Tokyo: Issuing data about the amount of highly radioactive water that leaked into the Pacific Ocean from the crisis-hit Fukushima Daiichi nuclear plant in early April, Tokyo Electric Power Co (TEPCO) has said that the leak contained an estimated 5,000 terabecquerels of radioactive substances, 20,000 times the annual allowable limit for the plant, adding that the leak amounted to 520 tons.

It is the first time that the plant operator has issued data on the level of radioactive materials contained in the leak, which the utility estimated to have lasted for six days through April 6. But the actual scale of the leak may have been larger because a greater amount of contamination was detected in the sea from late March.

The country's worst nuclear crisis, triggered by the massive March 11 earthquake and tsunami, continued to affect residents near the plant in Fukushima Prefecture, with the government deciding the same day to legally enforce a no-entry zone within a 20-kilometre radius of the plant.

But the government said it would basically allow residents living in the 3 to 20 km-radius zone to return home for up to around two hours, given that many are hoping to collect belongings and inspect their homes after being forced to evacuate.

The science ministry announced the same day that radiation levels of over 100 microsieverts per hour were measured at four locations 2 to 3 km from the plant from late last month. The readings compare with the annual allowable limit of 1,000 microsieverts, excluding medical care, set by the Japanese government for ordinary people.

Many residents are likely to continue their life as evacuees for a prolonged period, although workers at the six-reactor nuclear complex are trying hard to bring under control the troubled number 1 to 4 reactors, which have lost their key cooling functions.

To make headway in efforts to restore the plant, the utility firm known as TEPCO on Tuesday started a nearly one-month operation to move highly radioactive water from in and around the No. 2 reactor turbine building to a nuclear waste disposal facility at the site.

Hidehiko Nishiyama, a spokesman of the government's Nuclear and Industrial Safety Agency, told a press conference on Thursday that the work is proceeding "smoothly." The operation to remove the contaminated water is also important to prevent it from further leaking into the sea.

The leak of highly radioactive water detected earlier in the month was confirmed as flowing from near the No. 2 reactor water intake on April 2 and was plugged on April 6 by means of a chemical agent.

TEPCO assumed that the contaminated water flowed into the sea at about 4.3 tons per hour between April 1 and April 6.