South Korean activists have criticised Japan's plan, saying it will contaminate the ocean By Tessa Wong

Japan’s controversial plan to release treated waste water from the Fukushima nuclear plant into the Pacific Ocean has sparked anxiety and anger at home and abroad.

Since the 2011 tsunami which severely damaged the plant, more than a million tonnes of treated waste water has accumulated there. Japan began discharging it on 24 August, in a process that will take 30 years to complete.

Despite an endorsement from the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), the plan has been deeply controversial in Japan with local communities expressing concerns about contamination.

Fishing industry groups in Japan and the wider region are also worried about their livelihoods, as they fear consumers will avoid buying seafood.

China has accused Japan of treating the ocean as its “private sewer”, and criticised the IAEA of being “one-sided”. While South Korea’s government has said it has no objections to the plan, many of its citizens are opposed to it.

So what is Japan’s plan and how exactly has it churned the waters?

What is Japan doing with the nuclear waste water?

Since the disaster, power plant company Tepco has been pumping in water to cool down the Fukushima nuclear reactors’ fuel rods. This means every day the plant produces contaminated water, which is stored in massive tanks.

More than 1,000 tanks have been filled, and Japan says that it needs the land occupied by the tanks to build new facilities to safely decommission the plant. It has also pointed out concerns that the tanks could collapse in a natural disaster.

Releasing treated waste water into the ocean is a routine practice for nuclear plants – though critics have pointed out that the amount from Fukushima is on an unprecedented, far vaster scale.

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.

Many scientists have also backed the plan.

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.