A worker at Japan's crippled Fukushima nuclear plant died Tuesday after falling into a water tank, the country's nuclear operator said, the second fatal accident to blight efforts to stabilise the tsunami-battered facility.

Separately on Tuesday, another worker died because of an incident at the Fukushima Daini nuclear power plant, which is located several kilometres (miles) south of the damaged plant, operator Tokyo Electric Power Co. (TEPCO) said.

The victim at the crippled Fukushima Daiichi plant, reportedly in his 50s, was inspecting an empty water tank with two other workers when he fell from the top of the 10-metre (33-feet) container, according to TEPCO.

The worker was trying to remove the lid of the giant tank by himself when he fell into the unit.

"He was wearing a harness, but the hook was found tucked inside the harness. This means the harness was not being used," said a TEPCO spokesman.

"We are investigating whether safety measures were appropriately observed," he added.

The tank is one of thousands at the site used to store rain water that may have picked up radiation at the battered site.

The worker is the second to die during efforts to stabilise the Fukushima Daiichi nuclear power plant after reactors went into meltdown in the aftermath of a huge tsunami in March 2011.

Last March, a worker died after being buried in earth and rubble while digging a hole.

While the earthquake and tsunami it caused four years ago killed more than 18,000 people, no one is officially recorded as having died as a direct result of radiation released by Fukushima's broken reactors.

In the unrelated incident at the Fukushima Daini plant, which was largely unscathed by the huge natural disaster, a worker died after suffering a severe head injury after being caught in equipment, a TEPCO spokesman said.

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Tokyo Electric Power