Tokyo Electric Power Co (TEPCO) said that the hole was probably caused by a meltdown of fuel rods after the plant was struck by a powerful earthquake and tsunami in March that knocked out the cooling system.
Workers have been pumping in water to cool down the reactors since the disaster, but discovered earlier this week that the water level in the vessel containing the uranium fuel rods at the No. 1 reactor had plummeted well below the amount needed to cover the fuel.
Junichi Matsumoto, a general manager at TEPCO, told a news conference yesterday that there was probably a leak several centimeters across and that it was very likely the rods had been entirely exposed to the air.
"There must be a large leak," he said. "The fuel pellets likely melted and fell, and in the process may have damaged ... the pressure vessel itself and created a hole."
Matsumoto explained that pumping in water had kept the temperature between 100 and 120 degrees Celsius, and that TEPCO will continue with this policy even at the risk of more radioactivity leaks.
Around 10,400 tonnes of water have been pumped into the reactor so far, but the authorities are unable to assess where any leaked water is going because of the high radiation levels at the site.