Fukushima: Radioactive water leak explained
21 August 2013 Last updated at 13:49 BST
Japan's nuclear agency wants to raise the severity level of a radioactive water leak at the Fukushima plant from one to three on an international scale.
Fukushima Daiichi nuclear plant continues producing 400 tonnes of contaminated water every day, and that excess water is stored in tanks on the site.
The plant operator Tokyo Electric Power Company (Tepco) suspects that 300 tonnes has probably leaked out of one of the tanks into the ground.
Joanna Gosling reports on the scale of the problem.