TOKYO: A worker at Japan's crippled Fukushima Daiichi nuclear plant has died of a heart attack, the operator said Thursday. It is the fifth death at the power station since it was hit by the tsunami of March 2011.

Tokyo Electric Power Company (TEPCO) said the man, who was in his 50s, suffered a cardiac arrest on Wednesday while working on the installation of a tank to store contaminated water.

He was confirmed dead by hospital doctors, company spokesman Jun Oshima said, adding it was not believed radiation from the broken reactors had played a part.

“As far as we know, he is the fifth person to have died after falling sick during work at the plant since the accident,” Oshima said.

“It doesn't seem that there was a causal link between his death and radiation because he died of a heart attack,” he said.

The cumulative radiation dose the worker received was measured at 25.24 millisieverts, Oshima said.

Under Japanese regulations, nuclear plant workers can be exposed to a maximum 50 millisieverts annually and 100 millisieverts total in five years.

When asked whether the fatality rate at Fukushima Daiichi was higher than at other nuclear plants, Oshima said direct comparisons were difficult to make, citing the large number of employees and the different nature of the work.

About 3,000 workers are engaged in decommissioning the crippled plant. Much of the work is physically demanding construction work, in contrast to the less exacting operation and inspection required at functioning nuclear plants, the spokesman said.

The quake-sparked tsunami of March 2011 knocked out cooling systems at the Fukushima Daiichi nuclear power plant, causing three reactors to go into meltdown in the world's worst atomic disaster for 25 years.

Nobody is officially recorded as having died as a direct result of the radiation released in the disaster.