https://www.quora.com/Who-had-the-worst-death-in-historyHisashi Ouchi.
Essentially, him and his fellow workers were pouring some uranium salts into a mixer not designed for it. Not only was it a just above critical amount of salts, but the mixer basically swirled it together, driving it to criticality.
2 of the other workers died of radiation poisoning. Hisashi survived somehow past them. He had recieved 17 seiverts of radiation: fatal dosage is 8 seiverts.
He received fluids, blood transfusions, numerous treatments. His white cell count was essentially zero, his blood became closer to water than actual blood, and due to the way the radiation caused damage, pain relief wasn't initially given, and when it was, his body essentially didn't get affected by it.
After all his skin fell off, the muscles in his chest stop and he needs a tracheostomy. His mucus membranes die and because his DNA is basically gone, his body can't really heal or replace it, so his intestines, mouth, eyes throat etc begin to liquify.
He lasted 83 days. And a reminder.. your body naturally kills off old cells and replaces them with new cells built from your DNA. Except he hadn't got DNA left in all his cells. His body was recycling his cells with nothing to replace them with.
He died of multiple organ failure.