A worker at Japan's wrecked nuclear power plant 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 worker, 55, died of multiple injuries early Tuesday after falling through an opening atop a 10-meter (33-foot) high tank. He was one of three men inspecting the tank. Tokyo Electric Power Co., which operates the plant, says 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 April-November totaled 40 last year, compared to 12 a year earlier, underscoring growing concern about sloppy safety measures.