The Fukushima nuclear plant has more than 330,000 tonnes of water stored in pits, basements and tanks with varying levels of toxicity. The 300+ tonne water leak last week was from a tank similar to the others across the site. The Japanese Nuclear Regulation Authority is concerned that further spills may occur from the badly constructed tanks.
Why Fukushima is worse than you think – CNN
Q&A Fukushima Leak Problems – BBC
Fukushima’s toxic water pool grows as Tepco dithers – the Age
The operator of the plant (Tokyo Electric Power Company (TEPCO) have stated that 300 tonnes of radioactive water has leaked from a tank at the Fukushima Daiichi nuclear power plant. There is serious concern that it may contaminate the ground water and flow into the sea. Nuclear Regulators have raised the assessment of the leak to level 3 on the UN’s International Nuclear Event Scale (INES).
Information on the original Fukushima Daiichi nuclear disaster: Wikipedia