CAUSES

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.