EFFECTS

Tepco filters the Fukushima water through its Advanced Liquid Processing System (ALPS), which reduces most radioactive substances to acceptable safety standards, apart from tritium and carbon-14.

Tritium and carbon-14 are, respectively, radioactive forms of hydrogen and carbon, and are difficult to separate from water. They are widely present in the natural environment, water and even in humans, as they are formed in the Earth's atmosphere and can enter the water cycle.

Both emit very low levels of radiation, but can pose a risk if consumed in large quantities.

The filtered water goes through another treatment, and is then diluted with seawater to reduce the remaining substances' concentrations, before it is released into the ocean via a 1km underground tunnel. Tepco will monitor the radioactivity of the processed water at various stages as well as the ocean water at the discharge site.

A system of emergency valves will ensure no undiluted waste water is accidentally released, says Tepco, and staff can also manually shut down the discharge quickly in case of a tsunami or earthquake.

Japan's government says the final level of tritium - about 1,500 becquerels per litre - is much safer than the level required by regulators for nuclear waste discharge, or by the World Health Organization for drinking water. Tepco has said the carbon-14 level would also meet standards.

Tepco and the Japanese government have conducted studies to show the discharged water will present little risk to humans and marine life.

Mark Foreman, an expert in nuclear chemistry with Chalmers University of Technology in Sweden, said the water would not make the sea much more radioactive than it already is.

He said the impact of the annual radiation doses from the discharge into the ocean were lower than dental x-rays or mammograms - even for those who eat a lot of seafood.

"The water released will be a drop in the ocean, both in terms of volume and radioactivity. There is no evidence that these extremely low levels of radioisotopes have a detrimental health effect," said molecular pathology expert Gerry Thomas.