FUKUSHIMA DAIICHI ACCIDENT
Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident beginning on 11 March 2011. All three cores largely melted in the first three days.
The nuclear power cycle uses water in three major ways: extracting and processing uranium fuel, producing electricity, and controlling wastes and risks. In addition to cooling the steam, nuclear power plants also use water in a way that no other plant does: to keep the reactor core and used fuel rods cool. To avoid potentially catastrophic failure, these systems need to be kept running at all times, even when the plant is closed for refueling.
CONTAMINATED WATER
Tepco has been struggling with the build-up of contaminated water since bringing three reactors under control after a 2011 earthquake and tsunami knocked out electricity and cooling. The company has been using a makeshift system of pumps and piping to inject water into damaged reactor vessels to keep melted uranium fuel rods cool.
Efforts to tackle the problem have included building an "ice wall" around the damaged reactors and wells to draw groundwater away before it reaches the reactors. These measures have slowed, but not halted, the buildup of contaminated water.
WATER RELEASE
Tepco plans to filter the contaminated water again to remove isotopes, leaving only tritium, a radioactive isotope of hydrogen hard to separate from water. Tepco will then dilute the water until tritium levels fall below regulatory limits, before pumping it directly into the ocean from the coastal site.
Water containing tritium is routinely released from nuclear plants around the world and releasing the Fukushima water to the ocean is supported by regulatory authorities. Tritium is considered to be relatively harmless because it does not emit enough energy to penetrate human skin. But when ingested it can raise cancer risks, a Scientific American article said in 2014.
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