The nuclear plant was a boiling-water reactor with an output of 0.54 million kW. The component affected was a steam piping (inside diameter: 15 cm; thickness: 1.1 cm) of a heat exchanger used for removing residual heat from the reactor. The piping was broken during a test of a high pressure core injection system for cooling water provided for an emergency core cooling system. The incident resulted in a leakage of radioactive gases. The part of the piping broken was located on the 2nd floor of the building, but there were no workers there. However, there were 36 workers in the radioactivity control area inside the building. A heat insulating material was wrapped round the piping and was not part of the periodical inspections. Nuclear safety and security workers reported that the part of the piping was turned up from inside and completely broken. From the broken surface, a hydrogen gas remaining in the piping apparently got ignited spontaneously by the catalyst action of platinum particles in the system, thus resulting in an explosion.