The operator of a Japanese nuclear plant that went into a tsunami-triggered meltdown knew the risks from highly radioactive water at the site but sent in crews without adequate protection or warnings, a worker says in a legal complaint.

The actions by Tokyo Electric Power Co led to radiation injuries, said the contract worker, who was with a six-member team working at the crippled Fukushima Dai-ichi plant's Unit 3 reactor in the early days of the 2011 crisis.

The worker gave a rare public account of what happened at the plant during the accident. He spoke on the condition that he be identified only as Shinichi, his given name.

Shinichi, 46, described a harrowing scene of darkness and fear, wading with headlamps into a flooded basement through steaming radioactive water that felt warm even through workers' boots. "It was outrageous. We shouldn't even have been there," he said.

He said his six-member team was sent to lay electric cables in the basement of the Unit 3 turbine on March 24, 10 days after its reactor building exploded, spewing massive amounts of radiation.

Their mission was to restore power to pumps to inject cooling water into its overheating spent-fuel pool.

Shinichi said TEPCO and its primary subcontractor never warned them even though water leaks had been found elsewhere at the plant.