constructing this nuclear marvel had lead me to believe that I had acquired mastery over the power of the atom and so, heedless of my ignorance, I went off to do some side tasks around my base.
Nature, of course, punished me for this entirely unwarranted hubris.
My first awareness that I had aroused the wrath of the atomic Furies was when the screen went a shade of green. That was odd, I thought, I don't see any status effects active. At first this green mist was merely a minor visual hindrance, but when my suspicions drove me to check on the reactor, I was greeted with a devastating sight. The entire reactor assembly had disappeared, and in that moment I knew the height of my arrogance which had lead to the depth of this mistake. In a daze, I carelessly wandered over the site of the former reactor. Yes indeed it was entirely gone, this was no glitch or bug. I looked around and observed that the meltdown had taken five Ultimate Energy Cubes along with the reactor, along with a good chunk of connecting pipes and wires. Strangely enough, the walls and floor of the room had not been structurally compromised at all. At least I would be spared the chore of fixing that up.
It was at this point that nature's radiological anger at my careless stewardship really began to bite, and I started taking damage from the radiation poisoning. I fled to an older base, taking a hefty stack of hearty steaks with me, in order to keep my health up as the radiation ate away at it, like the cancer it would most likely give me in several decades. I didn't think to try drinking milk or an immunizer potion, so I just kept my face topped up with beef for ten minutes. Finally I decided it would be quicker and less cow-intensive to just die and respawn by my bed.
I've made a geiger counter, and will be walking back to the site of the meltdown in order to establish the extent of the radiologically hazardous area. Once that is complete I will gather whatever resources that can be safely recovered from the site and establish a camp nearby where I can wait for the place to cool off without further exposing myself, and rebuild the reactor and energy storage of my tower base. More importantly I will need to construct a mechanism which shuts off the to-be-newly-rebuilt reactor in order to prevent meltdowns. Otherwise the gameplay will become less Promethean and more Sisyphean.
TL;DR I done fucked up in modded Minecraft and destroyed a few days' work while despoiling the virtual environment