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The workers were ordered to leave the facility after the level of radiation at the plant soared to 10 millisievert per hour – above the level considered harmful to human health Read the latest updates in the Guardian’s live blog Workers battling to prevent nuclear meltdown at the stricken Fukushima Daiichi plant were temporarily evacuated on Wednesday morning after radiation levels became too dangerous for them to remain. The withdrawal hampered efforts to secure the safety at the atomic power plant and avert a major radiation leak. Its operator, Tokyo Electric Power [Tepco] said it was considering using helicopters to spray the crippled No. 4 reactor with water and boric acid – a fire retardant – in an attempt to prevent more radiation leaks. The 50 or so engineers, working around the clock in harsh conditions, spent the morning trying to put out a fire at one reactor and to cool others at risk of overheating and reaching criticality. The government’s chief spokesman, Yukio Edano, said Japan was considering seeking help from the US military. All six of the plants reactors are experiencing problems following last Friday’s earthquake and tsunami, in which an estimated 10,000 people have died. The workers were ordered to leave the facility after the level of radiation at the plant soared to 10 millisievert per hour – above the level considered harmful to human health – possibly as a result of radioactive substances being emitted from the No. 2 reactor. The reading later fell to around 6 millisievert per hour, reports said. The evacuation followed another day of crisis at the plant, which has become the focus of the world’s attention, even as rescue workers sift through the damage caused by the tsunami along a vast stretch of Japan’s northeast coast. Earlier, officials from the nuclear and industrial safety agency said that 70% of fuel rods at the No. 1 reactor had been significantly damaged, as well as 33% of rods at the No. 2 reactor. The cores of both reactors are believed to have partially melted, Kyodo news agency said. “We don’t know the nature of the damage,” said Minoru Ohgoda, spokesman for the country’s nuclear safety agency. “It could be either melting, or there might be some holes in them.” Before they were moved to safety the workers had been trying to cool spent nuclear fuel pools at the No. 5 and No. 6 reactors, where temperatures have risen above normal levels. To compound the workers’ problems, a fire broke out at the No. 3 reactor, where a fuel storage pool has overheated and may have let off radioactive steam. Live TV footage showing a large cloud of light grey smoke rising above the plant. Edano said that there was “a possibility that the No. 3 reactor’s containment vessel is damaged”. A blaze also broke out again at the No. 4 reactor, which was already feared to be at risk of leaking radioactivity. The nuclear safety agency reported that flames and smoke were no longer visible half an hour later, but were unable to confirm that the fire had been extinguished. The No. 4 reactor is an increasing cause for concern. Tepco believes that the storage pool may be boiling, raising the possibility that exposed rods will reach criticality. “The possibility of re-criticality is not zero,” a Tepco spokesman said. Yesterday the government ordered 140,000 people living within a 30-kilometre radius of the plant to remain indoors after a spike in radiation levels. A further 70,000 residents had already been moved to safe distances. The government said it had no immediate plans to widen the evacuation zone. The crisis unfolding in Fukushima continued to raise anxiety levels in Tokyo, 150 kilometres to the south. Radiation levels in the capital was 10 times higher than normal on Tuesday evening, but posed no health hazard, the government said. The meteorological agency said winds near the power plant would blow from the northwest and out into the Pacific Ocean on Wednesday. The winds were expected to strengthen in the afternoon, the agency added. Japan earthquake and tsunami Japan Nuclear power Nuclear waste Natural disasters and extreme weather Justin McCurry guardian.co.uk