A worker at Japan's wrecked nuclear power plant has died after falling into a water storage tank, the latest in a growing number of accidents at the site still struggling to clean up the mess from the 2011 Fukushima disaster.

The operator of the Fukushima Dai-ichi plant said the 55-year-old man died of multiple injuries after plunging through an opening on top of a 10-metre high tank.

He was one of three workers inspecting the tank. Tokyo Electric Power Co, which operates the plant, said the cause of the accident is under investigation.

Currently, the plant's decades-long decommissioning involves nearly 7,000 workers. The number of injuries in the April to November period totalled 40 last year, compared with 12 a year earlier, underscoring growing concern about sloppy safety measures.

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