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Steve Connor: Contamination of water is the big headache for Tepco

Analysis: Pumping in fresh water has led to further build-ups of contaminated water

When fighting a fire, you don't worry about your water bills. When the Japanese power company Tepco was fighting a potential nuclear meltdown in its stricken reactors at Fukushima, it was not particularly worried about the huge volume of radioactively contaminated water created by the attempts to keep the reactors cool.

The priority in the aftermath of the devastating tsunami, a 14-metre-high wave that washed over the Fukushima site, was to avert a critical nuclear accident involving soaring temperatures within the steel-lined reactor vessels. Although some of the nuclear fuel rods in the three affected reactors have almost certainly melted, they do not seem to have breached the reactors themselves.

Tepco's priority now is to achieve a "cold shutdown", when the temperatures within the reactors are kept below 95C at atmospheric pressure. This prevents the reactors' coolant water from boiling even when the coolant systems are depressurised, a safer condition than an enforced hot shutdown.

Tepco has said that it would take about three months to stabilise the coolant systems of the three overheated Fukushima reactors. At the same time, the company must prevent the spent fuel rods within the four storage ponds from heating up. It would then take another three to six months to achieve cold shutdown.

This would be difficult enough on its own without having to contend with thousands of tonnes of contaminated water that has accumulated on the site. Some of the water used to keep the reactors cool has drained into the underground trenches used for carrying cables and pipes, which were flooded by the tsunami.

Ideally, the cooling water would be pumped around the reactors in a closed circuit but the company has not fixed the coolant systems completely, relying instead on pumping in fresh water, which leads to further build-ups of contaminated water, some of which has been discharged into the sea along with worryingly high levels of radiation.

The ultimate aim is to cool the reactors down, plug any radiation leaks and deal with the contaminated water. Only then can the serious business of decommissioning begin – a job likely to take at least 10 years.