A partial meltdown caused high levels of radioactive materials detected in water at a reactor at an earthquake- and tsunami-damaged nuclear power plant in north-eastern Japan, the government said today.

The partial meltdown of spent fuel rods was believed to be temporary at reactor 2 at the six-reactor plant in Fukushima, 250km north-east of Tokyo, chief cabinet secretary Yukio Edano said without elaborating.

The level of the radiation-contaminated water found in an adjacent turbine building of reactor 2 measured more than 1,000 millisieverts per hour.

japan nuclear fukushima no 3 reactor explosion image The legal limit of exposure to radiaction for plant workers is 100 millisieverts per shift although it has been raised to 250 millisieverts during the ongoing crisis, which began March 11 when a magnitude-9 quake and tsunami caused the cooling systems of all six reactors to fail.

Radiation-contaminated water had been found in the adjacent turbine buildings of three other reactors: 1, 3 and 4.

The contamination hampered the plant operator's work to restore power and cooling functions at the reactors to avert a potential disaster.

Tokyo Electric Power Co (Tepco) was trying to clear the radiation-contaminated water as quickly as possible to reduce the risks of more workers being exposed to radiation. Workers must access

the overheating reactors to restore key cooling systems.

On Thursday, three engineers working on the cable system at reactor 3 were exposed to radiation levels in water that were 10,000 times above normal.

The National Institute of Radiological Sciences said yesterday that the three workers would be reportedly discharged from a radiation treatment facility affiliated with the institute as early as this afternoon.

No health problems had been found in the three workers, the institute said.

The number of workers who have been exposed to radiation exceeding 100 millisieverts at the plant came to 19, the Kyodo News agency reported, citing Tepco.

Meanwhile, a magnitude-6.5 aftershock jolted north-eastern Japan early today, but Tepco said the tremor was not likely to affect its work at the plant.

The official death toll from the March 11 quake and tsunami stood at 10,872 today with 16,244 people listed as missing, Japan's National Police Agency said.

- dpa