TOKYO -- A new estimate claims it will take around 17 years to send treated radioactive water, which is currently stored at the Fukushima Daiichi Nuclear Power Station site as a consequence of the 2011 nuclear disaster, into the sea.

The figures came from calculations by Hiroshi Miyano, visiting professor in nuclear engineering at Hosei University, and head of the investigative committee on the decommissioning of the Fukushima Daiichi Nuclear Power Station at the Atomic Energy Society of Japan.

While the Ministry of Economy, Trade and Industry intends to resume discussions on the methods for disposal of treated water at a subcommittee of experts as early as next month, Miyano called for his estimate to be utilized in calm and quick discussions on the issue.

Miyano drew his attention to auxiliary cooling seawater pumps based on the premises of the nuclear power station, which was severely damaged by the March 2011 earthquakes and tsunamis and the subsequent meltdown they caused.

There are three operational pumps each at the No. 5 and 6 reactors, which are for pumping up seawater to cool apparatus at the nuclear plant. They can individually process 1,800 metric tons of water per hour at the No. 5 reactor, and 2,800 tons of water per hour at the No. 6 reactor.

At nuclear plants across the country, radioactive water containing tritium, which is difficult to remove, is released after being diluted to contain 60,000 becquerels or less of radioactive substances per liter of water, in accordance with criteria set under the Act on the Regulation of Nuclear Source Material, Nuclear Fuel Material and Reactors.

At the Fukushima Daiichi Nuclear Power Station, where decommissioning work is underway, however, stricter standards are being applied, with water such as rainwater accumulated in trenches outside reactor buildings released after being diluted to contain less than 1,500 becquerels of radioactive substances per liter of water.

At present, the nuclear power station site is host to over 950 storage tanks, which together hold more than 1.05 million tons of treated water containing tritium. Were the stricter rules on dilution to be applied, the treated water would amount to some 699 million tons.

The calculations by Miyano were based on the pumps at the No. 5 and 6 reactors working alternately but without interruption for 24 hours a day, 365 days a year. Under such conditions, it is expected that all of the water would be released after about 17 years and 4 months.

The Ministry of Economy, Trade and Industry is investigating a range of potential disposal methods, from releasing the water into the sea, to vaporizing the liquid or storing it for an extended period of time.

In 2016, the ministry came up with an estimate that releasing the water into the sea would take somewhere between 7 years and one month and 7 years and four months, using different preconditions from those employed by Miyano.

Miyano said, "At some point the tanks will deteriorate. This estimate is ultimately just one example, and it could change depending on conditions. But it does show that processing the water is going to take a long time. It's time to have a calm discussion."

(Japanese original by Suzuko Araki, Riki Iwama and Tomoki Okuyama, Science & Environment News Department)