Though only revealed on the weekend, the actual accident occurred on Thursday, May 23, 2013 at a lab in Tokaimura, Ibrakai-ken, according to the Japan Atomic Energy Agency (JAEA).
According to the JAEA, the researchers were performing an experiment whereby they fires proton beam at gold to generate particles. That''s when something went wrong , and the four were exposed up to 1.6 millisieverts of radiation.
Now... the good news is that no one needed medical attention (yet?) even though a safe limit of one millisievert per year is an accepted maximum, according to the International Commission of Radiological Protection.
That same Institute, however also says that exposure to less than 100 millisieverts per year presents no statistically significant increase in cancer risk.
For reference, just one CT (Computed Tomography or CAT - Computer Axial Tomography) scan at a hospital will give the patient about 10 millisieverts.
Despite the JAEA saying there was no widespread radiation concern, it notes that there were 51 other researchers and workers at the facility and may also have been exposed to the radiation.
Radiation leaked from the facility to the outer atmosphere after workers used fans to lower the radiation levels in the laboratory. That's called 'venting', by the way.
I suppose in this case, 'no widespread radiation' implies it did not leave the facility (it did!) and affect anyone nearby outside the whole area. The JAEA says that the fans should not have been used, just in case it posed a problem to the outside world.
Jeez... doesn't anyone know how to behave when there's a radiation problem?
Regulations stipulate that all incidents be reported to the Nuclear Regulation Authority (NRA) and the prefectural government within 24 hours.
It should be noted, however, that despite the accident occurring on Thursday, it was not reported until very late on Friday - well past the 24 hour mark.
An investigation by the NRA is pending into the cause and handling of the accident.