Two dudes work at plant that process radioactive materials in Tokaimura, Japan, to make them usable again.
The dudes were working on purifying a batch, but didn't understand shit about loads and critical mass. Basically tried to put too much of one thing in one pot.
Shit goes critical.
One dude (Ouchi) falls over the tank (not into, just on the top of) and gets the biggest dose when shit goes critical.
Second dude was close enough by to also get heavy dose of radiation.
There was a third tech who got a livable dose and was not duly harmed from the incident.
There wasn't shit all they could do for the two who had the lethal doses. So docs just watched 'em slowly melt away. Ouchi wanted to be killed after the first week, but the doctors would not comply. Partially from curiosity, partially from fucked up laws that the life of the patient is more paramount than the quality of life or the patient's wishes.
To have an idea of how much radiation Ouchi got, his chromosomes broke apart. What made him him was erased from existence, shattered apart by radiation.