The level of iodine-131, reported a few hundred metres (yards) south of its southern water outlet has risen in a series of tests since last week, carried out by plant operator the Tokyo Electric Power Company (TEPCO).

Previous readings there were 1,250 times the legal maximum on Friday, 1,850 times the limit on Saturday and 3,355 times the limit on Tuesday. A 9.0 magnitude earthquake and tsunami on March 11 knocked out the cooling systems of the Fukushima plant's six reactors -- triggering explosions and fires, releasing radiation and sparking global fears of a widening disaster.