On 12 May, TEPCO engineers confirmed that a meltdown occurred, with molten fuel having fallen to the bottom of the reactor’s containment vessel.
The utility said that fuel rods of the No. 1 reactor are fully exposed, with the water level 1 meter (3.3 feet) below the base of the fuel assembly. According to a Japanese press report, there are holes in the base of the pressure vessel, and most of the fuel has probably melted.
The nuclear fuel has possibly leaked into the containment vessel, which was damaged in an explosion during the crisis. This caused both the Japanese government and TEPCO to be criticized for consistently underestimating the severity of the situation.
The operator found the basement flooded with 4.2 meters of water. Workers were unable to observe the flooding situation due to high levels of radiation from the water.
TEPCO estimates the nuclear fuel was exposed to the air less than five hours after the earthquake struck. Fuel rods melted away rapidly as the temperature inside the core reached 2,800 °C within six hours.
In less than 16 hours, the reactor core melted and dropped to the bottom of the pressure vessel, burning a hole through the vessel. By that time, water was pumped into the reactor in an effort to prevent the worst-case scenario – overheating fuel melting its way through the containment and discharging large amounts of radionuclides in the environment.
In June the Japanese government confirmed that Unit 1 reactor vessel containment was breached, and pumped cooling water continues to leak months after the disaster.