A small quantity of radioactive water has leaked from a storage tank at Japan's tsunami-crippled Fukushima Daiichi nuclear power plant, Tokyo daily Asahi Shimbun reported Saturday.

A worker at the facility, located in northeastern Japan, found a wet patch measuring 20 sq. centimeters (3 sq. inches) under a storage tank for radiation-contaminated water on Friday morning, Tokyo Electric Power Co. the plant's operator, said.

Seventy microsieverts per hour of beta-ray-emitting radioactivity, far exceeding the recommended maximum exposure of 0.11 microsieverts per hour, were detected on the surface where the water had leaked.

Workers placed sandbags around the tank to prevent the radioactive water from spreading to other areas, the newspaper said.

The leak was detected on the same day that tests were launched in preparation for the eventual construction of a 1.5-kilometer-long (0.9-mile) frozen soil wall around the No. 1 to No. 4 reactor buildings, a project aimed at preventing further leaks of radioactive water into the sea.

A March 11, 2011, earthquake and subsequent tsunami crippled the Fukushima facility and triggered the worst nuclear disaster since the 1986 meltdown of the Chernobyl plant in Ukraine.

More than four years after the nuclear disaster, 70,000 people who lived near the Fukushima plant still cannot return to their homes due to high levels of radiation, which also has severely affected local farming, cattle-raising and fishing. EFE