Radioactive Material Leak Incident at the Hadron Experimental Facility of J-PARC

Abstract

In the Hadron (HD) Experimental Facility of J-PARC, a radioactivity leak incident occurred on May 23, 2013 because of a malfunction of the slow extraction system of the main ring (MR: 50 GeV synchrotron). At the time, a proton beam with abnormally high instantaneous power density was delivered to the target, and caused partial melting and evaporation of a gold target for experiments, and resulted in exposure of thirty-four workers in the experimental hall, and release of radioactive materials to outside of the controlled area of the HD hall. The incidence was, therefore, an event that should be notified to relevant authorities and local governments. The reporting, however, was delayed due to inappropriate emergency responses. The issues of the incident were summarized as follows: 1) Release of radioactive material to the environment outside of the controlled area, 2) Delay in reporting the incident to relevant authorities and local governments, and 3) Exposure of workers in the HD hall. A direct cause of the incident was a malfunction of the slow extraction system of MR. As described later, there were also failures in taking measures to prevent exposure and diffusion of radioactive materials, although the exposure and influence of the incident to the environment was not serious as noted in section 3. For these reasons, the incident was evaluated as Level 1 incidence in the International Nuclear and radiological Event Scale (INES), which means “Anomaly” with the words “lack of safety culture” by the Nuclear Regulation Authority of Japan (NRA). Accordingly, we thereafter employ the term “incident” in place of “accident” which has been used in the report thus far publicized from J-PARC [1-4]. For thoroughly reviewing the causes of the incident and the emergency responses in J-PARC, an external experts panel (EEP) was set-up by the two parent organizations of J-PARC, Japan Atomic Energy Agency (JAEA) and High Energy Accelerator