An additional 24 researchers were exposed to an exotic soup of radioactive isotopes at the Hadron Experimental Facility in Tokai, Ibaraki Prefecture, last week, raising the tally to 30, the state-run Japan Atomic Energy Agency said late Sunday.
According to medical tests run on the 55 people involved in the troubled experiment Thursday at JAEA's Japan Proton Accelerator Research Complex (J-PARC), at least 19 escaped exposure, the agency said. Six others are waiting to be examined.
The radioactive substances reportedly included sodium, iodine and more exotic elements.
The highest dose delivered was 1.7 millisieverts. Japan's annual limit for nuclear workers is 50 millisieverts.
The accident occurred around noon Thursday, but the JAEA didn't report it to the nuclear regulatory authorities until Friday night.
The JAEA said Saturday it initially thought the leak was minor and had been confined to the laboratory when the alarm went off. Workers then switched on the ventilation system, sending the radioactive contaminants outside the building.
The experiment conducted by the High Energy Accelerator Research Organization was intended to generate elementary particles by zapping a gold-covered object with a proton beam. Officials at the group said they suspect one of the electromagnets used to control the protons malfunctioned, allowing an excessively strong beam to melt the gold and release the exotic particles.