The operator of a troubled nuclear power plant in north-eastern Japan started to transfer highly radioactive water to a nearby storage vessel, news reports said Wednesday.

A series of strong temblors on Monday and Tuesday caused a delay in the pumping operation, but Tokyo Electric Power Co, which runs the Fukushima Daiichi Nuclear Power Station, started at 7:30 pm (1030 GMT) Tuesday to pump out of a reactor building, where the radiation-contaminated water has hindered workers' efforts to contain the crisis.

The plant was crippled by the March 11 earthquake and ensuing tsunami. Some 700 tons are to be moved into a condenser where, in normal operations, steam created from the reactor is cooled down. The pumping is expected to take 40 hours, Kyodo News reported.

The operations require considerable time as workers need to transfer some 60,000 tons of contaminated water, collected in the basements of the turbine buildings of reactors 1, 2 and 3, as well as the trenches linked to them.

The government decided to ban shipments of shiitake mushrooms grown outdoors in 16 municipalities near the plant as high levels of radioactivity were detected in the produce, Chief Cabinet Secretary Yukio Edano told a news conference. Edano added that all shiitake mushrooms grown indoors in the areas are safe.

Japan's Jiji Press reported that Gregory Jaczko, chairman of the United States Nuclear Regulatory Commission, said the plant was not yet stable.

External power sources to water pumps at reactors were lost about 50 minutes after a strong quake on Monday, Jaczko said. Jaczko warned that if the ability to cool the reactor cores is lost, Japan will have to face the possibility of “a further degradation in the fuel, which could lead to possibly a greater release (of radioactive substances) than what's going on.”

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