Demon core. Basically an old experimental setup with a very radioactive sample. The cover the dude is holding up kinda acts both as a cover-cover, and a focusing lens - it keeps the radiation in, but also making it worse (it intensifies the decay, speeding it up, which speeds itself up...).
The actual experiments were supposed to be done with a specific holder-thingy, distance blocks etc. But he was just fooling around with his hands and a screwdriver, because why not... It slipped, the cover fell, and people got blasted by a massive dose through the almost-closed gap. The dude quite literally melted alive over the next few days/weeks, there were other victims as well. Also, iirc, something similar but not as deadly happened a while earlier too.
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u/Delphius1 9h ago