Japan's Nuclear Regulation Authority has upgraded the severity of the situation at Fukushima, reopening questions about how to deal with the radioactivity?
The contaminated water problem at Fukushima comes from the mountains. Every day, 400 tonnes of groundwater flows down from peaks overlooking the complex, invades the stricken reactor halls and is contaminated. At present, the Tokyo Electric Power Company (Tepco), which runs the plant, redirects the water over the reactor cores to keep them cool. After filtering to remove radioactive caesium, the water is stored in tanks. Huge volumes are being placed in 1060 tanks, each holding up to 1000 tonnes.
Tepco has drilled wells in the mountains to pump out and divert groundwater before it reaches Fukushima. It is even considering creating an "ice wall" around the complex by freezing water in soil.
More prosaically, in March, the company installed new filtering equipment. The advanced liquid processing system (ALPS) filters out caesium and 60 other isotopes. The IAEA says such filtering offers the best hope for cleaning water to a standard fit for dumping at sea. The tanks would then be used for more concentrated waste.
But Tepco halted tests on ALPS this month after corrosion holes developed in an associated tank. It says tests won't resume until December.
"Anything they can do to remove the more dangerous compounds and dilute the others is almost the only solution," says Ken Buesseler of Woods Hole Oceanographic Institution in Massachusetts. Ken says the Kanda estimate is probably the best he is aware of, and closely matches figures released on 21 August by Tepco, of 0.1 to 0.6 TBq per month for caesium-137 and 0.1 to 0.3 for strontium.
He points out that the north Pacific contains an estimated 100,000 TBq of caesium-137 from H-bomb testing in the 1960s, so the fallout from Fukushima is adding only a fraction of that. Total discharges from the Sellafield nuclear plant in the UK released 39,000 TBq over 40 years, he says.
Buesseler says that during his own sampling survey in waters 30 to 600 kilometres from Fukushima in June 2011, three months after the meltdown, the highest levels he found were 3 Bq of caesium-137 per litre of seawater. By comparison, the natural weathering of rocks results in about 10 Bq of radioactive potassium-40 making it into each litre of seawater.
On an international level, even if all the waste from Fukushima was dumped neat into the Pacific, dilution would eliminate any radiation risks to distant countries like the US, says Simon Boxall of the National Oceanography Centre in Southampton, UK.