Japan Atomic Energy Agency (JAEA), who operates J-PARC, didn't bother to notify the Nuclear Regulatory Agency (secretariat of the Nuclear Regulatory Authority, mostly made of people from NISA) for 2 days, because they did not think there was any leak of radioactive materials.
According to Asahi, the researchers suffer internal radiation of exposure of 1.7 millisievert, and the contamination of the facility is 40 becquerels per square centimeter.
Apparently, the Ibaraki prefectural government was not too happy about the incident and the fact that JAEA didn't bother to notify the authority and the prefectural government, and held an angry press conference at 2AM on May 25.
The official story of the accident, from Asahi Shinbun (5/25/2013, 1:37AM):
Radioactive materials leak at JAEA facility, 4 researchers suffer internal radiation exposure
Japan Atomic Energy Agency disclosed on May 25 that there was a leak of radioactive materials outside the nuclear and particle research facility at J-PARC in Tokai-mura in Ibaraki Prefecture. At least 4 researchers inside the facility suffered internal radiation exposure. The leak has stopped, and there is no danger of the leak spreading further. The maximum radiation exposure was 1.7 millisievert, according to JAEA. JAEA is currently investigating how much radioactive materials have leaked outside the facility.
According to JAEA and the Nuclear Regulatory Authority, the leak took place at 11:55AM on May 23. They were conducting an experiment of generating elementary particles by hitting gold with proton beam. The equipment malfunctioned, and the output of the proton beam increased to 400 times the normal strength, and gold was vaporized. Radioactive materials [radioactive gold vapor] leaked [from the equipment], and were inhaled by the researchers. There were about 30 researchers when the accident happened.
Contamination of the facility is about 40 becquerels per square centimeter maximum. Currently the facility is off-limit. At the time of the accident, JAEA didn't think there was a leak of radioactive materials, and did not report to the government. J-PARC is a facility to study elementary particles with large accelerators.
Equipment malfunctioned. That's the official story.
Let us now turn to "baseless rumors" among knowledgeable researchers (like this one, from Professor Hayano of Tokyo University) for what may really have happened. According to the unverified rumors, it was not the equipment that malfunctioned. Researchers made a mistake, and instead of hitting the target (gold) with low, continuous proton beam over a long period of time they accidentally hit it with high-intensity proton pulses.
According to NHK quoting the Ibaraki prefectural government, the monitoring post outside the facility showed a slight increase in radiation level between 3PM and 6PM on May 23. The radiation level is normally between 70 to 130 nanogray/hour, but it rose by 10 nanogray/hour on May 23.
Never trust the Japanese with a gigantic facility like a proton accelerator. Or a nuclear reactor. They are not only incapable of operating them properly, but when an accident happens they don't tell you.
(How do you get rid of radioactive gold in the body? Does anyone know?)