Shinohara, 40, an employee at JCO Co.’s uranium-processing plant in Tokai, Ibaraki Prefecture, where the criticality accident occurred on Sept. 30, died at 7:25 a.m., doctors said.

Hisashi Ouchi, 35, another worker at the JCO plant exposed to a massive dose of radiation in the same accident, died Dec. 21 from multiple organ failure at the same hospital.

Doctors said Shinohara’s condition deteriorated earlier this week when his organs, including his lungs and kidneys, started to malfunction.

Shinohara was put on a respirator in late February when pneumonia coupled with radiation damage to his respiratory system left him unable to breathe. Doctors then placed him on a dialysis machine in mid-April when his kidneys failed.

Shinohara was exposed to an estimated 10 sieverts of radiation, about 10,000 times the maximum annual permissible exposure in Japan for people not working in nuclear or related facilities.

He and Ouchi reportedly sidestepped safety procedures and used a bucket to pour too much uranium into a processing tank, triggering a self-sustaining nuclear chain reaction.

Their supervisor, Yutaka Yokokawa, 55, who was in a room adjacent to where the two were working, was discharged from a hospital in December.

After the accident, Shinohara was first taken to the hospital of the National Institute of Radiological Sciences in Chiba but was later transferred to the University of Tokyo’s Institute of Medical Science in Minato Ward, Tokyo. ,soon afterward.

On Oct. 9, he underwent the world’s first transfusion of umbilical cord blood for someone suffering from radiation exposure, in an attempt to restore the blood-producing functions that had been damaged by the radiation. Umbilical cord blood contains stem cells that produce blood cells.

In early February, doctors at the Minato Ward institute, where Shinohara also received several skin grafts, said he had begun sitting up in bed and moving around in a wheelchair after regaining almost full capacity to produce blood.

But his condition worsened later that month when he developed respiratory problems and began to bleed in the stomach. He was transferred April 10 to the University of Tokyo Hospital in Bunkyo Ward where he was treated in an intensive care unit.

At least 439 people, including 207 local residents, were exposed to higher-than-normal levels of radiation in the accident. Only Ouchi, Shinohara and Yokokawa required immediate, long-term hospitalization.

Police are investigating JCO and its parent company, Sumitomo Metal Mining Co., in relation to the accident.

“We will redouble efforts to strengthen safety and disaster prevention measures,” Ibaraki Gov. Masaru Hashimoto said.

Prime Minister Yoshiro Mori promised Thursday to make every effort to ensure the safety of nuclear facilities, saying Shinohara’s death was “extremely regrettable.”

Science and Technology Agency chief Hirofumi Nakasone said he is “filled with deep regret.” The government will “renew its resolve and make more of an effort than it has been to prevent such an accident from ever happening again,” he said.