Water that the Japanese Government is planning to release into the Pacific Ocean from the crippled Fukushima nuclear plant contains radioactive material well above legally permitted levels, according to the plant's operator and documents.

Authorities are running out of space to store contaminated water that has come into contact with fuel that escaped from three nuclear reactors after the plant was destroyed in the earthquake and tsunami that struck north-east Japan in March 2011.

The plan to release just over a million tons of water, currently stored in 900 tanks, into the Pacific has angered local residents and environmental organisations, as well as groups in South Korea and Taiwan fearful that radioactivity might wash up on their shores.

Tokyo Electric Power Co (Tepco), which runs the plant, has until recently claimed that the only significant contaminant in the water is safe levels of tritium, which can be found in small amounts in drinking water, but is dangerous in large amounts.

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The Government has promised that all other radioactive material is being reduced to "non-detect" levels by the sophisticated Advanced Liquid Processing System (ALPS) operated by the nuclear arm of Hitachi Ltd.

However, documents shown to the Daily Telegraph by a Japanese government source suggest the ALPS has consistently failed to eliminate a cocktail of other radioactive elements.

Hitachi declined to comment on the reports on the performance of its equipment. The Japanese Government did not reply to requests for comment.

A document passed to the Daily Telegraph from the Japanese Government arm responsible for responding to the Fukushima collapse indicates authorities were aware that the ALPS facility was not eliminating radionuclides to "non-detect" levels.

That adds to reports of a study by Kahoko Shinpo, a regional newspaper, which it said confirmed levels of iodine-129 and ruthenium-106 exceeded acceptable levels in 45 samples out of 84 in 2017.

Storage tanks for contaminated water at the tsunami-crippled Fukushima nuclear power plant.
Storage tanks for contaminated water at the tsunami-crippled Fukushima nuclear power plant.

Iodine-129 has a half-life of 15.7 million years and can cause cancer of the thyroid; ruthenium-106 is produced by nuclear fission and high doses can be toxic and carcinogenic when ingested.

In late September, Tepco was forced to admit that around 80 per cent of the water stored at the Fukushima site still contains radioactive substances above legal levels.

It admitted that levels of strontium-90, for example, are more than 100 times above legally permitted measure in 65,000 tons of water that has been through the ALPS cleansing system and are 20,000 times above levels set by the government in several storage tanks at the site.

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Dr Ken Buesseler, a marine chemistry scientist with the US Woods Hole Oceanographic Institution, told the Daily Telegraph: "Until we know what is in each tank for the different radionuclides, it is hard to evaluate any plan for the release of the water and expected impacts on the ocean."