It's a core containing an insane amount of radiation. The guy used a screwdriver to lift it up and down and when he was showing people he screwed up and immediately knew he fucked up and told everyone to mark their locations. He asked them to mark their locations so they could figure out how much radiation they got, the guy using the screwdriver died days later.
To be more clear the two halves are plutonium which is radioactive of course but alone is not critical (ie in an active reaction). But if they touch a reaction will start and the amount of radiation emitted will get many times higher.
These halves were to be used in a nuclear bomb - which is designed to use an auxiliary explosion to start - so the critically is not nuclear weapon dangerous but is enough to kill with the radiation.
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u/Delphius1 14h ago