The operator of Japan's crippled Fukushima nuclear plant said yesterday that four tonnes of rainwater contaminated with low levels of radiation leaked during an operation to transfer the water between tank holding areas. Tokyo Electric Power Co, or Tepco, has been trying to contain contaminated water at the Fukushima site after it found 300 tonnes of radioactive water had leaked from a tank at the plant. Fukushima suffered triple nuclear meltdowns and hydrogen explosions after a March 2011 earthquake and tsunami. Heavy rain during a recent typhoon flooded one of the tank holding areas where Tepco stores excess water flushed over damaged reactors to keep them cool, a spokesman said. After tests last month showed the rainwater contained 160 becquerels per litre of radiation, a relatively low level, Tepco officials decided to transfer the water to another holding area for tanks, he said. During the transfer a worker found the leak, which the company estimated to be four tonnes and was absorbed into the ground, the spokesman said. The company faces the prospect of more heavy rain as another storm approaches Japan . Tropical Depression Sepat is forecast to gain strength overnight and arrive in the vicinity of Fukushima by noon today.