A 300-metric-ton leak from a tank filled with contaminated water has been labeled the most severe incident at Fukushima Daiichi since its damaged reactors were brought under control in late 2011.

Developments in the last week at the Tokyo Electric Power Co. plant severely damaged by the Japanese earthquake and tsunami include:

Tank leak at Fukushima. Source: TEPCOStorage Tank Leaks

On Monday, TEPCO announced a worker had found water pooled at the base of an above-ground tank. In a release, the company said the drop in the tank's water level indicated about 300 metric tons had leaked, although it could not identify where the leak was located.

The tank held water that had been used to cool damaged reactors, the Asahi Shimbun reported, and readings detected a dose of 100 millisieverts per hour at the surface of water on the ground.

INES Level 3

Japan's Nuclear Regulation Authority initially classified the leak as a level-1 anomaly on the International Nuclear and Radiological Event Scale. But two days later the NRA upped the severity classification to a level-3 serious incident. (The 2011 accidents at Fukushima were rated level 7.) The Asahi reported that NRA estimates placed the total amount of contamination in the leaked water at 24 trillion becquerels. Later in the week, TEPCO also acknowledged it found evidence some of the water might have reached the sea via a nearby ditch.

Inspections Show Radiation Hotspots, No Leaks in Other Tanks

The faulty tank is among 1,000 assembled near unit 4. On Thursday, TEPCO said in a release that workers had inspected tanks with the same design and did not find visible signs of other leaks. They did, though, find elevated radiation rates at 70 millisieverts per hour and 100 millisieverts per hour near the bottom flanges of two tanks.