What is tritium?

TOKYO • Tritium, an isotope of hydrogen, exists naturally and is found in rainwater, seawater and even the inside of the human body in the form of "tritiated water".

The International Atomic Energy Agency (IAEA) said that such tritiated water poses little to no risk to human health even if one drinks it, so long as the concentration is low, because the tritium would soon be excreted.

There is also no risk if the water comes into contact with skin.

Tritiated water is routinely discharged into the oceans as a by-product of nuclear plants in many countries, including the United States, China, France and South Korea.

But Japan, wary of the fears linked to the Fukushima Daiichi nuclear power plant, is taking extra steps. It has pledged to dilute the water to be released into the Pacific Ocean to 2.5 per cent of the tritium limits required under international standards for sea discharge.

This level is equivalent to one-seventh of World Health Organisation guidelines for tritium levels in safe drinking water. Drinking up to two litres of tritiated water a day is said to still be well below the amount of radiation exposure that would affect one's health.

The discharge process in Japan is expected to take three decades. But United Nations calculations show that even if the entire volume of treated water is discharged in one year, the impact would be no more than 0.1 per cent of the background natural radiation in Japan, at 2.1 millisieverts per year.

This is lower than the global average of 2.4 millisieverts per year.

IAEA director-general Rafael Grossi has stressed the scientific analysis behind the decision to discharge the water, adding that it would not affect the environment.

The move marks a key step in the plan to complete the decommissioning of the Fukushima Daiichi plant between 2041 and 2051.