The Tokyo Electric Power Company says radioactive substances that leaked into the sea at the damaged Fukushima plant over six days in April exceed the annual allowable limit by 20,000 times.
The plant operator said it calculated the total amount of leaked water assuming that the leak began on April 1st. Some 520 tonnes of the high-level radioactive water have likely reached the sea during the period, reports Japan's Broadcasting Corporation NHK.
The announcement comes as the government brought into effect the entry ban for the 20-kilometer evacuation area around the paralyzed nuclear complex. Police found more than 60 families still living within the contaminated area.
However, the authorities would arrange brief visits for residents, allowing them to return for up to two hours to collect essential belongings.
Meanwhile TEPCO has released multiple videos showing two PackBots navigating inside Fukushima reactors measuring radiation levels.
The plant, crippled by both the earthquake and massive tsunami, has been hit by a series of explosions and radiation leaks, for nearly six weeks.