A nuclear facility worker suffered what many experts have called the most excruciating death ever documented after a standard procedure went catastrophically wrong.

Hisashi Ouchi, 35, was exposed to an incomprehensible level of radiation when colleagues accidentally added excessive uranium into a processing tank, sparking an uncontrolled nuclear chain reaction, on September 30, 1999.

The unfortunate Ouchi was nearest to the tank, resulting in his exposure to 17,000 millisieverts of radiation, equivalent to 200,000 X-rays.

The amount he received was 850 times the safe annual limit for nuclear facility workers, 140 times what people near Chernobyl experienced after the 1986 catastrophe, and the highest recorded exposure in human history.

Within seconds and minutes of his exposure, Ouchi became severely unwell. Whilst most people exposed to such levels would perish within days, Ouchi survived, reports the Mirror.

He was transported to hospital conscious but in critical condition, as his white blood cell count had been virtually eliminated, leaving him completely without immune defences.

Medical staff moved him to the University of Tokyo Hospital, where they tried various experimental procedures in a frantic attempt to save his life.

What followed was 83 days of torment for the nuclear facility worker.

Radiation had completely obliterated Ouchi’s capacity to heal and regenerate cells, causing his skin to gradually deteriorate, his blood vessels to fail, and his eyelids to fall away. Fluids persistently seeped from his exposed tissue and accumulated in his lungs, necessitating doctors to keep him on a ventilator.

To compound matters, his digestive system completely shut down, resulting in severe pain and daily loss of litres of fluid. Despite numerous skin grafts and stem cell transfusions, nothing enabled his body to recover.

He was unable to breathe without assistance and could only consume food via a tube. The pain escalated to such an extent that, two months into his ordeal, Ouchi’s heart stopped, yet doctors chose to resuscitate him.

His wife reportedly hoped he would survive until at least January 1, 2000, so they could usher in the new millennium together.

Occasionally, he was conscious and aware of his what was happening. According to reports, Ouchi eventually reached breaking point, saying he couldn’t take it anymore and uttered six chilling words to staff: “I am not a guinea pig.”

Doctors were forced to administer up to ten blood transfusions a day ust to keep him alive. Pain relief appeared futile, and at one juncture, he is reported to have pleaded for the treatment to cease.

Ouchi passed away on December 21, 1999, succumbing to multiple organ failure, nearly three months post-accident. The official cause of death was recorded as multiple organ failure.

Four months later, in April 2000, his colleague Shinohara also died due to multiple organ failure, aged 40.

Supervisor Yokokawa, who had been at his desk when the criticality accident transpired, survived.

An inquiry by the Japanese government determined the incident had resulted from a lack of regulatory supervision, an insufficient safety culture and inadequate worker training.

Six officials from the company running the facility were subsequently charged with professional negligence and breaching nuclear safety regulations. In 2003, they received suspended prison sentences for their fatal negligence.